Surface water recharge in karst : Edwards-Trinity Aquifers-Nueces River system

Abstract

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 as surface water streams, including the Nueces River, cross the Recharge Zone (RZ). Recharge models use data from two stream gauges, located above and below the RZ. These gauges are used to compute recharge into the aquifer; this may underestimate recharge volume because the actual water balance 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 be occurring in the Contributing Zone (CZ). From these synoptic studies, a 10-km reach of the Nueces River near Montell, TX, was identified that loses 100% of flow over the CZ during low stream flows. In this study reach, Candelaria Creek runs parallel to the dry segment of the Nueces River for 2.5 km; the creek contributes 52-64% of flow measured at the USGS recharge index gauge. The main sources of flow to the creek are two springs, hypothesized as possibly being sourced from: underflow from the Nueces River, a combination of Trinity Aquifer groundwater and river underflow, or solely groundwater from the Trinity Aquifer. To investigate recharge in the CZ and the source water for springs that contribute flow to Candelaria Creek, a variety of methods were used including: hydrograph and gain/loss analyses, potential evapotranspiration calculations, and interpretation of specific conductance, temperature, chemical, isotopic, and near surface geophysical data. The data suggest that the springs are likely sourced from both Nueces River underflow and Trinity Aquifer groundwater. Defining the source of the springs that contribute to Candelaria Creek is important to understand the complex water balance in the Nueces River and the role of underflow/storage in this system. It was found that underflow was a significant source of spring flow, but could not account for the total amount of spring flow; this suggests the Trinity Aquifer also contributes flow to the springs. A water balance estimates that recharge in the CZ at 6,213,048-9,814,814 m3 per year, which is between 0.9 to 2% of total recharge to the Edwards Aquifer and 4 to 11% of Nueces Basin recharge may be unaccounted for over the CZ during low hydrologic flow conditions. This water balance suggests that there is significant recharge occurring over the CZ and some recharge may be unaccounted for based on the current method used to calculate recharge.