Arsenic mobility in groundwaters at contaminated sites

Arsenic is a toxic metalloid found dispersed throughout the earth's crust. It is often found at elevated concentrations in groundwaters because it is more soluble and sorbs more weakly under reducing conditions. Naturally occurring arsenic can be mobilized from aquifer materials by landfill-induced reducing conditions, as observed in Winthrop, Maine. At Winthrop, landfill leachate influence, including elevated dissolved arsenic concentrations, is seen predominantly in areas with relatively high hydraulic conductivities. Measurements of hydraulic conductivity (K) at the Winthrop site by numerous methods generally agree with one another with respect to the value of K, but not with respect to the spatial pattern of heterogeneity. The high degree of hydrogeologic heterogeneity, in addition to the widely dispersed arsenic in the aquifer, make this a difficult site to remediate. The solubility of arsenic is high under a relatively narrow range of redox conditions; therefore, perturbations of redox conditions either due to natural gradients or to redox manipulations could potentially remove arsenic from solution. Redox conditions become increasingly oxic and show progressively decreasing concentrations of leachate tracers (primarily chloride) with distance from the landfill; dissolved arsenic and iron concentrations also decrease with distance. Observations indicate that decreases in arsenic and iron concentrations are somewhat decoupled and are not due only to dilution by regional groundwaters. Attempted in situ field manipulation of the aquifer towards more oxic conditions with magnesium peroxide failed to appreciably lower dissolved arsenic concentrations due to the very large redox buffer capacity of aquifer solids. Laboratory incubation experiments indicate that in situ manipulations of the aquifer towards more reducing conditions (i.e. by addition of acetate or other electron donors) may be more feasible and could remove arsenic from solution by formation of arsenic-sulfide minerals. These experiments led to decreases in both readily accessible sediment arsenic and in dissolved arsenic concentrations. In these laboratory incubation experiments, evidence was also seen for temporary formation of aqueous thioarsenites, which stabilize high concentrations of dissolved arsenic. The presence of aqueous thioarsenites was also inferred in porewaters of stream sediments at the Vineland Chemical Superfund site in southern New Jersey, a site highly contaminated with arsenic due to the manufacture of arsenical biocides. (Abstract shortened by UMI.)...