PH EFFECTS ON METAL CONTAMINANT SPECIATION DURING IN SITU BIOREMEDIATION USING SULFATE REDUCING BACTERIA

During biogenic sulfate reduction in groundwater, pH increases due to consumption of hydrogen ions to make H2S. Because pH changes have significant effects on mineral solubilities and sorption/desorption reactions on solid surfaces, bioremediation strategies based on using sulfate-reducing bacteria (SRB) should consider potential positive and negative effects of increasing pH on the contaminants of interest at a site. At our field test site at Troy, Alabama, we have tracked the geochemical behavior of a suite of dissolved metals in groundwater impacted by a car-battery recycling plant. Sulfuric acid containing Zn, Cu, Pb, Cd, Cr contaminate a shallow water-table aquifer resulting in a low-pH (3-3.5) metal-sulfate plume at the site that is currently 1 km in length. A decade-long pump and treat process has proven largely ineffective and we have conducted pilot-scale tests of an in-situ bioremediation process by in injecting nutrients to stimulate SRB activity. These tests show that indigenous SRB effectively remove metals from groundwater and a large-scale bioremediation effort is planned for the entire plume. Solid sulfide phase precipitation apparently removes most Zn, Cd, and Cu, whereas modeling shows that both sorption (due to pH increase) and sulfide formation were important for removing lead. Intermittent rainfall events apparently caused short-term pH decreases and conductivity increases, suggesting that very soluble Fe-sulfate salts may be forming due to evaporation at the water table during dry periods. Thus it is important to consider pH changes when evaluating the fate, transport, and long term stability of metals at shallow contaminated sites.