Geochemical Changes in Ground Water Due to Landfill Gas Effects

Abstract

AbstractConcentrations of dissolved inorganic constituents commonly monitored in ground waters at landfills were evaluated during and after a period of landfill gas effects on the ground water. Landfill gas can potentially act as an acid or as a reducing agent (Lewis base) due to its carbon dioxide and methane content, respectively. Ground water data from a single landfill gas‐affected well were used to evaluate the correlation of the total volatile organic compound (VOC) concentration (as a general measure of landfill gas effects) with bicarbonate alkalinity, ammonia, calcium, iron, magnesium, manganese, sodium, chloride, and sulfate concentrations. Bicarbonate alkalinity, calcium, and magnesium concentrations were correlated with total VOC concentrations. The correlation with calcium and magnesium concentrations is attributed to increased dissolution of carbonate minerals by carbonic acid from the landfill gas carbon dioxide. Total manganese concentrations also increased with increasing VOC content. This is attributed to reduction of manganese (IV) in aquifer minerals by methane in the landfill gas. No detectable iron was observed during the landfill gas effects or after successful corrective action, suggesting that the redox potential of the ground water was not sufficiently low to reduce iron (III) minerals. There was no correlation observed between total VOC concentrations and chloride, sodium, or sulfate concentrations, and there were insufficient ammonia detections to evaluate. The observed effects of landfill gas are expected to depend on the particular mineralogy and ground water quality of a site. These results and basic chemical principles, however, suggest that landfill gas effects on ground water could cause an increase in bicarbonate alkalinity, calcium, and magnesium concentrations, without increases in sodium or chloride concentrations at many sites. Because municipal solid waste landfill leachate is typically characterized by concentrations of chloride and sodium that are significantly elevated relative to background ground water concentrations, landfill gas effects on ground water could potentially be differentiated from leachate effects by a lack of increases in sodium or chloride concentrations accompanying VOC detections.