Groundwater chemistry and 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 samples collected by the U.S. Survey from more than 100 wells between March 2015 and November 2017 in Hinkley Valleys, Mojave Desert 80 miles northeast of Los Angeles, California, were analyzed for field parameters, major ions, nutrients, selected trace elements, including hexavalent chromium, Cr(VI). most was alkaline oxic. The pH ranged 6.9 water-table near recharge areas along River to 9.4 deeper farther downgradient northern subarea.Hexavalent chromium concentrations measured ion chromatography using Environmental Protection Agency Method 218.6 a version that method used detection Cr(VI) as low 0.06 micrograms per liter (μg/L), produced results comparable speciation with subsequent analyses graphite furnace atomic absorption spectroscopy (coefficient determination, R2, 0.97). Hexavalent less study reporting level 0.10 2,500 μg/L. highest within October–December (Q4 2015) regulatory plume compressor station. outside Q4 high 11 water distributed narrow redox potential band overlapping chromate ion, CrO42−(aqueous), manganese-3, Mn(III)(solid), stability fields. some completed carbonate-rich mudflat/playa deposits approached oxic manganese-4, Mn(IV)(solid), field. However, porewater pressure-extracted Mn(IV)-containing eastern subarea did not exceed 3.3 μg/L, does appear be source greater this concentration subarea.On basis comparison California-wide data, at higher expected uncontaminated (1) plume, (2) nominally crossgradient station upgradient (3) shallow leading edge plume. Valley ranges elsewhere given their trace-element composition. on co-occur subareas margins. 4 μg/L domestic sampled Valleys generally groundwater composition.Interpretations derived pH, summative-scale analysis determine extent (chapter G). background G) calculated data January 2018.