UNUSUAL δ18O VALUES FROM THE EDWARDS AND EDWARDS-TRINITY AQUIFERS IN CENTRAL TEXAS: INTERPRETATIONS AND POTENTIAL FRACTIONATION MECHANISMS

The isotopic composition of karstic groundwater can provide information about processes affecting water prior to recharge, as well as within an aquifer system. Groundwater values typically fall near local meteoric water lines or along deviations that can be attributed to seasonal or climatic factors such as source of precipitation, evaporation, and rain-out. These processes affect both δD and δ¹⁸O values of water. However, a number of geochemical and biological processes may fractionate either oxygen or hydrogen isotopes without affecting the other isotope. Although this is well documented in hydrothermal waters, it is poorly studied in ­non-thermal groundwater systems.

Here, we present preliminary data from an ongoing isotopic characterization of surface and groundwaters in the karstic Edwards and Edwards-Trinity Aquifers in central Texas. Between February 2009 and January 2010, nearly 500 water samples were collected from diverse locations including springs (at least five unique flowpaths), three streams, dripwater from two caves, a residential well, and an artesian well. The isotopic composition of many of these samples falls along a nearly horizontal trend line showing significant variability in δ¹⁸O values with no concomitant change in δD values. This represents a significant deviation from the local meteoric water line (p<0.05, df = 119) that is not reflected in most cave drip or surface waters (excluding the local groundwater dominated streams). This pattern suggests that processes occurring at ambient groundwater temperatures within the phreatic zone may be responsible for observed groundwater isotopic compositions. Potential fractionation mechanisms and processes that may be responsible for the observed patterns can be hydrological (variable groundwater flowpaths), geochemical (dissolution/ precipitation or rock-water oxygen exchange), and biological (respiration, nitrogen cycling, sulfate reduction) in nature.

Although samples were collected on a weekly to monthly basis over the 11 month period, no temporal trend in isotopic composition is discernable. Currently, twice-daily samples are being collected from an artesian well to elucidate any high frequency trends in groundwater isotopic composition that may have been missed with the longer sample intervals.