Simulation of flow in the Edwards Aquifer, San Antonio region, Texas, and refinement of storage and flow concepts

Abstract

The Edwards aquifer is a complexly faulted, carbonate aquifer lying within the Balcones fault zone of south-central Texas. The aquifer is recharged mainly by streamflow losses in the outcrop area of the Edwards aquifer and is discharged by major springs located at considerable distances, as much as 150 mi, from the areas of recharge, and by wells. Groundwater flow within the Edwards aquifer of the San Antonio region was simulated to investigate concepts relating to the storage and flow characteristics. A general purpose, finite difference model, modified to provide the capability of representing barrier faults, was used to simulate groundwater flow and storage in the aquifer. The simulations investigated the effects of complex geologic structures and significant changes in transmissivity, anisotropy, and storage coefficient, with initial values based on concepts developed in previous studies. Results of the simulations confirmed the original estimates of transmissivity values (> 100 sq ft/sec) in the confined zone of the aquifer between San Antonio and Comal Springs. A storage coefficient of 0.05 in the unconfined zone of the aquifer produced the best simulation of water levels and springflow. A major interpretation resulting from the simulations is that two essentially independent areas of regional flow were identified in the west and central part of the study area. Flow from the two areas converges at Comal Springs. The directions of computed flux vectors reflected the presence of major barrier faults which locally deflect patterns of groundwater movement. The most noticeable deflection is the convergence of flow through the geologic structural opening, the Knippa gap, in eastern Uvalde County. A second significant interpretation is that groundwater flow in northeastern Bexar, Comal, and Hays Counties is diverted by barrier faults toward San Marcos Springs, a regional discharge point. (Lantz-PTT)