Lithologic and redox controls on hexavalent chromium in vadose zone sediments of California's Central Valley

In agricultural regions of California where ultramafic sediments containing naturally occurring Cr(III) are present, correlations between Cr(VI) and nitrate in groundwater have been attributed to oxidation of Cr(III) in vadose sediments and mobilization by areal recharge, including irrigation return. However, the distribution of Cr and nitrate through the vadose zone have yet to be evaluated together to investigate the controls on geogenic Cr(VI) occurrence and resulting Cr(VI) production rates and export fluxes to groundwater. To develop a framework for evaluating geogenic Cr(VI) contamination, we analyze vadose zone sediment cores from the southwestern Sacramento Valley of California at high spatial resolution. In the sandy, oxic, ultramafic, Cr-rich Holocene alluvial sediment, Cr(III) is oxidized to Cr(VI), resulting in increasing Cr(VI) concentrations with depth up to 79 mu g/kg. Oxidation is likely associated with l-meter scale co-located Mn(IV)-oxides. Within the fine-grained Pleistocene sediments beneath the historic high water table (5-18 m), Cr(VI) concentrations decrease with depth to <30 mu g/kg due to subsequent reduction. Patterns in Cr(VI) concentration parallel nitrate due to the similar depth of production zones, oxidation-reduction potential and geochemical behavior. Field evidence in the shallow profile also supports Cr (VI) production by enhanced Cr(III) dissolution due to nitrification-induced acidification and subsequent oxidation by Mn-oxides. From Cr(VI) and nitrate concentration gradients with depth through the vadose zone (similar to 20 m), we calculate field-based net production and removal rates, quantify vadose zone storage (156-1168 kg Cr(VI)/km(2); 1 x 10(5)-2.6 x 10(5) kg N/km(2)), and estimate export fluxes to groundwater (40-1314 kg Cr(VI)/km(2)/yr; 5-487 kg N/km(2)/yr). The framework we present for evaluating vadose zone geogenic Cr(VI) contamination highlights the compounding effects that vadose zone lithology and hydrology can have on solute production, accumulation, development of redoxclines, and subsequent distribution of redox sensitive elements in alluvial sediment and groundwater. (C) 2019 Published by Elsevier Ltd.