Several major geothermal anomalies are present in Trans-Pecos Texas because of warm to hot water springs and wells, silica geochemical indicators, and shallow thermal gradient measurements.
Forty-four thermal water (30°C or above) occurrences have been located in the area. The largest concentration of thermal waters is in southern Hudspeth and western Presidio Counties adjacent to the Rio Grande. The thermal waters range from slightly fresh to salty; total dissolved solids range from 275 to 11,500 ppm. Major ions in the high dissolved solid waters include sodium and chloride.
As indicated by the silica geothermometer temperatures, subsurface waters above 125°C exist at seven locations in northeast El Paso, western and southeastern Jeff Davis, western and northern Presidio, and southern Brewster Counties. The areas having highest silica temperatures are (1) west of the Hueco Mountains in northeastern El Paso County and extending northward into southwestern Otero County, New Mexico; here maximum silica temperatures are 151°C and maximum measured water temperatures reach 71°C; and (2) north of Candelaria in western Presidio County where maximum silica temperatures reach 177°C and maximum measured water temperatures are 72°C.
Geothermal gradients have been measured in over 70 wells and boreholes, most of these in Presidio County. Average gradient is 77°C/km in the Rio Grande Valley, with 30°C/km in the highlands east of the valley; this would seem to reflect the possible boundary of the Basin and Range and Great Plains tectonic provinces. In addition, a drilling program has been underway in northeastern El Paso County to investigate the geochemical anomaly west of the Hueco Mountains. Twelve to 14 holes drilled yielded gradients of over 100°C/km, and two of these penetrated the limestone bedrock, where the elevated gradients continued to increase with depth. Heat-flow values of 9 and 11 HFU were calculated for these latter two holes. On the basis of these gradients a geothermal resource may exist close to El Paso.