RATES OF IRON OXIDATION AND ARSENIC SORPTION DURING GROUND WATER-SURFACE WATER MIXING AT A HAZARDOUS WASTE SITE

The fate of arsenic discharged from contaminated ground water to a pond at a hazardous waste site is controlled, in part, by the rate of ferrous iron oxidation-precipitation and arsenic sorption. Laboratory experiments were conducted using site-derived water to assess the impact of these coupled processes on the removal of dissolved arsenic from the water column. The measured concentration of total organic carbon (TOC) from shallow and deep water sampled from the pond was approximately 15 and 200 mg C/L, respectively. Experiments conducted with these samples along with synthetic controls containing no organic carbon demonstrated that observed rates were dependent on the chemical composition of the discharging ground water. Increasing dissolved organic matter did not significantly interfere with ferrous iron oxidation, but inhibited precipitation of hydrous ferric oxide and subsequent sorption of arsenic. For experiments with 200 mg C/L TOC, there was a strong relationship between the fraction of precipitated iron and the fraction of sorbed arsenic. Laboratory- and field-derived hydrous iron oxide precipitates were characterized to evaluate mineralogy and arsenic distribution. Precipitates formed in the presence of organic carbon were consistent with formation of a 2-line ferrihydrite. These data were used to aid interpretation of field observations at the ground water-surface water interaction zone with respect to observed arsenic removal. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.