NOBLE GAS GEOCHEMISTRY OF THE FRESHWATER/SALINE WATER TRANSITION ZONE OF THE EDWARDS AQUIFER - IMPLICATIONS FOR GROUND WATER FLOW AND MIXING

The ability to define active ground-water flow regimes or the mixing of fresh and saline waters occurring along the transitional interface is critical in understanding the possible effects of saline water intrusion into the freshwater zone of the Edwards aquifer. Analysis of noble gas and geochemical data obtained from several monitoring well transects across the freshwater/saline water transition zone of the Edwards aquifer reveal a complex system of hydrologically active and inactive ground water regimes with mixing occurring between freshwaters and saline pore waters down-dip of the active freshwater flow zone. The composition of the saline pore waters results from the mixing of a sodium-chloride rich solution that contains a fractionated atmospheric noble gas composition with large amounts of excess helium (³He and ⁴He) and minor amounts of excess ⁴⁰Ar and nitrogen. Helium isotopic data reveal a distinctive end-member isotopic ratio of ~0.22 R/R_A for the saline pore waters. Helium data obtained from bulk rock samples of the carbonate aquifer and the intrusive igneous rocks indicate that the host rock contains typical crustal ratios of ~0.02 R/R_A. However, analysis of the olivine separates from the intrusive basanites indicates the olivine separates have helium isotopic ratios comparable to those observed in the saline waters. Because the amount of excess ³He cannot be accounted for solely by in-situ production in the aquifer or by contributions from host rocks of the system, the source of helium to the saline pore waters is likely to be an extrinsic source that is fluxing up along the Balcones Fault System from a deeper, more homogeneous crustal source. By combining the knowledge of the helium flux and its relation to the chloride concentrations, the active mixing and inactive water regimes can be delineated by the coupling or decoupling of the helium concentration to the chloride concentrations.