Abstract

The Lipan Aquifer, centered on Tom Green County in west central Texas, is 1 of the 21 minor aquifers in Texas. The study area is 3,850 square miles in size and encompasses the majority of Tom Green County and parts of Coke, Concho, Glasscock, Irion, Runnels, Schleicher, and Sterling counties. It lies entirely within the Region F Regional Water Planning Area and Groundwater Management Area 7, and contains parts of six groundwater conservation districts.

The Lipan Aquifer is composed of a diverse set of geological units capable of producing brackish groundwater with total dissolved solids between 1,000 and 9,999 milligrams per liter, defined by Winslow and Kister (1956) as slightly saline to moderately saline groundwater. We found that a thin cover of less than 200 feet of Quaternary and Neogene sediments and up to 400 feet of Cretaceous limestone overlie a series of westerly dipping geological formations composed of Permian limestone, dolomite, shale and siltstone. Groundwater can be found in the weathered portions of most of the Permian formations and throughout younger geological units where wells have been drilled below the static water level.

The Lipan Aquifer has approximately 6.22 million acre-feet of groundwater that can be quantified within three salinity zones: (1) 0.17 million acre-feet of fresh groundwater (0 to 999 milligrams per liter of total dissolved solids), (2) 4.44 million acre-feet of slightly saline groundwater (1,000 to 2,999 milligrams per liter of total dissolved solids), and (3) 1.61 million acre-feet of moderately saline ground Water (3,000 to 9,999 milligrams per liter of total dissolved solids). With the available data, we could not accurately define the bottom depth of very saline groundwater zone (10,000 to 34,999 milligrams per liter of total dissolved solids), but it is expected to be between 500 and 900 feet below ground surface.

The fresh water zone is restricted to the Quaternary-Neogene and Cretaceous formations. The slightly and moderately saline zones transect all Permian formations. A significant portion of any brackish groundwater in the Lipan Aquifer study area is within weathered Permian formations overlain by Cretaceous formations. Although there are only a few instances where water wells have been completed into Permian formations in these areas, the development of brackish groundwater in these areas may represent an untapped resource for entities interested in brackish groundwater desalination.

We found that despite the large numbers of wells drilled for water and hydrocarbons over the last 70 years within the study area there are very few records on the water quality and aquifer hydraulic properties of the brackish groundwater intervals. Future efforts to obtain this information from wells drilled through the brackish groundwater formations would be extremely useful in more accurately quantifying this resource.

Study deliverables include a peer-reviewed report, Geographic Information System map files, BRACS Database and data dictionary, and water well and geophysical well log files. All data used for the study is readily available to the public and downloadable from the Texas Water Development Board website.