ASSESSING THE STABILITY OF CHROMIUM AT CONTAMINATED SITES BY PROXY OF CHROMIUM GEOCHEMISTRY IN SERPENTINE SOILS

Weathering of ultramafic rocks particularly rich in serpentine at Jasper Ridge Biological Preserve (JRBP), Stanford, CA, produces soils with high concentrations of chromium and other biologically toxic elements. Chromium, an inorganic, carcinogenic contaminant, persists in the environment as either Cr(III), a mild toxin, and/or Cr(VI), a known human carcinogen causing both chronic and acute toxic effects. Chromium in serpentine soils is initially released in the form of Cr(III) and is considered chemically immobile; however, Cr(III) may oxidize to form Cr(VI) in the presence of high valence manganese oxides. By evaluating the sources of chromium and the processes governing its release, transport, and attenuation, these soils can be used as a proxy for assessing the long-term behavior of chromium in Cr-contaminated sites.

Analyses of total chromium within JRBP soils demonstrate that chromium concentrations are comparatively enriched with respect to the bedrock and range from 1725 ppm to 4760 ppm. Variable concentrations of chromium with depth provide evidence that the chromium is mobile and a maximum typically occurs near the boundary of the A-B horizon. Total organic carbon and organic acids present in each soil horizon may increase the solubility of chromium-bearing phases accounting for the maximum at the A-B horizon interface. Although high concentrations of chromium and manganese are present, Cr(VI) was not detected in any of the samples. Using SEM, XRD, and XAS, the overall abundance and depth distribution of chromium reflects detrital chromite/quartz present in sand size fractions , chromium sorption onto mineral surfaces, and chromium precipitated as (oxy)hydroxides and clay minerals.