Southern Arizona gets about 43% of its water by pumping groundwater aquifers. The geology is well-suit for this because Southern Arizona is in the Basin and Range province which contains very deep, fault-bounded valleys. In some places, bedrock is as much as 15,000 feet below the surface. Portions of the Tucson and Avra valleys are over 8,000 feet down to bedrock. Such valleys are filled with alluvium and water.
Currently, water for drinking exploits aquifers down to a depth of about 1,200 feet. Generally water below that depth is too salty for drinking.
Following up on two previous studies, Estimated Depth to Bedrock in Arizona and Preliminary evaluation of Cenozoic Basins in Arizona for CO2 sequestration Potential, the Arizona Geological Survey in a new study, examined the salinities of Arizona groundwater. The study is A Summary of Salinities in Arizona’s Deep Groundwater, Arizona Geological Survey Open-File Report, OFR-12-26.
As part of that study, geologists of the Arizona Geological Survey (AZGS) reviewed geophysical well logs to catalog the concentration of total dissolved solids (TDS, i.e., salinity) of 270 water wells. This included all water wells that penetrated deeper than about 2,600 feet, which is the minimum depth necessary to sequester carbon dioxide.
Among the results of that study, AZGS found that on the Colorado Plateau and in the Basin and Range province, there are some areas where “Fresh water can extend as deep as 5,000 feet (1,500 m), but below 6,600 feet (2,000 m) only brackish or saline groundwater was encountered..” Water is considered “fresh” if it contains less than 1,000 ppm (parts per million) TDS. Water is “brackish” if TDS are 1,000- to 30,000 ppm. “Saline” water contains greater than 30,000 ppm TDS. Sea water is about 35,000 ppm TDS.
This means that we may be able to extract drinking water from deeper aquifers in some areas.