Prepared in cooperation with Lahontan Regional Water Quality Control Board
Natural and Anthropogenic Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California By John A. Izbicki, Krishangi D. Groover, Whitney A. Seymour, David M. Miller, John G. Warden, and Laurence G. Miller The Pacific Gas and Electric Company (PG&E) Hinkley compressor station (fig. 1), in the Mojave Desert, 80 miles northeast of Los Angeles, California, is used to compress natural gas as it is transported through a pipeline from Texas to California. Between 1952 and 1964, cooling water was treated with a compound containing hexavalent chromium, Cr(VI), to prevent corrosion of machinery within the compressor station. Cooling wastewater containing Cr(VI) was discharged to unlined ponds and released into groundwater. Since 1964, cooling-water management practices have been used that do not contribute chromium to groundwater. A 2007 PG&E-funded study estimated the natural Cr(VI) background concentration in Hinkley Valley to be 3.1 micrograms per liter (µg/L; CH2M Hill, 2007); this value was adopted as the interim Cr(VI) background concentration for regulatory purposes by the Lahontan Regional Water Quality Control Board (RWQCB) in 2007. The Lahontan RWQCB requested that the 2007 background Cr(VI) concentration study be updated by the U.S. Geological Survey (USGS) to (1) address limitations with the 2007 study methodology (Lahontan Regional Water Quality Control Board, 2011) and (2) include increases in the mapped extent of Cr(VI) concentrations greater than 3.1 µg/L and increases in the regulatory Cr(VI) plume extent between 2008 and 2012. The purpose of the updated study was to estimate Cr(VI) background concentrations in unconsolidated deposits that compose the upper aquifer underlying Hinkley and Water Valleys (Izbicki and Groover, 2016). Rock and aquifer deposits within Hinkley Valley have chromium concentrations commonly less than 25 milligrams per kilogram (Izbicki and others, 2023). These concentrations are typical of the region and less than the average bulk continental crustal concentration for chromium of 185 mg/kg (Reimann and de Caritat, 1998). With the exception of hornblende diorite that crops out in Iron Mountain along the western margin of Hinkley Valley, chromium-containing rocks in the area are either (1) not consistently high in chromium; (2) have limited areal extent; or (3) in the case of basalt, are present only in Water Valley. More than 90 percent of the chromium in aquifer deposits is contained within unweathered mineral grains and is comparatively unavailable to groundwater. Consequently, natural Cr(VI) in water from wells within Hinkley and Water Valleys is related to factors other than chromium abundance, including (1) mineralogy and weathering rates of chromiumcontaining minerals; (2) accumulation of chromium weathered from chromium-containing minerals within iron- and manganese-oxide surface coatings on mineral grains and subsequent oxidation of accumulated trivalent chromium, Cr(III), to Cr(VI) in the presence of manganese oxides; (3) texture of aquifer deposits, with finer-textured deposits having greater surface area and more abundant iron- and manganese oxide coatings; and (4) pH-dependent desorption of Cr(VI) from iron- and manganese oxide surface coatings on mineral grains into groundwater under appropriate aqueous geochemical conditions. Once oxidized to Cr(VI), desorption of Cr(VI) from sorption sites on the surfaces of mineral grains increases with increasing pH. During timespans of several thousand years, pH increases with groundwater age (time since recharge) as silicate minerals that compose aquifers weather, and natural Cr(VI) concentrations may increase in older groundwater within Hinkley and Water Valleys as long as that older water remains oxic (contains dissolved oxygen). U.S. Department of the Interior U.S. Geological Survey
Figure 1. Pacific Gas and Electric Company (PG&E) compressor station, Hinkley, California, March 2009. (Photograph by Steven Perry, ARCADIS, Inc., courtesy of PG&E). The extent of anthropogenic (human-made) Cr(VI) released from the Hinkley compressor station was estimated using a summative-scale analysis of geologic, hydrologic (including groundwater source and age), and geochemical data collected from more than 100 wells between March 2015 and November 2017 (Izbicki and others, 2023). Sampled wells were selected by the USGS with input from a Technical Working Group (TWG) consisting of local community members, the Independent Review Panel (IRP) Manager, the Lahontan RWQCB, PG&E, and PG&E consultants (fig. 2). The summative scale developed from data collected as part of this study consisted of eight questions requiring binary (yes or no) answers for each sampled well (table 1). The questions were intended to provide (1) a transparent framework for data interpretation in which all stakeholders participated; (2) unbiased interpretation of data traceable to numerical measurements; (3) a framework in which geologic, hydrologic, and geochemical data could be interpreted collectively; and (4) a framework to consolidate different types of data into simple, easy-to-understand illustrations. A score of -1 was assigned for answers to questions within the summative scale that were consistent with natural Cr(VI); a score of +1 was assigned for answers consistent with anthropogenic Cr(VI). Scores for each question in the summative scale were summed to create a single score for each sampled well. Possible scores ranged from –8 for wells having all answers consistent with natural Cr(VI), to +8 for wells having all answers consistent with anthropogenic Cr(VI). Data were not available to score every question within the scale for every sampled well. Consequently, summative-scale scores were evaluated as the percent of the total possible score for each well, with possible scores ranging from –100 to +100 percent for natural and anthropogenic Cr(VI), respectively. When data from each well were scored using the questions and metrics Open-File Report 2023–1043 Version 1.1, June 2023
Start: study begins January 2015 Summative-scale questions (Table 1)
Groundwater model results (Jacobs Engineering Group, 2019)
Define plume margins (March 2015 through November 2017, figure 3)
Step as proposed by Izbicki and Groover, (2016 and 2018) not included
Evaluate/modify plume margins
Hexavalent chromium data outside summative-scale mapped plume (April 2017 through January 2018, figure 4)
Calculate background hexavalent chromium concentration (Table 2, figure 4)
Finish: study published (Izbicki and others, 2023)
Figure 2. Data collection, summative-scale analyses and interpretation, background calculation, and report preparation for U.S. Geological Survey background hexavalent chromium, Cr(VI), study, Hinkley and Water Valleys, western Mojave Desert, California. Data are modified from Izbicki and others (2023). within the summative scale, all stakeholders would score each well the same way and would draw the same summative-scale Cr(VI) plume extent. The summative-scale scores were used to draw the boundary of the summative-scale Cr(VI) plume around the lowest magnitude positivepercentage score that produced a contiguous plume extent (percentage scores greater than 50 percent). The summative-scale Cr(VI) plume extent of 5.5 square miles (mi2; fig. 3) was larger than the 2.2 mi2 extent of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume (ARCADIS, 2016) but smaller than the 8.3 mi2 maximum mapped extent of Cr(VI) concentrations greater than the interim regulatory Cr(VI) background concentration of 3.1 µg/L (fig. 3). The summative-scale Cr(VI) plume is within unconsolidated “Mojave-type” aquifer deposits transported to the area by the Mojave River that are composed of low-chromium alluvium and near shore lake (beach) deposits (Miller and others, 2020). Most groundwater within the summative-scale Cr(VI) plume was neutral to slightly alkaline (regulatory pH values less than 7.2; ARCADIS, 2016), contained tritium (a radioactive isotope of hydrogen having a half-life of 12.3 years), and was recharged from the Mojave River after 1952 (Izbicki and others, 2023). The summative-scale Cr(VI) plume is within the area covered by the PG&E monitoring-well network. Although the summative-scale Cr(VI) plume includes wells identified as containing anthropogenic Cr(VI), the summative-scale Cr(VI) plume may include wells that have Cr(VI) concentrations below regulatory concern and does not define the Cr(VI) plume extent for regulatory purposes. Hexavalent chromium concentrations in water from wells outside the summative-scale Cr(VI) plume extent were
used to calculate background Cr(VI) concentrations that can be used to update the regulatory Cr(VI) plume extent. The selected study design (fig. 2; Izbicki and Groover, 2016, 2018) compared the summative-scale Cr(VI) plume extent with particle-track simulations calculated using an updated groundwater-flow model of Hinkley Valley prepared by PG&E consultants (Jacobs Engineering Group, Inc., 2019). Differences among measured groundwater-age data, Cr(VI) regulatory data, and particle simulations (Izbicki and others, 2023) were not reconciled within the timeframe of this study; consequently, the updated groundwater-flow model was not used to iteratively evaluate and refine the summative-scale Cr(VI) plume extent. Outside the summative-scale Cr(VI) plume extent, naturally occurring Cr(VI) concentrations greater than the interim Cr(VI) regulatory background concentration of 3.1 µg/L were identified in water from wells completed in (1) fine-textured materials, including mudflat/playa deposits; (2) materials having visually abundant iron- and manganese-oxide surface coatings; (3) weathered hornblende diorite bedrock in the western subarea; and (4) weathered Miocene (5.3 to 23 million years old) materials underlying the western subarea, parts of the northern subarea downgradient from the Mount General fault, and Water Valley (fig. 3). Naturally occurring Cr(VI) concentrations in groundwater within these materials differ but can exceed 10 µg/L in areas where older, oxic (contains dissolved oxygen) groundwater is strongly alkaline with pH values greater than 8.0. Hexavalent chromium concentrations as high as 10 µg/L (Izbicki and others, 2023) were measured in water from wells downgradient from the “western excavation site” (fig. 3). The western excavation site, on property owned by PG&E, was used as an illegal disposal site by unknown parties. The western excavation site has a different hydrologic history from the Hinkley compressor station and is managed separately for regulatory purposes. Chemical and mineralogic data did not indicate high natural abundance, unusual mineralogy, unusual sorptive properties, or unusual aqueous geochemistry for chromium within unconsolidated deposits that would contribute to natural high Cr(VI) concentrations in water from wells downgradient from the western excavation site (Izbicki and others, 2023). Although Cr(VI) releases have not been confirmed at the western excavation site, Cr(VI) concentrations in water from downgradient wells were not used for the calculation of background Cr(VI) values. Background Cr(VI) concentrations near the margins of the Cr(VI) plume can be used for regulatory purposes including updating the regulatory Cr(VI) plume extent, plume management, and establishment of cleanup goals. Background Cr(VI) concentrations were calculated using the computer program ProUCL 5.1 (Singh and Maichle, 2015) as the upper 95-percent tolerance level, UTL95. The UTL95 is the value below which 95 percent of measured concentrations are expected to fall 95 percent of the time. The UTL95 controls for false positive and false negative results (statistical errors) in estimates of background. Hexavalent chromium data from 81 wells completed in undifferentiated, unconsolidated deposits outside the summative-scale Cr(VI) plume, collected quarterly between April 2017 and March 2018, were used to calculate an overall UTL95 value of 3.8 µg/L. The overall UTL95 value is similar to the maximum Cr(VI) concentration of older groundwater in contact with Mojave-type deposits of 3.6 µg/L (Izbicki and others, 2023). For regulatory purposes, including plume management near the summative-scale Cr(VI) plume margin, UTL95 values of 2.8, 3.8, and 4.8 µg/L were calculated for the eastern and western subareas and the northern subarea upgradient from the Mount General fault, respectively (table 2). A separate UTL95 value of 5.8 µg/L was calculated for wells near mudflat/playa deposits in the eastern subarea near Mount General. A UTL95 value of 2.3 µg/L was calculated for Cr(VI) concentrations that may have been present in Mojave-type deposits within the updated regulatory Cr(VI) plume if Cr(VI) had not been released from the Hinkley compressor station. This value is lower than values elsewhere in Hinkley Valley because of
Table 1. Summative-scale questions used to determine the extent of natural and anthropogenic (human-made) hexavalent chromium, Cr(VI), Hinkley and Water Valleys, western Mojave Desert, California. Data are modified from Izbicki and others (2023). [Items in the scale are formulated as questions requiring a binary, yes or no, answer. Based on the answers to each question, a score of –1 is consistent with a natural source and a score of 1 is consistent with an anthropogenic source. Abbreviations: USGS, U.S. Geological Survey; PG&E, Pacific Gas and Electric Company; mg/kg, milligrams per kilogram; GAMA, Groundwater Ambient Monitoring Assessment Project]
Question
Data source
Chapter where data are discussed
USGS lithologic descriptions of core material (PG&E Chapters B, C lithologic descriptions from well logs or drillers and E logs used if core material was not available)
Answer and score Yes
No
−1
1
1.
Are geologic materials at the well screen fine textured (predominately silt or finer)?
2.
Do geologic materials at the well screen contain more Portable (handheld) X-ray fluorescence (HXRF) than 85 mg/kg chromium? measurements of core material
Chapter B
−1
1
3.
Do geologic materials at the well screen contain more Portable (handheld) X-ray fluorescence (HXRF) than 970 mg/kg manganese? measurements of core material
Chapter B
−1
1
4.
Are Cr(VI) concentrations trended upward, downward or no trend with time?
Regulatory Cr(VI) data collected between July 2012 Chapter D and June 2017, interpreted using the Mann-Kendall test for trend (Helsel and others, 2020)
1
−1
5.
Is there an excess of Cr(VI) with respect to pH, with the probability of natural Cr(VI) occurrence at the measured pH less than 30 percent?
pH-dependent sorption evaluated on the basis of pH and Cr(VI) concentrations in California-wide GAMA data
Chapter E
1
−1
6.
Is there an excess of Cr(VI) with respect to other trace elements?
Principal component analyses (PCA; Helsel and others, 2020) of Cr(VI), arsenic, vanadium, uranium, iron, and manganese.
Chapter E
1
−1
7.
Was the water recharged from the Mojave River?
delta oxygen-18, δ18O, and delta deuterium, δD, data Chapter F
1
−1
8.
Does the water contain measurable modern, post1952, water (with measurable tritium) and a carbon-14 activity greater than 84 percent modern carbon?
Tritium, helium-3, and carbon-14 data.
1
−1
coarser textured, low-chromium deposits and proximity to recharge areas along the Mojave River that results in younger, less alkaline (near-neutral pH) groundwater compared to wells farther downgradient. The value may be a suitable cleanup metric for wells within the updated regulatory Cr(VI) plume. The UTL95 values calculated for undifferentiated deposits in the northern subarea downgradient from the Mount General fault and in Water Valley were 9.0 and 6.1 µg/L, respectively. These values define background Cr(VI) concentrations in areas farther downgradient from the plume margins (fig. 4). Hexavalent chromium concentrations in water from more than 70 domestic wells sampled in Hinkley and Water Valleys between January 27 and 31, 2016, did not exceed 4.0 µg/L (Izbicki and Groover, 2018). Hexavalent chromium concentrations in water from domestic wells were within background ranges expected for native (uncontaminated) groundwater within the various subareas in Hinkley Valley. However, domestic wells in former residential areas within the community of Hinkley having Cr(VI) concentrations as high as 8.6 µg/L had been destroyed by PG&E based on guidance from the Lahontan RWQCB and were not available for sample collection. Water from 47 percent of sampled domestic wells had arsenic, uranium, or nitrate concentrations above drinking water limits (maximum contaminant levels, MCLs) for these constituents. Remediation of anthropogenic Cr(VI) within groundwater downgradient from the Hinkley compressor station is accomplished using a number of techniques, including bioremediation using ethanol as a
Chapter F
reductant injected within a volume of aquifer known as the in situ reactive zone (IRZ). Laboratory-microcosm studies showed that soluble Cr(VI) was rapidly reduced to Cr(III) with additions of ethanol. Reduced Cr(III) was sorbed and then sequestered into crystalline iron and manganese oxides on the surfaces of mineral grains within the microcosms during a period of several months. Sequestration of chromium with manganese oxides facilitated reoxidation of Cr(III) back to Cr(VI) within 14 days after oxic conditions were established within laboratory microcosms. The amount of reoxidation of Cr(III) to Cr(VI) increased with manganese (Mn) concentration, and as much as 20 percent of the added Cr was oxidized to Cr(VI) in microcosms prepared as part of this study. Although much of the reoxidized Cr(VI) remained sorbed to mineral grains, aqueous Cr(VI) was present within the microcosms. Microcosm studies are not directly analogous to reactions that occur within aquifers; however, maintenance of anoxic (does not contain oxygen) conditions within the IRZ could ensure future sequestration of chromium within treated aquifer materials as Cr(III). Results of the USGS Cr(VI) background study are presented by Izbicki and others (2023). Hexavalent chromium background concentrations can be used for regulatory purposes to define and manage the Cr(VI) plume margins, identify unusual Cr(VI) concentrations outside the Cr(VI) plume margins, and establish cleanup goals within the updated regulatory Cr(VI) plume. The Lahontan RWQCB has the sole authority to establish and update Cr(VI) background concentrations for regulatory purposes.
117°10'
117°05'
EXPLANATION Harper Lake (dry)
Maximum extent of 3.1 micrograms per liter hexavalent chromium, Cr(VI), October–December 2015 (Q4 2015); queried where approximate (ARCADIS, 2016)
MW-184S1
?
Q4 2015 regulatory Cr(VI) plume (ARCADIS, 2016)
MW-205S1, S3
?
?
Summative-scale Cr(VI) plume
28M-08
27N-01
28N-04
Mojave River active channel, 2005 (Mojave Water Agency, 2014)
MW-193S1, S2, S3
Predevelopment (1930) saturated unconsolidated deposits
Water Valley 35°00'
Pacific Gas and Electric Company Hinkley compressor station
MW-186S3
Fault—location accurate; dashed where approximately located or concealed (Miller and others, 2018)
Hinkley Gap
MW-174S1
Western excavation site
MW-212S1, S2 MW-131S1
Well type and identifier
MW-133S1
MW-154S1
MW-136S1, S2
MW-207S1, S2
MW-200S1
HINKLEY ROAD
Lynx Cat Mountain
M
MW-137S1, S3
MW-173S1
OU NT
MW-104S1, D MW-106S MW-105S, D
GE NE RA L
—
—
—
—
Less than 25 to –100
LENWOOD ROAD
1 Shallow monitoring well identified by suffix S, S1, or A; middle monitoring well by suffix S2; deep monitoring well by suffix S3, D, D1, D2, or B; MW-128S1, S3 bedrock well by suffix C; replacement well by suffix R. THOMPSON ROAD MW-126S1, 2 MW-97S 30E-01 22-63 MW-79S, D MW-72S MW-129S BGS-48 22-09 Northern MW-96S MW-118S C-01 subarea FA Water and UL MW-119S, D 27-38 T MW-203S, D Valley MW-110S Water MW-50S, B Valley LO 27-03 MW-115S, D MW-45A CK MW-164S HA IW-03 MW-153S Western MW-199S1 RT OLD HIGHWAY 58 FA MW-49S, B subarea ULT MW-121S, D MW-38B MW-102S, D MW-192S, D Hinkley MW-158SR Valley MW-103D BGS-34 MW-155S, D MW-146S, D1 MW-209S MW-159S, D Hinkley MW-178S, D MW-208S MW-143S, D2 Eastern compressor MW-149S MW-163S, D Hinkley subarea COMMUNITY BLVD station
DIXIE ROAD
34°55'
—
50 25 to less than 50
MW-123S1 MW-172S1
58
Selected Summative-scale Monitoring well depth1 Domestic production well score, in percent Shallow Middle Deep well Greater than (>) — — 75 to 100 — — >50 to 75
MW-197S1
BG-0003A, B, C
Valley
MW-77S
BG-0005A, B
BW-01S, D MW-217S, D
BG-0006A, B
58
BG-0002
Mo jav e
BG-0004A, B, C
Iron Mountain
LOC KH AR T
r ve Ri
FA UL T
66
BG-0001
Base from U.S. Geological Survey and other Federal and State digital data, various scales; Lambert Conformal Conic projection, standard parallels are 34.0333° N. and 35.4667° N.; North American Datum of 1983
15 0 0
2
1 1
2
3 MILES
3 KILOMETERS
Figure 3. Summative-scale scores and summative-scale hexavalent chromium plume extent, Hinkley and Water Valleys, western Mojave Desert, California, March 2015 through November 2017. Summative-scale scores were calculated from data available in Izbicki and others (2023) chapter E (appendix E.1, table E.1.1), Groover and Izbicki (2018), and U.S. Geological Survey (2021). Selected data and scores are available in Izbicki and others (2023) chapter G (appendix G.1, table G.1.1). sac17-0650_fig03
117°12'
117°10'
Harper Lake (dry)
MW-184S1
Water Valley
117°06'
Summative-scale plume1 Pacific Gas and Electric Company (PG&E) Hinkley compressor station
Western subarea
27N-01
Mojave River active channel, 2005 (Mojave Water Agency, 2014)
Hinkley Valley
MW-186S3 Hinkley compressor station
MW-193S1, S2, S3
Predevelopment (1930) saturated unconsolidated deposits Cr(VI) background concentrations for undifferentiated unconsolidated deposits within the eastern, western, and northern subareas1 as the upper 95-percent tolerance limit (UTL95)— values assume a normal distribution to the data
Eastern subarea
Hinkley Gap
MW-174S1
Water Valley—UTL95 6.4 micrograms per liter (µg/L) Northern subarea downgradient from Mount General fault —UTL95 9.0 µg/L
MW-212S1, S2
MW-200S1
MW-207S1, S2 MW-137S1, S3
MW-173S1
M OU MW-142S1 NT
BGS-34
MW-159S, D MW-163S, D
BG-0005A, B BG-0002
BG-0004A, B, C
BG-0001
MW-199S1
MW-192S, D
Well used to calculate UTL95 Well not used to calculate UTL95 Overall normal UTL95 for all three subareas is 3.8 µg/L. O D HIGS,H S1, or A; middle Shallow monitoring well identified by Lsuffix WAY monitoring well by suffix S2; deep monitoring well by 5 8 suffix S3, D, D1, D2, or B; bedrock well by suffix C; replacement well by suffix R. COMMUNITY BLVD
2
MW-146S, D1 MW-143S, D2
BW-01S, D
BG-0006A, B
MW-217S, D
Base from U.S. Geological Survey and other Federal and State digital data, various scales; Lambert Conformal Conic projection, standard parallels are 34.0333° N. and 35.4667° N.; North American Datum of 1983
U.S. Geological Survey sampled well and identifier Monitoring well depth2 Domestic well Shallow Middle Deep
MW-115S, D
1
Hinkley Valley
MW-77S
BG-0003A, B, C
Iron Mountain
MW-110S
MW-208S
PG&E wells sampled for regulatory purposes used to calculate UTL95 for undifferentiated unconsolidated deposits
BGS-48
MW-96S
MW-103D MW-209S
MW-178S, D MW-149S
Western excavation site
General MW-128S1, S3 FA THOMPSON ROAD ULT MW-97S 30E-01 MW-79D
MW-102S, D
MW-155S, D
Fault—location accurate; dashed where approximately located or concealed (Miller and others, 2018)
Mount
MW-105S, D
MW-45A MW-49S, B
MW-38B MW-158SR
Mudflat/playa deposit within eastern subarea —UTL95 5.8 µg/L
GE NE MW-106S RAL
MW-172S1 MW-126S1, 2 22-63 MW-129S MW-72S 22-62 MW-118S MW-119S, D MW-50S, B
MW-153S MW-121S, D
58 34° 55'
MW-123S1
Western subarea—UTL95 3.8 µg/L Eastern subarea—UTL95 2.8 µg/L
Mo jav eR
KH AR MW-203S, D TF AU LT MW-164S
MW-104S1, D
Northern subarea upgradient from Mount General fault —UTL95 4.8 µg/L
ROAD
HINKLEY ROAD
Lynx Cat Mountain MW-139S1
LO C
MW-197S1
MW-154S1
r ive
LENWOOD ROAD
MW-133S1 MW-136S1, S2
DIXIE
MW-131S1
117°04'
EXPLANATION
October–December 2015 regulatory hexavalent chromium, Cr(VI), plume (ARCADIS, 2016)
Northern subarea and Water Valley
Water Valley
MW-205S1, S3
28M-08
35° 00'
117°08'
0 0
58
LOC KH 66 AR TF AU LT
2
1 1
15
2
3 MILES
3 KILOMETERS
Figure 4. Background hexavalent chromium, Cr(VI), concentrations within Hinkley and Water Valleys, western Mojave Desert, California, April 2017 through March 2018. Data are modified from Izbicki and others (2023).
Table 2: Background hexavalent chromium, Cr(VI), conentrations for plume management within Hinkley Valley, western Mojave Desert, California, April 2017 through March 2018. Locations of subareas identified in table 2 are shown in figure 4.
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., https://doi.org/10.3133/tm4a3. (Supersedes USGS Techniques of Water-Resources Investigations, book 4, chap. A3, version 1.1.)
[Background Cr(VI) concentrations calculated as the upper 95-percent tolerance limit, UTL95, using the computer program ProUCL 5.1 (Singh and Maichle, 2015). The number of wells is the number of wells used in the calculation. Abbreviation: µg/L, micrograms per liter]
Izbicki, J.A., and Groover, K.D., 2016, A plan for study of hexavalent chromium, Cr(VI), in groundwater near a mapped plume, Hinkley, California: U.S. Geological Survey Open-File Report 2016–1004, 12 p., accessed November 28, 2018, at https://doi.org/10.3133/ofr20161004.
Number of wells
UTL95, in mg/L
Maximum Cr(VI), in mg/L
Modeled distribution
Eastern subarea1 24
2.8
3.6
normal
Western subarea 27
3.8
3.8
normal
Northern subarea (upgradient of Mount General fault)2 30
4.8
4.0
normal
Overall (eastern and western subareas and the northern subarea upgradient of the Mount General fault) 81
3.8
4.0
normal
1 A separate UTL95 value of 5.8 µg/L calculated for mudflat/playa deposits and older groundwater near Mount General. An additional UTL95 value of 2.3 µg/L calculated for the Cr(VI) plume extent after regulatory updates. 2 Separate UTL95 values of 9.0 and 6.1 µg/L were calculated for the northern subarea downgradient from the Mount General fault and for Water Valley, respectively.
Acknowledgments This study was done by the U.S. Geological Survey (USGS) with input from a Technical Working Group (TWG) composed of community members, the Lahontan Regional Water Quality Control Board (RWQCB), the Independent Review Panel (IRP) Manager, Pacific Gas and Electric Company (PG&E), and consultants for PG&E. The study was funded cooperatively under an agreement between the Lahontan RWQCB and the USGS, with financial and logistical support from PG&E. The authors thank the many people involved in the design and implementation of this study including community members, the Lahontan RWQCB, Project Navigator, Ltd., PG&E, and their consultants. The authors also acknowledge and thank the many Hinkley community members who allowed access to their properties for sample collection and who collectively donated thousands of hours on behalf of the local community in support of this project and for resolution of issues related to anthropogenic hexavalent chromium, Cr(VI), releases from the Hinkley compressor station.
References Cited ARCADIS, 2016, Annual cleanup status and effectiveness report (January to December 2015), Pacific Gas and Electric Company, Hinkley Compressor Station, Hinkley, California: San Francisco, Calif., Pacific Gas and Electric Company, prepared by ARCADIS, Oakland, Calif., RC000699, [variously paged], accessed February 2016, at https://documents.geotracker.waterboards. ca.gov/esi/uploads/geo_report/5010230779/SL0607111288.PDF. CH2M Hill, 2007, Groundwater background study report—Hinkley compressor station, Hinkley, Calif.: Oakland, Calif., CH2M Hill, [variously paged], accessed January 12, 2018, at https://www.waterboards.ca.gov/lahontan/water_issues/ projects/pge/docs/2007_background_study_report.pdf.
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