Could deep groundwater help slake California’s thirst?

In the midst of California’s worst drought in recent history, researchers poring over gas and oil project data claim to have found a previously uncataloged mother lode of potential drinking water. Their study, published in the Proceedings of the National Academy of Sciences, looked to the depths of the Central Valley and concluded that there’s nearly three times as much fresh groundwater and four times as much usable groundwater than previously believed. However, the researchers warn that this water risks contamination from oil and gas extraction.

For the study, researchers gathered information about depth, salinity, and total dissolved solids (TDS) in underground water from 360 oil and gas fields in eight California counties. Previous estimates of drinking water only looked as far as 305 meters below the surface. By considering depths of up to 3,000 meters, researchers found that estimates increased from 1,020 km3 to 2,700 km3—and nearly 60 percent of this water was found shallower than 1,000 meters. In California, water considered “fresh” has fewer than 3,000 ppm TDS.

They also found 3,900 km3 of fresh and saline water that falls under the definition of Underground Sources of Drinking Water. USDW is water with fewer than 10,000 ppm TDS. (Water with 3,000 to 10,000 TDS is considered moderately saline.)

The authors point out that in California and in other parts of the country and world, desalinization plants are converting seawater, which has 35,000 ppm TDS, into drinking water. Desalinizing moderately saline water would be less energy intensive and cheaper, researchers say.

Of course, treating or desalinizing usable drinking water from these deeper sources is not without its own issues. These waters may be contaminated or contain naturally occurring radium, requiring additional treatment. What’s more, extracting deep groundwater could cause subsidence, or downward sinking of the land surface.

The authors also found that the analyzed counties are home to nearly 20,000 oil and gas wells that go down over 8,500 meters. Many of these are located where the deep underground sources of water lie. The wells pose not only a danger of leaks into the water sources, but the possibility that they may open access for deeper, more-saline water to infiltrate more superficial fresh water. Oil and gas operations also inject waste water deep underground to dispose of it or to use it to increase extraction of oil and gas, other potential means for contamination. Monitoring and testing of deep groundwater is rare.

Of course none of this should be an excuse to lessen efforts to conserve and protect the sources of water that Californians currently use. But when it comes to protecting potential and much needed sources of drinking water for the future, these conclusions are a clear call to think deep. —Catherine Elton | 08 July 2016

Source: Kang, Mary and R.B. Jackson. (2016) Salinity of deep groundwater in California: Water quantity, quality, and protection. Proceedings of the National Academy of Sciences doi:10.1073/pnas.1600400113

Header image: ©Enid Martindale

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