Analyses of regulatory water-quality data

First posted April 25, 2023 For additional information, contact: Director,California Water Science CenterU.S. Geological Survey6000 J Street, Placer HallSacramento, California 95819 Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas Electric Company (PG&E) Hinkley compressor station in Mojave Desert 80 miles northeast of Los Angeles, California. The has monitored near Hinkley, California, for Cr(VI) other constituents since late 1980s. By June 2017, more than 20,000 samples had been collected analyzed regulatory purposes. Most were using U.S. Environmental Protection Agency (EPA) Method 218.6 with a laboratory reporting level (LRL) 0.2 micrograms per liter (μg/L). July 2012 selected low-level concentrations modified version EPA an LRL 0.06 μg/L. Field-blank data duplicate during this period indicate study (SRL) μg/L most analyses SRL 0.12 analyses. overall precision by both methods at interim background concentration 3.1 0.09 μg/L, or about 3 percent.Hexavalent chromium trends calculated 564 monitoring wells through 2017. Upward present water 102 throughout Valleys. margins October–December 2015 (Q4 2015) plume (1) within strands Lockhart fault east southeast (2) shallow northern subarea consistent expansion these areas between widely distributed elsewhere Valleys outside Q4 commonly associated declining levels. These upward may result natural sources, including movement containing weathered bedrock, fine-textured deposits, (3) secondarily oxidized material aquifer deposits. Downward observed 146 wells. largely can be attributed to remediation activities downgradient station. Hexavalent also 219 domestic 8 downward 23 clustered former residential west plume. Results trend (including upward, downward, no trend) used as part summative-scale analysis (chapter G) define extent anthropogenic