GEOLOGIC CONTROLS ON GROUNDWATER MOVEMENT IN THE EDWARDS AQUIFER OF SOUTH-CENTRAL TEXAS

The Edwards Aquifer provides water to over 1.7 million people in south-central Texas and is used for municipal, industrial, agricultural, and recreational purposes. The Edwards Aquifer also provides habitat for seven endangered species, including the Texas Blind Salamander, the first species listed under the Endangered Species Act. Understanding groundwater flow paths are critical in managing this resource.

The Edwards Aquifer consists of karstified limestone with a wide range of permeability including matrix, fractures, and conduits. The Edwards Aquifer Authority has used multiple techniques to investigate the flow characteristics of the Edwards Aquifer. The current conceptual model of the aquifer is based on data collected from tracer tests, continuous water level and conductivity dataloggers, downhole and surface geophysical investigations, water quality analyses, and modeling. The tracer tests along with other data have provided insight into geologic controls on groundwater movement in the aquifer.

Faulting along the Balcones fault zone has dropped the Edwards Limestone in places, to depths of more than 1,000 meters below the surface. Much of the permeability is derived from hypogene speleogenetic processes. This creates a three-dimensional, heterogeneous flow system. Groundwater flow velocities have been measured at more than three kilometers per day in highly-transmissive conduits, and sub-meter distances per day in other settings. In the vicinity of the Comal Springs complex, which discharges an average of 8.2 cubic meters per second, the conduit system consists of discrete flow paths separated by only a few meters. Faulting plays many rolls in the aquifer depending on local conditions and may act as a barrier or pathway for groundwater flow.