Evaluation and mitigation of landfill gas impacts on cadmium leaching from native soils

This study evaluated the potential impact of carbon dioxide contained in municipal solid waste (MSW) landfill gas on the release of cadmium from uncontaminated native soils. The study was initiated to evaluate the potential sources of elevated cadmium concentrations detected in monitoring wells downgradient of the landfill but not in landfill leachate or monitoring wells upgradient. The study included field data evaluation, soil leaching experiments, and geochemical modeling using MINEQL(+) and Visual MINTEQ chemical speciation software. Bench-scale leaching experiments were conducted using uncontaminated soils with natural cadmium concentrations ranging from less than 0.10 to 2.6 mg/kg. Carbon dioxide gas was equilibrated with uncontaminated soil samples and ground water from the site to simulate field conditions. Carbon dioxide concentrations were varied from 10% to 100%. Dissolved cadmium concentrations measured over this range of carbon dioxide concentration increased from less than 0.0001 to 0.0047 mg/L, which is equivalent to the maximum contaminant level of 0.005 mg/L for cadmium (U.S. EPA). Geochemical equilibrium modeling using Visual MINTEQ demonstrated that cadmium carbonate, or otavite, is the primary cadmium mineral controlling cadmium solubility. Predicted dissolved cadmium concentrations ranged from 0.0221 to 0.664 mg/L, with carbon dioxide concentrations of 0.0316% (atmospheric concentration) to 100%. MINEQL+ sorption modeling using a surrogate soil system predicted that dissolved cadmium concentrations would increase from 0.00603 to 0.0737 mg/L, which is in the range of cadmium concentrations observed in field samples. In response to these findings, the landfill operator initiated soil vapor extraction (SVE) in the vadose zone around the impacted wells. Since startup of the SVE system, dissolved cadmium concentrations in the formerly impacted wells have decreased to below the method detection limits.