SURFACE WATER RECHARGE IN KARST: EDWARDS AQUIFER - NUECES RIVER SYSTEM

The karstic Edwards Aquifer is a primary source of water in south-central Texas for domestic, agriculture, and industrial uses. Significant recharge into the aquifer occurs through surface water streams, including the Nueces River. Recharge models use data from two stream gauges to compute recharge into the aquifer from the Nueces River, but this may underestimate volume because the actual water balance in the river valley is complex. Synoptic gain/loss studies show that: flow rates change significantly as the river passes through extensive unconsolidated alluvium, gain/loss in reaches varies temporally, and recharge may not be restricted to the defined recharge zone. From these synoptic studies, a 10-km reach near Montell, TX, was identified that loses 100% of flow during low base flow conditions over the contributing zone, upstream of the recharge zone. Below this point, Candelaria Creek runs parallel to the dry segment of the Nueces River; it contributes 52-64% of flow measured at the USGS recharge index gauge, located downstream of the creek. The main flow sources to the creek are two springs, hypothesized as being sourced from: underflow from the Nueces River, a combination of Trinity Aquifer groundwater and river underflow, or solely groundwater from the Trinity Aquifer. Synoptic conductivity analyses and continuous conductivity records (n=3) collected throughout Candelaria Creek under varying flow conditions that the Nueces River (369-385µS/cm) is fresher than the springs (420-448µS/cm). Stream flow measurements show that Candelaria Creek is gaining parallel to (8.9 cfs) and losing (5.6 cfs) as it joins the river. Farther downstream, the Nueces River is gaining after another significant spring contributes 6.4 cfs of flow. The results indicate that the springs are likely sourced from both Nueces River underflow and Trinity Aquifer groundwater. Defining the source of the springs on Candelaria Creek is important to understand the complex water balance in the Nueces River. With current estimates only utilizing the recharge zone as a source of recharge to the Edwards Aquifer, estimates of recharge may be significantly lower than actual recharge. This study indicates that a some recharge may occur over the contributing zone of the Edwards Aquifer, including contributions from major springs along the Nueces River.