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Mineral Buffering of Contaminated Ground Water Compositions at a Hazardous Waste Site in Southwestern Louisiana
Jeffrey S. Hanor (1), Kathleen M. McManus (1,2), Vishnu Ranganathan (1,3), Shaobing Su (1,4)
Thermodynamic evaluation of the composition of ground waters contaminated by chlorinated hydrocarbon and chloride wastes at a site in southwestern Louisiana provides strong evidence for the chemical buffering of pH, major cations, and dissolved silica in both contaminated and uncontaminated waters by metastable equilibrium with respect to the assemblage kaotinite-smectite-illite-calcite-dolomite, mineral phases known to be present from mineralogical analysis. Such buffering serves as a sink for acids released by the dehalogenation and reoxidation of carbon in organic wastes, as for example, carbon tetrachloride: CCl4 + 3H2OH2CO3 + 4HCl. Although such acids should eventually be consumed by mineral-fluid reactions, sediment porosity and permeability may be altered, as suggested by the enhanced authigenic mineral dissolution and precipitation observed in contaminated sediments at the site. By altering the permeability framework, the rates and pathways of ground water flow and contaminant transport could be modified in ways which complicate modeling waste migration rates and in planning remediation strategies. Contaminated ground waters at the site have lower values of pH and dissolved silica and elevated alkalinities relative to uncontaminated waters. The systematic use of these water quality parameters in waste site characterization studies at similar sites would provide clues as to whether mineral-water reactions and possible consequent changes in permeability have occured as a result of the presence of contaminants.
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