Will desalinated brackish groundwater become Arizona’s next big supply source?

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Drought is a popular topic in the media these days. Most efforts that are designed to mitigate the impacts of drought involve decreasing water demand. However, here in Arizona and other parts of the Southwest, we have a largely untapped resource — brackish groundwater — that could substantially augment water supplies for a few hundred years.

So what exactly is brackish groundwater? Essentially, it’s more saline than fresh water but less saline than seawater. Brackish groundwater contains total dissolved solids (TDS) in concentrations of 1,000 to 10,000 mg/L; anything less than 1,000 mg/L is considered fresh water, although the secondary (non-enforceable) standard for TDS in drinking water supplies is 500 mg/L.

Brackish groundwater offers one significant advantage over any other source in Arizona and the Southwest: it is extremely abundant. In Arizona alone, more than 600 million acre-feet (MAF) of recoverable brackish groundwater are stored underground; the map below shows how this resource is distributed. This equates to almost 100 times our current total water use. Furthermore, the impediments associated with utilizing this water source for drinking have diminished rapidly as desalination technologies have become economical for most water types[1].

Brackish groundwater resources in Arizona for potential desalination

Brackish groundwater resources in Arizona (click to enlarge)

Of course, the use of brackish groundwater does present some challenges. Some are not so different from those we face in pumping fresh groundwater supplies from our aquifers: We need to consider not only the sustainability of the brackish resource as land uses change but also the impacts of withdrawal on other groundwater users and the environment. Other challenges are related to water quality. For one thing, pre-treatment may be required to remove problematic constituents that are not addressed through desalination, depending on the source of the salinity and the groundwater chemistry. In addition, the desalination process produces a brine waste stream that must be disposed of. Fortunately, this waste stream can be greatly reduced through advanced technology.

Perhaps, however, the biggest impediments we face in using brackish groundwater are regulatory in nature. Check our future Hydro Notes for “Part 2” of this interesting topic.

—Ed McGavock

Ed McGavock is a hydrogeologist with M&A who conducted a statewide brackish water inventory several years ago. A 31-year veteran of the USGS, Ed served as Assistant District Chief of Water Resources Divisions in both Arizona and Washington prior to joining us in 1994. He frequently presents at conferences on the topic of brackish groundwater.

[1] The current cost to desalinate water ranges from about $1–$5 per 1,000 gallons. Compare this to the cost of bottled water, which is $1–$4 per gallon.