Hydrogeology of the Lobo and Ryan Flats area, Trans-Pecos Texas

Lobo Flat and Ryan Flat are the southern extent of the Salt Basin of Trans-Pecos Texas. The Salt Basin is a fault-bounded, northwest trending alluvial basin resulting from Basin-and-Range extension approximately 24 m.y.a. The study area covers approximately 700 square miles (1813 km²), is about 75 miles (120 km) long and ranges from 5 to 25 miles (8 to 40 km) wide. An unconfined aquifer occurs in the Cenozoic alluvial fill. The basin fill is absent at the margins of the basin, and thickens to over 1000 feet (305 m) near the center of the basin. It consists of sediments derived from the surrounding Wylie and Davis Mountains to the east, the Van Horn and Sierra Vieja Mountains to the west and the Oak Hills to the south. The alluvium consists of poorly-sorted coarse gravels and sands adjacent to the mountains, and grades into and is interlayered with silts and clays of the basin interior. The presence of interlayered finer-grained deposits and clay lenses results in localized semiconfined and perched water-table conditions. The groundwater is generally low in total dissolved solids, ranging from 128 mg/l to 1240 mg/l and is classified into three hydrochemical facies based on dominant ions. A Ca-HCO₃ water is found in a few wells near the margins of the basin and is similar to water found in the volcanic aquifers of the Davis Mountains. The dominant water type is a Na-HCO₃ water which occurs over the entire area. Several wells yield Na-SO₄ water high in total dissolved solids. The groundwater chemistry is controlled by the dissolution of calcium carbonate, gypsum, and by ion exchange processes as the groundwater moves along the flow path. The basin is through-flowing, with natural discharge occuring only by lateral flow to the north. Agricultural development of the Lobo area in the north part of the basin resulted in drawdown of the water table of up to 150 feet (45 m) between 1950 and 1980. Since about 1980, agricultural pumpage has decreased resulting in about 50 feet (15 m) of water table rebound. Recharge to the aquifer occurs primarily adjacent to the mountains in the alluvial fans. Results of 2-D computer modeling using MODFLOW and MODINV indicate a complex flow system influenced by the presence of clay lenses. Four hydraulic conductivity fields with a range of hydraulic conductivities from 1 ft/day (3.2 x 10⁻⁵ m/s) to 28 ft/day (9.9 x 10⁻⁵ m/s) were required to adequately simulate groundwater flow in the area