Background/Objectives. An electric power utility company conducted routine groundwater sampling in the surficial aquifer at its facility in Tampa, FL, ("Site") according to the facility's industrial waste water permit.
Results of this sampling identified an area of groundwater with elevated arsenic concentrations. Subsequent subsurface evaluations coupled with review of historic construction activities revealed that the arsenic impacted area was constructed in a portion of Tampa Bay with a heterogeneous mixture of dredge materials, imported fill, and coal combustion residuals (CCRs) and a hydraulic conductivity on the order of 10-6 cm/s. CCRs are a known source of arsenic in groundwater under appropriate conditions. Furthermore, literature showing successful remediation of arsenic impacted groundwater with a CCR source term is limited to pump and treat. Based on the reducing conditions of the aquifer in the target area and the desire to avoid pumping and treating groundwater, a feasibility study indicated that, while not previously reported in literature, biosequestration of arsenic was a viable remedial alternative. Approach/Activities. Previous studies by Geosyntec have shown that arsenic sulfide precipitates (e.g. orpiment, realgar, and arsenopyrite) can be formed under sulfate reducing conditions by in-situ biosequestration using an electron donor and, if needed, a source of sulfate. Once formed, the arsenic sulfide precipitates are stable and dissolution is negligible. At the subject Site a pilot test was conducted using an emulsified vegetable oil/lactate mixture as the electron donor and ferrous sulfate as a sulfate source. This mixture was pressure injected into the subsurface in a centrally located injection well within the 30 ft by 30 ft treatment area. Injection results were evaluated through groundwater sampling at four monitoring wells installed in the treatment area. Following the pilot test, a laboratory scale treatability test was performed on microcosms consisting of soil and groundwater collected from the Site and an emulsified vegetable oil/lactate mixture. For the purposes of the bench scale test both ferrous sulfate and potassium sulfate were used as a sulfate source for the sulfate reducing bacteria. Results/Lessons Learned. Evaluation of the pilot test data showed that while there were some initially encouraging results (i.e. initial decrease in oxidation-reduction potential [ORP], increase in total organic carbon [TOC]), arsenic concentrations within the treatment area showed a decrease in one monitoring well and little to no change at the three remaining monitoring wells. The analytical data along with field observations indicated that the pilot test was not successful due to either insufficient mixing of the electron donor/sulfate source in the subsurface and/or the presence of iron in CCRs which may have out-competed the arsenic for complexation with sulfides. This theory was tested during the bench scale study which showed the technology was successful in reducing arsenic concentrations in groundwater by 83% even in the presence of iron laden CCRs. However, it was critical to achieve relatively uniform mixing. To achieve good mixing in the low permeability sediments, a pilot scale to full scale soil blending administration of the electron donor/ferrous sulfate is in the initial planning stages and results may be available prior to the conference.