California Subsidence Program - Subsidence and Groundwater Monitoring Project

California Department of Water Resources (DWR) | June 21st, 2024

This Initial Study (IS) and Mitigated Negative Declaration (MND) has been prepared by the California Department of Water Resources (DWR) to address the environmental impa

This Initial Study (IS) and Mitigated Negative Declaration (MND) has been prepared by the California Department of Water Resources (DWR) to address the environmental impacts of the proposed California Aqueduct Subsidence Program (CASP) Subsidence and Groundwater Monitoring Project (proposed project) and to satisfy the requirements of the California Environmental Quality Act (CEQA). The DWR is planning installation of equipment to collect spatial information at approximately five (5) groundwater and subsidence monitoring stations along the California Aqueduct (Aqueduct) (approximately Mile Posts 213.0L, 230.6L, 259.6L, 271.2L, and 279.1L within the San Joaquin Field Division) (Figure 1). The proposed project would provide real-time data to monitor groundwater levels and other spatial information as they relate to ground subsidence. The data would be used to help inform how subsidence is affecting the Aqueduct and would assist in maintaining infrastructure of the State Water Project (SWP). The proposed project is described in more detail in Chapter 2.

California’s Stream Flow Monitoring System Is Essential for Water Decision Making

University of California, Berkeley (UC Berkeley) | April 1st, 2018

With California’s drought risk, flood risk, and demand for water all increasing, effective monitoring is more important than ever to water decision making. Str

With California’s drought risk, flood risk, and demand for water all increasing, effective monitoring is more important than ever to water decision making. Stream gages monitor the most basic vital sign of California’s waterways—stream flow.1 Stream flow data support day-to-day decisions about how to manage water and operate water infrastructure. In turn, those decisions have important implications for flood control and the water supplies upon which residential, industrial, agricultural, and environmental water users depend. Stream flow information also provides technical insights into basin hydrology, and those insights aid long-term water planning. As pressures on the state’s water systems intensify, the need for accurate and timely stream flow information will continue to grow. Opportunities for better water management created by groundbreaking legislation such as the Sustainable Groundwater Management Act (SGMA), the Open and Transparent Water Data Act, and Proposition 1 will be severely limited without an effective network of continuous stream gaging.

CASGEM Alternative Monitoring Plan Fact Sheet

California Department of Water Resources (DWR) | August 15th, 2023

Central Coast Groundwater: Seawater Intrusion and Other Issues

California Water Foundation | August 4th, 2014

The Central Coast area relies on groundwater for more than 80 percent of its water supplies, a greater percentage than any other region in the state. Over 40 percent of t

The Central Coast area relies on groundwater for more than 80 percent of its water supplies, a greater percentage than any other region in the state. Over 40 percent of the groundwater basins in the Central Coast region were ranked as either “high” or “medium” priority as part of the California Statewide Groundwater Elevation Monitoring (CASGEM) prioritization released in June 2014, indicating that they are among the most threated by present or future increases in groundwater demand. Central Coast groundwater supplies are seriously threatened by over-pumping that causes seawater intrusion, a challenge unique to coastal communities. Seawater intrusion results in saline groundwater that is unsuitable for irrigation or domestic use, directly threatening the Central Coast’s economy, where many high-value, salt-sensitive crops, such as strawberries, are grown. Groundwater basins in the Monterey and Salinas Valley area are in a state of long-term overdraft, resulting in falling groundwater levels, regional seawater intrusion, and reduced groundwater storage. Seawater intrusion reaches more than five miles inland in the Salinas Valley and three miles inland in the Pajaro Valley. In addition to salinity issues, several groundwater contaminants are present in these aquifers, including elevated levels of total dissolved solids, iron, manganese, nitrates, and other trace elements. The Paso Robles groundwater subbasin supplies water for 29 percent of the population and an estimated 40 percent of the agricultural production of San Luis Obispo County. Groundwater levels in the basin have been dropping rapidly in recent years, with many areas experiencing groundwater level declines of more than 70 feet. As a result, many wells have gone dry and groundwater users have been forced to deepen their wells or drill new wells. In July 2013, over 100 rural property owners complained to the San Luis Obispo County Board of Supervisors that their wells were running dry and in August 2013 the Board adopted the “Paso Robles Groundwater Basin Urgency Ordinance” as a temporary action to prevent new pumping from worsening overdraft conditions. Additionally, land surface subsidence has been noted in the Paso Robles area, and the recent rapid groundwater level declines may have triggered additional subsidence in this area. Although groundwater levels in the Oxnard Plain area have been relatively stable or have shown an increasing trend, in the coastal regions this stability is largely due to seawater intrusion and results in water of unusable quality replacing high quality groundwater. Rising groundwater levels in the Las Posas Valley are a result of active management to increase groundwater recharge beneath the Arroyo Las Posas. Seawater intrusion began in the Oxnard Plain area by 1930s and was widespread as early as the 1940s. Changes in groundwater management, including pumping reductions, shifting of pumping locations, construction of the Freeman Diversion, and the operation of the Pumping Trough and Pleasant Valley pipeline systems have significantly reduced seawater intrusion, but seawater intrusion conditions persist. Other contaminants present in the region include nitrates, iron, manganese and sulfate. In addition to causing seawater intrusion, over-pumping also caused irreversible land subsidence ranging from 2 .5 to 5 feet in several areas of the basin. The Central Coast is critically dependent upon groundwater, but this resource is under serious threat in a number of basins. In light of the limitations of existing state policies, the following measures would improve the effectiveness of groundwater planning and support sustainable management of California’s Central Coast’s groundwater resources . Establish a statewide policy to manage groundwater sustainably. Organize and empower local groundwater agencies to manage groundwater subbasins. Require the development and implementation of groundwater sustainability plans by local groundwater agencies. Provide local agencies with technical guidance and financial support from the state. Empower the State to oversee and ensure the implementation of groundwater sustainability plans.

Chapter B of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas and Electric Company (PG&E) compressor station in Hinkley, California, in the western Mojave Desert 80 miles northeast of Los Angeles, California. In 2015, the extent of anthropogenic Cr(VI) in groundwater in Hinkley and Water Valleys was uncertain, and some Cr(VI) in groundwater may be naturally occurring from rock and aquifer material. On the basis of more than 1,500 portable (handheld) X-ray fluorescence (pXRF) measurements on more than 250 samples of rock, surficial alluvium, and core material from selected wells in Hinkley and Water Valleys, chromium concentrations are commonly low compared to the average bulk continental abundance of 185 milligrams per kilogram (mg/kg). However, chromium concentrations are as high as 530 mg/kg in mafic hornblende diorite that crops out along the western margin of Hinkley Valley in Iron Mountain. Other 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. Chromium concentrations in core material adjacent to the screened intervals of wells sampled for water chemistry and isotopic composition as part of the U.S. Geological Survey Cr(VI) background study ranged from less than the study reporting level (SRL) of 5 mg/kg to 410 mg/kg, with a median concentration of 23 mg/kg. Chromium concentrations in core material were lower in the eastern subarea and higher in the western and the northern subareas of Hinkley Valley and in Water Valley. The highest chromium concentration in core material was in weathered hornblende diorite bedrock. Chromium concentrations in core material adjacent to the screened interval of sampled wells were log-normally distributed below a threshold of 85 mg/kg, and 3 percent of chromium concentrations were greater than 85 mg/kg. Manganese can oxidize trivalent chromium, Cr(III), to Cr(VI). Similar to chromium, manganese concentrations in core material also were log-normally distributed below a threshold of 970 mg/kg, and 5 percent of manganese concentrations were greater than 970 mg/kg. Both chromium and manganese concentrations were higher in fine-textured core material and in visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains. High concentrations of chromium and manganese in core material commonly co-occurred. Fine-textured core material, chromium concentrations greater than 85 mg/kg, and manganese concentrations greater than 970 mg/kg in core material adjacent to the screened interval of sampled wells were selected for use as metrics (threshold values) within a summative-scale analysis (SSA) developed to identify natural and anthropogenic Cr(VI) in water from wells later within this professional paper (chapter G). Principal component analysis (PCA) of 18 elements within surficial alluvium, rock, and core material measured using pXRF shows distinct elemental assemblages associated with (1) older and more recent “Mojave-type” deposits, including alluvium and lake-margin (beach) deposits sourced from the Mojave River, (2) alluvium eroded from mafic rock, including hornblende diorite that crops out on Iron Mountain, (3) alluvium eroded from felsic volcanic and hydrothermal rock that crops out on Mount General along the eastern margin of Hinkley Valley, (4) playa/mudflat and other fine-textured deposits, and (5) material with visually abundant iron- and manganese-oxide coatings. Most wells sampled as part of this study were completed in Mojave-type deposits. Portable (handheld) X-ray fluorescence data measured on core material from those wells do not appear to be different or unusual compared to the magnitude and range of data from the larger Mojave River groundwater basin, and the core material has a low-chromium, felsic composition consistent with a Mojave River origin. In general, the elemental composition of core material from wells was not measurably altered by admixtures with local mafic, felsic volcanic, or hydrothermal source materials; although, where present, admixtures with basalt may contribute chromium to core material.

Chapter C of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Chromium in Minerals and Selected Aquifer Materials

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from a Pacific Gas and Electric Company (PG&E) compressor station in Hinkley, California, in the western Mojave Desert 80 miles northeast of Los Angeles, California. In 2015, the extent of anthropogenic Cr(VI) in groundwater in Hinkley and Water Valleys was uncertain, but some Cr(VI) in groundwater may be naturally occurring from rock and aquifer material. To evaluate potential sources of natural Cr(VI), chromium and other selected trace-element concentrations were measured by inductively coupled plasma-mass spectrometry (ICP-MS), with multi-acid digestion, on 34 samples of surficial alluvium and core material from Hinkley and Water Valleys, California, and on 2 samples of alluvium from the mafic Sheep Creek fan to the southwest. Chromium concentrations in Hinkley and Water Valleys ranged from 2 to 110 milligrams per kilogram (mg/kg), with a median concentration of 14 mg/kg; concentrations were highest in weathered mafic hornblende diorite associated with Iron Mountain. High chromium concentrations also were present within fine-textured materials and visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains. For comparison, chromium concentrations as high as 170 mg/kg were measured in mafic alluvium from the Sheep Creek fan. In contrast, chromium concentrations were lowest in Mojave-type deposits (Mojave River stream and lake margin deposits), with a median of 6 mg/kg. Chromium concentrations measured by ICP-MS compared favorably with concentrations measured by portable (handheld) X-ray fluorescence (pXRF; chapter B), on the basis of least-squares regression results and a coefficient of determination (R2) of 0.97. Minerals in bulk samples and the heavy (dense) mineral fractions isolated from those samples were identified using optical techniques, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Quartz and feldspar were the most abundant minerals, especially within recent and older Mojave River deposits. Chromium concentrations were as high as 1,250 mg/kg in the heavy-mineral fraction, with specific gravity greater than 3.32. Chromium was not commonly detected in the light-mineral fraction, with specific gravity less than 2.85. Most chromium within the heavy-mineral fraction was substituted within magnetite mineral grains less than 100 micrometers (μm) in diameter, and almost no chromite was present within the heavy-mineral fraction. Although magnetite is resistive to weathering, weathering of magnetite to hematite was identified (1) in Miocene materials underlying unconsolidated deposits in the western subarea of Hinkley Valley and (2) in alluvium within Water Valley that contains weathered minerals eroded from Miocene rock. Less-dense, more easily weathered chromium-containing amphiboles, such as actinolite in older Mojave River alluvium and hornblende in locally derived alluvium from Iron Mountain, were identified optically. Magnetite was not identified in weathered hornblende diorite and was less abundant in locally derived materials and in Miocene materials than in Mojave-type deposits. A comparison of ICP-MS data and sequential extraction data shows that approximately 90 percent of chromium in aquifer material within Hinkley and Water Valleys was not extractable and was interpreted to reside within unweathered mineral grains. Most extractable chromium was within the strong acid extractable fraction. Chromium within the weakly sorbed, and specifically sorbed extractable fractions in oxide accumulations within the regulatory Cr(VI) plume is potentially mobile into groundwater with changes in ionic strength or pH. Although Hinkley and Water Valleys are regionally low in chromium, natural geologic sources of chromium may be present in aquifer materials penetrated by wells completed in (1) weathered hornblende diorite bedrock underlying the western subarea; (2) Miocene deposits underlying the western subarea and unconsolidated material in the northern subarea and Water Valley containing basalt or weathered minerals eroded from Miocene deposits; (3) unconsolidated material containing visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains that are present near the water table and near lithologic or geologic contacts; and (4) brown clay and mudflat/playa deposits in the northern subarea. Brown clay and mudflat/playa deposits in the eastern subarea near Mount General have a low-chromium, felsic mineralogy similar to Mojave River deposits and do not contain high concentrations of chromium; however, manganese(IV) oxides within these materials may facilitate oxidation of trivalent chromium, Cr(III), to Cr(VI). In general, admixtures of geologic material eroded from various source terrains that were identified within Mojave-type deposits do not appear to greatly alter the elemental composition of the unconsolidated material composing aquifers in Hinkley and Water Valleys. Chemical and mineralogic data did not indicate high natural abundance, unusual mineralogy, or unusual sorptive properties for chromium on the surfaces of mineral grains within older Mojave River alluvium downgradient from the “western excavation site” in the western subarea.

Chapter D of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California Hinkley Valley: Analyses of Regulatory Water-Quality Data

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station in the Mojave Desert 80 miles northeast of Los Angeles, California. The Pacific Gas and Electric Company has monitored groundwater near Hinkley, California, for Cr(VI) and other constituents since the late 1980s. By June 2017, more than 20,000 samples had been collected and analyzed for Cr(VI) for regulatory purposes. Most Cr(VI) samples were analyzed using the U.S. Environmental Protection Agency (EPA) Method 218.6 with a laboratory reporting level (LRL) of 0.2 micrograms per liter (μg/L). Between July 2012 and June 2017, selected samples were analyzed for low-level Cr(VI) concentrations using a modified version of EPA Method 218.6 with an LRL of 0.06 μg/L. Field-blank data and duplicate samples collected during this period indicate a study reporting level (SRL) of 0.2 μg/L for most analyses and a SRL of 0.12 μg/L for low-level Cr(VI) analyses. The overall precision for Cr(VI) data analyzed by both methods at the interim regulatory Cr(VI) background concentration of 3.1 μg/L was 0.09 μg/L, or about 3 percent. Hexavalent chromium concentration trends were calculated for 564 monitoring wells for the period from July 2012 through June 2017. Upward Cr(VI) concentration trends were present in water from 102 monitoring wells throughout Hinkley and Water Valleys. Upward Cr(VI) concentration trends in water from wells near the margins of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume (1) within strands of the Lockhart fault east and southeast of the Hinkley compressor station and (2) in water from shallow wells within the northern subarea were consistent with expansion of the Cr(VI) plume in these areas between 2012 and 2017. Upward Cr(VI) concentration trends were widely distributed elsewhere in Hinkley and Water Valleys outside the Q4 2015 regulatory Cr(VI) plume and were commonly associated with declining water levels. These upward trends may result from natural Cr(VI) sources, including movement of Cr(VI) containing groundwater from (1) weathered bedrock, (2) fine-textured deposits, or (3) secondarily oxidized material distributed throughout aquifer deposits. Downward Cr(VI) concentration trends were observed in 146 monitoring wells. Downward trends and can be attributed to remediation activities downgradient from the Hinkley compressor station. Hexavalent chromium concentration trends also were calculated for 219 domestic wells from July 2012 through June 2017. Upward Cr(VI) concentration trends in 8 domestic wells and downward trends in 23 domestic wells were clustered largely within former residential areas west of the Q4 2015 regulatory Cr(VI) plume. Results of Cr(VI) trend analyses (including upward, downward, and no trend) were used with other data as part of a summative-scale analysis (chapter G) to define the extent of anthropogenic Cr(VI) and natural Cr(VI) within Hinkley and Water Valleys.

Chapter E of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Groundwater Chemistry and Hexavalent Chromium

Publisher not available | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Water samples collected by the U.S. Geological Survey from more than 100 wells between March 2015 and November 2017 in Hinkley and Water Valleys, in the Mojave Desert 80 miles northeast of Los Angeles, California, were analyzed for field parameters, major ions, nutrients, and selected trace elements, including hexavalent chromium, Cr(VI). Water from most wells was alkaline and oxic. The pH ranged from 6.9 in water-table wells near recharge areas along the Mojave River to 9.4 in deeper wells farther downgradient in the northern subarea. Hexavalent chromium concentrations measured by ion chromatography using U.S. Environmental Protection Agency Method 218.6 and a version of that method used for detection of Cr(VI) concentrations as low as 0.06 micrograms per liter (μg/L), produced results comparable to field speciation with subsequent analyses by graphite furnace atomic absorption spectroscopy (coefficient of determination, R2, of 0.97). Hexavalent chromium concentrations ranged from less than the study reporting level of 0.10 to 2,500 μg/L. The highest concentrations were within the October–December 2015 (Q4 2015) regulatory Cr(VI) plume downgradient from the Hinkley compressor station. Hexavalent chromium concentrations outside the Q4 2015 regulatory Cr(VI) plume were as high as 11 μg/L. Hexavalent chromium concentrations in water from most wells were distributed in a narrow redox potential and pH band within the overlapping chromate ion, CrO42−(aqueous), and manganese-3, Mn(III)(solid), stability fields. The redox potential of water from some wells completed in carbonate-rich mudflat/playa deposits approached the more oxic manganese-4, Mn(IV)(solid), stability field. However, Cr(VI) concentrations in porewater pressure-extracted from Mn(IV)-containing deposits in the eastern subarea did not exceed 3.3 μg/L, and porewater does not appear to be a source of Cr(VI) concentrations greater than this concentration in water from wells in the eastern subarea. On the basis of comparison with California-wide data, Cr(VI) concentrations at the measured pH were higher than expected for uncontaminated water from wells (1) within the Q4 2015 regulatory Cr(VI) plume, (2) within the eastern subarea nominally crossgradient from the Hinkley compressor station and upgradient from the Q4 2015 regulatory Cr(VI) plume, and (3) from shallow wells in the northern subarea downgradient from the leading edge of the Q4 2015 regulatory Cr(VI) plume. Hexavalent chromium concentrations in alkaline water from wells in the northern subarea of Hinkley Valley and in Water Valley were within ranges expected for uncontaminated water elsewhere in California given their pH and trace-element composition. Hexavalent chromium concentrations were higher than expected on the basis of selected trace-element concentrations that co-occur with Cr(VI) in water from wells within the Q4 2015 regulatory Cr(VI) plume and from wells in the eastern and northern subareas near the plume margins. Hexavalent chromium concentrations did not exceed 4 μg/L in water from domestic wells sampled in Hinkley and Water Valleys and were generally within ranges expected for uncontaminated groundwater given their pH and trace-element composition. Interpretations derived from Cr(VI) and pH, and from Cr(VI) and selected trace-element concentrations collected between March 2015 and November 2017 were used within a summative-scale analysis to determine the Cr(VI) plume extent (chapter G). However, Cr(VI) background concentrations (chapter G) were calculated from regulatory data collected from selected wells between April 2017 and January 2018.

Chapter F of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Hexavalent chromium, Cr(VI), was discharged in cooling wastewater to unlined surface ponds from 1952 to 1964 and reached the underlying unconsolidated aquifer at the Pacific Gas and Electric Company (PG&E) Hinkley compressor station in the Mojave Desert, 80 miles northeast of Los Angeles, California. A suite of environmental tracers was analyzed in water samples collected from more than 100 wells to characterize the source, age, and geochemical evolution of groundwater within and near the Cr(VI) plume in Hinkley and Water Valleys. This information was used to help determine the extent of Cr(VI) associated with releases from the Hinkley compressor station and to identify Cr(VI) associated with natural sources. The source of water in most wells, indicated by stable oxygen and hydrogen isotope values for water, delta oxygen-18 and delta deuterium, was recharge by infiltration of intermittent surface flows in the Mojave River. With the exception of small flows in 1958, the Mojave River was largely dry between 1952 and 1969. This dry period spans the period of Cr(VI) releases from the Hinkley compressor station; 1952–69 also spans the period of high tritium levels in precipitation resulting from the atmospheric testing of nuclear weapons and, as a consequence, tritium concentrations in groundwater in Hinkley Valley were comparatively low. Groundwater ages (time since recharge) increased downgradient from the Mojave River and with depth. Tritium, measured by helium ingrowth with a study reporting level of 0.05 tritium unit, was detected in water from 51 percent of wells, with detectable tritium as far as 7 miles downgradient from the Mojave River. Tritium concentrations were higher, and tritium/helium-3 groundwater ages younger, in water from wells near the Mojave River and in water from shallower wells downgradient. Agricultural pumping has decreased groundwater levels as much as 60 feet since 1952. As a result of this pumping, some groundwater containing tritium, and presumably anthropogenic Cr(VI), has been removed from the aquifer. The distribution of wells having carbon-14 activities near or greater than 100-percent modern carbon, consistent with post-1952 recharge water, was similar to the distribution of wells containing detectable tritium. Carbon-14 activities as low as 8.9-percent modern carbon, with carbon-14 ages (unadjusted for reactions with aquifer materials) of almost 20,000 years before present (ybp), were sampled in water from some deep wells. Hexavalent chromium concentrations in older groundwater were as high as 11 micrograms per liter but did not exceed 3.6 micrograms per liter in older water from wells completed in “Mojave-type” deposits (composed of felsic Mojave River stream and near-shore lake deposits sourced from the Mojave River); this value may represent an upper limit on Cr(VI) concentrations in groundwater within Mojave-type deposits that likely approximates background Cr(VI) concentrations in the study area. Chlorofluorocarbons were released to the atmosphere and hydrologic cycle as a result of industrial activity beginning in the 1930s. Chlorofluorocarbon data were not generally suitable for groundwater-age dating in Hinkley and Water Valley because of nonatmospheric contributions from local sources. Strontium-87/86 isotope ratios and stable chromium isotopes, delta chromium-53, provide information on the geochemical evolution of groundwater in the aquifer. Highly radiogenic strontium-87/86 ratios greater than 0.71000 were present in water from wells completed in coarse-textured Mojave-type deposits having low chromium concentrations but were not diagnostic of these materials. Nonradiogenic strontium-87/86 ratios less than 0.70950 were diagnostic of weathered materials in the northern subarea of Hinkley and in Water Valley that were eroded from Miocene (23–5 million ybp) deposits east of the study area. Values for delta chromium-53 ranged from near 0 to 2.8 parts per thousand (‰) difference. The extent of reductive fractionation, mixing with native groundwater, and longitudinal dispersion within the October–December 2015 (Q4 2015) regulatory Cr(VI) plume can be estimated on the basis of the delta chromium-53 isotope composition of groundwater within the plume. Reduction of Cr(VI) to trivalent chromium, Cr(III), can occur in the presence of natural reductants in oxic groundwater. Although not diagnostic of anthropogenic chromium at the concentrations of interest near the Q4 2015 regulatory Cr(VI) plume margin, delta chromium-53 data indicate anthropogenic Cr(VI) within the plume is not conservative and has reacted with aquifer materials; these reactions have removed some anthropogenic Cr(VI) from groundwater. Environmental tracers, and the distribution of modern (post-1952) and premodern (pre-1952) groundwater, inform understanding of the extent of anthropogenic and naturally occurring Cr(VI) near the Q4 2015 regulatory Cr(VI) plume and the understanding of geochemical processes occurring in and near the margins of the Cr(VI) plume. The oxygen and hydrogen isotope compositions of water, tritium/helium-3 groundwater-age data, and carbon-14 data were used with mineralogy and chemistry data as part of a summative-scale analysis to determine the Cr(VI) plume extent later in this professional paper (chapter G).

Chapter G of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium

U.S. Geological Survey (USGS) | April 25th, 2023

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and

Documents in this series: Chapter A: Introduction to Study Area Hydrogeology, Chromium Sources, Site History, and Purpose of Study Chapter B: Survey of Chromium and Selected Element Concentrations in Rock, Alluvium, and Core Material Chapter C: Chromium in Minerals and Selected Aquifer Materials Chapter D: Analyses of Regulatory Water-Quality Data Chapter E: Groundwater Chemistry and Hexavalent Chromium Chapter F: Environmental Tracers of Groundwater Source, Age, and Geochemical Evolution Chapter G: Evaluation of Natural and Anthropogenic (Human-Made) Hexavalent Chromium Chapter H: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California Chapter I: Sequestration and Reoxidation of Chromium in Experimental Microcosms Chapter J: Summary and Conclusions Hexavalent chromium, Cr(VI), was released between 1952 and 1964 from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station, in the Mojave Desert about 80 miles northeast of Los Angeles, California. Geologic, geochemical, and hydrologic data from more than 100 wells collected between March 2015 and November 2017 were interpreted using a summative-scale analysis to define the extent of anthropogenic (human-made) Cr(VI) in groundwater. The summative scale consisted of eight questions requiring binary (yes or no) answers for each sampled well. The questions were intended to (1) provide a transparent framework for data interpretation in which all stakeholders participated; (2) provide unbiased interpretation of data traceable to numerical measurements; (3) provide a framework that enabled geologic, geochemical, and hydrologic data to be considered collectively; and (4) consolidate different types of data into a simple, easy-to-understand interpretation. When data from each well are scored using questions and metrics 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 areal extent of the summative-scale Cr(VI) plume was 5.5 square miles (mi2); this is larger than the 2.2-mi2 extent of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume but smaller than the 8.3-mi2 maximum mapped extent of Cr(VI) greater than the interim regulatory Cr(VI) background value of 3.1 micrograms per liter (μg/L). The summative-scale Cr(VI) plume is within the area covered by the PG&E monitoring well network and lies within “Mojave-type” deposits composed of low-chromium stream and near-shore lake deposits sourced from the Mojave River. The summative-scale Cr(VI) plume included all shallow wells within the footprint of the Q4 2015 regulatory Cr(VI) plume, but summative-scale scores indicate that anthropogenic Cr(VI) was not present in several wells within the footprint of the regulatory Cr(VI) plume that were screened within the deep zone of the upper aquifer. The summative-scale Cr(VI) plume extent was consistent with mineralogic and geochemical data collected as part of this study that were not used within the summative-scale analysis. Data from wells outside the summative-scale Cr(VI) plume collected for regulatory purposes from April 2017 through March 2018 were used to estimate Cr(VI) background concentrations as the upper 95-percent tolerance limit (UTL95) in different parts of Hinkley and Water Valleys. The UTL95 values were calculated using the computer program ProUCL 5.1 and are suitable for use by regulatory agencies in support of (1) updating the regulatory Cr(VI) plume extent and management of Cr(VI) near the plume margins, (2) establishing cleanup goals for Cr(VI) within the updated regulatory Cr(VI) plume, and (3) identifying unusual Cr(VI) concentrations outside the regulatory Cr(VI) plume. The nonparametric UTL95 values for wells screened in Mojave-type deposits in the eastern, western, and northern subareas of Hinkley Valley were 3.7, 3.9, and 4.0 μg/L, respectively. The normal UTL95 values for wells screened in undifferentiated, unconsolidated deposits in the eastern and western subareas and the northern subarea upgradient from the Mount General fault were 2.8, 3.8, and 4.8 μg/L, respectively. An overall normal UTL95 value of 3.8 μg/L was calculated for undifferentiated, unconsolidated deposits in these areas. This value is similar to the overall nonparametric UTL95 value of 3.9 μg/L calculated for Mojave-type deposits and similar to the maximum Cr(VI) concentration of older groundwater in contact with Mojave-type deposits of 3.6 μg/L. The provenance of most PG&E monitoring wells is not precisely known, and the UTL95 values for wells screened in undifferentiated, unconsolidated deposits in the different subareas may be more widely applicable for regulatory purposes than the UTL95 values for Mojave-type deposits. The UTL95 value of 2.8 μg/L for wells screened in undifferentiated, unconsolidated deposits in the eastern subarea is important for plume management because most of the summative-scale Cr(VI) plume is within the eastern subarea. A UTL95 value of 5.8 μg/L was calculated for older (pre-1952) groundwater associated with mudflat/playa deposits in the eastern subarea near Mount General. A UTL95 value of 2.3 μg/L was calculated for Mojave-type deposits within the Cr(VI) plume downgradient from the Hinkley compressor station after regulatory updates. This lower value is consistent with neutral to slightly alkaline, younger (post-1952) groundwater within coarse-textured, low-chromium Mojave-type deposits in this area and may be a suitable metric for Cr(VI) cleanup goals. The UTL95 value of 4.8 μg/L for wells screened in undifferentiated, unconsolidated deposits in the northern subarea upgradient from the Mount General fault provides for possible increases in Cr(VI) concentrations if water levels continue to decline. Downgradient from the Q4 2015 regulatory Cr(VI) plume and the summative-scale Cr(VI) plume, UTL95 values of 9.0 and 6.4 μg/L were calculated for wells screened in undifferentiated, unconsolidated deposits in the northern subarea downgradient from the Mount General fault and for Water Valley, respectively, consistent with different geologic and geochemical conditions in these areas. The UTL95 values calculated as part of this study provide scientifically defensible estimates of background Cr(VI) concentrations that differ with local geologic, geochemical, and hydrologic conditions in Hinkley and Water Valleys. The regulatory Cr(VI) plume extent can be updated on the basis of these values. The summative-scale Cr(VI) plume extent may contain wells having anthropogenic Cr(VI) concentrations less than the UTL95 values for their respective subareas that may not require regulatory attention, and an updated regulatory Cr(VI) plume extent may be less than the summative-scale Cr(VI) plume extent. The UTL95 values are not background Cr(VI) concentrations for regulatory purposes, and the authority to establish regulatory values resides solely with the Lahontan Regional Water Quality Control Board.