BIOREMEDIATION OF CHROMATE-CONTAMINATED GROUNDWATER BY REDUCTION AND PRECIPITATION IN SURFACE SOILS

A cost-effective method is needed for removing chromate from cooling water blowdown, wastewater effluent, and contaminated groundwater. Experiments were conducted to determine the effectiveness of treating Cr-contaminated water by using the water for irrigation, and that reduction of Cr(VI) to Cr(III) would occur in a soil amended with organic matter and irrigated to promote low oxidation/reduction status. The Cr(III) would then precipitate as oxides and hydroxides, and be immobilized and rendered plant unavailable. Samples of a field soil (mixed, thermic Typic Torripsamments) overlying a contaminated groundwater site were placed in pots and irrigated for 20 wk with water containing 1000 mu g L(-1) Cr(VI). Treatments included plants (alfalfa; Medicago sativa L.) vs. no plants, each at three organic matter loadings - 0, 12, and 50 Mg ha(-1) dried cattle manure (0, 5.5, and 21.8 g manure kg(-1) soil). The drainage waters were collected weekly and analyzed for total Cr, Cr(VI), pH, dissolved O-2 dissolved Organic C, and electrical conductivity. The removal percentages of Cr(VI) from the enriched water ranged from 51 to 98% and increased with increasing organic matter loading. Chromium concentrations in the drainage water were consistently <50 mu g L(-1) in the organic-amended soil. Daily irrigation yielded lower drainage water Cr concentrations than weekly irrigation (at a constant weekly volume) due to increased residence time of the water in the bioactive zone. Chromate adsorption accounted for <1% of the total immobilized Cr and the amount taken up by the alfalfa shoots was <0.5% of the total added. This method shows promise as a cost-effective treatment for Cr-contaminated groundwater.