Salinities of Arizona's deep groundwater

Arizona receives about 43% of its water supply from groundwater. A new statewide study documenting salinity concentrations of 270 deep groundwater wells in Arizona found that fresh water can extend as deep as 5,000 feet (1,500 m) in some areas but below 6,600 feet (2,000 m) only brackish or saline groundwater was encountered.

As part of a program to examine the suitability of sedimentary basins in Arizona as potential geologic repositories for industrial carbon dioxide (CO2) injection and disposal (called "geologic sequestration"), 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 (800 m), which is the minimum depth necessary to sequester carbon dioxide.  [Right, distribution of elevated salinity in groundwater wells]

Of the 56 wells that penetrated 2,600 feet (800 m) depth, 36 were on the Colorado Plateau and 18 were in sedimentary basins of Arizona's Basin and Range Province. Twenty-two deep wells (wells > 2,600 feet) had TDS values greater than 10,000 milligram per liter (mg/L), while 34 showed TDS concentrations less than 10,000 mg/L. The study concludes that fresh, brackish, saline, and brine water exist below 2,600 feet (800 m) depth in the Colorado Plateau and sedimentary basins. 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.
To further characterize salinity of Arizona's groundwater, AZGS geologists documented all shallower wells with elevated salinity values. This resulted in an additional 214 wells being analyzed: 115 wells with TDS greater than 10,000 mg/L, and 99 wells with TDS between 5,000 and 10,000 mg/L.
From these data AZGS geoscientists inferred that:

• Correlations between salinity and depth are difficult to discern regionally or even in individual basins.
• Brackish and saline conditions are present in both provinces below 6,500 feet (2,000 m) depth.
• Groundwater salinity of basins varies broadly and there is no obvious trend between basin-sediment volume and groundwater salinity.
• Basin groundwater is influenced by sampling methods, depth, borehole-screened intervals, drilling fluids at the time of sampling, and a variety of geologic factors (e.g. rock chemistry, faults, geothermal gradients, and salt domes).

Groundwater data was provided by the US Geological Survey – National Water Information System, Arizona Dept. of Environmental Quality, Arizona Department of Water Resources - Groundwater Site Inventory, Arizona Oil and Gas Conservation Commission, and consulting geologist James Witcher's Geothermal Resource Data Base. For well reports reporting conductivity in place of TDS, a conversion factor was used to calculate the equivalent TDS value.

The U.S. Department of Energy (DOE) funded this research as part of nationwide program to explore a variety of geologic repositories for their carbon dioxide sequestration potential. The report is available online for review or downloading at no cost.

Citation: A Summary of Salinities in Arizona's Deep Groundwater, 2012, Gootee, B.F., Mahan, M.K and Love, D.S. Arizona Geological Survey Open-File Report, OFR-12-26. 10 p.