PH and Redox Buffering Mechanisms in a Glacial Drift Aquifer Contaminated by Landfill Leachate

Abstract

AbstractThe KL Landfill in Kalamazoo County, Michigan was closed in June 1979 because of ground‐water contamination. Study of water analyses from a monitoring‐well network has provided insight into a variety of pH and pe buffering reactions within the contaminant plume. The reactions occur when contaminated ground water containing dissolved organic acids from the landfill reacts with carbonate minerals and other solid phases in a glacial drift aquifer. Concentrations of Total Inorganic Carbon (TIC) in the plume, measured as the difference between Total Carbon (TC) and Total Organic Carbon (TOC), are lower than those obtained by alkalinity titrations. The difference results from the dissociation of organic acids in solution, producing organic anions that are included in the erroneously high titration alkalinity values.Organic acids cause dissolution of carbonate minerals in the drift and increased concentrations of calcium, magnesium, and bicarbonate in the plume. Geochemical modeling using WATEQF indicates that the plume is supersaturated with respect to calcite, dolomite, and siderite. The dissolution reactions buffer pH in the plume at near background levels.Redox potentials are below the level of sulfate reduction in the contaminant plume. Therefore, redox buffering mechanisms, reactions that inhibit changes in redox potential in the presence of a strong reducing agent, are much less effective than pH buffering in this system. Possible redox reactions occurring along the flow path include oxidation of organic compounds coupled with reduction of nitrate to ammonium and reduction of ferric iron in oxyhydroxide phases to ferrous iron.WATEQF simulations indicate that dissolved iron concentrations in the plume are well above saturation levels with respect to sulfide phases, moderately above saturation with respect to siderite, and below saturation with respect to iron hydroxide. It is likely that iron precipitates as sulfide phases until the dissolved sulfide is consumed. The concentration of dissolved iron is subsequently controlled by precipitation of siderite, which occurs when moderate levels of supersaturation are reached.These redox buffering reactions are insufficient to maintain high pe levels, and pe values drop to the level of sulfate reduction in the contaminant plume. Together, pH and redox buffering reactions control the concentrations of most major chemical species in the KL Landfill contaminant plume.