Abstract: Hydrochemistry at the Falls City Uranium Mine Tailings Remedial Action Project, Karnes County, Texas
Timothy J. Jackson, Charles W. Kreitler
Acidic tailings and tailings solutions, created by sulfuric acid processing of uranium ores, were disposed on the outcrop of the Whitsett Formation (Eocene). These solutions have recharged the sandstones of the Whitsett since the 1960s. Previous work found a large, complex, and unexplained pattern of contamination. The present 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 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 due to 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 earlier estimated.
AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994