Presentations 2016

Cr(VI) and nitrate in groundwater and sediments of the southwestern Sacramento Valley, California, USA

Stanford University

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Naturally occurring hexavalent chromium (Cr(VI)), derived from leaching of Cr-rich ultramafic sediments in the unsaturated zone, is found at levels above California’s drinking water limit (10 µg/L) in groundwater of the southwestern Sacramento Valley. Increasing evidence suggests agricultural irrigation may be enhancing natural Cr(VI) release from the unsaturated zone, as evidenced by the correlation between Cr and nitrate in shallow groundwater. The unsaturated zone has been identified as a key location were Cr(VI) is generated. Nevertheless, the processes governing this depth distribution and the corresponding Cr(VI) loading to groundwater under different irrigation and groundwater pumping scenarios have not been quantified. We conducted a nested study investigating regional groundwater quality down to the micrometer-scale chemical concentration and speciation of solids and fluids from 25 m sediment cores that extend through the unsaturated zone, variably saturated zone, and into the shallow groundwater aquifer. Regionally, we found Cr levels to be highest in shallow groundwater, decreasing with depth and increasing over time; all of these patterns mirror nitrate. Chromium(VI) depth profiles in sediment cores suggest a geogenic source and appear to be governed by changes in lithology (e.g. ultramafic content, sand, clay) and redox reactions associated with changing water content. Chromium(VI) concentrations from sediment extractions are elevated relative to the drinking water limit (up to 75 µg/L), reaching a maximum from 2-6 m depth coinciding with an enrichment in magnetic susceptibility and elevated Cr(s) concentrations. Chromium-rich minerals were found to be co-located with Mn(III/IV)-oxides (based on in situ chemical mapping and speciation). To our knowledge this study reveals the first micron-scale evidence for geogenic Cr(VI) generated by oxidation on Mn(IV)-oxides in California sediments. In variably saturated sediments, from 5-25 m depth (where the water table fluctuates seasonally), Cr(VI) concentrations are lower (10 to 40 µg/L) than in overlying unsaturated sediments. In this depth interval, the presence of dissolved Mn and Fe and extensive mottling and gleying in sediments suggest reduction attenuates Cr(VI) levels. Based on calculations for native and agricultural land and water use scenarios, increased infiltration through Cr-Mn bearing sediments and water level fluctuations within the variably saturated zone have likely led to heterogeneous regional Cr(VI) levels in shallow groundwater that are above California’s drinking water limit (> 10 µg/L) and parallel patterns of nitrate contamination.