Identifying Hydraulic Interactions and Flow Systems Using Geochemical and Isotopic Indicators, South Central Texas Aquifers

Most of the drinking water supplies in south-central Texas are derived from the Edwards-Trinity (Plateau), Trinity, and Edwards (Balcones Fault Zone) aquifers. These aquifers mainly consist of carbonate reef and non-marine sands. These aquifers have been extensively used for water supplies resulting in a significant decline to the water levels. Frequent droughts, along with rapid urbanization, further complicate sustainable management of these aquifers.

 We used chemical maturity parameters, such as Sr/Ca and Mg/Ca ratios, in combination with the unconfined/confined aquifer characteristics to determine degrees of hydraulic interactions. We observed that high Sr/Ca and Mg/Ca ratios occur in areas where the aquifers are confined and low Sr/Ca and Mg/Ca ratios occur where the aquifers are semi-confined or outcrop. Furthermore, most of the groundwater shows a progressive increase in Sr/Ca and Mg/Ca ratios with depth. Spatial and vertical distributions of these ratios indicate their usefulness in determining groundwater residence time.

 Groundwater with enriched oxygen isotopes occurs in areas where the aquifers outcrop or are affected by faulting, suggesting that these waters were evaporated prior to recharge. High tritium values, along with high percent modern carbon, and their proximity to the outcrop suggest water originating from modern recharge. However, groundwater to the west, under confining conditions, contains depleted oxygen-18, tritium, and percent modern carbon, suggesting older water. Groundwater in these areas is slow-moving, either due to lower hydraulic conductivity of the aquifer materials or fault barrier, even though a hydraulic gradient exists for flow. Geochemical and isotopic data suggests that the eastern and the southern parts act as “water fairways” where the aquifers are replenished with modern recharge through the outcrop and fault zones. This investigation illustrates that combined use of geochemical and isotopic information can provide greater understanding of a groundwater flow system than conventional use of hydrogeological data alone.