ARSENIC ADSORPTION ONTO AQUIFER SEDIMENTS: IMPLICATION FOR REMEDIATION AT THE VINELAND SUPERFUND SITE, NEW JERSEY

At the Vineland Superfund Site, New Jersey, remediation activities have included pump-and-treat (up to 2 million gallons per day, with aquifer turnover ~ 200 days) since 2000, as well as vadose zone soil washing. Early analysis of groundwater showed that As(V) (arsenate), As(III) (arsenite), MMA and DMA all existed in groundwater with organic As <20%. Total As of most extraction wells has since decreased significantly, and sporadic speciation data showed rapid disappearance of MMA and DMA. This study uses batch and column experiments to compare the adsorption-desorption behavior of all four As species onto aquifer sediments. Understanding the sorption-desorption behavior of As species is essential to the accurate predication of residual groundwater As levels by pump-and-treat operation, whose main purpose has been to prevent further spread of the contaminant plume. Little differences in As adsorption were observed for sediment preserved in an anaerobic chamber and for sediment that was left in air for more than 2 months. Incubations either inside or outside the anaerobic chamber also yielded very similar results, suggesting that potential oxygenation of the aquifer probably would have minimal impact on the removal of As from the liquid phase. Among the four As species, As(V) adsorption was the most efficient while As(III) the least, in the order of As(V)>MMA>DMA>As(III). In a vial with 10-mL natural groundwater and 20 g sediment, within 36 hours only about 50% of As(III) was adsorbed, comparing to DMA (~75%), MMA and As(V) (both approaching 100%). Similarly, column experiments revealed that desorption rates followed the same order, with As(V) the slowest. The lab results are somewhat counter intuitive to the fact that As(III) still constitutes a large proportion of the total As in groundwater, and that MMA disappeared much quicker than As(III). XANES results showed that sediment-bound As is almost exclusively composed of arsenate; therefore the long term residual As level may be dictated by the release rates of As(V) due to its strong binding to the sediment and slow release into groundwater. The persistence of large amount of As(III) in groundwater may suggest mechanism in which arsenate is rapidly converted to arsenite, which could greatly accelerate pump-and-treat remediation.