Evaluation of natural and anthropogenic (human-made) hexavalent chromium

First posted April 25, 2023 For additional information, contact: Director,California Water Science CenterU.S. Geological Survey6000 J Street, Placer HallSacramento, California 95819 Hexavalent chromium, Cr(VI), was released between 1952 and 1964 from the Pacific Gas Electric Company (PG&E) Hinkley compressor station, in Mojave Desert about 80 miles northeast of Los Angeles, California. Geologic, geochemical, hydrologic data more than 100 wells collected March 2015 November 2017 were interpreted using a summative-scale analysis to define extent anthropogenic (human-made) Cr(VI) groundwater. The summative scale consisted eight questions requiring binary (yes or no) answers for each sampled well. intended (1) provide transparent framework interpretation which all stakeholders participated; (2) unbiased traceable numerical measurements; (3) that enabled geologic, be considered collectively; (4) consolidate different types into simple, easy-to-understand interpretation. When well are scored metrics within scale, would score same way draw plume extent.The areal 5.5 square (mi2); this is larger 2.2-mi2 October–December (Q4 2015) regulatory but smaller 8.3-mi2 maximum mapped greater interim background value 3.1 micrograms per liter (μg/L). area covered by PG&E monitoring network lies “Mojave-type” deposits composed low-chromium stream near-shore lake sourced River. included shallow footprint Q4 plume, scores indicate not present several screened deep zone upper aquifer. consistent with mineralogic geochemical as part study used analysis.Data outside purposes through 2018 estimate concentrations 95-percent tolerance limit (UTL95) parts Valleys. UTL95 values calculated computer program ProUCL 5.1 suitable use agencies support updating management near margins, establishing cleanup goals updated identifying unusual plume. nonparametric Mojave-type eastern, western, northern subareas Valley 3.7, 3.9, 4.0 μg/L, respectively. normal undifferentiated, unconsolidated eastern western subarea upgradient Mount General fault 2.8, 3.8, 4.8 An overall 3.8 μg/L these areas. This similar 3.9 concentration older groundwater contact 3.6 μg/L. provenance most precisely known, may widely applicable deposits.The 2.8 important because subarea. A 5.8 (pre-1952) associated mudflat/playa General. 2.3 downgradient station after updates. lower neutral slightly alkaline, younger (post-1952) coarse-textured, metric goals. provides possible increases if water levels continue decline. Downgradient 9.0 6.4 Valley, respectively, geologic conditions areas.The scientifically defensible estimates differ local can on basis values. contain having less their respective require attention, an extent. purposes, authority establish resides solely Lahontan Regional Quality Control Board.