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The Pacific Gas and Electric Company (PG&E) Hinkley compressor station (
Pacific Gas and Electric Company (PG&E) compressor station, Hinkley, California, March 2009. (Photograph by Steven Perry, ARCADIS, Inc., courtesy of PG&E.)
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;
Rock and aquifer deposits within Hinkley Valley have chromium concentrations commonly less than 25 milligrams per kilogram (
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 (
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
[Data are modified from
Question | Data source | Chapter where data are discussed | Answer and score | ||
Yes | No | ||||
1 | Are geologic materials at the well screen fine textured (predominately silt or finer)? | USGS lithologic descriptions of core material (PG&E lithologic descriptions from well logs or drillers logs used if core material was not available) | Chapters B, C and E | −1 | 1 |
2 | Do geologic materials at the well screen contain more than 85 mg/kg chromium? | Portable (handheld) X-ray fluorescence (HXRF) measurements of core material | Chapter B | −1 | 1 |
3 | Do geologic materials at the well screen contain more than 970 mg/kg manganese? | Portable (handheld) X-ray fluorescence (HXRF) 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 and June 2017, interpreted using the Mann-Kendall test for trend ( |
Chapter D | 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; |
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, post-1952, water (with measurable tritium) and a carbon-14 activity greater than 84 percent modern carbon? | Tritium, helium-3, and carbon-14 data. | Chapter F | 1 | −1 |
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 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 positive-percentage 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;
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).
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 (
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 (
Hexavalent chromium concentrations as high as 10 µg/L (
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 (
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 (
[Locations of subareas identified in
Number of |
UTL95, |
Maximum Cr(VI), |
Modeled |
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 |
|||
81 | 3.8 | 4.0 | normal |
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.
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.
Background hexavalent chromium, Cr(VI), concentrations within Hinkley and Water Valleys, western Mojave Desert, California, April 2017 through March 2018. Data are modified from
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 (
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 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
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.