A 2-D, Dual-Density, Numerical Model of Carbonate Subsurface Fluid Flow through the Edward’s Aquifer, New Braunfels, Texas: Mechanisms That Inhibit Flow across the Freshwater/Saline-Water Interface

The Edward’s Aquifer, composed of faulted carbonate bedrock, contains freshwater and saline water. In aquifers that are used as a source of drinking water and that contain fresh and saline waters, saline water intrusion can result in degradation of water quality. Yet, in the New Braunfels section of the Edward’s Aquifer, limited saline water intrusion has occurred. The focus of this research is to determine why there is less saline water intrusion than expected and if there is a trigger that will result in more extensive saline water intrusion across the freshwater/saline-water interface. Three hypotheses are being tested: (1) a fault that could be acting as a barrier between the freshwater and saline water zones, preventing movement of the saline water into the freshwater zone, (2) an extremely low permeability of the bedrock in the saline water zone, limiting movement of the saline water, and (3) an extremely high saline water density near the interface that prevents mixing. A 2-D, dual-density, numerical model of groundwater flow is currently being produced for each proposed hypotheses using Basin2 and data from five wells along a transect that crosses the freshwater/saline-water interface. Numerical modeling will be used to estimate the offset along a hypothetical fault, the position of the hypothetical fault, and the permeability of the hypothetical fault face that is needed to block saline water flow across a fault. The estimated parameters (offset, position, and permeability) will be compared to the offset and location of mapped faults and the permeability of fault faces as determined from geophysical well logs. Similarly, numerical modeling of the permeability structure of the fresh and saline water zones will be used to estimate the permeability of the saline water zone needed to prevent saline water flow across the interface. The estimated permeability will be compared to the measured permeability, obtained from aquifer pump test data. Finally, numerical modeling will be used to estimate the density of saline groundwater that limits movement of the saline water into the freshwater zone. The estimated density will be compared to the density of the saline water, obtained from geochemical samples of each well. To date, the third hypothesis has been disregarded as results indicate that the density of saline water that is needed to prevent flow across the interface is much higher than any of the measured saline water densities near the interface.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California