Chapter H of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Predevelopment Water Levels, Groundwater Recharge, and Selected Hydrologic Properties of Aquifer Materials, Hinkley and Water Valleys, California

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 Hydrologic and geophysical data were collected to support updates to an existing groundwater-flow model of Hinkley Valley, California, in the Mojave Desert about 80 miles northeast of Los Angeles, California. These data provide information on predevelopment (pre-1930) water levels, groundwater recharge, and selected hydrologic properties of aquifer materials. A predevelopment groundwater-level map, drawn using water-level measurements from 48 wells collected as early as 1918, showed groundwater movement from recharge areas along the Mojave River to evaporative discharge areas near the margin of Harper (dry) Lake in Water Valley. During predevelopment conditions, depth to water ranged from near land surface along the Mojave River to above land surface near Harper (dry) Lake, consistent with flowing wells in Water Valley at that time. Depths to water in much of Hinkley Valley downgradient from the Lockhart fault were less than 20 feet below land surface. By 2017, water-level declines as a result of agricultural pumping, were as much as 60 feet near the Hinkley compressor station. Areal recharge from infiltration of precipitation on the valley floor is negligible. Average annual recharge as infiltration of runoff from upland drainages to Hinkley and Water Valleys averages 64.7 acre-feet per year. In most years recharge does not occur; in years when it occurs, recharge to Hinkley Valley is typically about 296 acre-feet. In contrast, average recharge as infiltration of streamflow from the Mojave River from 1931 to 2015 was between 13,400 and 17,100 acre-feet per year; in some years recharge from the Mojave River exceeded 100,000 acre-ft. Estimates of predevelopment groundwater movement through Hinkley Gap and groundwater discharge to Harper (dry) Lake ranged from 570 to 1,900 and 820 to 2,460 acre-feet per year, respectively; at the time of this study in 2017, groundwater movement through Hinkley Gap was estimated to be about 83 acre-feet per year. Hydraulic-conductivity values estimated from slug-test data for 95 monitoring wells ranged from less than 0.1 to 680 feet per day (ft/d); values generally decreased with depth. Median hydraulic-conductivity values calculated from nuclear magnetic resonance (NMR) data for Mojave River alluvium and near-shore lake deposits were 73 and 11 ft/d, respectively; median hydraulic-conductivity values for locally derived alluvium and weathered bedrock were 6 and 2 ft/d, respectively. Hydraulic-conductivity values, estimated from NMR data for formerly saturated deposits overlying the 2017 water table, were as high as 300 ft/d near the Hinkley compressor station. Downgradient from the Hinkley compressor station, formerly saturated deposits had hydraulic-conductivity values of about 150 ft/d, which were higher than values in saturated material. Coarse-textured, permeable material in formerly saturated deposits above the 2017 water table may have allowed groundwater, released from the Hinkley compressor station that may have contained Cr(VI), to move rapidly downgradient. The Lockhart fault is an impediment to groundwater flow within Hinkley Valley. Groundwater-flow directions from horizontal point-velocity probe data were deflected to the west on the upgradient side of the fault compared to the nominal direction of groundwater flow estimated from water-level data. Younger groundwater was present on the upgradient and downgradient sides of the fault, and older groundwater with unadjusted carbon-14 ages as old as 5,650 years before present was in water from wells within splays of the Lockhart fault, consistent with limited groundwater movement across the fault. As a result, groundwater and Cr(VI) released from the Hinkley compressor station moved to the northwest along the downgradient side of the fault. Coupled well-bore flow and depth-dependent water-quality data show water from wells C-01 and IW-03 within the Q4 2015 (October–December 2015) regulatory Cr(VI) plume was yielded from thin layers within the aquifer that are composed of well-sorted lake-margin (beach) deposits that likely have high lateral and longitudinal connectivity. Collectively, data show highly permeable deposits above the regional water table and thin permeable deposits within saturated portions of the upper aquifer that may have conducted groundwater and Cr(VI) downgradient when releases from the Hinkley compressor station first occurred.

Chapter I of Natural and Anthropogenic (Human-Made) Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California: Sequestration and Reoxidation of Chromium in Experimental Microcosms

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 Groundwater containing hexavalent chromium, Cr(VI), downgradient from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station in the Mojave Desert, 80 miles northeast of Los Angeles, California, is undergoing bioremediation using added ethanol as a reductant in a volume of the aquifer defined as the in situ reactive zone (IRZ). This treatment reduces Cr(VI) to trivalent chromium, Cr(III), which is rapidly sequestered by sorption to aquifer particle surfaces and by co-precipitation within iron (Fe) or manganese (Mn) bearing minerals forming in place as reduction proceeds. Successful mitigation of the Cr(VI) plume is projected to require 10–95 years, at which time bioremediation with ethanol will likely cease. This projection assumes that Cr(VI) removal is permanent and that no Cr(III) will oxidize back to Cr(VI) in the event of changing hydrologic conditions that may cause oxygen-rich water to re-enter the IRZ. Laboratory microcosm experiments were done to explore the process of reductive sequestration of Cr(VI) to Cr(III) and the potential for reoxidation of Cr(III) to Cr(VI). In reductive sequestration experiments, batch microcosms were prepared with aquifer materials collected from sites upgradient of the Cr(VI) regulatory plume. Control microcosms were prepared using Fe- and Mn-coated quartz sand. Unfiltered Mojave River groundwater containing an added tracer of isotopically labeled chromium-50 were reacted with microcosm materials for up to 2 years; during this time, bio-reduction was stimulated by repeated additions of diluted ethanol to maintain reduced conditions within appropriate ranges, avoiding sulfate reducing or methanogenic conditions as much as possible while mimicking field conditions. Analysis of chromium-50, Fe, and Mn obtained by sequential extraction from microcosms harvested (incubation terminated and microcosm contents analyzed) at various times showed that some aqueous chromium (Cr) was sorbed to particle surfaces within hours; reduction to Cr(III) and incorporation into amorphous and crystalline solid phases occurred during the next few months. Amorphous Cr-containing fractions included Fe and Mn hydroxides and organic matter. Ultimately, most of the chromium-50 tracer was present in the less reactive crystalline phase. However, Fe and Mn were broadly distributed at later stages of reduction, and both were spatially co-located with Cr on a micrometer (μm) scale. Solid-phase data collected using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray absorption spectroscopy (XAS) indicated that some Cr(III) was associated with mixed valence Fe oxides like magnetite and Fe-Mn oxides like jacobsite. Additionally, Cr(III) was observed within several μm of Fe and Mn embedded in clays and in mineral coatings. To evaluate the potential for reoxidation of Cr(III) to Cr(VI), additional batch microcosms of aquifer materials and mixtures of Fe- and Mn-coated sand were first reduced for more than 1 year and subsequently oxidized for almost 2 years. Hexavalent chromium was formed and was available for release to the aqueous phase during oxidation of all materials; however, the timing and amount of Cr(VI) formed and released varied among substrates. Artificial substrates containing more Mn produced more Cr(VI). Site material characteristic of recent Mojave River deposits contained within the IRZ produced the least Cr(VI) during oxidation, while site materials composed of older Mojave River aquifer material (containing more Mn) produced more Cr(VI). Site material collected from within the IRZ contained more Cr but produced an intermediate amount of Cr(VI) following oxidation. The combined results of microcosm chemistry and solid-phase analyses showed that the nature and locus of Cr(III) sequestration influenced its vulnerability to reoxidation to Cr(VI). It was concluded that co-location of Cr with Mn at later stages of reduction influenced the susceptibility of Cr(III) to reoxidation in microcosms. Reoxidation of Cr(III) to Cr(VI) was observed in experiments with previously reduced material after just 14 days exposure to oxygen. As much as 10 percent of added Cr was oxidized to Cr(VI) in microcosms prepared using recent Mojave River aquifer material, and as much as 20 percent of added Cr was oxidized to Cr(VI) in microcosms prepared using older Mojave River aquifer material. Less Cr(VI) (less than 3 percent of Cr added before reduction) was released to the aqueous phase, and this release occurred following longer oxygen exposure. Site managers may need to plan for long-term monitoring and the possibility of active maintenance of anoxic conditions within the IRZ to ensure permanent sequestration of Cr after bioremediation with ethanol ceases.

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

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 Chromium concentrations in rock and aquifer material in Hinkley and Water Valleys in the Mojave Desert, 80 miles northeast of Los Angeles, California, are generally low compared to the average chromium concentration of 185 milligrams per kilogram (mg/kg) in the average bulk continental crust. Chromium concentrations in felsic, coarse-textured “Mojave-type” deposits, composed of Mojave River stream (alluvium) and lake-margin (beach) deposits sourced from the Mojave River, are as low as 5 mg/kg, with a median concentration of 23 mg/kg in aquifer materials adjacent to the screened intervals of sampled wells. The most abundant chromium-containing mineral within aquifer materials in Hinkley and Water Valleys is magnetite. Magnetite is resistant to weathering, and about 90 percent of chromium remains within unweathered mineral grains. However, chromium-containing hornblende diorite and basalt are present in surrounding uplands, and chromium-containing actinolite is present within some aquifer materials. Although geologic abundance of chromium is clearly important, hexavalent chromium, Cr(VI), concentrations in alkaline oxic groundwater are related to additional factors. Hexavalent chromium concentrations in groundwater are influenced by a combination of processes including (1) mineralogy and the weathering rates of chromium-containing minerals; (2) texture of aquifer deposits; (3) accumulation of chromium weathered from minerals within surface coatings on mineral grains; (4) oxidation of accumulated Cr(III) to Cr(VI) in the presence of manganese oxides (Mn oxides), including the abundance and oxidation states of those Mn oxides; (5) pH-dependent desorption of chromium from coatings on the surfaces of mineral grains into groundwater during appropriate aqueous geochemical conditions; and (6) age (time since recharge) of groundwater. The pH of groundwater increases with groundwater age (time since recharge) as a result of silicate weathering, and desorption of Cr(VI) from aquifer deposits increases with increasing pH as long as groundwater remains oxic. In the absence of the detailed geologic, geochemical, and hydrologic data collected as part of this study, pH-dependent sorption, evaluated as the Cr(VI) occurrence probability at the measured pH, is an effective indicator of natural or anthropogenic Cr(VI). The Pacific Gas and Electric Company (PG&E) Hinkley compressor station is used to compress natural gas as it is transported through a pipeline from Texas to California. Between 1952 and 1964, cooling water containing Cr(VI) was discharged to unlined ponds and released into groundwater in unconsolidated aquifers. The extent of groundwater containing evidence of at least some anthropogenic Cr(VI) was 5.5 square miles (mi2) and was estimated using a summative scale incorporating geologic, geochemical, and hydrologic data collected from more than 100 wells between March 2015 and November 2017. The summative-scale Cr(VI) plume extent is larger than the 2.2 mi2 extent of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume but is smaller than the 8.3 mi2 maximum mapped extent of Cr(VI) greater than the interim regulatory Cr(VI) background concentration of 3.1 micrograms per liter (μg/L). The summative-scale Cr(VI) plume is within felsic, low-chromium aquifer material deposited by the Mojave River described as Mojave-type deposits and is within the area covered by the PG&E monitoring well network. Background Cr(VI) concentrations were calculated using the computer program ProUCL 5.1 as the upper 95-percent tolerance limit, UTL95, using data from wells outside the summative-scale Cr(VI) plume extent collected between April 2017 and March 2018. The overall UTL95 for undifferentiated, unconsolidated deposits in the eastern and western subareas and the northern subarea upgradient of the Mount General fault in Hinkley Valley was 3.8 μg/L; this value is similar to the overall UTL95 value of 3.9 μg/L calculated for Mojave-type deposits in Hinkley and Water Valleys, and is similar to the maximum Cr(VI) concentration of older groundwater in contact with Mojave-type deposits of 3.6 μg/L. In most cases the overall UTL95 value may be an acceptable Cr(VI) background value near the Cr(VI) plume margin; however, UTL95 values for the various subareas in Hinkley and Water Valleys provide greater resolution of Cr(VI) background that may be important for some purposes. The UTL95 values for undifferentiated, unconsolidated deposits in the eastern, western, and northern subareas upgradient of the Mount General fault were 2.8, 3.8, and 4.8 μg/L, respectively. 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 the Hinkley compressor station and most of the summative-scale Cr(VI) plume are within the eastern subarea. A UTL95 value of 2.3 μg/L was calculated for Mojave-type deposits downgradient from the Hinkley compressor station. This value represents Cr(VI) concentrations that may have been present in that part of the aquifer had Cr(VI) not been released from the Hinkley compressor station, and it reflects coarser textured deposits in this area and the proximity of those deposits to recharge areas along the Mojave River that results in younger (post-1952), less alkaline groundwater than in wells farther downgradient. This value may be a suitable metric for Cr(VI) cleanup goals within the Cr(VI) plume after regulatory updates. A separate UTL95 value of 5.8 μg/L was calculated for mudflat/playa deposits and older groundwater near Mount General in the eastern subarea. The UTL95 values calculated for undifferentiated, unconsolidated deposits in the northern subarea downgradient from the Mount General fault and in Water Valley, including lacustrine (lake) deposits and material eroded from basalt and Miocene deposits, were 9.0 and 6.4 μg/L, respectively. Hexavalent chromium concentrations in more than 70 domestic wells sampled between January 27 and 31, 2016, ranged from less than the study reporting level of 0.1–4.0 μg/L, with a median concentration of 1.2 μg/L. Hexavalent chromium concentrations in water from domestic wells did not exceed UTL95 values within subareas where the wells were located. Water from 47 percent of domestic wells sampled between January 27 and 31, 2016, had arsenic, uranium, or nitrate concentrations above a maximum contaminant level. Anthropogenic Cr(VI) within groundwater downgradient from the Hinkley compressor station is treated by PG&E using bioremediation by adding ethanol as a reductant within a volume of aquifer known as the in situ reactive zone (IRZ). Laboratory microcosm studies showed that Cr(VI) is rapidly reduced to Cr(III) with additions of ethanol. Reduced Cr(III) is sorbed and is sequestered into crystalline iron and manganese oxides on the surfaces of mineral grains within the microcosms during a period of several months. Trivalent chromium was reoxidized back to Cr(VI) within 2 weeks of return to oxic (oxygen present) conditions within the microcosms. As much as 10 percent of added Cr was oxidized to Cr(VI) in microcosms prepared using recent Mojave River aquifer material, and as much as 20 percent of added Cr was oxidized to Cr(VI) in microcosms prepared using older Mojave River aquifer material. Less Cr(VI) (less than 3 percent of Cr added before reduction) was released to the aqueous phase, and this release occurred following longer time periods of oxygen exposure. Sequestration of chromium with manganese oxides during reduction facilitates reoxidation of Cr(III) to Cr(VI) under oxic conditions. Future maintenance of anoxic (oxygen absent) conditions would ensure continued sequestration of chromium as Cr(III) within IRZ treated portions of the Cr(VI) plume. Although Cr(VI) within the summative-scale Cr(VI) plume may have an anthropogenic history associated with releases from the Hinkley compressor station, Cr(VI) concentrations less than the UTL95 values for the various subareas may not require regulatory attention. The regulatory Cr(VI) plume can be updated using the UTL95 values calculated as part of this study. The updated regulatory Cr(VI) plume extent would lie within the summative-scale Cr(VI) plume extent. The authority to establish regulatory Cr(VI) background values, clean-up goals, and future site management practices resides with the Lahontan Regional Water Quality Control Board. Also see: Natural and Anthropogenic Hexavalent Chromium, Cr(VI), in Groundwater near a Mapped Plume, Hinkley, California

Characterization of brominated, chlorinated, and phosphate flame retardants in San Francisco Bay, an urban estuary

Science of the Total Environment (Elsevier) | February 20th, 2019

Flame retardant chemical additives are incorporated into consumer goods to meet flammability standards, and many have been detected in environmental matrices. A uniquely

Flame retardant chemical additives are incorporated into consumer goods to meet flammability standards, and many have been detected in environmental matrices. A uniquely wide-ranging characterization of flame retardants was conducted, including polybrominated diphenyl ethers (PBDEs) and 52 additional brominated, chlorinated, or phosphate analytes, in water, sediment, bivalves, and harbor seal blubber of San Francisco Bay, a highly urbanized estuary once considered a hot spot for PBDE contamination. Among brominated flame retardants, PBDEs remained the dominant contaminants in all matrices, though declines have been observed over the last decade following their phase-out. Hexabromocyclododecane (HBCD) and other hydrophobic, brominated flame retardants were commonly detected at lower levels than PBDEs in sediment and tissue matrices. Dechlorane Plus (DP) and related chlorinated compounds were also detected at lower levels or not at all across all matrices. In contrast, phosphate flame retardants were widely detected in Bay water samples, with highest median concentrations in the order TCPP > TPhP > TBEP > TDCPP > TCEP. Concentrations in Bay water were often higher than in other estuarine and marine environments. Phosphate flame retardants were also widely detected in sediment, in the order TEHP > TCrP > TPhP > TDCPP > TBEP. Several were present in bivalves, with levels of TDCPP comparable to PBDEs. Only four phosphate flame retardants were detected in harbor seal blubber: TCPP, TDCPP, TCEP, and TPhP. Periodic, multi-matrix screening is recommended to track contaminant trends impacted by changes to flammability standards and manufacturing practices, with a particular focus on contaminants like TDCPP and TPhP that were found at levels comparable to thresholds for aquatic toxicity.

Characterization of Sediment Contamination in Central Bay Margin Areas

San Francisco Estuary Institute (SFEI) | July 2nd, 2018

The Bay margins (i.e., mudflats and adjacent shallow areas of the Bay) are important habitats where there is high potential for wildlife to be exposed to contaminants. H

The Bay margins (i.e., mudflats and adjacent shallow areas of the Bay) are important habitats where there is high potential for wildlife to be exposed to contaminants. However, these areas are not routinely sampled due to logistical considerations. In 2015, the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) conducted a survey of margin areas in Central Bay to determine ambient concentrations of contaminants in sediment in these areas. The results generally confirmed the current conceptual model expectation that the margin sediments are often more contaminated that those in the subtidal open bay, with PCBs showing the largest differences. These differences between Central Bay margins and open water areas are likely the largest to be found in San Francisco Bay, as margins in other bay segments generally account for more of the total area and the adjacent land use is less heavily industrial. The ambient data provide a useful baseline context against which the severity of contamination at specific sites can be compared. The baseline data will also be useful in setting targets and tracking improvements in watershed loads and their nearfield receiving waters, or for appropriate re-use or disposal of dredged sediment. These spatially unbiased data can also improve estimates of mean concentrations and contaminant inventories in margins. For example, based on data from this study, contamination in the margin areas accounts for ~20% of PCBs in Central Bay which is disproportionately high compared to the margin area (5% of Central Bay). Given a large inventory of contaminants in the open bay compared to annual loads, changes may be difficult to see at the bay scale in the short- and mid-term. However, with margins’ smaller inventories and greater proximity to likely sources, improvements in margin sediments may be easier to observe. This ambient sampling effort also identified or verified one expected area of very high PCB concentrations which had not been previously measured. However, the randomized monitoring design used for the Central Bay study was not ideal for identifying “hot spots”. Future studies with a goal of detecting hot spots should use deterministic sampling designs.

Characterization of Sediment Contamination in South Bay Margin Areas

San Francisco Estuary Institute (SFEI) | November 15th, 2019

The Bay margins (i.e., mudflats and adjacent shallow areas of the Bay) are important habitats where there is high potential for wildlife to be exposed to contaminants. Ho

The Bay margins (i.e., mudflats and adjacent shallow areas of the Bay) are important habitats where there is high potential for wildlife to be exposed to contaminants. However, until recently, these areas had not been routinely sampled by the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) due to logistical considerations. In 2015, the RMP conducted a spatially-distributed characterization of surface sediment contamination and ancillary characteristics within the RMP-defined Central San Francisco Bay margin areas. This was repeated in 2017 within South Bay, which for this report refers to the area collectively encompassing Upper South Bay (usually just called the “South Bay” segment in the Bay RMP, “Upper” added here to distinguish from the combined area), Lower South Bay, and “Extreme” Lower South Bay (previously named “Southern Sloughs”) margin areas.  Findings from the 2015 survey of margin areas in Central Bay (Yee et al., 2017) showed contamination in the margin areas accounted for 20% of the PCB mass in Central Bay, which is disproportionately high compared to margin area (5% of Central Bay). Central Bay watersheds contains many of the oldest and most industrialized urban environments within the Bay Area. There was a significant difference in mean contaminant concentrations in sediment from margins and nearby open water areas. In contrast, for South Bay, margins are more proximate to open water areas and represent a greater percentage of the overall Bay surface area. Therefore, as hypothesized, in the South Bay more uniform distributions were found, likely in part due to these physical factors. Average concentrations in South Bay margins were lower than in Central Bay margins, which was also hypothesized, because South Bay watersheds contain more recent industrial development and relatively less older industrial areas as a percentage of total area (~2% vs 5% for Central Bay watersheds). Although combined loads of PCBs from the South Bay watersheds are about 40% higher than from Central Bay areas, their total area is over 5 times larger, and their total runoff volume is more than double.  Ambient margins data in South Bay provide a context against which the severity of contamination at specific sites can be compared. The baseline data could also be useful in setting targets and tracking improvements in watershed loads and their nearfield receiving waters, or for appropriate assessment of re-use or disposal of dredged sediment. These spatially distributed data also provide improved estimates of mean concentrations and contaminant inventories in margins. Based on data from this study, contamination in the margin areas accounts for 35% of PCB mass in the upper 15 cm of surface sediments in South Bay, which is approximately proportional to the relative area of the margin (34% of the region). In contrast, margins only contain 30% of the mercury mass in South Bay, somewhat less than their proportional area.  Given the large inventory of contaminants in the open Bay compared to annual loads, changes may be difficult to see in the open Bay in the short- and mid-term. However, due to their smaller inventories and closer proximity to likely sources and loading pathways, improvements in margin sediments may potentially be detected more easily or sooner as loads are reduced, even if South Bay margins are better connected and more similar in concentrations to adjacent open water areas than in Central Bay. 

Characterizing Nitrates in Central Coast Groundwater

Central Coast Groundwater Coalition (CCGC) | April 1st, 2016

The Central Coast Groundwater Coalition (CCGC) is a third-party cooperative groundwater monitoring program that was established for landowners/operators in the Ce

The Central Coast Groundwater Coalition (CCGC) is a third-party cooperative groundwater monitoring program that was established for landowners/operators in the Central Coast region as an alternative to the individual groundwater monitoring program. The cooperative program collects and synthesizes groundwater monitoring data on behalf of its members to fulfill requirements of the Irrigated Lands Regulatory Program. The Central Coast Regional Water Quality Control Board required that the CCGC characterize groundwater quality in the uppermost aquifer and identify and evaluate groundwater used for drinking water purposes. Specifically, the focus was on evaluating the spatial distribution of nitrate concentrations in ground- water basins used for drinking water and provide statistically-based estimates of where groundwater nitrate concentrations are likely to be above the maximum contaminant level (MCL) for nitrate.

Charter: Regional Monitoring Program for Water Quality in San Francisco Bay

San Francisco Estuary Institute (SFEI) | November 1st, 2017

This Charter describes the purpose and function of the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP). Established in 1993, the RMP is a collab

This Charter describes the purpose and function of the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP). Established in 1993, the RMP is a collaborative effort between the San Francisco Estuary Institute, the California Regional Water Quality Control Board, San Francisco Bay Region, and the regulated discharger community.

Clear Lake Watershed and Lake Remediation

University of California, Davis (UC Davis) | December 9th, 2024

The main goal of this joint effort between the University of California Davis, Tahoe Environmental Research Center (UCD-TERC), and the United States Geological Survey (US

The main goal of this joint effort between the University of California Davis, Tahoe Environmental Research Center (UCD-TERC), and the United States Geological Survey (USGS), is to develop the watershed and in-lake monitoring and modeling needed to:  Better understanding of the processes contributing to poor water quality;  Predict watershed and lake responses to changing environmental conditions; and,  Evaluate the impacts and unintended consequences of potential management and restoration activities. 

Climate change-resilient snowpack estimation in the Western United States

Nature Portfolio (Springer Nature) | June 18th, 2024