Geohydrology of the San Agustin Basin, Alamosa Creek Basin upstream from Monticello Box, and upper Gila Basin in parts of Catron, Socorro, and Sierra counties, New Mexico

The San Agustin Basin, the Alamosa Creek Basin upstream from Monticello Box, and the upper Gila Basin are located in parts of Catron, Socorro, and Sierra Counties in west-central New Mexico. Four major aquifers are within the study area: (1) the San Agustin bolson-fill aquifer; (2) the Datil aquifer; (3) the shallow upland aquifers; and (4) the Alamosa Creek shallow aquifer. Two minor aquifers, the Baca Formation at the northern edge of the San Agustin Basin and a basalt to basaltic andesite unit overlying the Datil Group, yield some water to wells.

Sixty-three vertical electrical- resistivity soundings were used to estimate the depth to bedrock and the saline/freshwater interface in the San Agustin bolson-fill aquifer. The dissolved-solids concentration of ground-water samples ranged from 74 to 23,500 milligrams per liter. The dominant cations varied; the dominant anion of freshwater generally was bicarbonate. Point-of-discharge temperatures of well or spring water that exceed 21 degrees Celsius are associated with faults in the areas of shallow or exposed bedrock. The dissolved-solids concentration of this warm water ranged from 120 to 1,200 milligrams per liter.