Located in the Lipan Flats area of the counties of Tom Green. western Concho, and southern Runnels, the Lipan is a minor aquifer according to classifications of the Texas Water Development Board (TWDB) the Lipan aquifer is used primarily for irrigation with limited consumption for livestock and rural domestic purposes (Lee 1986). However, the chemical quality of the Lipan aquifer water does not meet drinking water standards (Lee, 1986). Drinking water may contain radon at rates just above safety levels provided by the Environmental Protection Agency (EPA). Upwards of 125 feet of saturated alluvial deposits of the Leona Formation of Quaternary age make up the aquifer (Lee 1986). Part of the aquifer constitution (see Figure 1.) is comprised of the "updip portions of the underlying dolomites, limestones, and shale of Permian age" (Lee 1986). These structures, which are hydrologically continuous with the alluvial deposits of the Leona aquifer, contain slightly saline water. The ground water in the Leona Formation proper is very hard, ranging from fresh to slightly saline (Lee 1986). Natural discharge of the Lipan aquifer occurs by seepage into the Concho River and by evaptranspiration wherever the land surface is close to the water table. The yield from wells is generally 100 gallons per minute to in excess of 1,000 gallons per minute.
Groundwater usage in the aquifer has been heaviest in the Lipan Flats agricultural areas that lie in eastern Tom Green and western Concho counties. From the 1950s to the 1980s, well records show a moderate increase. Irrigation in the area began in the 1940s, and pivot irrigation systems were introduced in the late 1980s, increasing irrigation pumping to over 70,000 acre-feet (AFY) from 15,000. The number of well increased dramatically, expanding from about 200 wells in 1990 to an excess of 1,000 wells in 2000. Heavy irrigation pumping coupled with periods of drought brought about a significant decrease of water levels in some areas. The net result was that irrigation pumps in some areas could be operated throughout the season while, in other areas, irrigation pumping was substantively reduced.
Locating Drinking Water Wells in Lipan Aquifer
The well yields in the Lipan aquifer vary substantially across even short distances, with yields at less than 10 gpm to over 500 gpm (Collier 2011). Drillers report that the highest production is extracted from relatively small geographic zones (Collier 2011). Published capacity test results indicate less than 5 gpm/ft to over 100 gpm/ft (Collier 2011). These measures indicate that hydrolic conductivity of the aquifer is characterized by large variations (Collier 2011). Two areas of the Lipan aquifer demonstrate the highest productivity (Collier 2011). These areas are oriented parallel to the strike of the geologic units, suggesting that the underlying geology is a factor in the permeability of the aquifer units (Collier 2011).
Comparisons of potentiometric surface maps showing water-levels collected in 1950 and 2000 indicate that an overall decrease in the water levels at the center of the Lipan Flats area (Collier 2011). Interestingly, water-levels in the proximal Edwards-Trinity wells and in the edges of the Lipan aquifer did not change significantly during this period (see Figure 2). Theoretically, low groundwater levels near the Concho River may permit surface water to recharge the aquifer where it is near the stream. Further, hydrographs for wells outside the Lipan Flats show smaller water-level declines than do wells in the middle of the Lipan Flats area. The population in San Angelo grew from 52,093 in 1950 to 93,804 in 2010. The perfect storm of the 1990s -- increased irrigation pumping coupled with drought and increased population -- caused water levels in many wells to drop 30, 60, and even 100 feet (Collier 2011).
Years before the impoundment of reservoirs west of San Angelo and before mechanized irrigation -- between 1918 to 1925 -- two gain-loss studies were conducted along the Concho River between San Angelo and Paint Rock (Collier 2011). The studies show a net gain of 5.4 cubic feet/second (cfs) (3,912 acre-feet per year or AFY) and 5.3 cfs (3,767 AFY), respectively (Collier 2011). Spring Creek and Dove Creek, which emanate from the junction of the Edwards-Trinity and the Lipan aquifers, provide the Concho River…