The USGS report, commissioned by the Lahontan Regional Water Quality Control Board, showed how the valley’s geology affected background hexavalent chromium concentrations in groundwater.

Hexavalent chromium occurs naturally in groundwater in the Mojave Desert. Concentrations increased in Hinkley Valley beginning in 1952 when the Pacific Gas and Electric Company discharged it into unlined ponds. From there, hexavalent chromium entered the aquifer. Once in the ground, a plume of hexavalent chromium traveled with groundwater away from the Hinkley compressor station into Hinkley Valley.  

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This new work estimates naturally occurring hexavalent chromium concentrations in groundwater. The study identified an overall hexavalent chromium concentration of 3.8 micrograms per liter for Hinkley Valley and other background concentrations for specific areas within the Valley. The study was developed by the USGS with input from a technical working group of stakeholders. The report’s results may guide the Lahontan Regional Water Quality Control Board’s regulatory efforts to manage the human-introduced hexavalent chromium and Pacific Gas and Electric Company’s efforts to remediate Hinkley’s groundwater.

“The overall 3.8 microgram per liter value and values for specific areas within Hinkley Valley are scientifically defensible estimates of background hexavalent chromium concentrations in groundwater within deposits sourced from the Mojave River within Hinkley Valley,” said John Izbicki, a U.S. Geological Survey research hydrologist who led the study.

The Hinkley area geology is naturally low in chromium. The study found hexavalent chromium background concentrations differed within different parts of the study area. For example, the study found a higher value of 4.8 micrograms per liter in the northern part of Hinkley Valley, whereas in the eastern part of the valley, where the Hinkley compressor station and most of the plume is located, had a lower background value of 2.8 micrograms per liter.

To update the natural hexavalent chromium concentrations for this new report, USGS scientists worked with a technical working group that included local community members, the Lahontan Regional Water Quality Control, the Independent Site Manager, Pacific Gas and Electric Company and their consultants. The team came up with eight “yes or no” questions to apply to geologic materials and water samples from 100 different wells within the Hinkley Valley.

“The Lahontan Water Board is pleased to see the hard work of the USGS come to fruition with the completion of the much-anticipated hexavalent chromium background study,” said Mike Plaziak, executive officer of the Lahontan Regional Water Quality Control Board.

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The questions addressed the kinds of geologic material at the well site, chromium and manganese concentrations, whether chromium concentrations in water from wells went up or down with time, how chromium concentrations varied compared to the pH of the water and with other trace element concentrations, whether water was recharged from the Mojave River, and if the age of the water was consistent with recharge during the period of hexavalent chromium releases from the Hinkley compressor station after 1952. Answers to these questions were used to draw an updated extent of the human-introduced hexavalent chromium plume from the Pacific Gas and Electric Company discharge.

The team also studied more than 70 domestic wells within Hinkley Valley to determine if they had been impacted by the Pacific Gas and Electric company chromium-6 releases. Although almost half of the wells had arsenic, uranium or nitrate concentrations greater than drinking water limits, hexavalent chromium values for all wells were less than the background values for their respective subareas of the valley.   

“The extent of hexavalent chromium released from the Hinkley compressor station estimated from this study is larger than the current extent of the regulatory hexavalent chromium estimated by the Lahontan Regional Water Quality Control Board, especially in the eastern and northern parts of Hinkley Valley,” Izbicki said. 

Scientific-defensible estimates of background concentrations are not estimates of background for regulatory purposes. The Lahontan Regional Water Quality Control Board is the authority responsible for establishing regulatory values. 

“The report will inform the Lahontan Water Board of revisions that are necessary of its Cleanup and Abatement Order No. R6V-2015-0068 requiring PG&E to cleanup chromium-6 to background concentrations,” Plaziak said.

You can find the study here https://doi.org/10.3133/pp1885 

You can find more information on the study here https://www.usgs.gov/centers/california-water-science-center/science/occurrence-natural-and-anthropogenic-hexavalent