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).
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.