OXYGEN AND HYDROGEN ISOTOPIC EVOLUTION OF GROUNDWATER IN THE EDWARDS AND TRINITY AQUIFER SYSTEM, SOUTH-CENTRAL TEXAS

The conservative nature of stable isotopes of oxygen and hydrogen (e.g., ¹⁸O and ²H) in water is utilized as tracers to understand the geochemical evolution of highly karstified carbonate-hosted Edwards and Trinity aquifers, South-Central Texas. Our modeling is based on isotope data reported by USGS publication 2004-5201, and explains the variance in the data set due to possible evaporation effects and mixing of various sources. Their data includes oxygen and hydrogen isotopic compositions of groundwater in the Edwards unconfined, Edwards confined, the adjacent Trinity aquifers, and the Medina lake water. The Local Meteoric Water Line (LMWL) exhibits a linear trend (d²H=8.51d¹⁸O+15.74) in the d¹⁸O- d²H space (d is the relative deviation of the heavy to light isotopic ratio in the sample compared with the standard in per mil (‰)). Our calculations assume Rayleigh fractionation effects during evaporation (both kinetic and equilibrium effects). The Edwards unconfined water compositions plot away from the LMWL, and mostly lie on a linear trend (slope=7.2, relative humidity=0.8), and intersects the LMWL at d¹⁸O =-5.8 ‰, d²H =-33.3 ‰, which is the starting meteoric composition for the unconfined zone. An average of 11% evaporation explains deviation of the data from LMWL. The Trinity water compositions yield similar slope but with slightly lighter meteoric composition, which is likely due to precipitation at higher elevation.  The Edwards confined water plot on the high-end of both unconfined and Trinity compositions, suggesting derivation from the (latter) already evolved water. Medina water trend translates to relative humidity ~0.6, and evaporation of 29%. Simple binary mixing (average of unconfined and/or Trinity aquifer, and Medina end-members) accounts for water samples plotting along the mixing trend, and otherwise cannot be explained by an evaporation model alone. The relative contribution of Medina lake in this case lies in the range 5%-70%. Alternately, this mixing may be explained by a different end-member other than Medina lake having heavy isotopic signatures.  A likely source for such end-member may be normal summer rainfall with documented d¹⁸O value of -2.9 ‰. Finally, few unconfined samples show a horizontal trend, which suggests interaction between the groundwater and the surrounding carbonates.