Potential additional volumes of fresh groundwater (TDS < 3,000 ppm) and potential volumes of underground sources of drinking water (USDWs) (TDS < 10,000 ppm) by depth intervals in the Central Valley of California. The mean BFWs per county are all deeper than 305 m (1,000 ft) in the Central Valley counties considered (Kern, Fresno, Yolo, Colusa, and Solano). Therefore, we assume that the depth used for the groundwater volumes estimates in ref. 32 is 305 m (1,000 ft). Graphic: Kanga and Jackson, 2016 / PNAS

By Bobby Magill
27 June 2016

(Climate Central) – California’s Central Valley has three times more freshwater in underground aquifers than previously thought, drinking water that could help the state weather future drought and fortify itself against a changing climate, according to a new Stanford University study.

But tapping that water, locked thousands of feet beneath the ground, will be expensive and comes with an enormous risk  — it could cause the valley floor to sink, according to the study, published Monday in the Proceedings of the National Academy of Sciences. Sinking land in the Central Valley is threatening roads, homes and other infrastructure, and reduces the amount of water some aquifers can hold.

“It’s not often that you find a ‘water windfall,’ but we just did,” said study co-author Rob Jackson, an earth system science professor at Stanford University. “California’s already using an increasing amount of groundwater from deeper than 1,000 feet. Our goal was to estimate how much water is potentially available.”

Climate change is exposing the state to a greater threat of drought, reducing the amount of water available for farming and drinking as higher temperatures evaporate reservoirs. More precipitation is expected to fall as rain instead of snow in California as the world warms, forcing the state to find new ways to store rain water for municipal and agricultural use.

To stave off losses during its four-year drought, California has relied on groundwater to irrigate its farm fields. So much groundwater is being used that the water table has fallen by 50 feet in some places in the Central Valley, and the valley floor is sinking, or subsiding, as aquifers are depleted.

Land subsidence, which has been occurring in the valley for decades because of groundwater pumping, has accelerated to two inches per month in some places, according to NASA. Sinking land threatens roads, bridges, aqueducts, buildings and other infrastructure as the land collapses beneath them.

Most of the groundwater comes from aquifers less than 1,000 feet deep. Deeper aquifers are usually considered too salty to be used for drinking or irrigation, requiring costly desalination and drilling operations to access them.

Analyzing water data gathered from oil and gas wells across eight Central Valley counties, the Stanford researchers show that there are about 2,700 cubic kilometers of accessible fresh or brackish water locked in the Central Valley’s deep underground aquifers. That’s almost triple the 1,020 cubic kilometers of freshwater that had been previously estimated. […]

“We're not advocating running out and drilling lots more groundwater wells,” Jackson said. “The Central Valley's been in denial about groundwater overdrafts for years. We need to consider ground subsidence. We also need to think about oil and gas activities directly in and around freshwater aquifers. Is that the best use of the resource long term?” [more]

‘Water Windfall’ Found in Drought-Stricken California

ABSTRACT: Deep groundwater aquifers are poorly characterized but could yield important sources of water in California and elsewhere. Deep aquifers have been developed for oil and gas extraction, and this activity has created both valuable data and risks to groundwater quality. Assessing groundwater quantity and quality requires baseline data and a monitoring framework for evaluating impacts. We analyze 938 chemical, geological, and depth data points from 360 oil/gas fields across eight counties in California and depth data from 34,392 oil and gas wells. By expanding previous groundwater volume estimates from depths of 305 m to 3,000 m in California’s Central Valley, an important agricultural region with growing groundwater demands, fresh [<3,000 ppm total dissolved solids (TDS)] groundwater volume is almost tripled to 2,700 km3, most of it found shallower than 1,000 m. The 3,000-m depth zone also provides 3,900 km3 of fresh and saline water, not previously estimated, that can be categorized as underground sources of drinking water (USDWs; <10,000 ppm TDS). Up to 19% and 35% of oil/gas activities have occurred directly in freshwater zones and USDWs, respectively, in the eight counties. Deeper activities, such as wastewater injection, may also pose a potential threat to groundwater, especially USDWs. Our findings indicate that California’s Central Valley alone has close to three times the volume of fresh groundwater and four times the volume of USDWs than previous estimates suggest. Therefore, efforts to monitor and protect deeper, saline groundwater resources are needed in California and beyond.


Groundwater withdrawals are increasing across the United States, particularly in California, which faces a growing population and prolonged drought. Deep groundwater aquifers provide an alternative source of fresh and saline water that can be useable with desalination and/or treatment. In the Central Valley alone, fresh groundwater volumes can be increased almost threefold, and useable groundwater volumes can be increased fourfold if we extend depths to 3,000 m. However, some of these deep groundwater resources are vulnerable to contamination from oil/gas and other human activities. Our findings provide the first estimates, to our knowledge, of underground sources of drinking water depths and volumes in California and show the need to better characterize and protect deep groundwater aquifers.

Salinity of deep groundwater in California: Water quantity, quality, and protection



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