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Evaluation of Water Quality in the Langford Valley–Irwin Groundwater Subbasin, Fort Irwin National Training Center, California, 1993–2019

Jill N. Densmore, John A. Izbicki, Meghan C. Dick, Sandra Bond, T.B. Coplen, M.C. Dick · U.S. Geological Survey

USGS Publications · Papers · License: Public Domain

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usgs, geological survey, united states, science, government publication, usgs scientific investigations report, 2025, 5105, evaluation, water, quality, langford, valley, irwin, groundwater, subbasin, fort, national, training, center, california, 1993, 2019

Water Resources Mission Area—Water Availability and Use Science Program Prepared in cooperation with the U.S. Army Fort Irwin National Training Center

Evaluation of Water Quality in the Langford Valley–Irwin Groundwater Subbasin, Fort Irwin National Training Center, California, 1993–2019

Scientific Investigations Report 2025–5105

U.S. Department of the Interior U.S. Geological Survey

Cover. False color composite image of Fort Irwin created from National Agriculture Imagery Program images collected April 30, 2022. The bright red areas highlight vegetated and irrigated areas. Image created by Meghan Dick, U.S. Geological Survey.

Evaluation of Water Quality in the Langford Valley–Irwin Groundwater Subbasin, Fort Irwin National Training Center, California, 1993–2019 By Jill N. Densmore, John A. Izbicki, Meghan C. Dick, and Sandra Bond

Water Resources Mission Area—Water Availability and Use Science Program Prepared in cooperation with the U.S. Army Fort Irwin National Training Center

Scientific Investigations Report 2025–5105

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2026

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Densmore, J.N., Izbicki, J.A., Dick, M.C., and Bond, S., 2026, Evaluation of water quality in the Langford Valley–Irwin Groundwater Subbasin, Fort Irwin National Training Center, California, 1993–2019: U.S. Geological Survey Scientific Investigations Report 2025–5105, 45 p., https://doi.org/10.3133/sir20255105. Associated data for this publication: Dick, M.C., 2025, Water-quality data for treated drinking water and treated wastewater effluent at Fort Irwin National Training Center: U.S. Geological Survey data release, https://doi.org/10.5066/P9HQ9C4W. ISSN 2328-0328 (online)

iii

Acknowledgments The authors thank U.S. Army Fort Irwin National Training Center personnel Justine Dishart, Clarence Everly, Chris Woodruff, Paul Gregorio, and David Housman for providing pumpage data, practical knowledge about irrigation practices, and funding for this study. The authors also thank Edgar Ortusiastigue, Jacobs Solutions, Inc., for data about treated wastewater disposal and irrigation practices.

Contents Acknowledgments��iii Abstract��1 Introduction��1 Purpose and Scope��3 Accessing Data��3 Background��6 Hydrogeologic Setting��7 Groundwater Pumpage��13 Treated Wastewater and Groundwater Recharge��13 Groundwater Levels and Movement��18 Methods of Sample Collection and Analysis��21 Quality Assurance��21 Statistical Analysis��22 Water Quality��22 Major-Ion Composition��24 Temporal Changes in Dissolved Solids and Nitrate Concentrations��27 Selected Trace Elements��33 Isotope Composition of Groundwater��34 Stable Oxygen and Hydrogen Isotopes in Water��34 Isotope Composition of Water from Wells��34 Changes in Dissolved Solids and Nitrate Concentrations Associated with Evaporation��36 Tritium and Carbon-14��39 Study Limitations��40 Summary and Conclusions��40 References Cited��42

Figures 1. Map showing location of groundwater basins, faults, and weather stations at the U.S. Army Fort Irwin National Training Center, California��2 2. Map showing geology, aquifer system boundary, location of wells, selected land use, and geologic section locations in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California��5 3. Cross-sections showing generalized geologic structure of the Langford Valley– Irwin groundwater subbasin��8 4. Graph showing monthly precipitation data for weather stations during 2012–19, U.S. Army Fort Irwin National Training Center, California��12 5. Graphs showing groundwater pumping for water supply to U.S. Army Fort Irwin National Training Center, California��15 6. Graph showing treated wastewater discharged to ponds in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2011–19��16 7. Map showing water-level data, including location of wells with water-level hydrographs and 2019 water table elevation, and graphs showing measured water levels in wells, 1993–2019, in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California��19 8. Map showing location of selected wells sampled for water quality and water- quality shape diagrams, Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2015–19��23 9. Trilinear diagrams showing the major-ion composition of water��25 10. Map and chemographs showing dissolved solids and nitrate plus nitrite concentrations in water from selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019��28 11. Graph showing the delta oxygen-18 composition as a function of delta deuterium composition in water from selected wells in the Langford Valley– Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019��35 12. Graphs showing estimated evaporative effects on dissolved solids and nitrate concentrations in water from wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019��38

vii

Tables 1. State well number, abbreviated State well number, U.S. Geological Survey station number, well-construction data, data type, and 2019 water-level elevation for selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California��4 2. Summary of changes in groundwater and wastewater management practices between 1941 and 2016 and the effects of these changes on the groundwater system of the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California��6 3. Monthly precipitation data for selected weather stations, U.S. Army Fort Irwin National Training Center, California��11 4. Annual groundwater pumpage from the Bicycle Valley and Langford Valley groundwater basins, and estimated groundwater recharge to the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1941–2019��14 5. Summary of treated wastewater discharged to ponds and water used for irrigation in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2011–19��17 6. Method reporting levels, method precision, and relative standard deviation for replicate pairs of selected constituents, U.S. Army Fort Irwin National Training Center, California, 2014–19��22 7. Trends in dissolved solids, nitrate plus nitrite, arsenic, and fluoride concentrations in water from selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019��32

viii

Conversion Factors U.S. customary units to International System of Units

Multiply

To obtain

Length inch (in.)

2.54

centimeter (cm)

inch (in.)

25.4

millimeter (mm)

foot (ft)

0.3048

meter (m)

mile (mi)

1.609

kilometer (km)

Mass pound

0.4536

kilogram

Area acre

4,047

square meter (m2)

acre

0.4047

hectare (ha)

acre

0.4047

square hectometer (hm2)

acre

0.004047

square kilometer (km2)

square mile (mi2)

259.0

square mile (mi2)

hectare (ha)

2.590

square kilometer (km2)

Volume acre-foot (acre-ft)

1,233

acre-foot (acre-ft)

cubic meter (m3)

0.001233

cubic hectometer (hm3)

Radioactivity picocurie per liter (pCi/L)

0.037

becquerel per liter (Bq/L)

Flow rate acre-foot per year (acre-ft/yr)

1,233

gallon per minute (gal/min) million gallons per day (Mgal/d)

cubic meter per year (m3/yr)

0.06309 3,785

inch per year (in/yr)

liter per second (L/s) cubic meter per day (m3/d)

25.4

millimeter per year (mm/yr)

International System of Units to U.S. customary units

Multiply

To obtain

Length centimeter (cm)

0.3937

inch (in.)

Volume liter (L)

33.81402

ounce, fluid (fl. oz)

liter (L)

2.113

pint (pt)

liter (L)

1.057

quart (qt)

liter (L)

0.2642

gallon (gal)

liter (L)

61.02

cubic inch (in3)

Mass milligram (mg)

3.527×10−5

ounce, avoirdupois (oz)

microgram (µg)

3.527×10−8

ounce, avoirdupois (oz)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F = (1.8 × °C) + 32. Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F − 32) / 1.8.

Datums Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Elevation, as used in this report, refers to distance above the vertical datum (North American Vertical Datum 1988 [NAVD 88]), in feet.

Well-Numbering and Naming System Wells are identified and numbered according to their location in the rectangular township and range grid system for the subdivision of public lands. Identification consists of the township number, north or south; the range number, east or west; and the section number. Each section is divided into sixteen 40-acre tracts lettered consecutively (except “I” and “O”), beginning with “A” in the northeast corner of the section and progressing in a sinusoidal manner to “R” in the southeast corner. Within the 40-acre tract, numbers are assigned sequentially in the order the wells are inventoried. The final letter within the station name refers to the base line and meridian. California has three base lines and meridians—Humboldt (H), Mount Diablo (M), and San Bernardino (S). Wells in the study area are referenced to the San Bernardino base line and meridian (S). Well names consist of 15 characters and follow the format 013N003E10E003S. In this report, well numbers are abbreviated and written 13N/3E-10E3. Wells in the same township and range are referred to only by their section designation, 10E3. The following diagram shows how the number for well 13N/3E-10E3 is derived.

Supplemental Information Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C). Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) or micrograms per liter (µg/L). Activities for radioactive constituents in water are given in picocuries per liter (pCi/L). Results for measurements of stable isotopes of an element (with symbol E) in water, solids, and dissolved constituents commonly are expressed as the relative difference in the ratio of the number of the less abundant isotope (iE) to the number of the more abundant isotope of a sample with respect to a measurement standard. Delta (δ) notation ratios are given in units of parts per thousand (per mil). Concentrations in water-quality shape diagrams are given in units of milliequivalents per liter.

Abbreviations 14C

carbon-14

δD

deuterium to hydrogen ratio relative to the standard concentration in seawater

16O

oxygen-16

18O

oxygen-18

δ18O

oxygen-18 to oxygen-16 ratio relative to the standard concentration in seawater

Cr(VI)

hexavalent chromium

deuterium

hydrogen-1

deuterium

tritium

MCL

maximum contaminant level

nitrogen

NTC

U.S. Army Fort Irwin National Training Center

NWQL

National Water Quality Laboratory

pmc

percent modern carbon

SMCL

secondary maximum contaminant level

tritium unit

USGS

U.S. Geological Survey

WWTF

wastewater treatment facility

Abstract The U.S. Army Fort Irwin National Training Center (NTC), 120 miles northeast of Los Angeles in the Mojave Desert of California, obtains its potable water supply from the Bicycle Valley and Langford Valley groundwater basins; Langford Valley groundwater basin is further subdivided into the Langford Well Lake and Irwin groundwater subbasins. The Irwin groundwater subbasin contains younger, unconsolidated deposits that have a saturated thickness of as much as 200 feet (ft) and a lower aquifer within older unconsolidated deposits as thick as 650 ft. Groundwater recharge under predevelopment conditions (before 1941) occurred primarily from infiltration of intermittent streamflow in small washes that cross the Irwin groundwater subbasin. Since that time, groundwater recharge has increased because of growth of the NTC in recent years and as a result of other processes, including (1) infiltration of treated wastewater into the aquifer through ponds near the NTC wastewater treatment facility (WWTF) and (2) infiltration of imported water and treated wastewater used for landscape irrigation at base housing and athletic fields. Water samples were collected from 17 wells and analyzed for field parameters, chemical constituents, and isotope composition in the Irwin groundwater subbasin between 2014 and 2019. These data were supplemented with water-chemistry data collected during 1993–95 and at other times if available. Between 1993–95 and 2015–19, median dissolved solids and nitrate concentrations in water from wells in the Irwin groundwater subbasin increased from 620 to 1,030 milligrams per liter (mg/L) and from 2.8 to 4.5 mg/L as nitrogen, respectively. After 2014, dissolved solids and nitrate concentrations in water from wells near the NTC WWTF decreased as a result of changes in source water quality attributable to reverse osmosis of treated drinking water delivered within the Irwin groundwater subbasin and to increased levels of treatment at the NTC WWTF. Based on delta oxygen-18 and delta deuterium isotope data, increases in dissolved solids concentrations in water from

most wells were consistent with evaporation prior to recharge and mobilization of soluble salts from the unsaturated zone. Arsenic and fluoride concentrations in water from wells decreased between 1993–95 and 2015–19 as the basin filled with treated wastewater, but 2015–19 concentrations generally exceeded the California State Water Resources Control Board maximum contaminant levels of 10 micrograms per liter for arsenic and 2 mg/L for fluoride. Most groundwater in the Irwin groundwater subbasin has unadjusted carbon-14 ages ranging from 18,400 to 12,350 years before present. However, water from well 10E3, located along the wash near the subbasin outflow in the southeastern part of the Irwin groundwater subbasin, contained measurable tritium, which is consistent with infiltration of intermittent streamflow and groundwater recharge from the wash after 1952. Chemical and isotopic data indicate that treated wastewater is present in water from most wells in the upper aquifer that underlies the Irwin groundwater subbasin. Wells were not sampled to adequately determine the extent of treated wastewater and changes in water quality within the lower aquifer that underlies the Irwin groundwater subbasin.

Introduction The U.S. Army Fort Irwin National Training Center (NTC), 120 miles (mi) northeast of Los Angeles, is composed of 1,177 square miles within the Mojave Desert of California. Potable water for the NTC is pumped from the Bicycle Valley (basin number 6-025) and Langford Valley (basin number 6-036) groundwater basins; the Langford Valley groundwater basin is further subdivided into the Langford Well Lake (basin number 6-036.01) and Irwin (basin number 6-036.02; fig. 1; California Department of Water Resources, 2021) groundwater subbasins (henceforth referred to as the “Langford Well Lake subbasin” and the “Irwin subbasin,” respectively). Most of the water pumped for supply is used within the Irwin subbasin.

R46E

116°40'

116°20'

R47E

(and California Department of Water Resources basin number [2021])

T 27 S

MOJAVE DESERT

PILOT KNOB VALLEY 6–051

Bicycle Valley groundwater basin (6–025)

T. 15 N.

RIGGS VALLEY 8–023

RED PASS VALLEY 6–024

Fort Irwin National Training Center boundary Faults Approximately located Concealed

Goldstone weather station

Location certain

Goldstone Valley Weather 6–048 stations

Bicycle Lake

LIC

Tiefort Mountains

CRONISE VALLEY 6–035

T. 14 N.

Red Pass Lake

GAR

Study area

SUPERIOR VALLEY 6–049

? 35° 10'

AVAWATZ VALLEY 6–026

? Queried

T 31 S

T. 16 N.

BICYCLE VALLEY

Dry lake (playa)

T 30 S

Langford Well Lake groundwater subbasin (6–036.01)

Barstow

T. 17 N.

Los Angeles

Irwin groundwater subbasin (6–036.02)

T 29 S

Fort Irwin National Training Center

LEACH VALLEY 6–027

Langford Valley groundwater basin (6-036) 35° T 30' 28 S

T. 18 N.

6–027

T EXPLANATION 26LEACH SVALLEY Selected groundwater basin

SPR

ING

FAU LT Langford Well Lake

Noble Dome

T 32 S

0 COYOTE LAKE VALLEY 6–037

R. 3 W. R. 2 W. R. 1 W. R. 2 E. R. 1 E. Base map from U.S. Geological Survey and other Federal and State digital data, various scales; Universal Transverse Mercator projection, zone 11 north; North American Datum of 1983

5 5

10 MILES 10 KILOMETERS

Alvord Mountain

R. 3 E.

R.4 E.

R. 5 E.

R.6 E. R. 7 E. R. 8 E. Faults modified from Miller and others, 2014; Groundwater basin from California Department of Water Resources, 2021

Figure 1. Location of groundwater basins, faults, and weather stations at the U.S. Army Fort Irwin National Training Center, California.

T. 13 N.

T. 12 N.

2 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

117°00' R45E

Introduction 3 In the Irwin subbasin, groundwater pumping exceeded recharge beginning in the early 1940s, resulting in groundwater-level declines (Voronin and others, 2014). However, importation of water from the Bicycle Valley groundwater basin (beginning in the mid-1960s) and Langford Well Lake subbasin (beginning in the early 1990s), and subsequent groundwater recharge of treated wastewater reversed the water-level declines and water levels in Irwin subbasin began to rise (Voronin and others, 2014). Water- quality changes occurred in some parts of the Irwin subbasin as a result of groundwater recharge from treated wastewater and irrigation practices (Densmore and Londquist, 1997). The U.S. Geological Survey (USGS) has been studying water-resources issues at Fort Irwin since the early 1990s. One issue of concern is the effect of groundwater development, resulting from training and infrastructure expansion at the NTC, on water quality in the Irwin subbasin. In 2014, the USGS collaborated with the U.S. Army to monitor groundwater resources within the NTC. A groundwater-monitoring network was developed for the NTC that included water-level and water-quality data collection from selected wells in the Bicycle Valley and Langford Valley groundwater basins. This monitoring network was designed to provide data for evaluation of water-level and water- quality changes over time. Wells in the Irwin subbasin within this network include monitoring wells drilled by the USGS (Densmore and Londquist, 1997) and supply wells operated by the NTC (table 1; locations of wells are shown on fig. 2).

Purpose and Scope The purpose of this report was to evaluate changes in groundwater quality in the Irwin subbasin related to the disposal of treated wastewater and return water from

irrigation at base housing landscaping and athletic fields with treated wastewater. This report examines changes in groundwater levels, major-ion compositions, dissolved solids, nitrates, and trace elements in groundwater in the Irwin subbasin between 1993 and 2019. Stable isotopes of oxygen and nitrogen, tritium, and carbon-14 are used to identify sources of groundwater. This study includes interpretation of data collected as part of the USGS study in the Irwin subbasin by Densmore and Londquist (1997) and data collected for this study (U.S. Geological Survey, 2019).

Accessing Data The groundwater-level data presented in this report were accessed through the USGS National Water Information System Web service (NWISWeb) at (https://waterdata.usgs.gov/ca/nwis/gw/; U.S. Geological Survey, 2019). Water-quality data used in this study were retrieved from NWISWeb. Before publication of this report, the water level and water-quality data services on NWISWeb were decommissioned. The water-level data are now available at the USGS Water Data for the Nation website (https: //waterdata.usgs.gov).The water-quality data are now available at the National Water Quality Monitoring Council’s Water Quality Data Portal (https://www.waterqualitydata.us). The temperature, pumping, recharge, and irrigation data presented in this report and provided by the NTC and Jacobs Solutions, Inc. personnel were not published at the time of this report. Original copies of these data are available upon request from the respective owners.

[mm/dd/yyyy, month/day/year; NAVD 88, National Vertical Datum of 1988; USGS, U.S. Geological Survey; WL, water level; WQ, water quality; — no data]

State well number 013N003E04B003S 013N003E04C003S 013N003E04D003S 013N003E04G001S 013N003E04K004S 013N003E04Q002S 013N003E05G001S 013N003E05G002S 013N003E08B001S 013N003E09B001S 013N003E10D001S 013N003E10E003S 014N003E32B001S 014N003E32B002S 014N003E32B003S 014N003E32F002S 014N003E32F003S 014N003E32H001S 014N003E32K001S 014N003E32K005S 014N003E32K006S 014N003E32P004S 014N003E32P006S 014N003E32Q003S 014N003E33E003S 014N003E33F001S 014N003E33H002S 014N003E33J002S 014N003E33N001S 014N003E33R001S 014N003E33R002S 014N003E34D001S

Abbreviated State well number 4B3 4C3 4D3 4G1 4K4 4Q2 5G1 5G2 8B1 9B1 10D1 10E3 32B1 32B2 32B3 32F2 32F3 32H1 32K1 32K5 32K6 32P4 32P6 32Q3 33E3 33F1 33H2 33J2 33N1 33R1 33R2 34D1

USGS station number 351521116400203 351514116401403 351523116402803 351504116400301 351449116400503 351441116400401 351505116405701 351504116405501 351418116410201 351429116395401 351422116392501 351416116392203 351616116410701 351616116410702 351616116410703 351558116412101 351558116412102 351556116404601 351538116410301 351539116405605 351539116405606 351527116412503 351527116412505 351525116411301 351556116402402 351602116401101 351559116393701 351542116393702 351525116402901 351530116394401 351533116394401 351604116393101

Elevation of land surface, in feet above NAVD 88 2,393 2,402 2,422 2,388 2,390 2,384 2,462 2,462 2,502 2,381 2,347 2,345 2,532 2,532 2,532 2,524 2,534 2,464 2,486 2,475 2,475 2,519 2,519 2,497 2,427 2,419 2,407 2,392 2,427 2,382 2,389 2,440

Depth of well, in feet below land surface 130 150 170 140 135 90 450 450 255 120 65 60 630 540 300 460 290 490 551 290 230 580 270 750 175 274 190 116 330 112 107 201

Perforated interval depth, in feet below land surface 110–130 130–150 150–170 90–140 115–135 85–90 210–350; 380–440 210–440 235–255 70–120 15–65 40–60 610–630 520–540 280–300 440–460 270–290 220–475 375–535 270–290 210–230 560–580 250–270 215–450; 492–745 155–175 179–259 138–188 76–116 160–320 72–112 67–107 150–200

Type of data available

Date (mm/dd/yyyy)

Water-level elevation, in feet above NAVD 88

WL,WQ WL,WQ WL,WQ WL WL,WQ WL WL,WQ WL,WQ WL WL WL WL,WQ WL WL WL,WQ WL,WQ WL,WQ WL,WQ WL WL,WQ WL,WQ WL,WQ WL,WQ WL WL,WQ WL WL WL WL WL, WQ WL WL

11/05/2019 11/07/2019 11/07/2019 12/11/2019 12/11/2019 12/11/2019 12/19/2019 — 12/11/2019 12/11/2019 12/11/2019 12/11/2019 12/11/2019 12/11/2019 12/11/2019 11/01/2019 11/07/2019 4/16/2019 — 11/07/2019 11/07/2019 11/07/2019 11/07/2019 12/19/2019 12/11/2019 12/11/2019 12/12/2019 12/12/2019 12/11/2019 12/11/2019 12/11/2019 12/12/2019

2,320 2,317 2,315 2,319 2,322 2,323 2,301 — 2,295 2,323 2,327 2,321 2,313 2,313 2,314 2,317 2,317 2,314 — 2,315 2,315 2,321 2,321 2,316 2,315 2,315 2,319 2,326 2,315 2,329 — 2,315

4 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Table 1. State well number, abbreviated State well number, U.S. Geological Survey (USGS) station number, well-construction data, data type, and 2019 water-level elevation for selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California.

Introduction 5 116°42'

116°41'

116°40'

116°39'

ITAM

! 32B1-3

33A1

A' !

33B1 ! 33F1 ! 32H1

32F1 ?

! 33R1

4B1-3

T. 13 N.

! 4C1-3

5G1

RS BA

5G2

35°15'

33Q1

33N1 !! 4D1-3

5D1

T. 14 N.

ET RE ST

? 32Q3

H 5T

32Q1

34M2 !

! 32K3-6

? ? ?

A''

OOP

32J1 ?

32Q2

32P2-6 ! 32N1 !

! 33E2, 3

DPW606

32K1 32P1 ?

ROAD

32F3

35°16'

! 4K2-4

! 4Q2-6

6P1

10D1 !

10E1-3 ! 1 MILE

0.5

U FA

0 35°14'

0.5

1 KILOMETER

10Z1

! B'

R. 3 E. Base map from U.S. Geological Survey and other Federal and State digital data, various scales; Lambert Conformal Conic projection, standard parallels 34.0333 N. and 35.4667 N.; North American Datum of 1983

Aquifer boundary from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Selected land use Pond Duck pond Old biological evaporation pond

Unconsolidated deposits

Base housing 2005–191

Fault

Consolidated rock

Aquifer system boundary

Army ball field

Golf course and driving range

Baseball field

Center-pivot sprinkler field Fort Irwin wastewater treatment facility Original base housing 1941–841

Geology Base housing 1985–20051

Fritz field

32H1

Soccer field

33R1

Well and identifier for well or group of wells used

5G1

Test well and identifier

Wash 1

A A' Line of section

Date ranges show onset of initial construction and improvements within that period.

DPW606

Production well and identifier

Weather station and identifier

Figure 2. Geology, aquifer system boundary, location of wells (U.S. Geological Survey, 2019), selected land use (Voronin and others, 2014), and geologic section locations, shown on figure 3, in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California. Modified from Densmore and Londquist (1997); well information from U.S. Geological Survey (2019).

6 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Background The hydrogeology of the Irwin subbasin, including the effects of groundwater pumping and groundwater recharge with treated wastewater, on groundwater levels and movement are summarized in the following sections

of this report. Previous USGS reports document changes in pumping, groundwater use, and recharge as a consequence of groundwater development beginning in 1941 and how those changes in water-management activities affected groundwater in the Irwin subbasin (table 2).

Table 2. Summary of changes in groundwater and wastewater management practices between 1941 and 2016 and the effects of these changes on the groundwater system of the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California. Year

Water-management change

Effect(s) on basin

1941–55

Irwin subbasin (exclusively) pumped for municipal and irrigation uses; wastewater disposed of in old biological evaporation ponds

1955

Secondary treated wastewater from the Wastewater- Continued water-level declines Treatment Facility (WWTF) disposed of in percolation ponds and used to irrigate golf course/ driving range and sprinkler-pivot area southwest of the WWTF (eastside basin)

Densmore and Londquist (1997); Densmore (2003); Voronin and others (2014)

1967

Import of groundwater pumped from Bicycle Valley groundwater basin

Slight increase in water levels

Densmore and Londquist (1997); Densmore (2003); Densmore and others (2018)

1990, 1994–99

Highest basewide pumping rates since 1941 (3,016–3,250 acre-feet per year total)

Cone of depression developed near the center of the basin; pumping within Irwin subbasin reduced during this period

Densmore and Londquist (1997); Densmore (2003); Voronin and others (2014)

1992

Import of groundwater pumped from Langford Well subbasin

Water levels stabilize and begin to recover

Densmore and Londquist (1997); Densmore (2003); Voronin and others (2013)

1996

Sprinkler-pivot area, south of WWTF decommissioned

Recharge in sprinkler-pivot area ceases

Densmore and Londquist (1997); Densmore (2003); Voronin and others (2014)

2002–07

Landscaping changes to xeriscaping; most groundwater pumping is from Bicycle Valley groundwater basin

Reduced pumping from Irwin subbasin; total pumping continues to decline; groundwater levels continue to rise

Voronin and others (2014)

2010

Tertiary-treated wastewater is recycled and replaces some native groundwater resources for landscape irrigation

Reduced pumping of groundwater from Irwin subbasin

Voronin and others (2014)

2015

Soccer field converted to artificial turf and no longer irrigated

Less than 8 acres of irrigated area removed along west side of basin; water levels beginning to stabilize

See information in this report.

2016

Reduced pumping from Bicycle Valley groundwater basin;

Since 2017, the volume of groundwater See information in this pumped from Irwin subbasin increased report. relative to the total volume pumped from all basins (relative to the previous 20-year period)

New Water Works plant provides treated drinking water with low total dissolved solids in Irwin subbasin

Water levels decline over time; groundwater flows toward area of pumping

References Densmore and Londquist (1997); Densmore (2003); Voronin and others (2014)

Introduction 7

Hydrogeologic Setting The hydrogeologic setting of the Irwin subbasin was described by Densmore and Londquist (1997) and Densmore (2003). The Irwin subbasin contains unconsolidated deposits that consist of late Tertiary to Quaternary older alluvium and lacustrine (lake) deposits and Quaternary younger alluvium (Densmore, 2003). The deposits are unconsolidated at land surface and become partly consolidated with depth. Older and younger alluvium are the only water-bearing materials in the Irwin subbasin from which large amounts of groundwater can be pumped for water supply; these materials compose a lower aquifer and an upper aquifer in the Irwin subbasin, respectively. The upper aquifer is unconfined and consists of younger alluvium that has a maximum thickness of about 500 feet (ft) in the west-central part of the basin (fig. 3). Much of the younger alluvium is above the regional water table, and the maximum saturated thickness of the upper aquifer is about 200 ft (fig. 3). The lower aquifer consists of Quaternary- Tertiary deposits (older alluvium and lacustrine deposits) and is confined throughout most of the basin. The unconsolidated deposits that compose the upper and lower aquifers are underlain by a basement complex (fig. 3) composed of igneous and metamorphic rocks, which yield only small amounts of water to wells, except in areas where they are jointed, weathered, or fractured (Densmore and Londquist, 1997). In the Irwin subbasin, Tertiary deposits (unit QTa, including older alluvium and lacustrine deposits; fig. 3) are as thick as 650 ft (fig. 3A) and are composed of sand, gravel, and clay derived predominantly from granitic material, except in the northern part of the basin, where deposits eroded from volcanic material are present. Where older alluvium is composed predominantly of sand and gravel, it yields moderate amounts of water to wells. In the southeastern part of the basin, the Tertiary deposits are composed almost entirely of low-permeability lacustrine deposits of silt and clay that do not yield large amounts of water to wells (fig. 3C). These low-permeability deposits extend from northwest of well 33N1 near the center of the basin to the southeast, beyond wells 10E1–3 to the southwest. Younger alluvium (unit Qa, fig. 3) is composed primarily of loose, coarse sand and gravel with small amounts of clay. Most younger alluvium lies above the water table. Areas where the younger alluvium is saturated, primarily in the center of the basin, yield large quantities of water to

wells (as much as 1,000 gallons per minute). Wellbore-flow tests of selected production wells indicated that most of the water pumped from wells comes from the younger alluvium (Densmore and Londquist, 1997). Thin, discontinuous clay lenses within younger alluvium (not shown on fig. 3) impede downward movement of irrigation return water beneath the center-pivot sprinkler field in the southeastern part of the basin (fig. 3C) and beneath base housing and athletic fields to the northwest (fig. 3). These discontinuous clay lenses may result in a perched groundwater layer above the regional water table where present. The Garlic Spring fault on the northeast and an unnamed fault on the southwest extend through the Irwin subbasin (figs. 2, 3). Older alluvium is present at shallower depths to the northeast of the Garlic Spring fault, and younger alluvium is thicker to the southwest of the fault (fig. 3B). The faults impede the movement of groundwater through the subsurface (Densmore and Londquist, 1997). Traces of the faults are not present at land surface in the northeastern part of the basin where they have been covered by younger alluvium (Densmore and Londquist, 1997). Between 2012 and 2019, annual precipitation in the Irwin subbasin ranged from 0.66 to 4.85 inches (in.), with an average of 2.66 inches per year (David Housman, U.S. Army Fort Irwin National Training Center, unpub. data, 2019; table 3). Areal recharge to groundwater from infiltration of precipitation in the Mojave Desert does not occur under present-day (2019) climatic conditions (Izbicki and others, 2007). Natural recharge to the aquifer occurs as infiltration of occasional streamflows in small washes that cross the Irwin subbasin after periods of intense rain. Although they occur infrequently in the area, large storms (fig. 4; table 3) have resulted in flooding at the NTC. Flooding occurred in the Irwin subbasin after a monsoonal storm on August 25, 2013 (Justine Dishart, U.S. Army, written commun., 2013). Rainfall of 2.25 in. measured at the Goldstone weather station and 1.14 in. at the ITAM weather station during November 21–23, 2013 (table 3; David Housman, U.S. Army Fort Irwin National Training Center, unpub. data, 2019; at the time of publication, data were not available from the U.S. Army) also contributed to flooding in the Irwin subbasin (Clarence Everly, U.S. Army, written commun., 2013). There were similar large storms during the winters of 2016 and 2017 (table 3).

8 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

2,300 2,200

14N/3E-33B1

14N/3E-33A1

BEND IN SECTION 14N/3E-33F1

SOUTH LOOP ROAD

QTa

Boundary of aquifer system

BEND IN SECTION

Old biologicalevaporation ponds

14N/3E-33E2–3

5TH STREET

BEND IN SECTION 14N/3E-32J1

SECTION B–B'

NORTHEAST

2,400

14N/3E-32K1

Army ball field

14N/3E-32K3–6

2,500

Soccer field

BARSTOW ROAD

BEND IN SECTION

2,600

14N/3E-32P2–6

2,700

Boundary of aquifer system

14N/3E-32N1–3

13N/3E-6P1

SOUTHWEST

FEET

BEND IN SECTION

? ?

2,100

QTa

2,000

GARLIC SPRING FAULT

QTa

1,900 1,800 1,700 1,600 1,500

? HORIZONTAL SCALE APPROXIMATELY 1.4X VERTICAL SCALE GREATLY EXAGGERATED DATUM IS NORTH AMERICAN VERTICAL DATUM OF 1988

1 MILE

0.5

0 0

0.5

1 KILOMETER

Figure modified from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Water table (measured during 2019)

Geology Quaternary

Quaternary–Tertiary

QTa

Water table (measured during 1994)

Younger alluvium—Upper aquifer

Older alluvium—Lower aquifer Pre-Tertiary Weathered and fractured basement complex

Well diagram

14N/3E-33E2–3

Basement complex

? Contact—Queried where uncertain Fault—Queried where uncertain; arrows indicate direction of apparent movement

Well identifier Perforated interval (perforated intervals for wells 13N/3E-6P1 and 14N/3E-33B1 are not known) Bottom of well

}

Bottom of borehole

Figure 3. Generalized geologic cross sections of the Langford Valley–Irwin groundwater subbasin. A, southwest to northeast (A–A’ ); B, west to east (A–A” ); and C, north to south (B–B’ ), U.S. Army Fort Irwin National Training Center, California. Section lines are shown on figure 2. Modified from Densmore and Londquist (1997); well information from U.S. Geological Survey (2019).

Introduction 9 B

WEST

EAST

QTa

14N/3E-34M2

BEND IN SECTION 14N/3E-33R1

14N/3E-33Q1

GARLIC SPRING FAULT

2,100 2,000 1,900 1,800

13N/3E-4B1–3

QTa

BICYCLE LAKE FAULT

2,200

Boundary of aquifer system

2,300

BEND IN SECTION

SECTION B–B'

2,400

A''

Wastewatertreatment facility (projected)

13N/3E-4C1–3

5TH STREET

LANGFORD LAKE ROAD

2,600 2,500

BEND IN SECTION

2,700

SOUTH LOOP ROAD

Boundary of aquifer system

13N/3E-5G1

13N/3E-6P1

2,800

BEND IN SECTION

A FEET

HORIZONTAL SCALE APPROXIMATELY 1.4X VERTICAL SCALE GREATLY EXAGGERATED DATUM IS NORTH AMERICAN VERTICAL DATUM OF 1988

1 MILE

0.5

0 0

0.5

1 KILOMETER

Figure modified from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Water table (measured during 2019)

Geology Quaternary

Older alluvium—Lower aquifer

Lacustrine deposits

Well diagram

13N/3E-4B1–3

Quaternary–Tertiary

QTa

Water table (measured during 1994)

Younger alluvium—Upper aquifer

Pre-Tertiary Weathered and fractured basement complex

? Figure 3.—Continued

Fault—Queried where uncertain; arrows indicate direction of apparent movement

Perforated interval (perforated intervals for wells 13N/3E-6P1 and 5G1 are not known) Bottom of well

Basement complex

? Contact—Queried where uncertain

Well identifier

}

Bottom of borehole

BEND IN SECTION

2,100

? ?

2,000 1,900

Qa QTa

Boundary of aquifer system

? ?

13N/3E-10Z1

13N/3E-10E1–3

BEND IN SECTION

(projected)

? ?

2,200

Duck ponds

13N/3E-10D1

13N/3E-4Q2–6

2,300

13N/3E-4K2–4

5TH STREET

BEND IN SECTION

SECTION A–A"

Center-pivot sprinkler field

2,400

QTa

HORIZONTAL SCALE APPROXIMATELY 1.4X

VERTICAL SCALE GREATLY EXAGGERATED DATUM IS NORTH AMERICAN VERTICAL DATUM OF 1988

1 MILE

0.5

0 0.5

Figure modified from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

1 KILOMETER

EXPLANATION

Younger alluvium—Upper aquifer Quaternary–Tertiary

QTa

Water table (measured during 2019)

Well diagram

13N/3E-10E1–3

Geology Quaternary

Water table (measured during 1994)

? Contact—Queried where uncertain

Older alluvium—Lower aquifer

Lacustrine deposits Pre-Tertiary Weathered and fractured basement complex Basement complex

13N/3E-4Q2–6

1,800

13N/3E-4C1–3

? ?

2,500

5TH STREET

2,600

13N/3E-4D1–3

2,700

14N/3E-33N1

14N/3E-32B1–3

2,800

SECTION A–A'

Boundary of aquifer system

14N/3E-32K3–6

2,900

BEND IN SECTION

14N/3E-32F1

FEET

SOUTH

BEND IN SECTION

NORTH

Well identifier Perforated interval (perforated interval for well 13N/3E-10Z1 is not known) Bottom of well

}

Bottom of borehole Lysimeter and number

10 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Figure 3.—Continued

Table 3. Monthly precipitation data for selected weather stations, U.S. Army Fort Irwin National Training Center, California. [Precipitation data in inches. Locations of weather stations shown on figures 1 and 2. Data from David Housman, U.S. Army Fort Irwin National Training Center, unpub. data, 2019; at the time of publication, data were not available from the U.S. Army. Abbreviation: —, no data]

Year

January

February

March

April

May

June

July

2012

0.11

0.00

0.07

0.29

0.00

0.04

2013

0.44

0.03

0.42

0.00

2014

0.00

0.74

0.00

0.01

0.00

2015

0.84

0.70

0.06

0.01

0.09

2016

1.00

0.08

0.30

1.08

0.26

2017

3.78

0.24

0.00

2018

0.66

0.00

0.73

0.00

2019

1.68

1.45

1.13

2012

—

2013

—

2014

—

August

September

October

November

December

Total

0.02

0.00

0.03

0.01

0.37

0.94

0.00

0.02

0.04

0.08

2.25

0.00

3.28

0.06

0.70

0.24

0.00

0.01

1.06

2.82

0.00

0.72

0.11

1.86

0.48

0.06

0.07

5.00

0.00

0.11

0.00

0.91

0.13

1.46

5.32

0.00

0.14

0.22

0.00

4.38

0.34

0.00

0.23

0.01

0.00

1.13

0.18

0.35

3.63

0.82

0.50

0.00

0.23

0.00

0.25

0.00

—

6.06

—

Goldstone weather station

DPW606 weather station

2015

—

0.00

0.01

0.02

0.00

0.03

2016

0.77

0.12

0.07

0.52

0.00

0.13

0.00

0.93

0.02

0.95

3.51

2017

2.32

0.58

0.00

0.30

0.87

0.58

0.00

4.65

2018

0.59

0.00

0.40

0.00

0.06

0.00

0.02

0.00

0.64

0.00

0.32

2.03

2019

1.35

1.23

0.67

0.44

0.66

0.00

—

4.35

2012

0.03

0.02

0.11

0.08

0.00

0.01

0.15

0.03

0.00

0.23

0.66

2013

0.36

0.00

0.26

0.04

0.00

0.52

2.45

0.00

0.08

1.14

0.00

4.85

2014

0.00

0.52

0.01

0.00

0.03

0.00

0.01

0.60

0.00

0.66

1.83

2015

0.54

0.59

0.01

0.00

0.01

0.43

0.01

—

0.00

—

0.00

1.60

2016

0.75

0.13

0.06

0.49

0.00

0.11

0.00

0.87

0.02

1.13

3.56

2017

1.89

0.62

0.00

0.12

0.14

0.00

2.77

2018

0.58

0.01

0.43

0.00

0.12

0.00

0.53

0.16

0.29

2.12

2019

0.95

0.77

1.04

0.32

0.84

0.00

—

3.92

ITAM weather station

Introduction 11

EXPLANATION

3.5

Weather stations (location shown on figures 1 and 2)

Precipitation, in inches

3.0

DPW606 Goldstone

2.5

ITAM

2.0

1.5

1.0

0.5

0.0

2012

2013

2014

2015

2016

2017

2018

2019

Calendar year

Figure 4. Monthly precipitation data for weather stations during 2012–19, U.S. Army Fort Irwin National Training Center, California (data provided by David Housman, U.S. Army, Fort Irwin National Training Center, unpub. data, 2019. At the time of publication, data were not available from the U.S. Army).

12 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

4.0

Introduction 13

Groundwater Pumpage Groundwater development in the Irwin subbasin began in 1941 (table 4). From 1941 to 1992, most groundwater pumpage for the NTC was from the Irwin subbasin, which resulted in water-level declines as much as 30 ft in parts of the subbasin during this period (Voronin and others, 2014). In response to declining water levels and to supplement water supply, pumping and importation of water for supply from the Bicycle Valley groundwater basin began in 1967 and from the Langford Well Lake subbasin in 1992 (fig. 5A). The highest levels of groundwater pumping in the Irwin subbasin occurred between 1987 and 1990. Groundwater pumping in the Irwin subbasin occurred at lower levels after 1990 (fig. 5A; table 4). The combined pumping from the Bicycle Valley groundwater basin and the Langford Well Lake subbasin exceeded pumping in the Irwin subbasin until 2017 (fig. 5B) when well 5G2 was activated in the Irwin subbasin (fig. 2). From January 1993 to December 2019, an average of 1,184 acre-feet per year of groundwater was pumped from the Bicycle Valley groundwater basin and averages of 693 and 657 acre-feet per year of groundwater were pumped from the Langford Well Lake subbasin and the Irwin subbasin, respectively (table 4). The greatest pumping from the three basins was from the Bicycle Valley groundwater basin during the late 1990s and early 2000s; however, pumping in the Bicycle Valley groundwater basin has declined since 2013 (table 4; fig. 5A). Overall pumping from the three basins has been on a steady decline since 1999, and pumping patterns have shifted between basins and wells. Since 2016, pumping from the Irwin subbasin and water imported from the Langford Well Lake subbasin has met, almost equally, the water needs for the NTC within the Irwin subbasin.

Treated Wastewater and Groundwater Recharge Wastewater in the Irwin subbasin is treated at the NTC wastewater-treatment facility (WWTF) in the southeastern part of the basin (fig. 2). The NTC WWTF has a capacity of 2 million gallons per day (Mgal/d), with a peak capacity of 4 Mgal/d, and is designed to treat residential sewage (Shawn Anderson, Jacobs Solutions Inc., written commun., 2010). The NTC WWTF was upgraded from secondary treatment to tertiary treatment in 2007. After 2015, groundwater delivered for supply within the Irwin subbasin was treated using reverse osmosis to lower dissolved solids concentrations, resulting in a decrease in the dissolved solids concentration of treated wastewater from the NTC WWTF (Dick, 2025). The quality of treated wastewater from the NTC WWTF varied through time as a result of changes in source water, source water quality, and changes in wastewater treatment practices. Water-quality data for treated wastewater collected by Jacobs Solutions Inc. personnel during 2015–19 are published in a USGS data release (Dick, 2025).

Treated wastewater use and discharge to ponds and other locations in the Irwin subbasin changed over time. Between 1945 and 1955, treated wastewater was disposed of in biological evaporation ponds (fig. 2; Densmore and Londquist, 1997). Discharge to ponds near the NTC WWTF began in 1955. Irrigation with treated wastewater in the golf course/driving range area occurred from 1955 to 1971 and after 1981; irrigation of the larger golf course area ceased in the mid-2000s, while the smaller driving range area (known as Pitch and Putt, not shown on fig. 2) is still irrigated with treated wastewater. Beginning in 1986, treated wastewater was disposed through irrigation at the center-pivot sprinkler field in the southeastern part of the basin (fig. 2). This practice ceased in 1996, after a study by Densmore (2003) determined that groundwater degradation was due to leaching of salts beneath the center-pivot sprinkler field. Beginning in 2010, irrigation with treated wastewater replaced irrigation with groundwater on most athletic fields and base housing. Between 2011 and 2019 (the period detailed records are available), the NTC WWTF produced between 687 and 1,137 acre-feet (acre-ft) of treated wastewater annually (Edgar Ortusiastigue, Jacobs Solutions, Inc., written commun., 2020). During this period, the amount of treated wastewater discharged to ponds near the NTC WWTF, to duck ponds in the southeast part of the Irwin subbasin, or to ponds on the golf course ranged from a high of 996 acre-ft in 2011 to a low of 533 acre-ft in 2017 (table 5; fig. 6). The ponds where treated wastewater is discharged are unlined and most treated wastewater discharged to the ponds infiltrated into the subsurface; infiltration from the ponds is the largest source of groundwater recharge to the underlying aquifer. During 2011–19, between 127 and 215 acre-ft of treated wastewater was used to irrigate base housing (including the town area), parts of the golf course, and various athletic fields at the NTC (table 5). Irrigation return water in these areas would have reached the water table and recharged the underlying aquifer. Smaller amounts of recharge also may have occurred within the Irwin subbasin as leakage from water distribution or sewage pipes (Densmore and Londquist, 1997; Voronin and others, 2013). Between 2015 and 2019 (the period that detailed records are available; Dick, 2025), treated wastewater sampled weekly at the NTC WWTF had a median specific conductance of 780 microsiemens per centimeter, corresponding to a median dissolved solids concentration of about 515 milligrams per liter (mg/L). Based on available data (Dick, 2025), between 2015 and 2019 the median nitrate concentration in treated wastewater was 21 mg/L as nitrogen (N). Dissolved solids and nitrate concentrations in treated wastewater varied widely during this period, with an interquartile range (50-percent of the values calculated as the difference between the 25-percent and 75-percent quartiles) from 370 to 924 mg/L for dissolved solids and from 8.5 to 26 mg/L nitrate as N. Variations in treated wastewater quality presumably result from a combination of changes in drinking-water source and quality, drinking water treatment (including reverse osmosis after 2015), and NTC WWTF operations.

14 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 Table 4. Annual groundwater pumpage from the Bicycle Valley and Langford Valley Table groundwater 4. basins and estimated groundwater recharge to the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1941–2019. [Pumping and groundwater recharge values in acre-feet] Pumping1

Pumping1

Year

Bicycle Valley groundwater basin

Langford Valley groundwater basin Langford Well Lake subbasin

Irwin subbasin

Groundwater recharge, Irwin subbasin2

Year

Bicycle Valley groundwater basin

Langford Valley groundwater basin Langford Well Lake subbasin

Irwin subbasin

Groundwater recharge, Irwin subbasin2

1981

793

660

702

130

1982

758

630

666

350

199

1983

866

720

901

480

273

1984

689

1,675

1,558

182

102

1985

1,243

1,133

1,548

1986

1,329

1,315

1,006

1947

1987

822

1,927

1,257

1948

1988

1,033

1,700

1,173

1949

1989

829

1,696

1,073

1950

1990

1,312

1,868

1,390

1951

293

166

1991

1,380

1,331

1,022

1952

336

193

1992

1,134

619

1,110

1,146

1953

671

385

1993

757

1,114

997

1,196

1954

668

382

1994

964

1,006

1,180

1,276

1955

598

488

1995

1,051

816

1,270

1,645

1956

602

1996

1,226

663

1,138

1,568

1957

704

1997

1,780

656

580

1,343

1958

686

1998

2,292

328

484

1,460

1959

655

1999

2,075

394

781

1,460

1960

746

2000

1,904

582

451

1,462

1961

881

195

2001

1,896

234

332

1,640

1962

1,119

344

2002

1,984

384

333

1,716

1,948

618

201

1,781

1941

1942

1943

1944

1945 1946

1963

1,147

370

2003

1964

1,202

408

2004

1,938

495

334

1,671

1965

1,305

607

2005

1,541

683

402

1,701

1966

1,509

790

2006

1,275

903

634

1,733

1967

822

827

928

2007

1,360

133

280

1,690

1968

820

764

864

2008

868

855

853

1,385

1969

954

727

947

2009

1,180

792

769

1,376

1970

896

549

742

2010

1,090

899

541

1,391

1971

608

364

421

2011

1,158

685

544

1,141

1972

480

399

785

2012

1,587

481

166

1,113

1973

157

321

447

2013

1,207

963

1,078

1974

170

200

352

2014

930

528

744

1,101

1975

210

236

421

2015

399

981

772

941

1976

393

236

569

2016

685

961

237

843

1977

123

224

2017

826

1,111

678

1978

493

283

702

2018

823

818

772

1979

462

502

883

2019

656

836

739

1980

866

721

1,388

1,184

693

657

1,330

Average 1993–2019

1Pumping data for 1940–2010 from Voronin and others (2014); data for 2011–19 provided by Edgar Ortusiastigue, Jacobs Solutions Inc., unpub. data, 2019. 2Groundwater recharge data for 1941–2010 are from Voronin and others (2014). Groundwater recharge data for 2011–19 are derived from records of treated wastewater discharged to ponds (table 5) and records of irrigation at base housing and athletic fields (table 5), provided by Edgar Ortusiastigue, Jacobs Solutions Inc., unpub. data, 2019; at the time of publication, data were not available from Jacobs Solutions Inc.

Introduction 15

3,500

3,000

2,500

2,500 Base temporarily deactivated

2,000

1,500

2,000

1,500

Base on standby

1,000

500

0 1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

2020

Recharge, in acre-feet

Pumpage, in acre-feet

Calendar year EXPLANATION Groundwater basin and number (California Department of Water Resources, 2021)

Groundwater recharge to the Langford Valley— Irwin groundwater subbasin

Bicycle Valley groundwater basin (6–025) Langford Valley—Irwin groundwater subbasin (6–036.02) Langford Valley—Langford Well Lake groundwater subbasin (6–036.01)

Figure 5. Groundwater pumping for water supply to the U.S. Army Fort Irwin National Training Center, California. A, Annual groundwater pumpage from Bicycle Valley groundwater basin and the Langford Valley–Irwin and Langford Well Lake groundwater subbasins, 1941–2019; and B, annual groundwater pumpage, 1941–99, and pumpage by well, 2000–19, from the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California. Data for 1940–2010 are from Voronin and others (2014); data for 2011–19 are from Edgar Ortusiastigue, Jacobs Solutions, Inc., unpub. data, 2019. At the time of publication, data were not available from Jacobs Solutions, Inc.

16 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 B 3,500

EXPLANATION 3,000

Well pumped for supply 014N003E32F001S 014N003E32K001S

Pumpage, in acre-feet

2,500

014N003E32H001S 013N003E5G002S

2,000

Undifferentiated wells

1,500

1,000

500

0 1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Calendar year

Figure 5.—Continued

Treated wastewater discharged to ponds, in acre-feet

1,000

800

600

400

200

2011

2012

2013

2014

2015

2016

Calendar year

2017

2018

2019

Figure 6. Treated wastewater discharged to ponds in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2011–19. Data provided by Edgar Ortusiastigue, Jacobs Solutions, Inc., unpub. data, 2019. At the time of publication, data were not available from Jacobs Solutions, Inc.

2005

2010

2015

2020

Table 5. Summary of treated wastewater discharged to ponds and water used for irrigation in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2011–19. [Selected land use shown on figures 2, 7A, 8, and 10A. Town area and base housing 2005–19 includes irrigation water use in the town area (not shown on figures 2, 7A, 8, and 10A), base housing 2005–19, and soccer field. Barracks, 2005–19 included in base housing 2005–19. Date ranges show onset of initial construction and improvements within that period. Data provided by Edgar Ortusiastigue, Jacobs Solutions, Inc., unpub. data, 2019; at the time of publication, data were not available from Jacobs Solutions, Inc. Groundwater recharge from irrigation estimated from total water use, assuming 50-percent recharge efficiency]

Irrigation by selected land use, in acre-feet

Year

Wastewater discharged to ponds, in acre-feet

Treated wastewater used for irrigation Golf course and driving range

Army ball field

Baseball field

Fritz field

Base housing, 2005–19 (includes town area)

Groundwater used for irrigation

Total

Base housing, 1941–84

Base housing, 1985–2005

Barracks, 2005

Total

Total irrigation water use

Groundwater recharge from infiltration

2011

996

127

163

290

145

2012

922

215

168

382

191

2013

911

183

151

334

167

2014

934

177

158

335

167

2015

785

158

154

312

156

2016

685

160

156

316

158

2017

533

154

136

290

145

2018

610

182

141

323

162

2019

596

184

101

285

143

Total

6,971

349

159

369

153

509

1,538

420

425

483

1,328

2,866

1,434

Introduction 17

18 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Groundwater Levels and Movement Beginning in 1941, groundwater development in the Irwin subbasin resulted in a lowering of the water table (Densmore and Londquist, 1997; Densmore, 2003; Voronin and others, 2014). Since the mid-1990s, reduced pumping and recharge from treated wastewater and irrigation return caused water levels to rise or stabilize throughout the Irwin subbasin (fig. 7; U.S. Geological Survey, 2019). Since the mid-1990s, water levels rose as much as 45 ft in the northern and central part of the Irwin subbasin (wells 32B3, 32F3, 32K6, 5G1, and 32P6), about 35 ft in wells near the NTC WWTF (wells 4B3 and 4D3), and about 10 ft near the subbasin outflow in the southeast part of the subbasin (well 10E3, fig. 7B). The pumping depression centered near well 32K6 during the mid-1990s (Densmore and Londquist, 1997) diminished by 2019, and the water table flattened out in the area extending from wells 33N1, 32K5, and 32K6 in the center of the subbasin, north to well 32B3, and east to wells 32H1 and 34D1 (fig. 7A, well 32K5 at multiple well site 32K3–6 not shown). In 2019, the highest water-level elevation (2,329 ft) was in well 33R1 near the ponds at the NTC WWTF (table 1; fig. 7B). Water-level elevations in wells 32F3, 32P6, and 32Q3 were 2,317, 2,321, and 2,316 ft, respectively (table 1; fig. 7B). High water levels in these wells are consistent with recharge from infiltration of irrigation return water at base housing and athletic fields in this part of the Irwin subbasin.

In 2019, the water-level elevation in well 5G1 was 2,301 ft (table 1; fig. 7B; U.S. Geological Survey, 2019). The water- level elevation declined in well 5G1 after 2016 as pumping increased from well 5G2 (figs. 2, 7B). A water-table map (fig. 7A) prepared from 29 discrete measurements collected in November and December 2019 and water-level changes in selected wells within the Irwin subbasin between 1990 and 2020 (fig. 7B) are consistent with movement of treated wastewater infiltrated from ponds at the NTC WWTF (table 5) and with movement of smaller amounts of treated wastewater and groundwater recharged as irrigation return from base housing and athletic fields (table 5) toward the center of the Irwin subbasin. Water-level contours (fig. 7A) also are consistent with impediment to groundwater flow through faults that are oriented to the southeast and northwest and subsequent movement of groundwater to the southeast toward the subbasin outflow along the alluviated channel of the wash at the downgradient end of the Irwin subbasin (fig. 7A). Water-level data for wells in unconsolidated deposits in the southwest part of the Irwin subbasin are not available, and water-level contours in this area are not shown (fig. 7A). Where water-level data are available at multiple well sites, water-level elevations generally were similar in shallower monitoring wells completed in the upper aquifer and in deeper wells completed in the lower aquifer (table 1; U.S. Geological Survey, 2019).

Introduction 19 A

116°42'

116°41'

116°40'

116°39'

32B3

ITAM

32F3

? 32H1 32K6

33E3

2,320

DPW606 32K6

32P6

16 2,3 32Q3

2,3

4D3

4B3

2,324 33J2

T. 14 N. T. 13 N.

33R1

4C3

5G1

4G1 ?

2,300

5G1

35°15'

2,3

1 2,3

33H2

33R1

4D3

33N1

2,3

32P6

34D1

33F1 2,316

32F3

35°16'

4K4 4Q2

9B1 ?

10D1

RL NG LT

U FA

10E3

1 MILE

0.5

10E3 0 35°14'

0.5

1 KILOMETER

Aquifer boundary from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Selected land use Pond Duck pond Old biological evaporation pond Golf course and driving range

Geology Base housing 1985–20051

Unconsolidated deposits

Base housing 2005–191

Fault

Army ball field

Aquifer system boundary

Baseball field

Center-pivot sprinkler field

Fritz field

Fort Irwin wastewater treatment facility

Soccer field

Original base housing 1941–841

Wash

Consolidated rock

2,300

2019 water-table elevation, in feet above North American Vertical Datum of 1988 (NAVD 88)— Contour interval 4 feet, queried where uncertain

32H1

33R1

Production well and identifier Well and identifier for well or group of wells used

5G1 DPW606

32B3

Test well and identifier Weather station and identifier Thumbnail of hydrograph shown on figure 7B

Date ranges show onset of initial construction and improvements within that period.

Figure 7. Water-level data, including A, location of wells with water-level hydrographs and 2019 water table elevation; and B, measured water levels in wells, 1993–2019, in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California (U.S. Geological Survey, 2019).

20 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

32B3

1995

Water-level elevation, in feet above NAVD 88

Water-level elevation, in feet above North American Vertical Datum of 1988 (NAVD 88)

2000

2005

2010

2015

2020

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

4D3

1995

2000

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

2020

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

2020

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

33R1

1995

Water-level elevation, in feet above NAVD 88

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

2000

2005

Calendar year

2010

2015

5G1

1995

2000

2005

2010

2015

Water-level elevation, in feet above NAVD 88

Figure 7.—Continued

2000

2005

2010

Calendar year

2015

2020

1995

2000

2005

2010

2015

2020

2005

2010

2015

2020

2015

2020

Calendar year

32K6

1995

2000

Calendar year

32P6

1995

2010

10E3

Calendar year 2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

2005

Calendar year

Water-level elevation, in feet above NAVD 88

Calendar year

2015

2020

2,350 2,340 2,330 2,320 2,310 2,300 2,290 2,280 2,270 2,260 2,250 1990

32F3

1995

2000

2005

Calendar year

2010

Methods of Sample Collection and Analysis 21

Methods of Sample Collection and Analysis Groundwater samples were collected and processed by USGS personnel following the protocols established by the USGS National Field Manual for the Collection of Water-Quality Data (U.S. Geological Survey, 2015). Most sampled wells were monitoring wells installed by the USGS (Densmore and Londquist, 1997). Wells were purged before sample collection and sampled using portable piston pumps. Field parameters (including water levels, pH, specific conductance, and temperature) were measured during well purging, and samples were collected after three casing volumes of groundwater were removed and field parameters stabilized. These sample collection protocols were designed to ensure a representative sample of groundwater is collected and potential contamination of samples during collection and handling is minimized. A detailed description of sampling methods is available in Kjos and others (2014). Groundwater samples were sent to the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, for analysis of major ions, nutrients, and selected trace elements, using methods by Fishman and Friedman (1989), Fishman (1993), Garbarino and others (2002, 2006), and Patton and Kryskalla (2011). Stable isotopes were analyzed by the USGS Stable Isotope Laboratory using methods described by Epstein and Mayeda (1953), Coplen and others (1991), and Coplen (1994). Oxygen-18 (18O) and deuterium (D or 2H) abundances are reported as ratios with the more abundant isotopes oxygen-16 (16O) and hydrogen-1 (1H), relative to those ratios in the Vienna Standard Mean Ocean Water–Standard Light Antarctic Precipitation scale (Coplen, 1994). The ratios are reported in delta (δ) notation as delta oxygen-18 (δ18O) and delta deuterium (δD), in units of parts per thousand (per mil) differences relative to the isotopic ratios in the standards. Selected samples were sent to the Woods Hole Oceanographic Institute, National Ocean Sciences Accelerator Mass Spectrometry Facility in Woods Hole, Massachusetts, for analysis of carbon-14 (14C) and the stable isotope ratio of carbon-13 to carbon-12 using accelerator mass spectrometry and mass spectrometry, respectively (Vogel and others, 1987; Donahue and others, 1990; McNichol and others, 1992, 1994; Gagnon and Jones, 1993; Schneider and others, 1994). Tritium (3H) was analyzed with a detection limit of 0.3–0.5 picocurie per liter, dependent on sample volume, using electrolytic enrichment-gas scintillation at the University of Miami Tritium Laboratory or electrolytic enrichment-liquid scintillation at the USGS Isotope Tracer Laboratory in Menlo Park, California (Oestlund and Werner, 1962; Thatcher and others, 1977; Beukens, 1992). Water-quality data are available in U.S. Geological Survey (2019).

Quality Assurance Replicate data provide information on variability resulting from laboratory, field sample collection, and sample handling. Replicate samples were collected for about 9 percent of samples from wells after the monitoring network was established in 2014; replicate data are unavailable before 2014. For constituents analyzed for trends in this report (dissolved solids, nitrate plus nitrite, arsenic, and fluoride), precision, calculated as the relative standard deviation of replicate pairs, is presented in table 6. Precision of replicate samples from this study was comparable to precision calculated from replicate data available for the NWQL (table 6; U.S. Geological Survey, 2024) and within the limits of acceptable reproducibility that would not be expected to interfere with interpretation of data presented in this report. Because variability also results from sample collection and sample handling in addition to analysis within the NWQL, precision data from other studies (Gross and others, 2012; Medalie and Bexfield, 2020) that were collected during similar periods, using similar techniques, and analyzed using the same methods at the NWQL also are provided for comparison (table 6). In general, relative standard deviations were within similar ranges (Gross and others, 2012; Medalie and Bexfield, 2020). Precision for major-ion data analyzed between 2014 and 2019 as part of this study ranged from 1.2 percent for magnesium to 3.2 precent for sulfate (not shown in table 6); precision for these constituents also are similar to precision of data collected during similar periods at the NWQL (U.S. Geological Survey, 2024) and from other studies (Gross and others, 2012; Medalie and Bexfield, 2020) and is not expected to interfere with interpretation of data presented in this report. The median differences between δ18O or δD values in replicate samples collected as part of this study were 0.04 and 0.6 part per thousand, respectively. Laboratory acceptance criterion for these constituents is ±0.15 per mil for δ18O (Révész and Coplen, 2008b) and within ±1.0 per mil for δD (Révész and Coplen, 2008a). Four of five replicate pairs for δ18O were within 0.07 per mil, and one replicate pair differed by 0.17 per mil; all replicate pairs for δD were within 1.0 per mil. Replicate data were not collected for 3H and 14C data. Blank samples are used to identify systematic issues associated with sample collection, handling, and analytical procedures that may introduce constituents into a sample. Introduction of constituents into a sample may affect study detection limits and thereby limit interpretation of low concentrations in samples. Blank samples were not collected as part of this study; however, constituent concentrations in environmental samples were sufficiently high, so bias associated with this issue was not a concern.

22 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 Table 6. Method reporting levels, method precision, and relative standard deviation for replicate pairs of selected constituents, U.S. Army Fort Irwin National Training Center, California, 2014–19 (U.S. Geological Survey, 2019). [Method description from National Environmental Methods Index (NEMI; https://nemi-test.er.usgs.gov/home/). Method precision reported as percent if laboratory precision varies with concentration or reported as a concentration if laboratory precision is independent of concentration. Abbreviations: mg/L, milligrams per liter; mg/L as N, milligrams per liter as nitrogen; NWQL, National Water Quality Laboratory; µ/L, micrograms per liter; <, less than]

Relative standard deviation, in percent Constituent

Method

Method reporting level

Method precision

This study (2014–19)

NWQL (period of record, and range1 of replicate data)

Other studies

Dissolved solids

I-1739

1 mg/L

3–8 percent

1.4

1.8 (1991–2006, 400–1,000 mg/L)

23.2

Nitrate plus nitrite

I-2547-11

0.04 mg/L as N

<0.02 mg/L as N

1.2

2.9 (1994–2006, 1–4 mg/L as N)

31.6

Arsenic

I-4472-97

1 µg/L

5 percent

2.7

3.0 (2001–06, 10–30 µg/L)

31.6

Fluoride

I-4327-85

0.1`mg/L

0.07 mg/L

3.5

4.7 (1993–2006, 1–2 mg/L)

23.3

1Range of replicate data used to calculate relative standard deviation presented in table. Relative standard deviation commonly increases as the concentration of replicates decreases. Laboratory data do not include variation associated with field sample collection and handling. Additional data are available from the U.S. Geological Survey Quality Systems Branch at https://qsb.usgs.gov/ibsp/charts.php (U.S. Geological Survey, 2024). 2Data collected 1992–2010 from Gross and others (2012). 3Data collected 2013–17 from Medalie and Bexfield (2020).

Statistical Analysis Maximum and minimum values reported for 1993–95 and 2015–19 were the highest and lowest values for wells sampled during those periods. Median values for 1993–95 and 2015–19 were calculated from the median values for each well. Replicate data were not used for the calculation of median values. Comparison of median values calculated for each period were done using the median test (Neter and Wasserman, 1974); differences in median concentrations were considered statistically significant at a confidence criterion (α) of α=0.05, unless otherwise stated. When a different confidence criterion is used, the probability of significance (p- value) also is provided. Regression analysis, including calculation of slope, intercept, and coefficient of determination, was done using the method of least squares (Neter and Wasserman, 1974; Helsel and others, 2020). Monotonic (consistently upward or consistently downward) trends in concentrations in water from selected wells were evaluated using the Mann-Kendall test (Kendall, 1938; Mann, 1945; Helsel and Hirsch, 2002; Helsel and others, 2020). Kendall’s tau calculated by the test is a nonparametric statistic that is less affected by outlier values than more commonly used parametric statistics. Temporal trends of increasing concentrations or upward trends (positive Kendall’s tau values) and decreasing concentrations

or downward trends (negative Kendall’s tau values) were identified as statistically significant or insignificant based on a two-tailed significance level of α=0.05, unless otherwise noted. The magnitudes of the trends are not reported because the different number of data available from sampled wells and the nonuniform temporal distribution of data available from the wells makes comparison of the magnitude of trends calculated for sampled wells difficult. Maximum, minimum, and median values presented in this report were calculated using the computer program Excel (Microsoft, Inc., Redmond, Washington). Results of statistical tests and regression statistics presented in this report were calculated using software from the Statistical Analysis System (SAS Institute, Cary, North Carolina).

Water Quality Water-quality samples were collected from 17 selected wells in the Irwin subbasin (table 1; fig. 8) as part of a groundwater monitoring network established in 2014. For the purpose of this report, these data were supplemented with water-quality data collected during 1993–95 (Densmore and Londquist, 1997; U.S. Geological Survey, 2019) to provide a more than 20-year period to evaluate changes in groundwater quality in the Irwin subbasin.

Water Quality 23 116°42'

116°41'

116°40'

ITAM

32B3 641 2019

32F3 1,310 2019

35°16'

32H1 596 2015 ?

DPW606

33E3 757 2018

32K6 1,580 2019

32P6 1,080 2018 32P4 539 2018

35°15'

116°39'

4D3 1,270 2018

32K5 687 2017

5G1 454 2015

T. 14 N.

33R1 1,910 2017

4B3 1,170 2017

T. 13 N.

4C3 1,260 2018

5G2 499 2019

4K4 1,070 2019

IC NG

SP LT

U FA

1 MILE

0.5

10E3 1,100 2019

35°14'

0.5

1 KILOMETER

Aquifer boundary from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Selected land use Pond Duck pond Old biological evaporation pond Golf course and driving range

Geology Base housing 1985–20051

Unconsolidated deposits

Base housing 2005–191

Fault

Army ball field

Aquifer system boundary

Baseball field

Center-pivot sprinkler field

Fritz field

Fort Irwin wastewater treatment facility

Soccer field

Original base housing 1941–841

Wash 1

Date ranges show onset of initial construction and improvements within that period.

Consolidated rock

4C3 1,260 ! 2018

Monitoring well—Black text is the well identifier, blue text is dissolved-solids concentration, in milligrams per liter, and red text is the year sample was collected

32H1

? Production well and identifier

5G1

Test well and identifier

DPW606

Weather station and identifier

Water-quality shape diagram Na+K Ca Mg

Cl HCO3 +CO3 SO4

15 10 5 0 5 10 15 Cation Anion milliequivalents per liter Color of diagram corresponds to major-ion composition groups presented in figure 9: group1 wells shown in red, group 2 wells having a composition similar to group 1 wells shown in pink; other group 2 wells shown in orange; group 3 wells shown in yellow; lower aquifer wells shown in bold. Na, Sodium; K, Potassium; Ca, Calcium; Mg, Magnesium; Cl, Chloride; HCO3, Bicarbonate; CO3, Carbonate; SO4, Sulfate.

Figure 8. Location of selected wells sampled for water quality and water-quality shape diagrams, Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 2015–19 (U.S. Geological Survey, 2019).

24 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 Eleven of the sampled wells were the shallowest wells at selected multiple-well monitoring sites (Densmore and Londquist, 1997; U.S. Geological Survey, 2019). Seven of these shallow wells (4C3, 4D3, 4K4, 32B3, 32F3, 32K6, and 32P6; fig. 3; 32F3 is not shown on fig. 3) were completed in younger alluvium within the main part of the Irwin subbasin; three wells (4B3, 33E3, and 33R1; fig. 3) were completed in older alluvium. Well 10E3 was completed in younger alluvium near the subbasin outflow in the southeast part of the Irwin subbasin. Three deeper wells at multiple well monitoring sites also were sampled; well 32K5 was completed in younger alluvium, and wells 32F2 and 32P4 were completed in the underlying older alluvium (fig. 3; well 32P4 was not sampled in 2015–19 and is not shown on fig. 8). In addition to wells at multiple-well monitoring sites, three wells (5G1, 5G2, and 32H1; fig. 3) also were sampled. Well 5G1 is a test well screened in older alluvium. Wells 5G2 and 32H1 are production wells; well 5G2 is screened in older alluvium and well 32H1 is screened in younger and older alluvium. Wells 5G1 and 5G2 are screened in similar depths; data from well 5G2 were used in place of data from well 5G1 after 2016. The number of samples collected at each well ranged from 2 at well 32F2 to 11 at well 33R1. In general, shallow wells near the NTC WWTF were sampled more frequently and deeper wells were sampled less frequently. Some samples were analyzed for fewer constituents. Changes in the major-ion composition, dissolved solids and nitrate concentrations, and selected trace element concentrations in water collected from wells between 1993–95 and 2015–19 are discussed in this section. Changes in the stable oxygen and hydrogen isotope composition of water, indicative of the source of water to wells, and changes in the radioactive isotopes of hydrogen (3H) and carbon (14C) activity, indicative of the age (time since recharge) of water from wells, also are discussed.

Major-Ion Composition The major-ion composition of water from sampled wells was evaluated using trilinear and water-quality-shape diagrams. Trilinear diagrams were plotted using a method described by Piper (1944) to graphically show differences between the major-ion composition of water from selected wells and changes in the major-ion composition of water from selected wells through time. Water-quality-shape diagrams were plotted using a method developed by Stiff (1951) and show differences in the major-ion composition of water from wells. Trilinear diagrams (fig. 9) show the relative contribution of major cations as a percentage of the total positive charge in the triangle to the left and major anions as a percentage of the total negative charge in the triangle to the right. The central diamond, with cations on the upper right and lower

left and the anions on the upper left and lower right sides, integrates data from the left and right triangles. The position of a sample on the diagram shows the major-ion composition of water and allows comparisons to be made between samples collected at different times and from different wells. The major-ion composition identified on the diagram is named for the cations and anions that contribute more than 50 percent of the respective charge. In general, the major-ion composition of water from most sampled wells in the Irwin subbasin is composed of more than 50-percent sodium on the cationic side, with differing percentages of the two most abundant anions, chloride and sulfate, on the anionic side of the diagram. For the purpose of comparing changes in the major-ion composition of water from wells over time, the earliest sample collected from each well (1993–95) is shown in red, the latest sample (2015–19) is shown in blue; samples collected between the earliest and latest samples are shown as open green circles (fig. 9A). Samples were subsequently divided into three groups based on their location and depth within the Irwin subbasin: (1) group 1 (fig. 9B) includes samples from monitoring wells 4B3, 4C3, 4D3, and 33R1 closest to the NTC WWTF; (2) group 2 (fig. 9C) includes samples from shallow monitoring wells in the northern and central part of the Irwin subbasin (wells 4K4, 32B3, 32F3, 32K6, 32P6, 33E3, deeper well 32K5 was included in group 2 because it is screened in younger alluvium); (3) group 3 (fig. 9D) includes samples from deeper wells, including monitoring wells 32F2 and 32P4 completed in older alluvium, samples from test well 5G1, and production wells 5G2 and 32H1. Monitoring well 10E3, completed in younger alluvium near the basin outflow, also was included in group 3. Only the central diamond of the trilinear diagram is shown for the three groups (figs. 9B–9D). Most group 1 wells, closest to the NTC WWTF, (fig. 9B) had a relatively uniform sodium/chloride-sulfate composition, with only comparatively small changes in major-ion composition measured between 1993–95 and 2015–19. Water from these wells presumably contains the highest fraction of treated wastewater. In April 2019, water from well 33R1, closest to the NTC WWTF discharge ponds (fig. 2), had a strongly sodium/carbonate composition that differed from other group 1 samples collected near the WWTF. The difference in major-ion composition was characterized by lower dissolved solids (693 mg/L) and lower nitrate concentrations (1.4 mg/L as N), compared to the average concentrations of other samples of water from well 33R1 (1,050 mg/L and 6 mg/L as N, respectively; U.S. Geological Survey, 2019). Water sampled from well 33R1 in April 2019 may have been altered by precipitation in the Irwin subbasin that accumulated within ponds near the NTC WWTF and subsequently infiltrated to groundwater (the data and the source of water from well 33R1 at that time are discussed in the “Stable Oxygen and Hydrogen Isotopes in Water” section).

) lor

)+ 4

T 20

HC O3 )

Bic arb ona te (

3) +

100

nat

rbo

100

) (C l

20 60 80

Ch )+ 4

(S O Su

lfa

100

Su 0

i de

(S O lfa

lor

lor Ch )+

100

) (C l i de

100

g) (M um esi gn 100

) (C l i de 4

(S O te

100

60 80

EXPLANATION Ending (2015–19) composition and well identifier

10E3 32H1 32F2 5G1 5G2 10E3 5G1 32H1 10E3, 2014–16 32P4 32P4

20 40

32F3

lfa

)

siu

gne

Most samples from group 1

gne

lciu

60 100 4C3

100

)

siu

32K6 Most samples 32P6 from group 1 32P6 33E3 32K5 4K4 32B3 Mixing line 32B3 32K6 33E3 32K5

Initial (1993–95) composition and well identifier

32F3 4K4

gne

33R1

D. Group 3: deep wells, production wells, and other wells

lor

Chloride (Cl) ANIONS

lciu

Ma 4C3

100

O 4) 20

C. Group 2: shallow wells in the northern and central subbasin

lciu

e (S

PERCENT

100

4C3 4D3 4D3 Most samples from group 1 4B3 4B3

lfat Su

100

PE RC EN

T 0

100

siu 20

100

Calcium (Ca) CATIONS

B. Group 1: wells closest to the wastewater treatment facility

4C3

tas Po

33R1

EN RC PE

0 20 60

)+ Na

SO 4

m( diu So

All other compositions

) Mg

100 80

Ending (2015–19) composition

(S O lfa

Initial (1993–95) composition

m( siu gne Ma

EXPLANATION

i de

(C l

)+ Ca m( lciu Ca

100

A. All wells 1993–2019

100

Water Quality 25

4C3

All other compositions and well identifier

Figure 9. Major-ion composition of water in A, water from wells sampled between 1993 and 2019; B, water from wells closest to the wastewater treatment facility; C, water from shallow wells; and D, water from deep wells, production wells, and other wells, Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California (U.S. Geological Survey, 2019). Figures 9B–9D identify wells that have water-quality samples in both the initial evaluation period (1993–95) and the ending evaluation period (2015–19) to show how ion composition changed over time in each well grouping. Hollow green dots represent samples from wells that do not have pairs in both the initial and ending evaluation periods or are outside of those timeframes. A dashed circle represents the general composition of wells in group 1 to show how ion composition changes as you move away from the wastewater treatment facility.

26 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 Group 2 wells (generally shallow wells in the northern and central Irwin subbasin, fig. 9C), revealed larger changes in major-ion composition between 1993–95 and 2015–19 than group 1 wells closer to the NTC WWTF (fig. 9B). In general, the major-ion composition of water from group 2 wells changed as chloride and sulfate percentages increased and sodium percentages decreased between 1993–95 and 2015–19. By 2019, water from four wells (4K4, 32F3, 32K6, and 32P6) was within the range of major-ion composition of group 1 wells closest to the NTC WWTF—consistent with increasing amounts of treated wastewater in water from these wells. Wells 32F3 and 32P6 are farther from the NTC WWTF than other wells in group 2 and are upgradient from the pumping depression near the center of the subbasin (fig. 7A). These wells are near base housing, and given their major- ion composition, water from these wells has been affected by past septic discharges in this area and ongoing irrigation of base housing and athletic fields with treated wastewater (Groover and others, 2020). Water from three wells (32B3, 32K5, and 33E3) plot along a mixing line extending from their initial (1993–95) composition to the composition of water in group 1 but do not have a major-ion composition in 2015–19 within the range of treated wastewater indicated by group 1 wells (fig. 9C). Wells 32B3 and 33E3 are in the northern part of the Irwin subbasin, in areas less affected by NTC WWTF discharges, past septic discharges, or irrigation using treated wastewater at base housing. Although completed in younger alluvium, well 32K5 (screened from 270 to 290 ft below land surface) is deeper than well 32K6 at the same site (screened from 210 to 230 ft below land surface). The major- ion composition in water from well 32K5 is consistent with groundwater within younger alluvium that contains small amounts of treated wastewater. Most group 3 wells (deep wells, production wells, and other wells, fig. 9D) generally show only small changes in major-ion composition between 1993–95 and 2015–19 and do not plot within the range of the major-ion composition of wells near the NTC WWTF. Well 32F2 (fig. 9D) was sampled in 1995 and 2019 but was not analyzed for major-ion chemistry in 2019, so it was not possible to determine if the composition of water changed in the older alluvium at this site during the study period. Production well 32H1 shows changes in major- ion composition that differ from other sampled wells. Well 32H1 shows an increase in sodium percentages, whereas chloride and sulfate percentages remain relatively unchanged between 1993 and 2019. Well 32H1 is screened in both younger and older alluvium and seems to have yielded a larger fraction of water from older alluvium by 2019. Well 10E3 is completed in the shallow aquifer near the basin outflow in the southeast part of the basin. There was little difference in the major-ion composition of water from well 10E3 between the earliest sample (1993) and the latest sample (2019), and the major-ion composition of most samples of water from well 10E3 differed only slightly from the composition of water from wells near the NTC WWTF (figs. 9B, 9D). However, during 2014 and 2016, the sodium percentage increased, and the sulfate-chloride percentage decreased

in water from well 10E3 (U.S. Geological Survey, 2019). These samples were collected after large storms and resulting runoff in 2013 produced streamflow in the nearby wash that potentially recharged the aquifer along the wash (recharge along the wash is discussed in the “Isotope Composition of Groundwater” section). To assist with visualizing the spatial distribution of the major-ion composition of water from sampled wells at the end of the study period (2015–19), water-quality-shape diagrams, prepared using a method developed by Stiff (1951), are shown (fig. 8). The diagrams show the cations to the left (sodium plus potassium, calcium, and magnesium) and anions to the right (chloride, bicarbonate plus carbonate, and sulfate) in milliequivalents per liter; the area on the left side of the diagram equals the area on the right side. Similarly shaped diagrams indicate water that has similar major-ion composition; the widths of the diagrams differ according to the total ionic charge of the dissolved constituents, which is proportional to the dissolved solids concentration. For this comparison, data from the sample from well 33R1 in group 1, collected in March 2017, was used instead of the sodium/carbonate water sample collected in April 2019. Samples are grouped in the manner as in the trilinear diagrams (figs. 9B–9D). Water samples from wells in group 1, closest to the NTC WWTF, are shown in red (fig. 8). The diagrams representing the major-ion composition of water from these wells have a similar shape, showing the most abundant cation sodium (plus potassium) paired with the anions chloride and sulfate, with lower abundances of magnesium and bicarbonate plus carbonate (fig. 8). As previously discussed, water from these wells contains the highest fraction of treated wastewater. Water samples from shallow monitoring wells within the Irwin subbasin in group 2 that had a major-ion composition similar to the group 1 wells in 2015–19 are shown in pink (fig. 8). The diagrams that represent the major-ion composition of water from these wells are similar in size and shape to diagrams for group 1 wells, although the sodium and chloride abundances are greater. The group 2 wells include shallow monitoring wells 32F3 and 32P6 near the northwestern margin of the basin and upgradient from the pumping depression in the center of the Irwin subbasin. Water from wells 32F3 and 32P6 was recharged from sources other than infiltration from ponds at the NTC WWTF, including past infiltration of treated septic from base housing and return water from irrigation at base housing and athletic fields using groundwater and treated wastewater. As previously discussed, changes in the major- ion compositions of water from these wells are consistent with increasing fractions of treated wastewater (fig. 9B). Other shallow wells in group 2 that do not have a major-ion composition within the range of treated wastewater (32B3, 32K5, and 33E3) are shown in orange (fig. 8). These wells are downgradient from areas that received recharge from past septic discharges or recharge as return from irrigation with treated wastewater at base housing or athletic fields and seem to have some history of recharge from treated wastewater.

Water Quality 27 Water samples from deep monitoring well 32P4, test well 5G1, and production wells 5G2 and 32H1 in group 3 are shown in yellow (fig. 8). The water-quality-shape diagrams for water from these wells differ in shape and size from the group 1 and group 2 wells. The shapes of the diagrams show sodium (plus potassium) as the most abundant cation, but the diagrams show similar abundances of the anions chloride, bicarbonate plus carbonate, and sulfate. The diagrams are smaller than those in group 1, which is consistent with lower ionic strength and lower dissolved solids concentrations in water from these wells. It is likely that these diagrams show the composition of native water from wells that have been less affected by treated wastewater discharge or infiltration of treated septic from base housing and return water from irrigation using groundwater and treated wastewater. Water from well 10E3, classified in group 3, near the southeastern margin of the Irwin subbasin, is the result of mixing from different source waters near the subbasin outflow. Based on its major-ion composition (fig. 8), water from well 10E3 also may include a large fraction of treated wastewater.

Temporal Changes in Dissolved Solids and Nitrate Concentrations The concentrations of dissolved solids and nitrates in water used for public supply are regulated by the State of California; therefore, changes in these constituents are important to water resource managers. The secondary maximum contaminant level (SMCL) for dissolved solids in drinking water is 500 mg/L, with an upper limit of 1,000 mg/L and a short-term limit of 1,500 mg/L (California State Water Resources Control Board, 2021). The SMCLs are based on taste, aesthetics, or scale-forming properties (hardness) of the water and are not enforceable by law. The maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L as N (California State Water Resources Control Board, 2021) and is enforceable by law. In 1993–95, dissolved solids concentrations in 16 sampled wells ranged from 453 to 1,530 mg/L, and nitrate concentrations ranged from less than the laboratory reporting level of 0.02–12 mg/L as N (U.S. Geological Survey, 2019). Median dissolved solids and nitrate concentrations in water from sampled wells were 620 mg/L and 2.8 mg/L as N, respectively. By 2015–19, dissolved solids concentrations in water from 17 sampled wells ranged from 454 mg/L (well 5G1) to 1,910 mg/L (well 33R1), and nitrate concentrations ranged from less than the laboratory reporting level of 0.04 mg/L (well 32F2) to 28 mg/L as N (well 33R1). Median dissolved solids and nitrate concentrations in water from sampled wells were 1,030 mg/L and 4.5 mg/L as N, respectively. Although dissolved solids and nitrate

concentrations increased between 1993–95 and 2015–19, the change in the overall median concentration of dissolved solids in the Irwin subbasin was not statistically significant based on the results of the median test (Neter and Wasserman, 1974) at a confidence criterion of α=0.05; however, the change in nitrate concentrations was statistically significant at a confidence criterion of α=0.10 (p-value 0.06). Chemographs for dissolved solids and nitrate concentrations for samples from 15 wells with water-quality data are presented for spatial comparison between wells (fig. 10). Adjacent wells 5G1 and 5G2, perforated in similar intervals, are presented on one chemograph, with data for well 5G1 shown for 1993–2015 and data for well 5G2 shown for 2016–19. Between 1993–95 and 2015–19, eight wells indicated statistically significant upward trends in dissolved solids concentrations at a confidence criterion of α=0.05 (table 7); well 32K5 had a statistically significant upward trend at a confidence criterion of α=0.10 (p-value 0.051; table 7). Well 32K5 is completed in the upper aquifer at a greater depth than well 32K6 at the same site. Wells with water having significant upward trends in dissolved solids concentrations included wells 32F3 and 32P6 near base housing and irrigated fields. Dissolved solids in water from well 32K6 in the same area also increased from 864 to 1,580 mg/L between 1993 and 2019, but the increase was not statistically significant (table 7). Five other wells, including wells 5G1–2 and 32H1 that yield water primarily from the lower aquifer, indicate statistically significant upward trends in dissolved solids concentrations. Well 32P4, completed in the underlying Tertiary deposits, had a statistically significant downward trend in dissolved solids concentrations at a confidence criterion of α=0.10 (p-value 0.099; table 7). Dissolved solids concentration trends were not statistically significant in water from two wells in group 1 nearest the NTC WWTF (wells 4B3 and 4C3, table 7), largely because water from these wells had high dissolved solids concentrations as a result of treated wastewater discharges before 1993. Dissolved solids concentrations in water from well 4B3 decreased from 1,520 mg/L in 1994 to 1,170 mg/L in 2017, and concentrations in water from well 4C3 decreased from a maximum concentration of 1,340 mg/L in 2014 to 1,260 in 2018. Although downward concentration trends were not statistically significant, decreases in dissolved solids concentrations in water from wells near the NTC WWTF may result from changes in source water quality attributable to reverse osmosis of treated drinking water delivered within the Irwin subbasin beginning in 2016. Decreases in dissolved solids concentrations are likely to continue and become measurable in other wells if reverse osmosis of the source water supply continues.

28 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 A

116°42'

116°41'

116°40'

116°39'

32B3

32H1

32F2

35°16'

ITAM

32B1-3

32F3

33E2, 3

32H1

32F2

32F3

! 33E3

DPW606 32K3-6

32K6

33R1

32P6

T. 14 N.

33R1

32P2-6

4D3

! 4B1-3

4D1-3

32P4

4B3

T. 13 N.

! 4C1-3 4C3 5G1, 2

35°15'

5G1 ? 5G2 4K2-4 ! 4K4

GA NG

10E3

U FA

1 MILE

0.5

10E1-3 ! 0 35°14'

0.5

1 KILOMETER

Aquifer boundary from Densmore and Londquist, 1997; Geology modified from Miller and others, 2014

EXPLANATION Selected land use

Geology Base housing 1985–20051

Pond Duck pond Old biological evaporation pond Golf course and driving range

Unconsolidated deposits

Fault

Army ball field

Aquifer system boundary

Baseball field

32H1

Center-pivot sprinkler field

Fritz field

33R1

Fort Irwin wastewater treatment facility

Soccer field

Well and identifier for well or group of wells used

5G1

Test well and identifier

Original base housing 1941–841

Thumbnail of chemograph shown on figure 10B

Consolidated rock

Base housing 2005–191

Wash

32B3

DPW606

Production well and identifier

Weather station and identifier

Date ranges show onset of initial construction and improvements within that period.

Figure 10. Dissolved solids and nitrate plus nitrite concentrations in water from selected wells. A, Map of well locations; and B, chemographs of dissolved solids and nitrate plus nitrite concentrations in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019 (U.S. Geological Survey, 2019).

Water Quality 29

1,400

1,200 1,000

800

600 400

200 0

1990

1995

2000

2005

2010

2015

2020

1,600 1,400

1,200 1,000

Estimated from specific conductance

800 600 400

1995

2000

2005

2010

Calendar year

2015

2,000 1,800

0 2020

33E3

1,600 1,400

1,200 1,000

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

0 2020

Nitrate plus nitrite, in milligrams per liter

30 Irrigation using recycled water begins

32F2

Nitrate plus nitrite, in milligrams per liter

Dissolved solids, in milligrams per liter

1,800

1,000

800

600 400 200 0 1990

1995

2000

2005

Figure 10.—Continued

Dissolved solids

2010

Calendar year

2015

2,000

0 2020

32H1

1,800

1,600 1,400

1,200 1,000

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

2,000

0 2020

30 Irrigation using recycled water begins

32K6

1,800 1,600 1,400

25 20

1,200 1,000

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

EXPLANATION Nitrate plus nitrite

25 20

1,200

Calendar year

2,000

Irrigation using recycled water begins

32F3

1,800

? Concentration uncertain

2015

0 2020

Nitrate plus nitrite, in milligrams per liter

1,600

2,000

Nitrate plus nitrite, in milligrams per liter

32B3

Nitrate plus nitrite, in milligrams per liter

1,800

Dissolved solids, in milligrams per liter

2,000

Nitrate plus nitrite, in milligrams per liter

30 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

2,000

Dissolved solids, in milligrams per liter

1,800

4B3

1,600 1,400

1,200 1,000

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

2,000

Dissolved solids, in milligrams per liter

1,800

0 2020

4K4

1,600 1,400

1,200 1,000

800

600 400

200 0 1990

0 2020

1995

2000

2005

2010

Calendar year

2015

0 2020

1,400

1,200 1,000

800

600 400

200 0 1990

1995

Figure 10.—Continued

2005

2010

Calendar year

2015

2,000 1,800

4C3

1,600 1,400

1,200 1,000

800

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

2,000 1,800

0 2020

5G1, 2

1,600 1,400

1,200 1,000

15 Data before 2016 are 5G1

800 600 400

10 5

200 0 1990

0 2020

1995

EXPLANATION Nitrate plus nitrite

2000

Dissolved solids

2000

2005

2010

Calendar year

2015

0 2020

Nitrate plus nitrite, in milligrams per liter

1,600

Nitrate plus nitrite, in milligrams per liter

1,000

4D3

Nitrate plus nitrite, in milligrams per liter

1,200

Dissolved solids, in milligrams per liter

1,400

1,800

Dissolved solids, in milligrams per liter

Reverse osmosis of drinking water begins

1,600

2,000

Dissolved solids, in milligrams per liter

33R1

Nitrate plus nitrite, in milligrams per liter

Dissolved solids, in milligrams per liter

1,800

Nitrate plus nitrite, in milligrams per liter

2,000

Nitrate plus nitrite, in milligrams per liter

B —Continued

Water Quality 31

1,600 1,400

Irrigation using recycled water begins

1,200

Irrigation at this location stopped when athletic field converted to artificial turf

1,000

600 400

200 1995

2000

2005

2010

Calendar year

2015

2,000

Dissolved solids, in milligrams per liter

1,800

0 2020

10E3

August 25, 2013, flood

1,600 1,400

Reverse osmosis of drinking water begins

1,200

1,000

800

600 400

200 0 1990

20 15

800

0 1990

1995

2000

2005

2010

Calendar year

2015

0 2020

2,000 1,800

30 Irrigation using recycled water begins

32P4

1,600 1,400

Irrigation at this location stopped when athletic field converted to artificial turf

1,200 1,000 800

20 15 10

600 400

200 0 1990

1995

2000

2005

2010

Calendar year

2015

0 2020

Nitrate plus nitrite, in milligrams per liter

32P6

Dissolved solids, in milligrams per liter

1,800

Nitrate plus nitrite, in milligrams per liter

2,000

Nitrate plus nitrite, in milligrams per liter

B —Continued

EXPLANATION Nitrate plus nitrite

Dissolved solids

Figure 10.—Continued

Only water from production well 32H1 indicated a statistically significant upward trend in nitrate concentrations between 1993–95 and 2015–19 at a confidence criterion of α=0.05 (table 7); water from well 32B3 indicated a significant upward trend at a confidence criterion of α=0.10 (p-value=0.062; table 7). Water from well 4B3, in group 1 near the NTC WWTF, indicated a statistically significant downward trend in nitrate concentrations (table 7). The decrease in nitrate concentrations in water from well 4B3 may have resulted from changes in wastewater treatment practices that remove nitrate from treated wastewater. Water from well 10E3 in the southwestern part of the Irwin subbasin also indicated statistically significant downward trends in nitrate concentrations (table 7). As previously discussed, water from well 10E3 is a mixture of water from different sources near

the basin outflow and, based on its major-ion composition, may contain a large fraction of treated wastewater (fig. 8). Downward nitrate concentration trends in water from well 10E3 may result from changes in several wastewater management practices (table 2) and from natural recharge along the wash that drains the Irwin subbasin. The lowest nitrate concentrations were in water from well 32K6 near the center of the Irwin subbasin; low nitrate concentrations were measured when dissolved oxygen concentrations in water from well 32K6 were at or less than the reporting limit of 0.2 mg/L (U.S. Geological Survey, 2019). Low dissolved oxygen concentrations are consistent with reducing conditions and indicate that nitrate in wastewater may be reduced and removed from groundwater in this area.

California, 1993–2019. [Wells grouped by similarity in major-ion composition (fig. 9). Linear trends were calculated using the Mann-Kendall test (Kendall, 1938; Mann, 1945): U and u are upward concentration trends at a confidence criterion of α=0.05 and α=0.10, respectively; D and d are downward concentration trends at a confidence criterion of α=0.05 and α=0.10, respectively. The p-value is the probability of significance calculated using the Mann-Kendall test. Data are available in U.S. Geological Survey (2019). Values are rounded for presentation. Abbreviations: N, number of samples; —, no statistically significant trend; <, less than]

Well

Dissolved solids,

Nitrate plus nitrite,

Arsenic,

Fluoride,

in milligrams per liter

in milligrams per liter as nitrogen

in micrograms per liter

in milligrams per liter

Range

Trend

p-value

Range

Trend

p-value

Range

Trend

p-value

Range

Trend

p-value

4B3

1,150–1,530

—

0.251

0.72–9.2

0.050

10–19

—

0.999

2.8–5.2

0.022

4C3

744–1,330

—

0.245

1.4–12

—

0.102

11–21

0.095

3.2–5.0

0.048

4D3

534–1,270

0.016

0.77–8.2

—

0.105

11–59

—

0.453

3.1–6.8

—

0.138

133R1

754–1,910

0.022

2.9–28

—

0.472

15–35

—

0.348

3.2–7.3

0.041

4K4

528–1,070

0.039

2.6–7.2

—

0.453

5.9–15

—

0.128

1.8–3.4

—

0.129

32B3

564–676

—

0.901

2.2–5.0

0.062

27–54

—

0.881

4.0–6.5

—

0.621

32F3

516–1,310

0.014

2.9–23

—

0.624

17–44

—

0.327

5.8–10

0.0143

32K5

474–687

0.051

2.4–9.2

—

0.761

25–44

0.0558

3.6–8.5

0.0558

32K6

864–1,580

—

0.453

<0.05–2.2

—

0.227

4.8–30

0.005

4.1–6.9

—

0.652

32P6

708–1,150

0.011

5.3–8.5

—

0.176

9.9–31

0.011

1.8–4.6

—

0.176

33E3

530–757

0.011

4.1–5.4

—

0.543

18–81

0.051

4.4–8.3

0.098

5G1-2

453–499

0.024

1.5–3.6

—

0.881

8.0–14

0.024

3.1–4.6

0.051

10E3

519–1,110

—

0.982

1.6–7.6

0.037

14–23

0.033

<0.10–7.2

—

0.421

32H1

488–602

0.050

2.1–4.4

0.050

31–39

—

0.497

8.4–11

—

0.624

32P4

533–566

0.099

0.37–0.45

—

0.881

20–26

—

0.881

11–14

0.068

1Sample collected on April 3, 2019, has anomalous chemistry and was potentially affected by precipitation (fig. 11); it was not included.

32 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Table 7. Trends in dissolved solids, nitrate plus nitrite, arsenic, and fluoride concentrations in water from selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center,

Water Quality 33

Selected Trace Elements Arsenic and fluoride in groundwater may be present naturally at concentrations of concern for public health and are known as geogenic contaminants. In groundwater worldwide, arsenic and fluoride are the two geogenic contaminants of greatest public health concern (Howard and others, 2006; Johnson and others, 2017). Other geogenic constituents of potential public health concern in groundwater in the western Mojave Desert include chromium, uranium, and vanadium (Izbicki and others, 2023). Arsenic and other trace element concentrations in groundwater often increase as groundwater levels decline. This increase is a result of pumping as wells yield older, more alkaline water from deeper deposits (González and others, 2022; Izbicki and Seymour, 2023; Izbicki and others, 2023). Between 1993 and 95, arsenic concentrations in water from wells in the Irwin subbasin ranged from 8 to 81 micrograms per liter (µg/L; U.S. Geological Survey, 2019), with a median concentration of 28 µg/L. At that time, water from 15 of 16 sampled wells exceeded the MCL of 10 µg/L (U.S. Environmental Protection Agency, 2023), with water from 3 wells exceeding the MCL of 50 µg/L that was mandated at that time (U.S. Environmental Protection Agency, 2001). By 2015–19, arsenic concentrations in water from 16 sampled wells ranged from 4.8 µg/L (well 32K6) to 37 µg/L (well 32B3), with a median concentration of 16 µg/L; water from 14 of 16 wells had concentrations that exceeded the MCL of 10 µg/L (U.S. Geological Survey, 2019). Statistically significant downward trends in arsenic concentration were measured between 1993–95 and 2015–19 using the Kendall test for trend in four shallow monitoring wells and in deeper well 32K5 (confidence criterion α=0.05 and α=0.10; table 7). These wells all indicated changes in major-ion composition, consistent with wastewater disposal from ponds or return from irrigation with treated wastewater (fig. 9). Downward trends in arsenic concentrations measured in water from wells between 1993–95 and 2015–19 were likely the result of dilution of native groundwater that contains arsenic by treated wastewater that has lower arsenic concentrations. Statistically significant downward trends in arsenic concentrations during this period were not measured in deeper wells 32H1 and 32P4 (table 7) that yield water from Tertiary deposits. In contrast to other wells, a statistically significant upward trend in arsenic concentrations was measured in water from well 10E3 near the southeast corner of the Irwin subbasin and in water from well 5G1–2 (table 7). Between 1993 and 95, fluoride concentrations in water from wells in the Irwin subbasin ranged from less than the laboratory reporting limit of 0.10 to 21 mg/L (U.S. Geological Survey, 2019), with a median concentration of 5.4 mg/L. During that time, fluoride concentrations in water from all 16 sampled wells exceeded the MCL set by the State

of California of 2 mg/L (California State Water Resources Control Board, 2023). By 2015–19, fluoride concentrations in water from wells in the Irwin subbasin ranged from 1.8 mg/L (well 4K4) to 14 mg/L (well 32P4), with a median concentration of 4.3 mg/L. During that period, concentrations of fluoride in water from 15 of 16 wells sampled exceeded the California MCL of 2 mg/L (U.S. Geological Survey, 2019). Statistically significant downward trends in fluoride concentration were measured between 1993–95 and 2015–19 using the Kendall test for trend in six shallow monitoring wells, in deeper well 32K5 completed in the upper aquifer, and in water from well 32P4 completed in underlying Tertiary deposits (table 7). Fluoride concentrations in water from wells 32K6 and 32P6, which indicated significant downward trends in arsenic concentrations, decreased from maximum concentrations of 6.9 and 4.6 during 1993–95 to 4.1 mg/L and 1.8 mg/L during 2015–19, respectively, but fluoride concentration trends were not statistically significant (table 7). These changes are consistent with dilution of groundwater with treated wastewater that has a lower fluoride concentration. Downward trends in fluoride concentrations in water from well 32P4 are consistent with small amounts of treated wastewater present within the underlying Tertiary deposits. Chromium, uranium, and vanadium are geogenic trace elements that also occur naturally at high concentrations in groundwater in some areas of the western Mojave Desert (Izbicki and others, 2023). In 2015–19, chromium concentrations (presumably in the form of hexavalent chromium [Cr(VI)]) ranged from less than the laboratory reporting level of 0.3 to 9.9 µg/L (U.S. Geological Survey, 2019); no wells exceeded the proposed California MCL for Cr(VI) of 10 µg/L (California State Water Resources Control Board, 2022). In 2015–19, uranium concentrations ranged from 0.13 to 106 µg/L (U.S. Geological Survey, 2019), with a median concentration of 8.0 µg/L. At that time, water from wells 4B3, 10E3, 32P6, 32K6, and 33R1 exceeded the MCL for uranium of 30 µg/L (U.S. Environmental Protection Agency, 2023). Statistically significant increasing uranium concentration trends were measured in water from wells 4K4, 4D3, and 4C3 near the NTC WWTF from 2015 to 2019, although none of these wells exceeded the MCL for uranium (not shown in table 7). In 2015–19, vanadium concentrations ranged from 3.0 to 40 µg/L (U.S. Geological Survey, 2019), with a median concentration of 14 µg/L; no wells exceeded the California notification level for vanadium of 50 µg/L (California State Water Resources Control Board, 2023). Low chromium concentrations and high uranium concentrations in water from wells in the Irwin subbasin are consistent with the local granitic geology and the regional occurrence of these constituents in alkaline, oxic groundwater in the western Mojave Desert (Izbicki and others, 2023).

34 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Isotope Composition of Groundwater The stable isotopes of oxygen and hydrogen (reported as δ18O and δD, respectively) are tracers of the source and movement of water and are used to understand the source, movement, and hydrologic history of water from sampled wells. The radioactive isotopes of hydrogen and carbon (3H and 14C, respectively) are used to determine the age (time since recharge) of groundwater, locate sources of recharge, and identify geologic controls on the movement of groundwater. In this study, δ18O, δD, 3H, and 14C were analyzed in water samples from wells to determine their source and age.

Stable Oxygen and Hydrogen Isotopes in Water Oxygen-18 and D are natural stable isotopes of oxygen and hydrogen, respectively. The ratios of oxygen isotopes (18O to the more abundant isotope 16O) and hydrogen isotopes (D to the more abundant isotope hydrogen, 1H) in groundwater are indicators of the source, movement, and hydrologic history of the water. The stable water isotope ratios (δ18O and δD) are expressed in delta notation (δ) as per mil differences, relative to the standard known as Vienna Standard Mean Ocean Water (Gonfiantini, 1978). The linear relation between δ18O and δD in natural precipitation throughout the world (Craig, 1961) is known as the global meteoric water line (GMWL; fig. 11). The GMWL exists because most precipitation is derived from the evaporation of ocean water, which undergoes fractionation during evaporation from the ocean surface to the vapor phase. Further fractionation occurs as precipitation condenses from the atmosphere, leaving the remaining water vapor relatively depleted in the heavier isotopes of 18O and D compared to the isotope composition of seawater. Latitude, elevation, and air temperature affect the isotope composition of atmospheric water. Precipitation from a given storm becomes isotopically lighter (more negative) as the storm moves inland and over higher elevations with cooler temperatures (Fournier and Thompson, 1980). These effects combine to give precipitation in a given area a range of δ18O and δD compositions distributed along a global meteoric water line that has a slope of 8. The δ18O and δD composition of groundwater derived from precipitation in a given area is distributed along a line known as the local groundwater line. In the Mojave Desert area, the local groundwater line is commonly subparallel (also with a slope of 8) to the GMWL (fig. 11; Izbicki, 2004; Groover and others, 2024). More negative values of δ18O and δD distributed along the local groundwater line represent enrichment in the lighter isotopes of oxygen and hydrogen, respectively (or a depletion in the heavier isotopes), and less negative values represent enrichment in the heavier isotopes. As the lighter isotopes of oxygen and hydrogen are preferentially removed during evaporation, the remaining water is enriched in the heavier isotopes relative to its original composition. The remaining partly evaporated water has δ18O

and δD values that plot below and to the right of the local groundwater line along a line known as the evaporative trend line (fig. 11). Evaporative trend lines commonly have a slope between 3 and 6 (Gat and Gonfiantini, 1981). The slope of the evaporative trend line is less for evaporation under drier, less humid conditions and higher for evaporation under wetter, more humid conditions. At the low temperatures within most groundwater systems, there is no change in the isotope composition of groundwater after infiltration below the depth of evaporation; therefore, any subsequent differences in the isotopic composition of groundwater along a flow line generally reflect either mixing within the aquifer system or fractionation by evaporation in groundwater discharge areas. The δ18O and δD composition of groundwater, relative to the GMWL, the local groundwater line, and the isotopic composition of water from other sources can be an indicator of the source, movement, and hydrologic history of groundwater (Gat and Gonfiantini, 1981; Izbicki, 2004).

Isotope Composition of Water from Wells The δ18O and δD isotope composition of 107 water samples from 17 wells in the Irwin subbasin collected between 1993 and 2019 ranged from −5.98 to −12.12 per mil for δ18O and from −67.8 to −98.1 per mil for δD (fig. 11; U.S. Geological Survey, 2019). Most samples plotted along a local groundwater line subparallel, with a slope of 8 and 13 per mil below the GMWL, or along evaporative trend lines intersecting the local groundwater line (fig. 11). Only one sample, from well 33R1 collected in April 2019, plotted close to the GMWL near the volume-weighted average δ18O and δD composition of −10.9 and −77 for recent (1994–97) Mojave Desert winter precipitation (Izbicki, 2004). It is possible that water sampled from well 33R1 in April 2019 is related to precipitation from large storms in the Irwin subbasin in earlier years. All other samples plotted either along the local groundwater line or along an evaporative trend line, with unevaporated δ18O and δD compositions more negative than those of recent (1994–97) precipitation (fig. 11). In 1993–95, the δ18O and δD composition of water from wells 4B3, 4C3, 4D3, and 33R1 near the NTC WWTF (in major-ion group 1, fig. 9) initially plotted along the local groundwater line with δ18O and δD compositions more negative than −10.7 and −88 per mil, respectively (fig. 11). By 2011, water from well 33R1 indicated changes in δ18O and δD composition consistent with evaporation, and, by 2014, water from wells 4B3, 4C3, and 4D3 also indicated changes consistent with evaporation. Evaporation likely occurred in ponds where treated wastewater from the NTC WWTF was discharged and allowed to infiltrate and recharge groundwater. Wells were not consistently sampled and analyzed for δ18O and δD between 1993–94 and 2011–14; consequently, the timing of the evaporative changes in water from these wells is not precisely known. Data from these wells were used to estimate an evaporative trend line that has a slope of 4.7.

Water Quality 35

delta Deuterium, in per mil

33R1; April 2019

lin e , ine

e lop

ce en d i f n ive co .7 at nt e=4 or e p p c a er slo Ev 5-p .99, 9 er 2 >0 R ow d l tion, n a a r n pe rmi e up ti h f det w o ine nt d l ficie n re ef e t co nd

ls,

r nte

ati

p va

tre

–80

–90

Lo ca lg rou nd wa ter

–70

Glo ba wa l me ter teo lin ric e

–60

10E3; June 2016 10E3; December 2014 –100 –13

–11

–9

–7

delta Oxygen-18, in per mil EXPLANATION Wells 4C3, 4B3, 4D3, and 33R1 (closest to wastewater ponds) Well 4K4 Wells 32F3, 32K6, and 32P6 (closest to base housing and athletic fields) Well 10E3 All other wells Volume-weighted composition (1994–97) of western Mojave Desert winter precipitation (Izbicki, 2004)

Figure 11. The delta oxygen-18 composition as a function of delta deuterium composition in water from selected wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019. Data summarized from U.S. Geological Survey (2019).

–5

36 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 The comparatively low slope of the evaporative trend line estimated from these data is consistent with evaporation under dry conditions typical of the Mojave Desert and with a hydrologic history of water sourced from the NTC WWTF and infiltrated through ponds. The oxygen and hydrogen isotope composition of water from well 4K4 (in major-ion group 2, fig. 9C) also was fractionated, which is consistent with evaporation of treated wastewater infiltrated through ponds. Additionally, water from well 4K4 indicated changes in major-ion chemistry consistent with the presence of treated wastewater (fig. 9C). The δ18O and δD compositions from well 4K4 plotted within the 95-percent confidence interval about the evaporative trend line estimated for wells 4B3, 4C3, 4D3, and 33R1; however, the evaporative fractionation for well 4K4 was less than the fractionation measured in water from wells closer to the ponds where water from the NTC WWTF is infiltrated. The change in the δ18O and δD composition in water from well 4K4 attributable to evaporative fractionation is about 25 percent of the maximum fractionation measured in wells 4B3 and 33R1 and is consistent with a smaller fraction of treated wastewater present in water from wells farther from the NTC WWTF. Although evaporative fractionation of water from well 4K4 was apparent in 2018, the timing of the evaporative shift in δ18O and δD composition of water from this well is not precisely known. Water from wells 32F3, 32K6, and 32P6 (in major- ion group 2; fig. 9C), farther from the NTC WWTF and closer to base housing, had changes in major-ion chemistry between 1993–95 and 2015–19 consistent with the presence of treated wastewater (fig. 9B). Although partly evaporated, water from these wells has a different evaporative history than water from wells closer to the NTC WWTF; more than half of the measured δ18O and δD values for these wells do not plot within the 95-percent confidence interval about the evaporative trend line estimated for wells closer to the WWTF (fig. 11). The data indicate less evaporative fractionation, and the evaporative trend line has a greater slope (about 5.5) than water from wells closer to the NTC WWTF. The greater slope is consistent with evaporation under higher humidity likely present during irrigation at base housing and athletic fields. Water from wells 32B3, 32K5, and 33E3 (in major-ion group 2, fig. 9) plot along the local groundwater line, and the δ18O and δD composition of water from these wells did not change in a manner consistent with evaporation (not identified on fig. 11). The major ion composition of water from these three wells changed but was not within the range of treated wastewater by 2015–19 (fig. 9). Changes in the δ18O and δD composition consistent with evaporation were not measured in water from deeper monitoring well 32P4, wells 5G1 and 5G2 completed in older alluvium (major-ion group 3, fig. 9; not identified on fig. 11), and production well 32H1 (major- ion group 3, fig. 9; not identified on fig. 11). Based on their

major-ion composition, water from these wells is relatively unaffected by infiltration of treated wastewater from the NTC WWTF. Water from well 10E3, along the wash in the southeastern part of the Irwin subbasin (major-ion group 3, fig. 9), is evaporated and plots within the 95-percent confidence interval about the evaporative trend line estimated for wells 4B3, 4C3, 4D3, and 33R1 near the NTC WWTF (fig. 11). Although consistent with treated wastewater discharges from the NTC WWTF, some water from this well may have originated from ponds on the base golf course. These ponds were used to infiltrate treated wastewater in the past before upgrades at the NTC WWTF (Densmore and Londquist, 1997), and evaporative fractionation of the δ18O and δD composition of water from well 10E3 was measured as early as 1993–94. Water from well 10E3 sampled in December 2014 and June 2016 plots along the local groundwater line and does not indicate a δ18O and δD composition consistent with evaporative fractionation. Although the δ18O and δD compositions of these samples differ from the composition expected for regional precipitation (Izbicki, 2004) and water from well 33R1 affected by precipitation (April 3, 2019; fig. 11), these samples have been attributed to infiltration of stormflow along the wash that occurred after large storms in 2013 and 2016. It is likely that the δ18O and δD composition of precipitation may differ greatly in different storms (Izbicki, 2004). In the Mojave Desert, precipitation from large storms that produce intermittent streamflow in desert washes have a δ18O and δD composition similar to native groundwater from wells within the Irwin subbasin that were recharged as much as 18,400–12,500 years before present.

Changes in Dissolved Solids and Nitrate Concentrations Associated with Evaporation Increases in dissolved solids and nitrate concentrations in treated wastewater that occur as a result of evaporation before infiltration from ponds and irrigation at base housing and athletic fields were estimated based on changes in δD composition using graphical techniques developed by Gonfiantini (1986). The approach estimates the fraction of water remaining in a water sample with a given change in the δD composition; the dissolved solids and nitrate concentration of the remaining water was then calculated assuming no chemical reactions with aquifer materials or constituents dissolved in the water. Differences between the calculated and measured concentrations were used to evaluate the relative contribution of mixing with native groundwater, evaporation, mobilization of saline water from the unsaturated zone, and dissolution of mineral salts to dissolved solids and nitrate concentrations in groundwater. Interpretation of data using this approach is limited because detailed information on wastewater composition is not available before 2015; however, Densmore and Londquist (1997) reported higher dissolved solids concentrations in treated wastewater discharged to the Irwin subbasin in the past.

Water Quality 37 The dissolved solids concentrations of water from wells in the Irwin subbasin increases as water becomes progressively more evaporated (fig. 12A). Assuming only evaporation of treated wastewater from ponds before recharge (at a relative humidity of 25 percent), the median dissolved solids concentration in treated wastewater between 2015 and 2019 of 515 mg/L (Dick, 2025) would increase linearly to 770 mg/L as δD values change from −90 to −68 per mil (fig. 12A). Similarly, the dissolved solids concentrations for the upper and lower interquartile range, representing 50 percent of treated wastewater samples, would increase linearly from 358 and 896 to 550 and 1,380 mg/L, respectively (fig. 12A). These data do not capture the range of treated wastewater concentrations (Dick, 2025), but concentrations in groundwater would be expected to approach the median value over time and with distance from the NTC WWTF. Most data plot above the median 2015–19 dissolved solids concentration expected for treated wastewater and seem to contain a large amount of dissolved solids from other sources, potentially including mobilization of chloride and other soluble salts from the unsaturated zone as wastewater infiltrated to the water table. Although historical data are limited, Densmore and Londquist (1997) reported dissolved solids concentrations in treated wastewater discharge of 826 mg/L. Given changes in the water supply and NTC WWTF operations that occurred since 2015, dissolved solids concentrations in treated wastewater in the past would have been higher than concentrations in 2015–19. A dissolved solids concentration of 826 mg/L would increase to a concentration of 1,270 mg/L as a result of evaporation prior to groundwater recharge (fig. 12A). Most water from wells in the Irwin subbasin plot near concentrations expected from evaporation of treated wastewater sampled in 1993 (Densmore and Londquist, 1997) and seem largely consistent with evaporation of treated wastewater prior to recharge. Some water from wells 4B3 (May 26, 1994, and August 24, 1994), 33R1 (March 19, 2017), and 32K6 (February 8, 2019) have higher dissolved solids concentrations than would be expected solely from evaporation of treated wastewater (fig. 12A). Examination of the data indicate a nonlinear trend (fig. 12A), with more

rapid initial increases in dissolved solids concentrations than expected solely from evaporation. The data indicate stable, or even decreasing, dissolved solids concentrations as water becomes progressively more evaporated (fig. 12A). The nonlinear trend in the data is consistent with initial mobilization of dissolved salts from the unsaturated zone during recharge of treated wastewater from ponds. In contrast to dissolved solids, most nitrate concentrations plot below the concentration range expected solely from evaporation of treated wastewater (fig. 12B). Nitrate concentrations in treated wastewater declined beginning in July 2019 (Dick, 2025), and lower than expected nitrate concentrations in some samples of water from wells near the NTC WWTF can be attributed to changes in operations at the WWTF. However, most samples were collected before July 2019, and the nitrate data are consistent with mixing of treated wastewater with native groundwater that has lower nitrate concentrations and with removal of nitrate prior to groundwater recharge. Mixing of treated wastewater with native groundwater was not obvious for changes in dissolved solids with evaporation (fig. 12A) because dissolved solids concentrations of treated wastewater and native groundwater were more similar than nitrate concentrations. Nitrate removal in water reaching wells 4C3,4B3, 4D3, and 33R1 may have occurred through assimilation into microbial biomass within ponds before infiltration. Less nitrate removal was measured in water from wells 32F3 and 32P6 (fig. 12B), which was not infiltrated from ponds near the NTC WWTF and was recharged from infiltration of irrigation return from base housing and athletic fields. Almost complete nitrate removal occurred in water from well 32K6 (fig. 12B). Most groundwater within the Irwin subbasin is oxic (contains dissolved oxygen), and denitrification would not be expected to occur. However, water from well 32K6 in the center of the Irwin subbasin has dissolved oxygen concentrations less than the reporting limit of 0.2 mg/L, and denitrification is possible in this area. Before upgrades at the NTC WWTF, most N was in the form of ammonia or organic N (Densmore and Londquist, 1997). As a result, increases in nitrate concentration associated with historical wastewater discharges were not evaluated.

38 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 A

2,500

EXPLANATION

Dissolved solids, in milligrams per liter

Evaporation 32K6; February 2019

2,000

1,500

4B3; May and August 1994

Changes in dissolved solids concentrations from the evaporation of treated wastewater at a relative humidity of 25 percent. Lines show expected change in dissolved solids concentrations from the upper quartile, median, and lower quartile of wastewater concentrations, 2015–19

33R1; March 2017

Coefficient of determination R²=0.66

Changes in dissolved solids concentrations from the evaporation of wastewater at a relative humidity of 25 percent for a concentration of 826 milligrams per liter (Densmore and Londquist, 1993)

1,000

Least-squares regression line showing change in dissolved solids concentration with evaporation 500

Wells 4C3, 4B3, 4D3, and 33R1 (closest to wastewater ponds) Well 4K4

0 –100

–90

–80

–70

–60

delta Deuterium, in per mil

Wells 32F3, 32K6, and 32P6 (closest to base housing and athletic fields) Well 10E3 All other wells

Nitrate plus nitrite as nitrogen, in milligrams per liter

Evaporation

EXPLANATION Changes in nitrate concentrations from the evaporation of treated wastewater at a relative humidity of 0.25. Line shows evaporation from the upper quartile, median, and lower quartile of wastewater nitrate concentrations, 2015–19

Wells 4C3, 4B3, 4D3, and 33R1 (closest to wastewater ponds)

32F3

Well 4K4 Wells 32F3, 32K6, and 32P6 (closest to base housing and athletic fields)

32P6

Well 10E3 32F3 0 –100

32K6 –90

–80

All other wells –70

–60

delta Deuterium, in per mil

Figure 12. Estimated evaporative effects on A, dissolved solids; and B, nitrate concentrations in water from wells in the Langford Valley–Irwin groundwater subbasin, U.S. Army Fort Irwin National Training Center, California, 1993–2019. Data summarized from U.S. Geological Survey (2019).

Water Quality 39

Tritium and Carbon-14 Tritium is a naturally occurring radioactive isotope of hydrogen that has a half-life of 12.4 years (Michel, 1976). For the purposes of this report, the concentration of 3H is reported in tritium units (TU). One TU is equal to one atom of 3H in 1018 molecules of water. About 1,760 pounds of 3H was released into the atmosphere as a result of the atmospheric testing of nuclear weapons between 1952 and 1962 (Michel, 1976). As a result, 3H concentrations in precipitation and groundwater recharge increased during that time. Tritium concentrations are not affected by chemical reactions other than radioactive decay because 3H is part of the water molecule. Therefore, 3H can be used as a tracer of the movement and relative age of water on timescales ranging from about 70 years before present (post-1952) to present. For the purposes of this report, groundwater that had 3H concentrations less than the detection limit of 0.2 TU was interpreted as water recharged before 1952. Samples that had measurable 3H concentrations were interpreted as containing at least some water recharged after 1952. Carbon-14 is a naturally occurring radioactive isotope of carbon that has a half-life of about 5,730 years (Mook, 1980). Carbon-14 data are expressed as percent modern carbon (pmc) by comparing 14C activities to the specific activity of National Bureau of Standards oxalic acid: 13.56 disintegrations per minute per gram of carbon in the year 1950 equals 100 pmc (Kalin, 1999; Mook and van der Plicht, 1999). Carbon-14 and 3H was produced by the atmospheric testing of nuclear weapons (Mook, 1980). As a result, 14C activities may exceed 100 pmc in areas where groundwater contains 3H. Carbon- 14 activities are used to determine the age of a groundwater sample on timescales ranging from more than 20,000 years before present to recent. Carbon-14 is not part of the water molecule and, therefore, 14C activities may be affected by chemical reactions that remove or add carbon to solution. Izbicki and Michel (2004) reported an initial 14C activity for recently (less than 50 years before present) recharged groundwater in alluvial aquifers within the Mojave Desert of 85 pmc before atmospheric testing of nuclear weapons. Groundwater that has a 14C activity greater than 85 pmc may contain some fraction of recent water—even if 3H is less than the study reporting level. In addition to reactions with aquifer materials, 14C activities are affected by the mixing of younger water that has high 14C activity with older water that has low 14C activity. Tritium and 14C were not measured consistently as part of this study and were not measured in water samples collected from all wells. However, where measured, 3H and 14C data provide information on the timing of groundwater recharge to wells. Tritium was measured at least once in water from 11 of the 17 wells sampled as part of this study; 14C was measured in water from 9 of the 17 sampled wells (U.S. Geological Survey, 2019). Tritium concentrations in six wells were less than the reporting level of 0.2 TU; 14C activities in water from these wells ranged from 10.8 to 22 pmc, which is consistent

with older groundwater that have ages unadjusted from reactions that may occur in the aquifer between 18,400 and 12,500 years before present. Samples from wells 4K4 and 32K6 had 3H values near the reporting level of 0.5 and 0.7 TU; small amounts of precipitation containing 3H may have infiltrated to the water table near these wells with irrigation return water. However, when interpreted with 14C values of 43.9 and 21.9, these values are not consistent with a large fraction of post- 1952 recharge water in water from wells 4K4 and 32K6, respectively, although some post-1952 water may be present. Water from well 32K6 is reduced (does not contain dissolved oxygen), and reactions with aquifer material may have lowered the 14C activity in water from the well. Water from wells 10E3, 32P6, and 33R1 had 3H concentrations of 2.0, 2.4, and 2.0 TU, respectively. Carbon- 14 values of 90.7 and 83 pmc in water from wells 10E3 and 33R1, respectively, are consistent with a large fraction of post-1952 recharge. Carbon-14 was not measured in water from well 32P6. Tritium and 14C activities in water from well 10E3, located along the wash near the subbasin outflow in the southeastern part of the Irwin subbasin, are consistent with major-ion and stable water isotope data that indicate large changes in composition after precipitation and resulting stormflows in the wash. Consistent with δ18O and δD data (fig. 11A), small amounts of precipitation may have entered ponds where treated wastewater is infiltrated into the ground, contributing to post-1952 water in well 33R1 and to a lesser extent in water from well 4K4. Similarly, small amounts of precipitation may have infiltrated into the subsurface along with treated wastewater used for irrigation at base housing and athletic fields near wells 32K6 and 32P6. Areal groundwater recharge from infiltration of precipitation did not occur during predevelopment. It is not known if development and runoff from impermeable road surfaces may have increased streamflow in washes within the Irwin subbasin and increased recharge to well 10E3. Most 14C data, where available, are consistent with older groundwater that has unadjusted ages ranging from 18,400 to 12,350 years before present. Consistent with water-level declines from pumping, these data indicate that natural groundwater recharge to the Irwin subbasin is small under present-day climatic and hydrologic conditions. It also is likely that younger groundwater that may have been present within the subbasin has been largely removed by pumping. Under present-day conditions, groundwater recharge to the Irwin subbasin occurs as infiltration of treated wastewater from ponds near the NTC WWTF and as irrigation return from landscape watering near base housing and athletic fields in the study area. Data from well 10E3 indicate that some groundwater recharge may occur in the Irwin subbasin, even under present-day climatic conditions, as infiltration of intermittent streamflow in washes after large storms. The δ18O and δD composition of water infiltrated from these storms was similar to the composition of older groundwater recharged 18,400–12,350 years before present and differed from the composition of present-day precipitation.

40 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

Study Limitations This study is based on data from 17 wells in the Irwin subbasin collected between 1993 and 2019. Before the establishment of a groundwater monitoring network in 2014, water-level and groundwater-quality data were not consistently collected from all wells of interest in the study area. Similarly, water-chemistry and isotope data for some wells in the older alluvium and Tertiary deposits that underlie recent alluvium are unavailable because only two wells (32F2 and 32P4) completed in the older alluvium and Tertiary deposits were included in the 2014 well network. The lack of well data limits the ability to assess possible movement of treated wastewater into these underlying deposits. In addition, ancillary data, including chemistry (specific conductance, dissolved solids, and nitrate) data for water discharged from the wastewater treatment facility and ponds used to infiltrate treated wastewater, are not available for the earlier part of this study (1993–95). For many constituents (including major ion and stable water isotope [δD and δ18O] data), treated wastewater data are not available. As a result of water-level rises as much as 45 ft in parts of the Irwin subbasin, shallow wells designed to sample the shallowest groundwater during periods when the water table was lower are now below the water table and do not sample the shallowest groundwater. Groundwater near the 2019 water table may contain higher fractions of treated wastewater infiltrated from ponds at the NTC WWTF or return water from landscape irrigation at base housing and athletic fields than data discussed in this study. Future data collection, including expansion of the 2014 well network to include deeper wells, installation and sample collection of water table monitoring wells, and increased sample collection from the NTC WWTF, may resolve some of these limitations.

Summary and Conclusions The U.S. Army Fort Irwin National Training Center (NTC), 120 miles northeast of Los Angeles in the Mojave Desert of California, obtains its potable water supply from the Bicycle Valley and Langford Valley groundwater basins; the Langford Valley groundwater basin is further subdivided into the Langford Well Lake and Irwin subbasins. The Irwin subbasin contains unconsolidated deposits that constitute an upper aquifer that has a saturated thickness of as much as 200 feet (ft) and a lower aquifer as thick as 650 ft. Groundwater pumping in excess of natural recharge beginning in 1941 resulted in water-level declines in the Irwin subbasin; by 1990, water-level declines were as much as 30 ft in some parts of the subbasin. Water was imported from the Bicycle Valley groundwater basin into the Irwin subbasin for potable supply beginning in the late 1960s and from the Langford Well Lake subbasin beginning in the early 1990s, reducing pumpage in Irwin subbasin. In Irwin subbasin, treated wastewater was recharged as infiltration through

ponds near the NTC wastewater treatment facility (WWTF) in the southeastern part of the subbasin and as return flow from landscape irrigation at base housing and athletic fields in the northwestern part of the subbasin. This combination of recharge activities and reduced pumpage has offset water-level declines in the Irwin subbasin. Localized water-quality changes occurred in parts of the Irwin subbasin from infiltration of treated wastewater through ponds and use of treated wastewater for landscape and athletic field irrigation. In 2014, a groundwater-monitoring network was developed that included collection of water-level and water-quality data from selected wells in the Irwin subbasin. This report presents an evaluation of water-level and water- quality conditions in the Irwin subbasin and documents changes that occurred between 1993 and 2019. Since the early 1990s, reduced pumping, recharge of treated wastewater, and irrigation return from landscape irrigation at base housing and athletic fields in the Irwin subbasin has caused water levels to rise or stabilize throughout the subbasin. Water levels rose as much as 45 ft near the center of the basin (wells 32B3, 32F3, and 32K6), about 35 ft in wells north of the NTC WWTF, and about 10 ft south of the duck ponds in well 10E3 near the basin outflow in the southeast part of the basin. Water-quality samples were collected from 17 wells in the Irwin subbasin between 2014 and 2019. These data were supplemented with water-quality data collected during 1993–95 and at other times, if available. Samples were analyzed for (1) selected chemistry, including major-ion composition, dissolved solids, nitrate, and selected trace elements and (2) selected stable and radioactive isotopes for groundwater source and age-dating purposes. The major-ion composition of groundwater in the Irwin subbasin differs spatially and with depth; the composition of water from wells ranges from sodium-sulfate and sodium- chloride-sulfate to sodium-bicarbonate compositions. Water from wells near ponds used to infiltrate treated wastewater (4K4, 4C3, 4D3, and 33R1) has a sodium-chloride-sulfate composition. The major-ion composition of water from these wells has not changed greatly between 1993–95 and 2015–19; this composition likely is consistent with the long-term major- ion composition of treated wastewater discharged from the NTC WWTF and infiltrated in ponds near the WWTF. Water from shallow wells 4K4, 32F3, 32K6, and 32P6, farther from the NTC WWTF, indicated changes in major-ion composition between 1993–95 and 2015–19 and resembled the composition of water from wells near the WWTF. In the northwestern part of the subbasin, wells 32F3 and 32P6 are upgradient from lower water levels in the center of the basin, and the composition of water in these wells was likely affected by recharge from ongoing landscape and athletic field irrigation with treated wastewater. Water from deep wells completed in the lower aquifer and production wells are a sodium mixed- anion composition and generally indicated only small changes in major-ion composition between 1993 and 2019.

Summary and Conclusions 41 By 2015–19, dissolved solids concentrations in the 17 sampled wells ranged from 454 milligrams per liter (mg/L) in water from well 5G1 to 1,910 mg/L in water from well 33R1. Between 1993–95 and 2015, the change in the overall median concentration of dissolved solids in the Irwin subbasin from 620 to 1,030 mg/L was not statistically significant; however, data from nine wells used to calculate trends had statistically significant upward trends in dissolved solids concentrations. Between 2014 and 2019, dissolved solids concentrations in water from wells 4B3 and 4C3 near the NTC WWTF decreased; these decreases may result from changes in source water quality attributable to reverse osmosis of treated drinking water delivered within the Irwin subbasin. Decreases in dissolved solids concentrations are likely to continue and become measurable in other wells if reverse osmosis of source water supply continues. By 2015–19, nitrate concentrations ranged from less than the reporting level of 0.2 mg/L as nitrogen (N) in water from well 33K6 to 28 mg/L as N in water from well 33R1 near the NTC WWTF. Between 1993–95 and 2015–19, the change in the overall median nitrate concentration in the Irwin subbasin from 2.8 to 4.5 mg/L as N was statistically significant (at a confidence criterion of α=0.10, p-value=0.06); however, only two wells indicated statistically significant upward trends in nitrate concentration, whereas well 4B3 near the NTC WWTF indicated a statistically significant downward trend in nitrate concentrations. Between 1993 and 1995, arsenic concentrations in water from wells in the Irwin subbasin ranged from 8 to 81 micrograms per liter (µg/L), with a median concentration of 28 µg/L. By 2015–19, arsenic concentrations in samples of water from wells ranged from 4.8 to 37 µg/L, with a median concentration of 16 µg/L; 14 of 16 samples exceeded the maximum contaminant level (MCL) of 10 µg/L. Between 1993 and 1995, fluoride concentrations in water from wells in the Irwin subbasin ranged from 0.5 to 21 mg/L, with a median concentration of 5.4 mg/L. By 2015–19, fluoride concentrations in water from wells in the Irwin subbasin ranged from 1.8 to 12 mg/L, with a median concentration of 4.3 mg/L; 15 of 16 samples exceeded the MCL of 2 mg/L. No samples exceeded the proposed California MCL for hexavalent chromium of 10 µg/L. Water from wells 4B3, 10E3, 32P6, 32K6, and 33R1 exceeded the MCL for uranium of 30 µg/L; no wells exceeded the California notification level for vanadium of 50 µg/L. The stable isotopes of oxygen and hydrogen (δ18O and δD) were used as tracers of the source, movement, and hydrologic history of water in the Irwin subbasin. Water from wells 4B3, 4C3, 4D3, 4K4, and 33R1 indicated changes in δ18O and δD compositions consistent with evaporation of treated wastewater infiltrated from ponds at the NTC WWTF, whereas water from wells 32F3, 32K6, and 32P6 closer to base housing and athletic fields in the northwestern part of

the subbasin indicated changes consistent with infiltration of return water from irrigation of treated wastewater. Examination of δ18O, δD, and water-chemistry data indicated increases in dissolved solids concentrations in water from wells in the Irwin subbasin are within ranges consistent with evaporation of treated wastewater prior to recharge and mobilization of soluble salts from the unsaturated zone. In contrast, nitrate data are consistent with mixing with native groundwater that has lower nitrate concentrations, evaporation, and removal of nitrate, likely through assimilation within microbial biomass within ponds prior to infiltration and groundwater recharge; less nitrate removal was measured in water from wells recharged from infiltration of landscape irrigation return water than in water infiltrated from ponds near the NTC WWTF. The radioactive isotope of hydrogen (tritium, 3H) and carbon-14 (14C) are used for determining the age (time since recharge) of recent (post-1952) and older groundwater, locating sources of recharge, and identifying geologic controls on the movement of groundwater. Most 14C data are consistent with older groundwater in the Irwin subbasin that has unadjusted ages ranging from 18,400 to 12,350 years before present. However, water from wells 10E3, 32P6, and 33R1 had 3H concentrations consistent with a large fraction of post- 1952 recharge water. Post-1952 water from well 10E3, located along the wash near the subbasin outflow in the southeastern part of the Irwin subbasin, is consistent with infiltration of stormflow in the wash. Small amounts of precipitation may have entered ponds where treated wastewater is infiltrated into the ground, contributing to post-1952 water in well 33R1; small amounts of precipitation may have infiltrated into the subsurface along with treated wastewater used for irrigation at base housing and athletic fields near wells 32K6 and 32P6. Consistent with water-level declines from pumping, these data indicate that natural groundwater recharge to the Irwin subbasin is small under present-day climatic and hydrologic conditions. Ongoing water-management practices in the Irwin subbasin, including reduction in demand, importation of water from the Bicycle Valley groundwater basin and Langford Well Lake subbasin, reverse osmosis of treated drinking water, and increased wastewater treatment from secondary to tertiary, has resulted in increases in groundwater levels and changes in water quality in the Irwin subbasin compared to 1993–95 conditions. Although between 1993–95 and 2015–19 dissolved solids indicated upward concentration trends, declines in dissolved solids and nitrate concentrations were observed in water from wells near the NTC WWTF beginning in 2014. The continuation of water-management practices in place at the time of this report may result in additional improvement as water recharged from ponds near the NTC WWTF propagates throughout the Irwin subbasin.

42 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019

References Cited Beukens, R.P., 1992, Radiocarbon accelerator mass spectrometry—Background, precision, and accuracy, in Taylor, R.E., Long, A., and Kra, R.S., eds., Radiocarbon after four decades—An interdisciplinary perspective: New York, N.Y., Springer-Verlag, p. 230–239, accessed November 19, 2024, at https://doi.org/10.1007/978-1-4757-4249-7_16. California Department of Water Resources, 2021, California’s groundwater update 2020—Full report: California Department of Water Resources Bulletin 118, Update 2020, 485 p., accessed June 5, 2023, at https://water.ca.gov/ Programs/Groundwater-Management/Bulletin-118. California State Water Resources Control Board, 2021, California safe drinking water laws: Sacramento, California, California Water Boards, 205 p., accessed October 10, 2022, at https://www.waterboards.ca.gov/laws_regulations/docs/ drinking_water_code_2021.pdf. California State Water Resources Control Board, 2022, Hexavalent chromium MCL: 2022 amendment to Title 22, Division 4, Chapter 15, Article 4, Section 64431: California Water Boards draft regulation, 13 p., accessed May 30, 2023, at https://www.waterboards.ca.gov/drinking_ water/certlic/drinkingwater/documents/chromium6/cr6_ draftregtextv2_2022.pdf. California State Water Resources Control Board, 2023, MCLs, DLRs, and PHGs for regulated drinking water contaminants: Sacramento, California, California State Water Resources Control Board, 11 p., accessed September 1, 2023, at https://www.waterboards.ca.gov/ drinking_water/certlic/drinkingwater/documents/mclreview/ mcls_dlrs_phgs.pdf. Coplen, T.B., 1994, Reporting of stable hydrogen, carbon, and oxygen isotopic abundances: Pure and Applied Chemistry, v. 66, no. 2, p. 273–276. [Available at https://doi.org/10.1351/pac199466020273.] Coplen, T.B., Wildman, J.D., and Chen, J., 1991, Improvements in the gaseous hydrogen-water equilibration technique for hydrogen isotope-ratio analysis: Analytical Chemistry, v. 63, no. 9, p. 910–912, accessed November 19, 2024, at https://doi.org/10.1021/ac00009a014. Craig, H., 1961, Isotopic variations in meteoric waters: Science, v. 133, no. 3465, p. 1702–1703, accessed November 19, 2024, at https://doi.org/10.1126/science.133.3465.1702.

Densmore, J.N., 2003, Simulation of ground-water flow in the Irwin Basin aquifer system, Fort Irwin National Training Center, California: U.S. Geological Survey Water- Resources Investigations Report 02–4264, 69 p., accessed November 19, 2024, at https://doi.org/10.3133/wri024264. Densmore, J.N., and Londquist, C.J., 1997, Ground- water hydrology and water quality of the Irwin Basin at Fort Irwin National Training Center, California: U.S. Geological Survey Water-Resources Investigations Report 97–4092, 159 p., accessed September 22, 2022, at https://doi.org/10.3133/wri974092. Densmore, J.N., Woolfenden, L.R., Rewis, D.L., Martin, P.M., Sneed, M., Ellett, K.M., Solt, M., and Miller, D.M., 2018, Geohydrology, geochemistry, and numerical simulation of groundwater flow and land subsidence in the Bicycle Basin, Fort Irwin National Training Center, California: U.S. Geological Survey Scientific Investigations Report 2018–5067, 176 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20185067. Dick, M.C., 2025, Water-quality data for treated drinking water and treated wastewater effluent at Fort Irwin National Training Center: U.S. Geological Survey data release, available at https://doi.org/10.5066/P9HQ9C4W. Donahue, D.J., Linick, T.W., and Jull, A.J.T., 1990, Isotope- ratio and background corrections for accelerator mass spectrometry radiocarbon measurements: Radiocarbon, v. 32, no. 2, p. 135–142, accessed November 19, 2024, at https://doi.org/10.1017/S0033822200040121. Epstein, S., and Mayeda, T.K., 1953, Variation of O18 content of waters from natural sources: Geochimica et Cosmochimica Acta, v. 4, no. 5, p. 213–224, accessed November 19, 2024, at https://doi.org/10.1016/0016-7037(53)90051-9. Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey Open-File Report 93–125, 217 p., accessed November 19, 2024, at https://doi.org/10.3133/ofr93125. Fishman, M.J., and Friedman, L.C., 1989, Methods for determination of inorganic substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water- Resources Investigations, book 5, chap. A1, 545 p., accessed November 19, 2024, at https://doi.org/10.3133/twri05A1. Fournier, R.O., and Thompson, J.M., 1980, The recharge area for the Coso, California, geothermal system deduced from δD and δ18O in thermal and nonthermal waters in the region: U.S. Geological Survey Open-File Report 80–454, 25 p., accessed November 19, 2024, at https://doi.org/10.3133/ofr80454.

References Cited 43 Gagnon, A.R., and Jones, G.A., 1993, AMS-graphite target production methods at the Woods Hole Oceanographic Institution during 1986–1991: Radiocarbon, v. 35, no. 2, p. 301–310, accessed November 19, 2024, at https://doi.org/10.1017/S0033822200064985. Garbarino, J.R., Bednar, A.J., and Burkhardt, M.R., 2002, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-Arsenic speciation in natural-water samples using laboratory and field methods: U.S. Geological Survey Water-Resources Investigations Report 2002–4144, 40 p., accessed November 19, 2024, at https://doi.org/10.3133/wri024144. Garbarino, J.R., Kanagy, L.K., and Cree, M.E., 2006, Determination of elements in natural-water, biota, sediment, and soil samples using collision/reaction cell inductively coupled plasma-mass spectrometry: U.S. Geological Survey Techniques and Methods, book 5, chap. B1, 87 p., accessed November 19, 2024, at https://doi.org/10.3133/tm5B1. Gat, J.R., and Gonfiantini, R., eds., 1981, Stable isotope hydrology—Deuterium and oxygen-18 in the water cycle: Vienna, Austria, International Atomic Energy Agency Technical Reports Series, no. 210, 339 p., accessed November 19, 2024, at https://inis.iaea.org/records/f7xsc-z1683.

Groover, K.D., Fram, M.S., and Levy, Z.F., 2024, Status and understanding of groundwater quality in the Mojave Basin Domestic-Supply Aquifer study unit, 2018—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2024–5019, 62 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20245019. Gross, E.L., Lindsey, B.D., and Rupert, M.G., 2012, Quality of major ion and total dissolved solids data from groundwater sampled by the National Water-Quality Assessment Program, 1992–2010: U.S. Geological Survey Scientific Investigations Report 2011–5153, 26 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20115153. Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in water resources: U.S. Geological Survey Techniques of Water-Resources Investigations, book 4, chap. A3, 522 p., accessed July 14, 2017, at https://doi.org/10.3133/twri04A3. Helsel, D.R., Hirsch, R.M., Ryberg, K.R., Archfield, S.A., and Gilroy, E.J., 2020, Statistical methods in water resources: U.S. Geological Survey Techniques and Methods, book 4, chap. A3, 458 p., accessed November 19, 2024, at https://doi.org/10.3133/tm4A3.

Gonfiantini, R., 1978, Standards for stable isotope measurements in natural compounds: Nature, v. 271, p. 534–536, accessed November 19, 2024, at https://doi.org/10.1038/271534a0.

Howard, G., Bartram, J., Pedley, S., Schmoll, O., Chorus, I., and Berger, P., 2006, Groundwater and public health, chap. 1 of Schmoll, O., Howard, G., Chilton, J., and Chorus, I., eds., Protecting groundwater for health— Managing the quality of drinking-water sources: London, IWA Publishing, p. 3–19, accessed November 19, 2024 at https://www.who.int/publications/i/item/9241546689.

Gonfiantini, R., 1986, Environmental isotopes in lake studies, chap. 3 of Fritz, P. and Fontes, J.-Ch., eds., Handbook of environmental isotope geochemistry: Amsterdam, Netherlands, Elsevier, accessed November 19, 2024, at https://doi.org/10.1016/B978-0-444-42225-5.50008-5.

Izbicki, J.A., 2004, Source and movement of ground water in the western part of the Mojave Desert, Southern California, USA: U.S. Geological Survey Water-Resources Investigations Report 2003–4313, 36 p., accessed November 19, 2024, at https://doi.org/10.3133/wri034313.

González, V.P., Avilés, R.M., Hernandez Anguiano, J.H., Knappett, P.S.K., Martinez, J.L.M., Alquiza, M.J.P., Naves, A., Bian, J., Liu, J., González, L.M.R., Céspedes,J.M.N., and Li, Y., 2022, Influence of geological faults on dissolved arsenic concentrations in an overexploited aquifer with shallow geothermal heat: Applied Geochemistry, v. 144. [Available at https://doi.org/10.1016/j.apgeochem.2022.105395.]

Izbicki, J.A., Groover, K.D., and Seymour, W.A., 2023, Arsenic, chromium, uranium, and vanadium in rock, alluvium, and groundwater, western Mojave Desert, southern California: U.S. Geological Survey Scientific Investigations Report 2023–5089, 96 p., accessed November 25, 2023, at https://doi.org/10.3133/sir20235089.

Groover, K.D., Densmore, J.N., Ely, C.P., and Brown, A.A., 2020, Irrigated field characterization at Fort Irwin National Training Center, San Bernardino County, California, 2018–2020: U.S. Geological Survey data release, accessed November 19, 2024, at https://doi.org/10.5066/P9T8OLSL.

Izbicki, J.A., Johnson, R.U., Kulongoski, J., and Predmore, S., 2007, Ground-water recharge from small intermittent streams in the western Mojave Desert, California: U.S. Geological Survey Professional Paper 1703–G, p. 157–184, accessed November 19, 2024, at https://doi.org/10.3133/pp1703G. Izbicki, J.A., and Michel, R.L., 2004, Movement and age of ground water in the western part of the Mojave Desert, Southern California, USA: U.S. Geological Survey Water- Resources Investigations Report 2003–4314, 42 p., accessed November 19, 2024, at https://doi.org/10.3133/wri034314.

44 Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019 Izbicki, J.A., and Seymour, W.A., 2023, Analyses of regulatory water-quality data, Chapter D in Natural and anthropogenic (human-made) hexavalent chromium, Cr(VI), in groundwater near a mapped plume, Hinkley, California: U.S. Geological Survey Professional Paper 1885–D, 28 p., accessed October 20, 2023, at https://doi.org/10.3133/pp1885D. Johnson, A., Johnson, R.B., and Bretzler, A., 2017, Geogenic contamination, chap. 2 of Johnson, C.A., and Bretzler, A., eds., Geogenic contamination handbook—Addressing arsenic and fluoride in drinking water: Dubendorf, Switzerland, Swiss Federal Institute for Aquatic Science and Technology, p. 16–24. Kalin, R.M., 1999, Radiocarbon dating of groundwater systems, chap. 4 of Cook, P.G., and Herczeg, A.L., eds., Environmental tracers in subsurface hydrology: Boston, Mass., Springer., p. 111–144. Kendall, M.G., 1938, A new measure of rank correlation: Biometrika, v. 30, nos. 1–2, p. 81–93, accessed March 15, 2018, at https://doi.org/10.1093/biomet/30.1-2.81. Kjos, A.R., Densmore, J.N., Nawikas, J.M., and Brown, A.A., 2014, Construction, water-level, and water-quality data for multiple-well monitoring sites and test wells, Fort Irwin National Training Center, San Bernardino County, California, 2009–12: U.S. Geological Survey Data Series 788, 140 p., accessed November 19, 2024, at https://doi.org/10.3133/ds788. Mann, H.B., 1945, Nonparametric tests against trend: Econometrica, v. 13, no. 3, p. 245–259, accessed November 19, 2024, at https://doi.org/10.2307/1907187. McNichol, A.P., Gagnon, A.R., Jones, G.A., and Osborne, E.A., 1992, Illumination of a black box— Analysis of gas composition during graphite target preparation—Proceedings of the 14th International Radiocarbon Conference: Radiocarbon, v. 34, no. 3, p. 321–329, accessed November 19, 2024, at https://doi.org/10.1017/S0033822200063499. McNichol, A.P., Jones, G.A., Hutton, D.L., Gagnon, A.R., and Key, R.M., 1994, The rapid preparation of seawater ΣCO2 for radiocarbon analysis at the National Ocean Sciences Ams Facility: Radiocarbon, v. 36, no. 2, p. 237–246, accessed November 19, 2024, at https://doi.org/10.1017/S0033822200040522. Medalie, L., and Bexfield, L.M., 2020, Quality of data from the U.S. Geological Survey National Water Quality Network for water years 2013–17: U.S. Geological Survey Scientific Investigations Report 2020–5116, 21 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20205116.

Michel, R.L., 1976, Tritium inventories of the world oceans and their implications: Nature, v. 263, p. 103–106, accessed November 19, 2024, at https://doi.org/10.1038/263103a0. Miller, D.M., Menges, C.M., and Lidke, D.J., 2014, Generalized surficial geologic map of the Fort Irwin area, San Bernardino County, California, chap. B of Buesch, D.C., ed., Geology and geophysics applied to groundwater hydrology at Fort Irwin, California: U.S. Geological Survey Open-File Report 2013–1024, 11 p., scale 1:100,000, accessed November 19, 2024, at https://doi.org/10.3133/ofr20131024B. Mook, W.G., 1980, Carbon-14 in hydrogeological studies, chap. 2 of Fritz, P., and Fontes, J., eds., Handbook of environmental isotope geochemistry, v. 1—The terrestrial environment, A: Amsterdam, Netherlands, Elsevier, p. 49–74. Mook, W.G., and van der Plicht, J., 1999, Reporting 14C activities and concentrations: Radiocarbon, v. 41, no. 3, p. 227–239, accessed November 19, 2024, at https://doi.org/10.1017/S0033822200057106. Neter, J., and Wasserman, W., 1974, Applied linear statistical models: Homewood, Ill., Richard D. Irwin Inc., 842 p. Oestlund, H.G., and Werner, E., 1962, The electrolytic enrichment of tritium and deuterium for natural tritium measurements, in Symposium on tritium in the physical and biological sciences, v.1, Vienna, Austria, May 3–10, 1961 [Proceedings]: Vienna, Austria, International Atomic Energy Agency, p. 95–104, accessed November 19, 2024, at https://inis.iaea.org/search/43116776. Patton, C.J., and Kryskalla, J.R., 2011, Colorimetric determination of nitrate plus nitrite in water by enzymatic reduction, automated discrete analyzer methods: U.S. Geological Survey Techniques and Methods, book 5, chap. B8, 34 p., accessed November 19, 2024, at https://doi.org/10.3133/tm5B8. Piper, A.M., 1944, A graphic procedure in the geochemical interpretation of water-analyses: Eos, Transactions American Geophysical Union, v. 25, no. 6, p. 914–928, accessed November 19, 2024, at https://doi.org/10.1029/TR025i006p00914. Révész, K., and Coplen, T.B., 2008a, Determination of the δ(2H/1H) of water—RSIL lab code 1574, chap. 1 of Stable isotope-ratio methods, sec. C of Révész, K., and Coplen, T.B., eds., Methods of the Reston Stable Isotope Laboratory: U.S. Geological Survey Techniques and Methods, book 10, chap. C1, 27 p., accessed November 19, 2024, at https://doi.org/10.3133/tm10C1.

References Cited 45 Révész, K., and Coplen, T.B., 2008b, Determination of the δ (18O/16O) of water—RSIL lab code 489, chap. 2 of Stable isotope-ratio methods, sec. C of Révész, K., and Coplen, T.B., eds., Methods of the Reston Stable Isotope Laboratory: U.S. Geological Survey Techniques and Methods, book 10, chap. C2, 28 p., accessed November 19, 2024, at https://doi.org/10.3133/tm10C2. Schneider, R.J., Jones, G.A., McNichol, A.P., von Reden, K.F., Elder, K.L., Huang, K., and Kessel, E.D., 1994, Methods for data screening, flagging, and error analysis at the National Ocean Sciences AMS Facility: Nuclear Instruments and Methods in Physics Research, Section B—Beam Interactions with Materials and Atoms, v. 92, nos. 1–4, p. 172–175, accessed November 19, 2024, at https://doi.org/10.1016/0168-583X(94)96000-3. Stiff, H.A., Jr., 1951, The interpretation of chemical water analysis by means of patterns: Journal of Petroleum Technology, v. 3, no. 10, p. 376–379. [Available at https://doi.org/10.2118/951376-G.] Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Methods for determination of radioactive substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources Investigations, book 5, chap. A5, 95 p., accessed November 19, 2024, at https://doi.org/10.3133/twri05A5. U.S. Environmental Protection Agency, 2001, Technical fact sheet—Final rule for arsenic in drinking water: U.S. Environmental Protection Agency Fact Sheet 815-F-00-016, accessed October 26, 2023, at https://archive.epa.gov/water/archive/web/html/regulations_ techfactsheet.html#area.

U.S. Environmental Protection Agency, 2023, Drinking water regulations: U.S. Environmental Protection Agency, accessed October 26, 2023, at https://www.epa.gov/ dwreginfo/drinking-water-regulations. U.S. Geological Survey, 2015, National field manual for the collection of water-quality data (ver. 7): U.S. Geological Survey website, accessed April 5, 2019, at https://water.usgs.gov/owq/FieldManual/. U.S. Geological Survey, 2019, USGS water data for the Nation: U.S. Geological Survey National Water Information System database, accessed December 20, 2019, at https://doi.org/10.5066/F7P55KJN. U.S. Geological Survey, 2024, Inorganic blind sample project: U.S. Geological Survey web page, accessed September 3, 2024, at https://qsb.usgs.gov/ibsp/. Vogel, J.S., Nelson, D.E., and Southon, J.R., 1987, 14C background levels in an accelerator mass spectrometry system: Radiocarbon, v. 29, no. 3, p. 323–333. [Available at https://doi.org/10.1017/S0033822200043733.] Voronin, L.M., Densmore, J.N., and Martin, P., 2014, Analysis of potential water-supply management options, 2010–60, and documentation of revisions to the model of the Irwin Basin Aquifer System, Fort Irwin National Training Center, California: U.S. Geological Survey Scientific Investigations Report 2014–5081, 34 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20145081. Voronin, L.M., Densmore, J.N., Martin, P., Brush, C.F., Carlson, C.S., and Miller, D.M., 2013, Geohydrology, geochemistry, and groundwater simulation (1992–2011) and analysis of potential water-supply management options, 2010–60, of the Langford Basin, California: U.S. Geological Survey Scientific Investigations Report 2013–5101, 86 p., accessed November 19, 2024, at https://doi.org/10.3133/sir20135101.

For more information concerning the research in this report, contact the Director, California Water Science Center U.S. Geological Survey 6000 J Street, Placer Hall Sacramento, California 95819 https://www.usgs.gov/centers/california-water-science-center Publishing support provided by the USGS Science Publishing Network, Sacramento Publishing Service Center

Densmore and others—Evaluation of WQ in the Langford Valley–Irwin GW Subbasin, California, 1993–2019—SIR 2025–5105

ISSN 2328-0328 (online) https://doi.org/10.3133/sir20255105

Evaluation of Water Quality in the Langford Valley–Irwin Groundwater Subbasin, Fort Irwin National Training Center, California, 1993–2019

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