Summary and conclusions

First posted April 25, 2023 For additional information, contact: Director,California Water Science CenterU.S. Geological Survey6000 J Street, Placer HallSacramento, California 95819 Chromium concentrations in rock and aquifer material Hinkley Valleys the Mojave Desert, 80 miles northeast of Los Angeles, California, are generally low compared to average chromium concentration 185 milligrams per kilogram (mg/kg) bulk continental crust. felsic, coarse-textured “Mojave-type” deposits, composed River stream (alluvium) lake-margin (beach) deposits sourced from River, as 5 mg/kg, with a median 23 mg/kg materials adjacent screened intervals sampled wells. The most abundant chromium-containing mineral within is magnetite. Magnetite resistant weathering, about 90 percent remains unweathered grains. However, hornblende diorite basalt present surrounding uplands, actinolite some materials.Although geologic abundance clearly important, hexavalent chromium, Cr(VI), alkaline oxic groundwater related factors. Hexavalent influenced by combination processes including (1) mineralogy weathering rates minerals; (2) texture deposits; (3) accumulation weathered minerals surface coatings on grains; (4) oxidation accumulated Cr(III) Cr(VI) presence manganese oxides (Mn oxides), states those Mn oxides; (5) pH-dependent desorption surfaces grains into during appropriate aqueous geochemical conditions; (6) age (time since recharge) groundwater. pH increases result silicate increasing long oxic. In absence detailed geologic, geochemical, hydrologic data collected part this study, sorption, evaluated occurrence probability at measured pH, an effective indicator natural or anthropogenic Cr(VI).The Pacific Gas Electric Company (PG&E) compressor station used compress gas it transported through pipeline Texas California. Between 1952 1964, cooling water containing was discharged unlined ponds released unconsolidated aquifers. extent evidence least 5.5 square (mi2) estimated using summative scale incorporating more than 100 wells between March 2015 November 2017. summative-scale plume larger 2.2 mi2 October–December (Q4 2015) regulatory but smaller 8.3 maximum mapped greater interim background 3.1 micrograms liter (μg/L). low-chromium deposited described Mojave-type area covered PG&E monitoring well network.Background were calculated computer program ProUCL 5.1 upper 95-percent tolerance limit, UTL95, outside 2017 2018. overall UTL95 for undifferentiated, eastern western subareas northern subarea upgradient Mount General fault Valley 3.8 μg/L; value similar 3.9 μg/L Valleys, older contact 3.6 μg/L.In cases may be acceptable near margin; however, values various provide resolution that important purposes. eastern, western, 2.8, 3.8, 4.8 μg/L, respectively. 2.8 management because subarea. A 2.3 downgradient station. This represents have been had not station, reflects coarser textured proximity recharge areas along results younger (post-1952), less farther downgradient. suitable metric cleanup goals after updates. separate 5.8 mudflat/playa Valley, lacustrine (lake) eroded Miocene 9.0 6.4 respectively.Hexavalent 70 domestic January 27 31, 2016, ranged study reporting level 0.1–4.0 1.2 μg/L. did exceed where located. 47 arsenic, uranium, nitrate above contaminant level.Anthropogenic treated bioremediation adding ethanol reductant volume known situ reactive zone (IRZ). Laboratory microcosm studies showed rapidly reduced additions ethanol. Reduced sorbed sequestered crystalline iron microcosms period several months. Trivalent reoxidized back 2 weeks return (oxygen present) conditions microcosms. As much 10 added Cr oxidized prepared recent material, 20 material. Less (less 3 before reduction) phase, release occurred following longer time periods oxygen exposure. Sequestration reduction facilitates reoxidation under conditions. Future maintenance anoxic absent) would ensure continued sequestration IRZ portions plume.Although history associated releases require attention. can updated study. lie extent. authority establish values, clean-up goals, future site practices resides Lahontan Regional Quality Control Board.