GEOCHEMICAL REDUCTION OF HEXAVALENT CHROMIUM IN THE TRINITY SAND AQUIFER

Ground water of the Trinity Sand aquifer at the Odessa I site has become contaminated with hexavalent chromium due to release of plating wastes during the 1970s. Measured hexavalent chromium partition coefficients for Trinity Sand aquifer sediments compare favorably with values derived from the literature, and with retardation factors calculated from observed plume migration rates between 1986 and 1991. Dissolved and adsorbed masses of hexavalent chromium were calculated from contoured plume maps using ground-water data collected between 1986 and 1991 to estimate the total mass of hexavalent chromium in the aquifer. Comparison of the calculations from each year indicated that only 30 percent of the hexavalent chromium present in the aquifer in 1986 remained in 1991. Decreasing hexavalent chromium mass in the Trinity Sand aquifer at the Odessa I site indicates that natural geochemical reactions are occurring to reduce mobile hexavalent chromium to the insoluble trivalent form. Thermodynamic calculations based on platinum electrode redox and pH measurements indicate that solid trivalent chromium hydroxide species are the stable phase in these ground waters. Reduction of hexavalent chromium to the trivalent state results in precipitation from solution of a solid chromium (III) hydroxide, probably in association with iron oxyhydroxides present in the aquifer. Two possible reducing agents in the Trinity Sand aquifer are naturally occurring organic carbon and ferrous iron. Laboratory measurements of the rate of hexavalent chromium reduction indicate that the reaction proceeds slowly at near-neutral pH values representative of the Odessa I site. Assuming first-order reaction kinetics, these laboratory reduction rates are consistent with the rate of hexavalent chromium loss at the Odessa I site between 1986 and 1991, and indicate a hexavalent chromium half-life of approximately 2.5 years. An analytical contaminant transport model was calibrated to simulate historical chromium concentrations for the aquifer, and then used to predict that hexavalent chromium will naturally decrease below the drinking water standard within approximately 10 years.