ABSTRACT

Lignite development will place major demands on ground-water supplies. The Simsboro Formation and the Calvert Bluff Formation (a major lignite-bearing unit) of the Wilcox Group between the Colorado and Trinity Rivers constituted a test case to evaluate the availability and quality of ground water. Aquifer geometry (sand) was determined by comparing environmental geologic maps with subsurface sand-percent and net-sand maps constructed from electric logs (Kaiser, 1978). The combined maps correlate well and show that the Calvert Bluff Formation consists of a complex interfingering of coarse channel sands and fine interchannel muds. Sand outcrop areas occupying approximately 10 square miles separate much larger interchannel areas with few and minor sands. The Simsboro Formation consists of two parts -- a thick multilateral sand (300 to 700 ft) in most of the southern outcrop belt and a series of channel sands (100 to 200 ft) interspersed with muds in the northern belt. Sands of the northern Simsboro belt are more like the Calvert Bluff channel sands than like the thick Simsboro sands.

Available hydrologic data suggest that Simsboro and Calvert Bluff sands have high hydraulic conductivity (6 to 20 m/day); interchannel muds have low hydraulic conductivity (1 to 2 m/day). Water compositions in the Simsboro and Calvert Bluff sands are similar and evolve similarly. Shallow ground water has a Ca-Mg-Cl-HCO₃ composition low in total dissolved solids (less than 500 mg/l). The water evolves over a depth range from 300 to 1,200 ft to become a Na-HCO₃ water. Change in composition probably results from ion exchange with clays (Ca⁺⁺ for 2Na⁺) and solution of calcite (which contributes more Ca ⁺⁺ for exchange and increases HCO₃ concentration). Poor quality water is largely restricted to shallow wells (less than 100 ft) in muddy parts of the Calvert Bluff Formation.