Under the Central Valley lies nearly three times as much fresh groundwater and four times as much drinking water than previously estimated, according to Stanford University researchers.
The scientists calculated that the Central Valley has 2,700 cubic kilometers of fresh groundwater if water sources deeper than 1,000 feet are included. Fifty-nine percent of that previously unknown freshwater is between 1,000 to 3,000 feet underground. Underground drinking water, which had not been quantified in the past, raise the groundwater total to 3,900 cubic kilometers.
Overall, 26 percent of underground freshwater and 42 percent of underground drinking water sits below 1,000 feet.
The study by Professor Robert B. Jackson and postdoctoral research fellow Mary Kang is based on data from 938 oil and gas fields and 34,392 oil and gas wells in eight counties, including neighboring Fresno County.
Northern counties, such as Fresno, had a larger proportion of deep freshwater than southern counties.
Unfortunately, some of that deep water will have higher amounts of salt than shallow water and would require treatment before being usable. Pumping the water will also be more expensive due to the greater depth, and would cause more land subsidence.
“Portions of the Central Valley have already dropped by tens of feet as shallower groundwater disappeared. Some locations dropped by a foot last year alone during the drought,” wrote Jackson and Kang in their research brief. “Subsidence permanently reduces the ground’s ability to hold water and can lead to costly upgrades for canals, buildings and other infrastructure.”
Another concern is the risk of groundwater contamination due to pressurized fluid injections as part of hydraulic fracturing (aka fracking) and other oil and gas activities. Such injections “occur below usable aquifers, but could drive saline water … upwards into them.” Deeper sources of drinkable water are especially vulnerable, the researchers said.
The oil and gas industry have drilled into as much as 35 percent of the deep drinking water reserves areas and 19 percent in freshwater zones. An exception was Kern County, where the highest percentage of oil and gas actions were in freshwater areas.
Further study is needed, the researchers wrote, to explore the risks of land subsidence and energy industry contamination so that deep groundwater can be both used and protected.
“What we are saying is that no one is monitoring deep aquifers. No one’s following them through time to see how and if the water quality is changing,” researcher Mary Kang in a public statement. “We might need to use this water in a decade, so it’s definitely worth protecting.”
The Stanford study was published this week in the journal Proceedings of the National Academy of Sciences.