REGIONAL CONTROLS ON THE GEOCHEMICAL EVOLUTION OF SALINE GROUNDWATERSIN THE EDWARDS AQUIFER, CENTRAL TEXAS

Geochemical and isotopic parameters are used to evaluate models for th e evolution of saline groundwaters in the Edwards aquifer, which lies on the northwestern margin of the Gulf of Mexico sedimentary basin. Sa line groundwaters, termed 'badwaters', rang in salinity from 1000 to 1 2 500 mg l(-1) total dissolved solids. Models for badwater evolution m ust account for complexities owing to: (1) the range in compositions o f saline basinal fluids that may migrate into the Edwards aquifer from its down-dip section and from underlying hydrostratigraphic units; (2 ) the range of depositional, diagenetic, and mineralogic variations in the host aquifer rocks; (3) volcanic and intrusive igneous rocks that locally crosscut the aquifer; (4) extensive faulting, which in places juxtaposes Edwards aquifer units with adjacent hydrostratigraphic uni ts. Calcium, Mg, Na, SO4, Cl, and HCO3 variations define six badwater hydrochemical facies. Strontium isotope values of freshwaters (Sr-87/S r-86 = 0.7077-0.7086), badwaters (0.7076-0.7094), and brines from aqui fer rocks down-dip (0.7078-0.7097) are nearly all higher than the rang e of most Edwards aquifer rocks (0.7074-0.7077) and volcanic and intru sive igneous rocks that crosscut the aquifer (0.7032-0.7055). Argillac eous carbonate rocks that underlie the Edwards aquifer are likely sour ces of Sr with high Sr-87/Sr-86 values. Transitions between the hydroc hemical facies in some locations correspond to changes in depositional environments in aquifer rocks and to changes in the intensity of faul ting through the aquifer. Major element, mineral saturation state, and Sr/Ca-Sr-87/Sr-86 variations in badwaters of the westernmost aquifer region indicate that incongruent dissolution of gypsum and recrystalli zation of calcite control the geochemical evolution of these badwaters . Sr/Ca-Na-Cl-Sr-87/Sr-86 variations in badwaters of extensively fault ed areas to the east of the westernmost aquifer region are consistent with fluid mixing processes involving at least five endmember fluids, including (1) saline groundwaters from two underlying hydrostratigraph ic units, (2) two endmember brines from down-dip Edwards aquifer equiv alent units, and (3) freshwaters that have interacted extensively with aquifer rocks. The compositions of badwaters from the north-easternmo st region reflect fluid mixing between freshwaters and saline groundwa ters from underlying hydrostratigraphic units.