Abstract: Mineral Buffering of Contaminated Ground Water Compositions at a Hazardous Waste Site in Southwestern Louisiana

Jeffrey S. Hanor, Kathleen M. McManus, Vishnu Ranganathan, Shaobing Su

A study has been made of natural geologic controls on preremediation water chemistry at a hazardous waste site where ground waters contained a variety of organic contaminants, including from <1µg/L to 3.5 g/L 1,2-dichloroethane.

Quartz, plagioclase feldspar, and K-feldspar are the most abundant non-clay minerals in the sand, slit, and clay beds at the site. Calcite and dolomite occur in smaller amounts. Smectite is generally the most abundant mineral in the <2µm fraction followed by illite and kaolinite. Authigenic illite/smectite having a webby appearance and forming intergranular pore fill and bridges was identified by SEM in several samples. The divalent cations Mg²⁺ and Ca²⁺ are preferentially adsorbed on the clays over the monovalent cations Na⁺ and K⁺. This exchange behavior has important implications for the fate of divalent contaminant cations such as Ba²⁺ and Ra²⁺. Authigenic minerals from contaminated sediments include gypsum, quartz, aragonite, and barite. The development of secondary porosity resulting from partial to complete dissolution of detrital silicate grains, most probably feldspars, exists in samples from the contaminated zone.

Thermodynamic evaluation of the ground water compositions provides strong evidence for the chemical buffering of pH, major cations, and dissolved silica in both contaminated and uncontaminated ground waters by metastable equilibrium with respect to the assemblage kaolinite-smectite-illite-calcite-dolomite, mineral phases known to be present at the site from our mineralogical analysis. Such buffering serves as a sink for hydrogen ion released by the dehalogenation and reoxidation of carbon in organic wastes, as for example, carbon tetrachloride by the reaction: CCl₄ + 3H₂O = HCO_3^- + 4Cl^- + 5H⁺, and could effect rates of degradation of these compounds.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana