Government Canyon Geologic and Hydrologic Assessment
[From the Introduction]
On March 1, 1993, the Board of Directors of the Edwards Underqround Water District requested that a geologic and hydrologic assessment of the Government Canyon property be performed by District Field Operations staff. This study was requested pursuant to the Board’s approval to participate in the purchase of the subject property. The following report is the result of that study.
A literature search and field study were performed to provide a brief overview of the geologic and hydrologic conditions in the Government Canyon area. This area consists of approximately 5152 acres located 3 miles west of Helotes, Texas in northwest Bexar County.
Available literature includes publications from the Texas Water development Board, the Texas Department of Water Resources, and the USGS, as well as an unpublished MS thesis from the University of Texas at Austin. In addition, unpublished data was obtained from HDR Engineering and from various individuals.
Field study methods included reconnaissance to quality check surface geology previously mapped by the USGS, locating wells on the property accessible and suitable for geophysical logging, water quality sampling, and measuring water levels in several wells in the study area.
The surface and subsurface geologic investigation of the Government Canyon property indicates that recharge to the Glen Rose Aquifer and the Edwards Aquifer occurs in the area. A portion of this recharge moves southwest along the Haby Crossing Fault complex into northeast Medina County. An unknown portion, as seen in water level and water quality data, may flow from the Glen Rose Aquifer across the fault into the Edwards Aquifer.
It is estimated the average annual potential recharge enhancement with a structure on Government creek at the southern Edwards recharge zone boundary would be 600 acre-feet per year. The District has operated the San Geronimo Creek recharge enhancement structure for 13 years in a drainage basin adjacent to the Government Canyon property with similar geology. Average annual recharge at this structure is estimated to be 775 acre-feet, which is similar to the amount of recharge calculated by HDR for Government Creek. Recharge during years of above average rainfall at the San Geronimo dam site contributed approximately 1400, 1100, 1200, 1600 and 2900 acre-feet during 1981, 1985, 1987, 1991 and 1992 respectively.
Since recharge to both the Glen Rose and Edwards aquifers may occur on the property it is imperative that the quality of the surface runoff entering the aquifers be maintained. Additional studies should include aquifer tests and possible tracer studies to determine the flow direction and the presence of communication between the two aquifers. Additional data from remote abandoned wells on the property should be obtained to further define the hydroloqic characteristics of the area.