Regional controls on the geochemical evolution of saline groundwaters in the Edwards aquifer, central Texas

Abstract

Geochemical and isotopic parameters are used to evaluate models for the evolution of saline groundwaters in the Edwards aquifer, which lies on the northwestern margin of the Gulf of Mexico sedimentary basin. Saline groundwaters, termed ‘badwaters’, range in salinity from 1000 to 12 500 mg l −1 total dissolved solids. Models for badwater evolution must account for complexities owing to: (1) the range in compositions of 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 hist 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 units. Calcium, Mg, Na, SO 4, Cl, and HCO 3 variations define six badwater hydrochemical facies. Strontium isotope values of freshwaters ( 87Sr 86Sr = 0.7077 – 0.7086 ), badwaters (0.7076 – 0.7094), and brines from aquifer rocks down-dip (0.7078 – 0.7097) are nearly all higher than the range of most Edwards aquifer rocks (0.7074 – 0.7077) and volcanic and intrusive igneous rocks that crosscut the aquifer (0.7032 – 0.7055). Argillaceous carbonate rocks that underlie the Edwards aquifer are likely sources of Sr with high 87Sr 86Sr values. Transitions between the hydrochemical facies in some locations correspond to changes in depositional environments in aquifer rocks and to changes in the intensity of faulting through the aquifer. Major element, mineral saturation state, and Sr Ca  87Sr 86Sr variations in badwaters of the westernmost aquifer region indicate that incongruent dissolution of gypsum and recrystallization of calcite control the geochemical evolution of these badwaters. Sr Ca NaCl 87Sr 86Sr variations in badwaters of extensively faulted 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 hydrostratigraphic units, (2) two endmember brines from down-dip Edwadrs aquifer equivalent units, and (3) freashwaters that have interacted extensively with aquifer rocks. The compositions of badwaters from the northeasternmost region reflect fluid mixing between freshwaters and saline groundwaters from underlying hydrostratigraphic units.