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Assessment of Water Levels, Nitrate, and Arsenic in the Carson Valley Alluvial Aquifer and the Development of a Data Visualization Tool for the Carson River Basin, Nevada

Ramon C. Naranjo, Anjela Bubiy, A.H. Welch, M.S. Lico, J.L. Hughes · U.S. Geological Survey

USGS Publications · Papers · License: Public Domain

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Water Resources Mission Area—Water's Cooperative Matching Funds Prepared in cooperation with Douglas County, Nevada

Assessment of Water Levels, Nitrate, and Arsenic in the Carson Valley Alluvial Aquifer and the Development of a Data Visualization Tool for the Carson River Basin, Nevada

Open-File Report 2024–1045 Version 1.1, September 2024

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

Cover. Carson River, looking upstream from gage 10311400: Carson River at Deer Run Road near Carson City, Nevada. Photograph by David Smith, U.S. Geological Survey, February 8, 2021.

Assessment of Water Levels, Nitrate, and Arsenic in the Carson Valley Alluvial Aquifer and the Development of a Data Visualization Tool for the Carson River Basin, Nevada By Ramon C. Naranjo, and Anjela Bubiy

Water Resources Mission Area—Water's Cooperative Matching Funds Prepared in cooperation with Douglas County, Nevada

Open-File Report 2024–1045

Version 1.1, September 2024 U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia First Release: 2024 Revised: September 2024 (ver. 1.1)

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: Naranjo, R.C., and Bubiy, A., 2024, Assessment of water levels, nitrate, and arsenic in the Carson Valley Alluvial Aquifer and the development of a data visualization tool for the Carson River Basin, Nevada (ver. 1.1, September 2024): U.S. Geological Survey Open-File Report 2024–1045, 29 p., https://doi.org/10.3133/ofr20241045. ISSN 2331-1258 (online)

iii

Acknowledgments The authors would like to thank Edward James of the Carson Water Subconservancy District for providing guidance and feedback on early stages of the project. We wish to thank Ron Kauble of the U.S. Geological Survey (USGS) for addressing database issues and for maintaining records, David Smith (USGS) for assistance with R-scripts for the statistical analysis, and Toby Welborn (USGS) for initial consultation on the Carson River Basin Hydro Mapper. We appreciate the input and reviews by David Smith, Philip Gardner, and Joseph Dolmagalski (USGS). We thank Rose Medina (USGS) for support on the map figures and Angela Paul (USGS) for technical insight on the report.

Contents Acknowledgments��iii Abstract��1 Introduction��2 Purpose and Scope��4 Description of Study Area��5 Methods��8 Water-Level and Nitrate Monitoring��8 Trends in Water Levels and Nitrate Concentration��8 Assessment of the Carson Valley Groundwater Monitoring Well Network��9 Carson River Basin Hydro Mapper��11 Results��14 Temporal Trends in Groundwater Levels��14 Spatial Distributions in Nitrate and Arsenic Concentrations��16 Temporal Trends in Nitrate Concentrations��17 Statistical Analysis of Water Levels and Nitrate Concentrations��21 Assessment of Monitoring Well Network��21 Carson River Basin Hydro Mapper��24 Summary��26 References Cited��27

Figures 1. 2. 3. 4. 5. 6. 7.

8. 9. 10.

Map showing location of major drainages, community areas, groundwater level, and nitrate monitoring within Carson Valley, Douglas County, Nevada��3 Graphs showing number of water-level, nitrate, and arsenic measurements in the alluvial aquifer in Carson Valley per year since 1980��10 Conceptual diagram showing nitrate and arsenic concentrations with aquifer depth in Carson Valley��11 Graphs showing water-level change observed in domestic wells within the community areas of Carson Valley��15 Time-lapse imagery showing unlined reservoirs in East Valley that were used to temporarily store reclaimed water��17 Maps showing spatial distribution and analytical results for nitrate and arsenic in groundwater within Carson Valley, Douglas County, Nevada��18 Boxplots showing nitrate and arsenic concentrations observed from the active and inactive monitoring well network within the community areas of Carson Valley, Nevada��19 Graphs showing discrete nitrate concentrations measured in the active monitoring well network within the community areas of Carson Valley��20 Image showing the visualization tool for the Carson River Basin with the boundaries of the hydrographic area��24 Images showing the Carson River Basin Hydro Mapper display of monitoring wells locations��25

Tables 1.

Site information for wells monitored for water-level and nitrate concentrations in the ongoing Carson Valley groundwater monitoring network��6 2. The total number of domestic wells, deepened wells, monitoring wells, and measurements from the community areas of Carson Valley for water levels, nitrate, and arsenic ��9 3. Datasets and data sources included in the Carson River Basin Hydro Mapper��12 4. Results of Mann-Kendall trend analysis for measured depth to water level and nitrate concentrations within the alluvial aquifer of community areas in Carson Valley��22

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)

Volume acre-foot (acre-ft) acre-foot (acre-ft)

1,233

cubic meter (m3)

0.001233 cubic hectometer (hm3) Flow rate

acre-foot per year (acre-ft/yr) 1,233

cubic meter per year (m3/yr)

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

0.001233 cubic hectometer per year (hm3/yr)

foot per year (ft/yr)

0.3048

meter per year (m/yr)

Hydraulic gradient foot per mile (ft/mi)

0.1894

meter per kilometer (m/km)

vii

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

Supplemental Information Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) or micrograms per liter (µg/L).

Abbreviations CBH

Carson River Basin Hydro Mapper

EPA

U.S. Environmental Protection Agency

MCL

maximum contaminant level

NDWR

Nevada Department of Water Resources

NWIS

National Water Information System

USGS

U.S. Geological Survey

Abstract Residents of Carson Valley, Douglas County, Nevada, rely on the basin-fill alluvial aquifer underlying the valley for drinking water. Since the 1980s, groundwater levels and water-quality data have been collected to monitor the status of the aquifer system and to assist in planning efforts to address current (2024) and future demand. The U.S. Geological Survey (USGS), in cooperation with Douglas County, Nevada, evaluated trends in water levels, nitrate, and arsenic concentrations from a network of monitoring and domestic wells in Carson Valley. This work also assessed the monitoring well network to determine the suitability of wells for characterizing the occurrence of arsenic in the groundwater. Monitoring of constituents, such as nitrate and arsenic concentrations, is needed to assess changes in contaminant distribution and to evaluate the effect that changing land use and groundwater pumping has on their occurrence and transport. Results of the trend analysis indicate water levels are declining (p<0.05) in 17 of 26 selected monitoring wells (65 percent). Areas with the largest change in water levels, with more than 20 feet of declines, were within the community areas of Johnson Lane, Ruhenstroth, South Agricultural, East Valley, and Fish Springs. Variations in water levels measured in wells from the Central Agricultural, Minden, Foothill, Alpine County (one well), and Gardnerville Ranchos areas show periods of increase and decrease over time, but they also maintain long-term declining trends (p<0.05). Increases in nitrate concentrations in groundwater samples collected from 9 out of 14 selected monitoring wells (64 percent) are statistically significant (p<0.05) within the Ruhenstroth, Gardnerville Ranchos, East Valley, Genoa, and Johnson Lane community areas. Samples collected from a well in Indian Hills/Jacks Valley indicated a decreasing trend in nitrate concentration over time. Nitrate concentrations in samples collected from wells in East Valley, Genoa, Johnson

Lane, and Indian Hills/Jack Valley were consistently low (less than 3 milligrams per liter [mg/L]) and stable. Nitrate concentrations from selected wells in Johnson Lane and Garnerville Ranchos exceeded the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) of 10 mg/L (as nitrogen) and have trends that are increasing over time. In 2022, a sample collected from Johnson Lane had a concentration (7.3 mg/L) below the MCL with an increasing trend over time. Temporal trend analyses for groundwater arsenic concentrations in Carson Valley could not be done because of a lack of temporal data. However, using available historical data, arsenic concentrations seem to be greater in groundwater from wells located on the eastern and northern areas of the valley than in wells located on the western or southern areas. Groundwater arsenic concentrations exceed 5 micrograms per liter (µg/L) in most samples collected from wells in Johnson Lane, Airport, Central Agricultural, and East Valley areas and in many cases exceed the U.S. Environmental Protection Agency (EPA) MCL of 10 µg/L. Data indicate that groundwater from domestic wells screened at deeper intervals are likely more vulnerable to elevated arsenic concentrations than shallower wells. A groundwater network evaluation for Carson Valley identified potential modifications in the sampling locations and frequency to better understand the effect of groundwater pumping in communities where municipal and domestic demand are greatest, potentially enhancing understanding of contaminant transport in these areas. Potential modifications to the active well network include reducing the frequency of sample collection from existing network wells (6 out of 11) that have consistently shown low and stable nitrate concentrations, adding wells in areas where data are sparse, and increasing the number of wells in areas with elevated groundwater nitrate concentrations. Including the analysis of arsenic in samples from the active groundwater monitoring well network will provide more detail on the temporal and spatial variability of arsenic concentrations.

2 Assessment of Water Levels, Nitrate, and Arsenic and the Development of a Data Visualization Tool A visualization tool for the Carson River Basin was developed to provide access to discrete and near real-time hydrologic and water-quality data. The Carson River Basin Hydro Mapper (CBH; U.S. Geological Survey, 2023b) shows active and historical discrete water levels measured by the USGS and the State of Nevada Division of Water Resources, discrete groundwater nitrate and arsenic concentration data collected by the USGS, near real-time streamflow, and surface water levels for select waterbodies. The hydrologic data in the CBH provides resource managers, the public, and the scientific community with an easily accessible tool to present and communicate the most up-to-date information available about local and basin-wide water resources.

Introduction The alluvial aquifer in Carson Valley (fig. 1) provides water resources for municipal, agricultural, and domestic uses. In 1982, the U.S. Geological Survey (USGS) and Douglas County developed a network of wells throughout the valley for monitoring depth to water and groundwater quality (Garcia, 1989). The network consists of both monitoring and domestic wells throughout many community areas of Douglas County, Nevada, with one well in Alpine County, California. The data are vital in interpreting changes in valley water levels, groundwater flow, and water quality that can be used to inform the effective management of groundwater resources (Maurer, 1986; Garcia, 1989; Thodal, 1996; Yager and others, 2012; Naranjo and others, 2013; Kitlasten and others, 2021). Long-term monitoring of water levels throughout Carson Valley provides essential information for the effective management of the aquifer. The long-term datasets show water-level declines because of groundwater pumping in community areas east of the Carson River (Maurer and Berger, 2007; Yager and others, 2012). Groundwater declines in areas of Carson Valley may have contributed to residents’ decision to deepen wells. Since 1980, 239 domestic wells have been deepened within select community areas of Carson Valley (State of Nevada Division of Water Resources, 2022). Screening domestic wells deeper in the aquifer may have unintended consequences for water quality. It has been previously documented that nitrate levels in groundwater are greater near the aquifer surface, whereas arsenic concentrations generally increase with depth (Naranjo and others, 2013; Paul and others, 2017). Although deepening domestic wells in response to declining water levels may provide a more reliable water supply and reduce the potential for nitrate exposure to the well owner, it could result in water being withdrawn from areas within the aquifer that contain higher concentrations of arsenic. Shallow groundwater is susceptible to nitrate contamination from anthropogenic sources at the land surface (Nolan and others, 2002). The U.S. Environmental Protection

Agency (EPA) has established a drinking-water criterion maximum contaminant level (MCL) for nitrate at 10 milligram per liter (mg/L; as nitrogen; U.S. Environmental Protection Agency, 2024) based on findings of methemoglobinemia in infants (Ward and others, 2018). However, epidemiological studies have reported that long-term exposure to water with nitrate concentrations below the MCL but greater than 2 mg/L may be associated with adverse health effects, such as cancer, birth defects, and preterm births (Ward and others, 1996, 2010; Bukowski and others, 2001; Weyer and others, 2001; De Roos and others, 2003; Chiu and others, 2007). Nitrate is a common contaminant in shallow groundwater in areas where fertilizers and treated effluent are applied for agricultural purposes and in areas of dense septic system use (Lico, 1997; Thomas and others, 1999; Paul and others, 2007; Naranjo and others, 2013). In focused investigations, elevated nitrate concentrations were identified in community areas of Johnson Lane, Ruhenstroth, Gardnerville Ranchos, (Naranjo and others, 2013) and in the foothill areas of Indian Hills/Jacks Valley (Thomas and others, 1999; fig. 1). These elevated concentrations are because of nitrogen exiting septic system leach fields and undergoing transformations in the unsaturated zone before entering the aquifer (Canter and Knox, 1985). Another potential pathway of nitrate transport to groundwater is the application of secondary treated effluent on fields for irrigation and in leaky sewage-effluent ponds (Lico, 1997; Alvarez and Seiler, 2004). In Carson Valley, the use of treated wastewater effluent is substantial, accounting for the second largest source of water, after the use of the Carson River, at approximately 10,000 acre-feet per year (acre-ft/yr) between 1990 and 2015 (Kitlasten and others, 2021). Wells in agricultural areas were observed to have lower nitrate concentrations than those in residential areas, but there were few wells sampled near effluent reuse areas (Naranjo and others, 2013). Stable isotopes can be used to identify sources of nitrate, but isotopic signatures can overlap, making interpretation challenging (Kendall and others, 2010). For example, effluent from septic systems has been identified as the primary source of nitrate in groundwater in the Indian Hills area of Douglas County (Thomas and others, 1999). Within the Johnson Lane and Ruhenstroth areas, stable isotopes revealed a mixture of effluent and soil nitrogen (Naranjo and others, 2013). Other sources, such as those derived from agriculture including livestock waste or synthetic fertilizers, have not been identified as potential sources to groundwater from a subset of samples analyzed for stable isotopic signatures (Thomas and others, 1999; Naranjo and others, 2013). Nutrients sourced from groundwater and surface water discharging to the Carson River have been the focus of investigations seeking to explain excessive algal blooms, increased biological oxygen demand, and nutrient cycling in the river system (Alvarez and Seiler, 2004; Alvarez and others, 2018).

Introduction 3 119°50'

119°45'

Indian Hills/ Jacks Valley

r Clea k e Cre

36 42

CARSON CITY DOUGLAS COUNTY

395

Pinenut Area Johnson Lane

34 24

Sierra Planning Area

Genoa

Central Agricultural

31 41

reek ye C 20

e Buck

For

rso

e re

Fish Springs

ive

Gardnerville

Ruhenstroth 27

Gardnerville Ranchos

3 38 395

4 26 39 37

Pinenut Area

38°50'

1 Base from U.S. Geological Survey, The National Map, 2023 Roads from U.S. Census Bureau TIGER digital line data, 2022 Shaded relief from Esri and its licensors, copyright 2023 Universal Transverse Mercator projection, zone 11 north North American Datum of 1983

Minden

Ind

Pinenut Area

ian

Cre

CA NE LI VA FO D RN A IA

South Agricultural

East Valley

iver

arso

Foothill

Ea s 1 1 t Fo 12

Minden

t Wes

Pinenut Area

Unlined reservoir area

207

38°55'

Airport

39°00'

CALIFORNIA

North Agricultural

NEVADA

Map area

Ri ve

39°05'

119°40'

DO AL UG PI LA NE S CO CO UN UN TY TY

34 13 38 0 0

EXPLANATION Community area boundary and name (Douglas County, 2022) Site by type of data collected — number from table 1 Nitrate Water level, active Water level, inactive

1 1

2 2

3 4

5 MILES

5 KILOMETERS

Figure 1. Location of major drainages, community areas, groundwater level, and nitrate monitoring within Carson Valley, Douglas County, Nevada, and Alpine County, California. Select inactive wells were included given the long period of record available at these locations. Boundaries of community areas defined in Douglas County (2021).

4 Assessment of Water Levels, Nitrate, and Arsenic and the Development of a Data Visualization Tool The transport and fate of nitrate in groundwater is affected by many factors associated with the physical, geochemical, and biological conditions in the aquifer. Depending on groundwater redox conditions, once introduced into the aquifer, nitrate is readily mobile and vulnerable to transformation (McMahon and Chapelle, 2008). If aquifer conditions are anoxic and contain labile carbon, nitrate can undergo transformation to nitrogen gas through denitrification (McMahon and Chapelle, 2008). In Carson Valley, stable isotopic analysis of groundwater samples collected in select monitoring wells indicated no evidence of denitrification (Naranjo and others, 2013). Arsenic is a common naturally occurring contaminant in groundwaters throughout the western United States (Thiros and others, 2015). Conditions associated with arsenic concentrations in groundwater exceeding the EPA drinking water MCL criterion of 10 micrograms per liter (µg/L; U.S. Environmental Protection Agency, 2024) include leaching from arsenic bearing volcanic rock and soil materials, long groundwater flow paths, desorption from the use of phosphate-based fertilizers (Welch and others, 1988; Welch and Lico, 1998; Busbee and others, 2009; Anning and others, 2012). Over time, exposure to elevated concentrations of arsenic has been associated with increased risk of diseases, including certain cancers (skin, lung, bladder, and kidney), peripheral vascular disease, hypertension, and diabetes (Hopenhayn, 2006; Saint-Jacques and others, 2014). Arsenic also causes increased incidences of infant mortality and low birth weight (Hopenhayn, 2006). The basin-fill aquifer underlying the Carson Valley aquifer is primarily composed of weathered granitic and volcanic material (Welch, 1994; Maurer and others, 2009). The vulnerability of groundwater to arsenic contamination depends on aquifer material, groundwater pumping rates, well screened intervals, and aquifer geochemical conditions (Focazio and others, 2002; Anning and others, 2012). Arsenic mobility and transport in groundwater is affected by groundwater interaction with aquifer materials (rocks and sediments), redox conditions in the aquifer, and the form or species of arsenic (Busbee and others, 2009). Although there are few focused studies on the occurrence and transport

of arsenic within Carson Valley, regional evaluations have provided insight on the spatial distribution of arsenic. For example, arsenic data collected from domestic wells in northern Nevada and northeastern California showed that 22 percent of the 174 domestic wells sampled had arsenic exceeding the MCL (Arienzo and others, 2022). Saftner and others (2023) determined that there is a greater than 50-percent probability of elevated arsenic in untreated well water for approximately 49,000 (64 percent) households in northern Nevada, northeastern California, and western Utah that rely on alluvial aquifers for domestic water supply. Water resource managers in Carson Valley have expressed concern about how the effects of a future development will affect water availability and quality. Concerns are focused on increased demand for municipal supply, pumping effects on water chemistry, groundwater-level declines, and managing elevated concentrations of nitrate and arsenic. To better understand the water availability and water quality, the USGS, in cooperation with Douglas County initiated this investigation to describe spatial and temporal trends in water levels, nitrate, and arsenic, to evaluate the groundwater monitoring network, and to develop an online tool for tracking changes in water quality and quantity within the Carson River Basin.

Purpose and Scope The objective of this report was to summarize the spatial and temporal trends in water levels, nitrate, and arsenic for the Carson Valley groundwater monitoring network, to evaluate the effectiveness of the monitoring network, and to develop a web tool to visualize hydrologic data in the Carson River Basin. The web mapping data visualization tool (U.S. Geological Survey, 2023b) also provides a portal to ongoing and historical arsenic and nitrate concentrations collected from wells, springs, and surface water, information stored within the USGS’s National Water Information System (NWIS) database (U.S. Geological Survey, 2022) and groundwater level data collected by the State of Nevada Division of Water Resources (2022).

Description of Study Area 5

Description of Study Area Carson Valley is in western Douglas County, Nevada, south of Carson City extending southward to Alpine County, California. The floor of the valley is roughly oval-shaped, approximately 20 miles (mi) long and 8 mi wide, and slopes from about 5,000 ft above sea level at the southern end of the valley to about 4,600 ft at the northern end (fig. 1). On the western side of Carson Valley, the Carson Range of the Sierra Nevada rises abruptly from the valley floor, with peaks ranging from 9,000 to nearly 11,000 ft above sea level. The valley is bordered on the east side by the Pine Nut Mountains, which rise more gradually to peaks ranging from 8,000 to nearly 9,500 ft above sea level. The dominant hydrologic features of Carson Valley are the East and West Forks of the Carson River, which join about 3 mi northwest of Minden (fig. 1). Other surface water features include small streams, including Indian Creek, Pine Nut Creek, and Buckeye Creek, that drain the Carson Range and Pine Nut Mountains and a network of irrigation channels and sloughs. Buckeye and Pine Nut Creeks only reach the valley floor and the Carson River during spring runoff in extremely wet years or large floods (Yager and others, 2012). Carson River streamflow is dependent on snowpack, groundwater discharge, and, to a lesser degree, local runoff from direct rainfall (Maurer and Berger, 2007). Irrigation diversions, return flows, and importation of sewage effluent can also have an effect on streamflow (Covay and others, 1996). Maurer and others (2009) describe groundwater near the center of the valley flowing parallel to the mainstem Carson River northward until it nears the northern terminus of the valley where it begins flowing northeast following the river. Groundwater hydraulic gradients decrease from about 100

feet per mile (ft/mi) in southwest valley areas to about 5 ft/mi in northern valley areas (Maurer and others, 2009). Annually, about 20 percent of river streamflow is lost through Carson Valley (Maurer and others, 2009). Groundwater inflow from the California part of Carson Valley ranges from 4,000 to 8,000 acre-ft/yr, with an average of 6,000 acre-ft/yr (Maurer and Berger, 2007). Water used to irrigate agricultural fields in Carson Valley includes East and West Fork Carson River surface water, pumped groundwater, and imported treated effluent (Maurer and Berger, 2007; Maurer and others, 2009). Along the western edge of the valley, groundwater flow is generally from the mountain-front areas toward the river (Maurer, 1986; Maurer and others, 2009). Groundwater underlying the southeastern area of the valley flows northwesterly toward the Carson River (Maurer and others, 2009). Groundwater-level and water-quality data obtained from the monitoring network have been used to estimate a water budget for the valley (Maurer and Berger, 2007), to understand water-quality patterns (Garcia, 1989; Thodal, 1996), and to predict water resource responses to different water use planning and climate change (Yager and others, 2012; Naranjo and others, 2013; Kitlasten and others, 2021). The groundwater-level and nitrate-concentration measurements are obtained from wells ranging in depth from 11 to 608 ft below land surface (table 1) and are sparsely distributed throughout the valley (fig. 1). The network of wells used for monitoring water levels and nitrate was established in the 1980s and has largely remained unchanged since 2010. In some places, domestic wells were removed from the monitoring network at the request of homeowners or when property ownership changed.

[Data collected for sites in this table can be accessed in the U.S. Geological Survey (USGS) National Water Information System database (U.S. Geological Survey, 2022) using USGS site identifiers. Coordinates are referenced to North American Datum of 1983 and North American Vertical Datum of 1988. Altitude is referenced to North American Vertical Datum of 1988. Community areas provided by Douglas County, Nevada, updated on December 21, 2022. Map identifier (ID) 1 is not in a community area of Douglas County. Abbreviation: ft, feet]

Map ID

USGS site ID

Longitude

Altitude (ft)

Well depth (ft)

384917119463101

105 N11 E20 18ABDA1 DBW

38.821

−119.775

4,995

Unknown

385304119460601

105 N12 E19 23CDBC1 USGS

38.885

−119.820

4,793

Foothill

3 4

385332119441501

105 N12 E20 21ACAA1 CHOS 7

38.892

−119.738

4,863

480

Gardnerville Ranchos

385357119404701

105 N12 E20 13DD 1

38.899

−119.680

4,988

240

Ruhenstroth

385452119464101

105 N12 E20 07DBCC1 USGS

38.914

−119.779

4,723

South Agricultural

385507119370301

105 N12 E21 10BCD 1

38.919

−119.619

5,360

190

Fish Springs

385548119385001

105 N12 E21 05DBDD1

38.931

−119.647

5,122

Fish Springs

385601119402901

105 N12 E21 06BCDB1 OLIVERS RANCH

38.934

−119.675

5,018

220

Fish Springs

USGS station name

Latitude

Community area

Active water level network Alpine County

385609119392201

105 N12 E21 05BCAB1 EFFD2

38.936

−119.656

5,095

210

Fish Springs

385612119382701

105 N12 E21 05AADD1

38.937

−119.642

5,131

120

Fish Springs

385612119464101

105 N12 E20 06ABCC1 USGS

38.937

−119.779

4,716

20.5

South Agricultural

385612119464401

105 N12 E20 06BADD1

38.936

−119.780

4,716

430

South Agricultural

385628119411901

105 N13 E20 36CDCA1

38.941

−119.689

4,974

350

East Valley

385637119504101

105 N13 E19 33DADD2

38.944

−119.845

4,787

220

Foothill

385657119385801

105 N13 E21 32BDAD1

38.949

−119.651

5,141

608

Fish Springs

385658119432001

105 N13 E20 34ACBC2

38.949

−119.723

4,791

East Valley

385724119382301

105 N13 E21 28CCBC1 USBLM FISH SPRING WINDMILL

38.957

−119.641

5,160

Fish Springs

385729119414501

105 N13 E20 26DADD1

38.958

−119.696

4,922

180

East Valley

385742119453101

105 N13 E20 29ACCC1 ME

38.962

−119.759

4,720

17.7

Minden

385834119395901

105 N13 E21 19CBBA1 BUCKEYE CREEK WELL

38.975

−119.676

5,148

140

East Valley

385834119464101

105 N13 E20 19ACCC1 USGS-Stockyard Rd

38.976

−119.779

4,695

Central Agricultural

390122119424701

105 N13 E20 03BCBB1

39.023

−119.733

4,761

108

Airport

390156119435801

105 N14 E20 33DADA1

39.033

−119.734

4,729

125

Johnson Lane

390208119444601

105 N14 E20 33BCDA1 UNRUH S

39.035

−119.747

4,683

218

Johnson Lane

390222119423001

105 N14 E20 35BABC1

39.039

−119.710

4,941

201

Johnson Lane

145

Ruhenstroth

Active nitrate wells 26

385321119405002

105 N12 E20 24ADCC2

38.889

−119.683

4,980

385352119455401

105 N12 E20 17CCDA1

38.897

−119.766

4,763

Gardnerville Ranchos

385509119414801

105 N12 E20 11ADD 1

38.919

−119.697

4,903

125

East Valley

6 Assessment of Water Levels, Nitrate, and Arsenic and the Development of a Data Visualization Tool

Table 1. Site information for wells monitored for water-level and nitrate concentrations in the Carson Valley groundwater monitoring network, 1970 to 2022.

Table 1. Site information for wells monitored for water-level and nitrate concentrations in the Carson Valley groundwater monitoring network, 1970 to 2022.—Continued [Data collected for sites in this table can be accessed in the U.S. Geological Survey (USGS) National Water Information System database (U.S. Geological Survey, 2022) using USGS site identifiers. Coordinates are referenced to North American Datum of 1983 and North American Vertical Datum of 1988. Altitude is referenced to North American Vertical Datum of 1988. Community areas provided by Douglas County, Nevada, updated on December 21, 2022. Map identifier (ID) 1 is not in a community area of Douglas County. Abbreviation: ft, feet]

Map ID

USGS site ID

Longitude

Altitude (ft)

Well depth (ft)

385801119421501

105 N13 E20 26ABBB1

38.969

−119.704

4,869

130

East Valley

390015119500101

390055119421902

105 N13 E19 10DBB 1

39.006

−119.836

4,680

115

Genoa

105 N13 E20 02CDAB2

39.015

−119.707

4,898

350

Johnson Lane

390106119424301

105 N13 E20 02CBB 1

39.019

−119.713

4,865

176

Johnson Lane

390208119433201

105 N14 E20 34BDBD1

39.035

−119.726

4,753

100

Johnson Lane

390232119443201

105 N14 E20 28CDC 1

39.042

−119.743

4,700

Johnson Lane

390457119491301

105 N14 E19 14BBD 1

39.082

−119.822

5,040

100

Indian Hills/Jacks Valley

390542119472002

105 N14 E19 12ADAB2

39.095

−119.789

4,911

350

Indian Hills/Jacks Valley

385305119404001

105 N12 E20 24DDAB1

38.885

−119.679

5,010

238

Ruhenstroth

385311119415301

105 N12 E20 23DACA1 USFWS-FH 2

38.886

−119.699

4,891

500

Ruhenstroth

385321119405002

105 N12 E20 24ADCC2

38.889

−119.683

4,980

145

Ruhenstroth

385413119405001

105 N12 E20 13DDBB1

38.899

−119.682

5,000

250

Ruhenstroth

390055119421901

105 N13 E20 02CDAB1

39.015

−119.707

4,900

176

Johnson Lane

390542119472001

105 N14 E19 12ADAB1

39.095

−119.789

4,909

155

Indian Hills/Jacks Valley

USGS station name

Latitude

Community area

Active nitrate wells—Continued

Inactive water level and nitrate wells

Description of Study Area 7

8 Assessment of Water Levels, Nitrate, and Arsenic and the Development of a Data Visualization Tool

Methods Groundwater-level data, nitrate, and arsenic concentrations were grouped by community area (Douglas County, 2021). The USGS actively measures water levels at 25 wells and nitrate at 11 wells annually in Carson Valley (fig. 1). All monitoring wells for which nitrate and arsenic concentration data were available within the USGS NWIS database were used to illustrate the spatial extent of nitrate and arsenic occurrence within the valley. Although periods of record differ between nitrate and arsenic concentration data, the most current concentration data for each well was used to map the spatial distribution of each constituent within Carson Valley.

Water-Level and Nitrate Monitoring Water-level measurements were made using standard USGS protocols for discrete water-level measurements using steel or electric tapes (Cunningham and Schalk, 2011). Water-quality samples were collected using USGS protocols for sampling wells (U.S. Geological Survey, 2018, 2023a). The discrete water-level and water-quality data collected from the groundwater monitoring network in Carson Valley are published online through the NWIS (U.S. Geological Survey, 2022). The active groundwater level and nitrate wells monitored by the USGS range in depth from 11 to 608 ft below land surface (table 1). Spatially, the wells are sparsely distributed throughout Carson Valley (fig. 1). The network of wells used for monitoring water levels and nitrate was established in the 1980s and has largely remained consistent since 2010 with added water-level wells in Fish Springs and Johnson Lane community areas. Ruhenstroth, Gardnerville Ranchos, Alpine County, and the Airport community areas have only one water level well. To supplement the trend assessment, selected inactive wells were included for Ruhenstroth, Johnson Lane, and Indian Hills/Jacks Valley because, although these well locations were removed from the network, the wells have long-term monitoring data (greater than 5 years) that are valuable for this analysis. The total number of domestic wells, deepened wells, water levels, and nitrate and arsenic measurements within each community area are shown in table 2. From 1980 through 2022, 171 out of 239 (70 percent) domestic wells deepened were located in the community areas of Johnson Lane, Ruhenstroth, East Valley, and Indian Hills/Jacks Valley. The listed water-level wells and measurements correspond to the monitoring network from 1970 to 2022. Additional water-level data are available from inactive wells throughout the valley. The highest number of groundwater samples collected for nitrate analysis are within the active monitoring network that include Johnson Lane, Indian Hills/Jacks Valley, Ruhenstroth, and Gardnerville Ranchos. These areas were also included in previous investigations to evaluate the extent, sources, trends,

and transport of nitrate (Thodal, 1996; Thomas and others, 1999; Rosen, 2003; Shipley and Rosen, 2005; Naranjo sand others, 2013). During a subset period from 1980 to 2022, the extent (number and distribution of sites) of monitoring varied (fig. 2). From 1985 to 1987, Thodal (1996) analyzed data from monitoring wells representing different land uses and sampled at varying frequencies (bimonthly, quarterly, and annually) to determine baseline groundwater chemistry conditions for Douglas County (including Carson Valley). Since that time, variation in sampling frequency has occurred because of updates to the monitoring well network, other monitoring efforts occurring within the valley, and changing well owner participation. In support of an investigation into water planning and modeling, water-level measurements increased from 2005 to 2006 (Yager and others, 2012). The increased number of nitrate measurements occurring from 2008 to 2009 was associated with a study designed to identify the source and fate of nitrate within the valley (Naranjo and others, 2013). Since the evaluation by Thodal (1996), there have been only discrete periods of arsenic monitoring (fig. 2C).

Trends in Water Levels and Nitrate Concentration Water-level measurement and nitrate concentration trend assessments used data from active and selected inactive wells of the Carson Valley monitoring well network, as available from the USGS NWIS database (U.S. Geological Survey, 2022). The period of trend analysis for water levels was from 1970 to 2022. Time-series (temporal) variations in water levels are presented graphically by computing water-level change from the first measurement at the onset of monitoring. For these analyses, the depth to water was used to evaluate trends. Therefore, if the trend in depth to water is decreasing, water levels are rising. If the trend in depth to water is increasing, water levels are declining. Water-level measurements made immediately after well construction were not used in calculations because water-level perturbations from the process of drilling or injection of drilling fluid often have not equilibrated. Trend analysis for nitrate concentrations used data from the monitoring network obtained from 1983 to 2022. Changes in the depth to water and nitrate concentrations over time were analyzed in the statistical programming language R (R Core Team, 2021) using the Mann-Kendall nonparametric statistical test (Helsel and others, 2020) and were consistent with data analyses used by others (Thodal, 1996; Rosen, 2003; Shipley and Rosen, 2005; Naranjo and others, 2013). With this test, we chose an alpha value of less than 0.05 to represent a statistically significant monotonic trend at the 95-percent confidence limit. P-values greater than 0.05 are not considered statistically significant. Positive tau values indicate increasing trends, and negative tau values indicate decreasing trends. Spatial patterns of arsenic concentrations were mapped across Carson Valley, but given the lack of data, statistical trend analysis for arsenic could not be done.

Description of Study Area 9 Table 2. The total number of domestic wells, deepened wells (1980 to 2022), monitoring wells, and measurements from the community areas of Carson Valley for water levels (1970 to 2022), nitrate (1983 to 2022), and arsenic (1960 to 2019). [The number of water level wel=ls and measurements listed are for the active monitoring network. The number of nitrate and arsenic wells and measurements include active and inactive wells. Data is available in National Water Information System (U.S. Geological Survey, 2022). Domestic well database obtained from Nevada Division of Water Resources (2022). Water level measured in Alpine County is not within a community area in Carson Valley. Abbreviation: mi2, square miles]

Community areas Airport

Area (mi2) 6.1

PercentDomestic Deepened age 47

Water levels

Nitrate

Arsenic

Wells

Measurements

Wells

Measurements

Wells

Measurements

139

Central Agriculture

6.3

160

East Valley

13.7

653

515

167

Fish Springs

19.1

328

424

Foothill

10.4

416

167

Gardnerville

4.1

Gardnerville Ranchos

10.5

317

117

Genoa

9.9

239

Indian Hills/ Jacks Valley

13.4

406

197

Johnson Lane

26.9

1,074

399

343

Minden

6.8

North Agricultural

14.7

Pinenut

347.4

149

Ruhenstroth

8.0

530

162

Sierra Planning Area

30.2

South Agricultural

25.3

192

520

Assessment of the Carson Valley Groundwater Monitoring Well Network According to Garcia (1989), the groundwater monitoring well network in Douglas County (including Carson Valley) was designed to (1) monitor areas where heavily pumped municipal wells could draw in contamination or sites in areas with high potential for contamination; (2) gather data for long-term trend assessment of water quality; and (3) supplement data gaps and help define new sources of contamination. The intent was to design a monitoring network with the flexibility to adjust to changing groundwater conditions to better define the causes for the changes. As information on the long-term status of nitrate concentrations in Carson Valley become available, it may be necessary to adjust the network to ensure monitoring data provides suitable information for water resource planning purposes. In the Carson Valley aquifer system, the well screen interval will affect the water quality drawn from wells. The general conceptual model shown on figure 3 illustrates the findings of Naranjo and others (2013) and Paul and others (2017) regarding the distribution of nitrate and arsenic in the alluvial aquifer of Carson Valley. Arsenic data assembled by Paul and others (2017) for the Gardnerville Ranchos, Minden, Gardnerville, Ruhenstroth, and Pinenut areas were shown to

increase in concentration with aquifer depth. Deeper aquifer conditions also reflect longer flow paths, older groundwater, and greater contact with sediments, increasing the solubility and mobility of arsenic (Anning and others, 2012). Samples collected from wells throughout the valley (Naranjo and others, 2013) have shown nitrate concentrations near the water table are higher where inputs from septic systems are focused and steeply decrease with aquifer depth. The sharp gradients in nitrate concentrations are affected by oxidation-reduction (redox) conditions of the aquifer and the denitrification process (Naranjo and others, 2013). The State of Nevada well inventory database indicates that 239 wells have been deepened in select community areas of Carson Valley since 1980 (State of Nevada Division of Water Resources, 2022). As domestic wells are deepened into the alluvial aquifer, they may have different water-quality characteristics. Given the risks to domestic and municipal water supply, the monitoring well network was reviewed to provide management considerations for potential updates to the nitrate and arsenic network in Carson Valley. The number of measurements per community area and the variability in concentrations and trends are important considerations in identifying needs for water resource planning. The assessment included a review of the existing and historical monitoring well data, knowledge gained from published reports, and a review of the monitoring frequency.

10 Assessment of Water Levels, Nitrate, and Arsenic and the Development of a Data Visualization Tool

Number of water level measurements

200

150

100

0 1980

Number of nitrate measurements

100

1990

1995

2000

2005

2010

2015

2020

1985

1990

1995

2000

2005

2010

2015

2020

1985

1990

1995

2000

2005

2010

2015

2020

80 60 40 20 0 1980

100

Number of arsenic measurements

1985

80 60 40 20 0 1980

Calendar year

Figure 2. Number of A, water-level; B, nitrate; and C, arsenic measurements in samples collected from monitoring wells in Carson Valley per year, 1980–2022 (U.S. Geological Survey, 2022). Water-level measurements were collected at wells in the active monitoring network. Groundwater samples collected for nitrate analyses were part of previous U.S. Geological Survey (USGS) investigations and the active network. Samples collected for arsenic analyses were taken periodically during USGS investigations and are not routinely sampled as part of the active network.

Description of Study Area 11 Shallow

EXPLANATION

Depth of aquifer

Nitrate

Well depth

Arsenic

Deep Low

High

Concentration

Figure 3. Conceptual diagram of nitrate and arsenic concentrations with aquifer depth in Carson Valley. Wells with well screens near the surface are more susceptible to high groundwater nitrate concentrations (Naranjo and others, 2013). Groundwater from wells with well screens deeper into the aquifer are less likely to have high nitrate concentrations detected and more likely to contain higher arsenic concentrations (Paul and others, 2017).

Carson River Basin Hydro Mapper A web-based data visualization tool was developed to improve the dissemination and presentation of USGS data and other hydrologic data relevant to the Carson River Basin. The hydrologic dashboard, called the Carson River Basin Hydro Mapper (CBH), provides near real-time climate (snow water equivalent, precipitation, soil moisture), streamflow, and surface-water level data (for example, Lahontan Reservoir storage and level). The CBH interfaces with the USGS NWIS and the USGS National Map Services for watershed extents, the Natural Resources Conservation Service for

snow measurement locations, the National Weather Service for rainfall and snow water equivalent, the National Oceanic and Atmospheric Administration for weather radar, and Douglas County, Nevada, for Carson Valley community area boundaries (table 3). The tool includes an interactive map viewer showing near real-time information and available discrete groundwater-level and water-chemistry data for select constituents. Assembling the integrated data into a single repository will allow for easier retrieval, analysis, and interpretation of data as they relate to important aspects of water resource management within the Carson River Basin.

[Three examples out of 16 basemap coverages are listed below. Descriptions and links to data sources are also available within the legend tab of the Carson Basin Hydromapper. Abbreviations: Esri, Environmental Systems Research Institute; GIS, Geographic Information System; USGS, U.S. Geological Survey]

Data type

Visualization

Source

Data Availability USGS observations

Streamflow gage

Real-time status

National Water Information System