Landscape surrounding a Pacific Gas and Electric Company compressor station in Hinkley, California

Hexavalent chromium, a known carcinogen under specific circumstances, occurs naturally in groundwater in the Mojave Desert. However, concentrations of hexavalent chromium increased in California’s Hinkley Valley beginning in 1952 when the Pacific Gas and Electric Company (PG&E) 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, CA compressor station into Hinkley Valley. In a April 2023 study, scientists mapped the extent of human-introduced hexavalent chromium in the Hinkley Valley. 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.