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The AAPG/Datapages Combined Publications Database
Lignite development will place major demands on groundwater supplies. The Simsboro Formation and the Calvert Bluff Formation (a major lignite host) of the Wilcox Group between the Colorado and Trinity Rivers constituted a test case to evaluate water availability and quality. Aquifer geometry (sand versus mud) was determined by comparing environmental geology maps and subsurface sand-percent and net-sand maps constructed from electric-logs. The combined maps correlate well and show that the Calvert Bluff consists of a complex interfingering of coarse channel sands and fine interchannel muds. Sand outcrop areas several tens of square kilometers separate much larger interchannel areas with few and minor sands. The Simsboro consists of two parts--a thick multilateral sand (300 to 700 ft o 90 to 212 m) in most of the southern outcrop belt and a series of channel sands (100 to 200 ft or 30 to 60 m) 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 m3/m2/day); interchannel muds have low hydraulic conductivity (1 to 2 m3/m2/day). Water compositions in the Simsboro and Calvert Bluff are similar and evolve similarly. Near-surface water has a Ca-Mg-HCO3 composition, low in total dissolved solids (<500 mg/l). The water evolves over a depth range from 300 to 1,200 ft (91 to 364 m) to a Na-HCO3 water (~1,000 mg/l). 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). Corr lation of composition with amount or percent sand has not been demonstrated. Poor-quality water is largely restricted to shallow wells (<100 ft or 30 m) in muddy parts of the Calvert Bluff.
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