RAPID CONVEYANCE OF WATER THROUGH A KARST CARBONATE AQUIFER IN A SEMI‑ARID ENVIRONMENT

The Devils River watershed in south-central Texas has been shown to be a complex groundwater and surface-water flow system in the karst limestone Edwards-Trinity Aquifer with the capacity to convey significant water (annual average of 263,000 acre-ft/yr) from its semi-arid headwaters (i.e., less than 20 in/yr average annual precipitation) to its entry into Amistad Reservoir. The Edwards-Trinity Aquifer within the Devils River watershed varies from a relatively thin water-table aquifer in the headwaters to a relatively deep semi-confined aquifer where it discharges to Amistad Reservoir. Water is conveyed in the watershed, first as groundwater in the headwaters and later as surface water in downstream reaches. The flow system has been characterized using hydraulic data based on well capacity, aquifer response using groundwater elevation monitoring, tracer test results, geophysical imaging, and numerical modeling. The aquifer is conceptualized as consisting of a network of preferential karstic conduits in bedrock beneath and coincident with major surface-water channels. This conceptual model is supported by numerical modeling that indicates that aquifer response is best replicated using high-conductivity zones aligned with the major river channels to allow for rapid transport of water through the watershed while maintaining a relatively large hydraulic gradient. The resulting numerical model provides an improved tool to evaluate the impact of periods of drought and possible groundwater extraction on the quantity of water that discharges to Amistad Reservoir.