Brine Discharge and Salinization, Concho River Watershed, West Texas

Discharge of naturally occurring brine from the Southern Great Plains regional ground-water flow system significantly affects water quality in local aquifers in the Concho River watershed in West Texas. Aquifers in outcropping Permian rocks locally contain brine and hydrocarbons at depths as shallow as 135 ft (41 m). Maps of hydraulic head, salinity, and hydrochemical facies and graphs of ionic ratios and stable isotopic composition locate where brackish to saline ground water occurs naturally as a result of mixing between locally recharged meteoric water and subsurface brine in the regional flow system. Br/Cl, CI/SO4, Ca/Na, and δD/δ18O ratios distinguish between brine sources in the mixing zone. For example, chemical composition of brackish-to-saline shallow ground waters beneath the Concho River watershed more closely resembles the composition of Permian than of Pennsylvanian formation brines. Other possible salinity sources include (1) seepage of salt water from rocks beneath oil-field brine-disposal p ts, which ceased operation in the late 1960 s, and (2) upward flow of artesian salt water across confining beds through decades-old abandoned oil-exploration holes. These sources are superposed on the naturally occurring mixing zone between brine and fresh water.