THE GEOCHEMISTRY OF CHROMIUM MIGRATION AND REMEDIATION IN THE SUBSURFACE

From a human health perspective, knowledge of Cr speciation in soils is important in assessing risk related to ingestion of Cr-contaminated soils, and in selecting appropriate remedial alternatives that are protective of ground-water quality. In this study, good agreement was found between Cr(VI) soil concentrations and the mass of Cr(VI) leached from columns over a range of Cr(III):Cr(VI) soil ratios (0.2-28) and Sigma Cr concentrations (27-290 mg/kg). In batch tests, adsorption of Cr(VI) to sandy soils was rapid and linear, with a Kd(Cr(VI)) of 0.2 liters/kg, suggesting Cr(VI)-contaminated vadose- and saturated-zone materials may be remediated by soil flushing or pump-and-treat methods. In column tests, soil flushing removed 85% of the soil-Cr(VI), while pump-and-treat leached only 17% of Cr attributable to aquifer contamination. Use of oxidizing agents increased Cr released from Cr (III)-contaminated soils, but only from 1.3% to 2.3% of Sigma Cr mass. Based on Cr speciation in soils, Cr(VI) was mobile, while Cr(III)was retained in the soil column, indicating that reduction of Cr(VI) to Cr(III), and the proportion of Cr(III) in the soil, will constrain remedial efficiency. Reduction of Cr(VI) to Cr(III), and subsequent precipitation of CrxFe1-x(OH)(3), occurred in a Cr(VI)-contaminated soil augmented with Fe+2, resulting in < 0.01 mg/l Cr in the supernatant, suggesting that in situ Cr fixation may mitigate Cr-contaminated ground water.