TRANSPORT OF CHROMIUM AND SELENIUM IN THE SUBOXIC ZONE OF A SHALLOW AQUIFER - INFLUENCE OF REDOX AND ADSORPTION REACTIONS

Breakthrough of Cr(VI) (chromate), Se(VI) (selenate), and O2 (dissolve d oxygen) was observed in tracer tests conducted in a shallow, sand an d gravel aquifer with mildly reducing conditions. Loss of Cr, probably due to reduction of Cr(VI) to Cr(III) and irreversible sorption of Cr (III), occurred along with slight retardation of Cr(VI), owing to reve rsible sorption. Reduction of Se(VI) and O2 was thermodynamically feas ible but did not occur, indicating conditions were unfavorable to micr obial reduction. Cr(VI) reduction by constituents of aquifer sediments did not achieve local equilibrium during transport. The reduction rat e was probably limited by incomplete contact between Cr(VI) transporte d along predominant flow paths and reductants located in regions withi n aquifer sediments of comparatively low permeability. Scatter in the amount of Cr reduction calculated from individual breakthrough curves at identical distances downgradient probably resulted from heterogenei ties in the distribution of reductants in the sediments. Predictive mo deling of the transport and fate of redox-sensitive solutes cannot be based strictly on thermodynamic considerations; knowledge of reaction rates is critical. Potentially important mass transfer rate limitation s between solutes and reactants in sediments as well as heterogeneitie s in the distribution of redox properties in aquifers complicate deter mination of limiting rates for use in predictive simulations of the tr ansport of redox-sensitive contaminants in groundwater.