OETTING, GREGG C., JAY L. BANNER, and JOHN M. SHARP, Jr., Department of Geological Sciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Geochemical and Isotopic Variations in Groundwaters of the Edwards Aquifer of Texas: Constraints on Diagenetic Fluid Compositions in Geochemical Models of Water-Rock Interaction

Quantitative studies of diagenesis in ancient carbonate platforms using isotopic and trace element constraints to determine extents of fluid-rock interaction require knowledge of trace element and isotopic compositions in both carbonate rocks and aquifer fluids. Results of geochemical and Sr isotopic analyses in the Edwards aquifer may provide constraints on diagenetic fluid compositions in models of carbonate diagenesis.

{87}Sr/{86}Sr values of eight Edwards aquifer groundwaters and two groundwaters from underlying hydrostratigraphic units range from 0.7087 to 0.7097. Host marine carbonates have {87}Sr/{86}Sr values of 0.7074 to 0.7076, similar to estimates of Fredericksburg age sea water. Groundwater Sr concentrations and Sr/Ca ratios range from 0.16 to 7.55 ppm and from 0.0008 to 0.25, respectively.

Results of mass balance modeling indicate that {87}Sr/{86}Sr and Sr/Ca variations in Edwards aquifer groundwaters are apparently controlled by fluid mixing processes with subordinate but significant effects from reaction with carbonate and evaporite aquifer minerals. Hydraulic head gradients and faulting in the Edwards aquifer and underlying units may provide pathways in which geochemically distinct groundwaters from underlying hydrostratigraphic units mix with groundwaters in the Edwards aquifer. Geochemical models of ion speciation and mineral saturation states indicate that the groundwaters are saturated to supersaturated with respect to both calcite and dolomite. These groundwaters would precipitate calcite cements with {87}Sr/{86}Sr values of 0.7087 to 0.7097, and Sr concentratio s of 30 to 9800 ppm. These calculations suggest that the geochemical variability of coexisting groundwaters may play a significant role in the origin of trace element and isotopic compositional variations in diagenetic carbonate phases.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.