REDUCTIVE DISSOLUTION AND REACTIVE SOLUTE TRANSPORT IN A SEWAGE-CONTAMINATED GLACIAL OUTWASH AQUIFER

Contamination of shallow ground water by sewage effluent typically con tains reduced chemical species that consume dissolved oxygen, developi ng either a low oxygen geochemical environment or an anaerobic geochem ical environment. Based on the load of reduced chemical species discha rged to shallow ground water and the amounts of reactants in the aquif er matrix, it should be possible to determine chemical processes in th e aquifer and compare observed results to predicted ones. At the Otis Air Base research site (Cape God, Massachusetts) where sewage effluent has infiltrated the shallow aquifer since 1936, bacterially mediated processes such as nitrification, denitrification, manganese reduction, and iron reduction have been observed in the contaminant plume, In sp ecific areas of the plume, dissolved manganese and iron have increased significantly where local geochemical conditions are favorable for re duction and transport of these constituents from the aquifer matrix, D issolved manganese and iron concentrations ranged from 0.02 to 7.3 mg/ L, and 0.001 to 13.0 mg/L, respectively, for 21 samples collected from 1988 to 1989, Reduction of manganese and iron is linked to microbial oxidation of sewage carbon, producing bicarbonate and the dissolved me tal ions as by-products. Calculated production and flux of CO2 through the unsaturated zone from manganese reduction in the aquifer was 0.03 5 g/m(2)/d (12% of measured CO2 flux during winter), Manganese is limi ted in the aquifer, however A one-dimensional, reaction-coupled transp ort model developed for the mildly reducing conditions in the sewage p lume nearest the source beds showed that reduction, transport, and rem oval of manganese from the aquifer sediments should result in iron red uction where manganese has been depleted.