U.S. groundwater supply is smaller than we thought

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The US groundwater supply is smaller than originally thought, a new study shows.

The findings offer important insights into the depths of underground fresh and brackish water in some of the most prominent sedimentary basins across the country.

“We found that potable groundwater supplies in the US do not go as deep as previously reported, meaning there is less groundwater for human and agricultural uses,” says Jennifer McIntosh, a professor of hydrology and atmospheric sciences at the University of Arizona.

“We show that there is potential for contamination of deep fresh and brackish water in areas where the oil and gas industry injects wastewaters into—or in close depth proximity to—these aquifers,” McIntosh says. “These potable water supplies are already being used up from the ‘bottom up’ by oil and gas activities.

“Groundwater is the primary source of domestic water supply for about half of the people living in the US,” she says. “About 40 percent of all of the water used in the US for irrigated agriculture comes from groundwater. In Tucson, Arizona, about half of our drinking water comes from groundwater.”

Many rural areas in Arizona and other parts of the US rely exclusively on groundwater for both agricultural and domestic use, she says.

Digging deeper

To find out how deep potable groundwater extends, scientists analyzed water chemistry data from the US Geological Survey for 28 key sedimentary basins in the US and looked at the correlation between water well depths and the depth to the transition between fresh and brackish water.

Until now, the focus has been on monitoring dropping water tables, says Grant Ferguson, principal investigator of the Global Water Futures project and lead author of the paper, which appears in Environmental Research Letters.

In parts of the western US known to geologists as the Basin and Range Province, fresh groundwater extends down an average of 3,400 feet, McIntosh says. The province includes Nevada, southern Arizona, and New Mexico, and extends into parts of California, Utah, Oregon, and Idaho.

The new research shows the average depth of transition from fresh to brackish groundwater in the US overall is about 1,800 feet, which contradicts previous studies suggesting that fresh groundwater extends down to 6,500 feet.

Especially in parts of the eastern US, the team found the transition from fresh to brackish water occurs at less than 1,000 feet. In such regions, drilling deeper wells is not a long-term solution to the need for additional fresh water, the researchers write.

“There are a number of cases where potentially you could go a kilometer or so deep for fresh groundwater, but there are other areas of the United States where in maybe a maximum of 200 or 300 meters you would run into saline groundwater—essentially you would be done in terms of water resources,” says Ferguson, an associate professor at the University of Saskatchewan in Saskatoon, Canada.

A global problem

In addition, the injection of water, chemicals, or sand that occurs with hydraulic fracturing—or “fracking”—or the injection of wastewater may drive waters containing hydrocarbons into adjacent areas that contain potable water.

“In some basins, injection wells are installed shallower than the transition from fresh to brackish water,” says coauthor Debra Perrone, assistant professor of environmental studies at the University of California, Santa Barbara.

Based on their findings for the US, the authors suggest suggest the amount of fresh groundwater available globally may also be less than previously thought. More than five billion people who rely on groundwater live where, in some cases, significantly more water is taken out of a groundwater basin than is coming in.

The Natural Sciences and Engineering Research Council of Canada, the US National Science Foundation, the Canada First Research Excellence Fund, and the W. M. Keck Foundation funded the research.

Source: University of Arizona