title: Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range.
abstract: Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km2reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km2/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km2/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.
authors.affiliation: Department of Geological Sciences, Stanford University , Stanford, California 94305, United States.
authors.forename: Cynthia N
authors.identifier: 0000-0002-1674-7323
authors.initials: CN
authors.lastname: McClain
authors.affiliation: Department of Earth System Science, Stanford University , Stanford, California 94305, United States.
authors.forename: Scott
authors.identifier: 0000-0002-9177-1809
authors.initials: S
authors.lastname: Fendorf
authors.affiliation: Stanford Synchrotron Radiation Lightsource , Menlo Park, California 94025, United States.
authors.forename: Samuel M
authors.initials: SM
authors.lastname: Webb
authors.affiliation: Department of Geological Sciences, Stanford University , Stanford, California 94305, United States.
authors.forename: Kate
authors.initials: K
authors.lastname: Maher
mesh_terms: D002857:Chromium; D060587:Groundwater; D010084:Oxidation-Reduction; D010087:Oxides; D012987:Soil; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical
chemical_list: D010087:Oxides; D012987:Soil; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D002857:Chromium