Hydrochemical facies in the badwater zone of the Edwards Aquifer, central Texas




Clement, Tonia Judith, 1944-

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The Cretaceous Edwards and Georgetown Formations and their stratigraphic equivalents form two extensive carbonate aquifers in central Texas. These faulted, cavernous, and highly transmissive formations are the sole source of water for many communities, including the City of San Antonio. At the badwater line, which delineates a hydrogeologic boundary from 1 to 21mi (2-34km) south and east of the outcrop, the total dissolved solids concentration of the groundwater changes abruptly from less than 500 mg/l to over 1000 mg/l. Although the boundary is relatively stable, some water from the badwater zone migrates updip toward major springs and well fields. Unlike the water in the freshwater zone which is predominantly Ca-HCO₃, the chemistry of the badwater zone changes along the strike of the aquifer. From west to east, the first hydrochemical facies is Ca-SO₄ with a low chloride concentration. Faulting is minimal, and chlorides previously trapped in the pores or adsorbed by the carbonates have been flushed. This facies is produced by the dissolution of anhydrite in the badwater zone. As the potentiometric surface declines and the intensity of faulting increases to the east, sodium and chloride concentrations also rise. The hydrochemical facies changes first to Ca-Mg-SO₄ with high sodium and chloride concentrations and then, in the area of maximum faulting, to Na-Cl with calcium, magnesium, and sulfate subordinate. Using PHREEQE computer modeling, the second facies can be simulated by mixing fresh water with a Na-Ca-Cl oilfield brine and saturating it to specific states with calcite, dolomite, and gypsum. The origin of the Na-Cl facies with Na/Cl molar ratios close to one is still problematical. The fourth hydrochemical facies borders the northern segment of the aquifer and is predominately a Na-Cl-SO₄-HCO₃ water, the anions being approximately coequal in milliequivalents per liter. This last facies is almost identical to groundwaters in the underlying Trinity Group which probably have provided some recharge to the aquifer. Leakage from the Trinity Group or updip migration of bad water also may affect the quality of fresh water in this area