TRANSPORT OF CHROMIUM AND SELENIUM IN A PRISTINE SAND AND GRAVEL AQUIFER - ROLE OF ADSORPTION PROCESSES

Field transport experiments were conducted in an oxic sand and gravel aquifer using Br (bromide ion), Cr (chromium, injected as Cr(VI)), Se (selenium, injected as Se(VI)), and other tracers. The aquifer has mildly acidic pH values and low concentrations of dissolved salts. Within analytical errors, all mobile Cr was present as Cr(VI). All mobile Se was probably present as Se(VI). Adsorption of Cr and Se onto aquifer sediments caused retardation of both tracers. Breakthrough curves for Cr and Se had extensive tails, which caused large decreases in their maximum concentrations relative to the nonreactive Br tracer after only 2.0 m of transport. A surface complexation model was applied to the results of laboratory studies of Cr(VI) adsorption on aquifer solids from the-site based on adsorption onto hydrous ferric oxide. The modeling results suggested that the dominant adsorbents in the aquifer solids have lower affinities for anion adsorption than pure hydrous ferric oxide. The steep rising limbs and extensive tails observed in most of the breakthrough curves are qualitatively consistent with the equilibrium surface complexation model; however, slow rates of adsorption and desorption may have contributed to these features. Variations during transport in the concentrations of Cr, Se, and other anions competing for adsorption sites likely gave rise to variations in the extent of adsorption. Adequate description of the observed retardation of Cr and Se would require a coupled transport-adsorption model that can account for these effects. Companion experiments in the mildly reducing zone of the aquifer (Kent et al., 1994) showed a loss of Cr mass, probably resulting from reduction to Cr(III), and little retardation of mobile Cr and Se during transport; this contrast illustrates the influence of aquifer chemistry on the transport of redox-sensitive solutes.