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Abstract: Hydrochemistry of the Falls City Uranium Mine Tailings Remedial Action Project, Karnes County, Texas
Timothy J. Jackson (1), Charles W. Kreitler (2)
Acidic tailings and tailings solutions, created by sulfuric acid processing of uranium ores, were disposed of on the outcrop of the Whitsett Formation (Eocene). These solutions have recharged the sandstones of the Whitsett since the 1960's. Previous workers found a large, complex, and unexplained pattern of contamination. Our study determined the extent and nature of contamination by (1) characterizing the geology and hydrology of the two shallow aquifers at the site, (2) determining the chemistry of the contaminant source (tailings solutions), and (3) identifying geochemical reactions that have altered the composition of contaminant plumes within each aquifer.
The tailings solutions are composed of sodium chloride and neutral sulfate salts of aluminum and ammonium, with lesser amounts of iron, calcium, magnesium, potassium, and sodium sulfate. Hydrolysis of aluminum sulfate produces an acid pH (3 to 4). Also, aluminum sulfate is a pH buffer, and it controls acidity of the tailings solutions. Cation exchange and neutralization by calcite modify the tailings solutions as they migrate through the aquifers. These reactions explain chemical patterns, which delineate five separate contaminant plumes in the aquifers. In the Deweesville sandstone, cation exchange has removed ammonium from acidic contaminant plumes. However, neutralization is incomplete because of the paucity of calcite in the Deweesville. In contrast, in the calcite-rich Conquista fossiliferous sandstone, cation exchange and complete neutralization by calcite have removed most contaminant ions. Those contaminant plumes are delineated by elevated concentrations of calcium and carbon dioxide. The amount of contamination in both aquifers is much smaller than that estimated previously.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78713
(1) Current address: Geraghty & Miller, Inc., 6400 Uptown Blvd. NE, Albuquerque, NM 87110
(2) Current address: LBG-Guyton Associates, 1101 S. Capital of Texas Highway, Austin, TX 78746
Copyright © 1999 by The Gulf Coast Association of Geological Societies