CONVEYANCE OF GROUNDWATER/SURFACE WATER THROUGH KARST LIMESTONE

The Devils River watershed conveys significant water (263,000 acre-ft in an average year) as surface flow where the Devils River discharges to Lake Amistad, equivalent to about ten percent of flow in the Lower Rio Grande. A complex system of groundwater and surface water flow has evolved in the karst limestone Edwards-Trinity Aquifer to convey this water from its semi-arid headwaters to its entry into Lake Amistad. Assessment of well capacity versus well location indicates a strong correlation between well capacity and proximity to river channels, indicating the development of preferential flow through limestone coincident with river channels. The development of preferential flow pathways by dissolution enhancement of fractures and high-permeability zones is a positive feedback system where preferential flow begets increased preferential flow through a carbonate aquifer, resulting in development of solution features coincident with river channels which are locations of localized and focused recharge. Characterization of the Edwards-Trinity Aquifer that does not account for this focused water conveyance system can result in inaccurate, if not misleading, assessment of the water resources of the region.