Many rural areas in the U.S. may be in danger of a groundwater shortage. Many parts of the country rely exclusively on groundwater for both agricultural and domestic use.

Additionally, digging deeper for water in the form of new, deeper wells may not be a good long-term solution to compensate for increasing demands on groundwater because there is potential for contamination of deep freshwater and brackish water in areas where the oil and gas industry injects wastewaters into or in close proximity to aquifers.

A new study may shed some light on use of underground freshwater and brackish water in some of the most prominent sedimentary basins across the U.S.

Research by scientists from the University of Saskatchewan, the University of Arizona and the University of California, Santa Barbara, was published Nov. 14 in Environmental Research Letters suggests this to be the case.

Groundwater is the primary source of domestic water supply for about half of the people living in the U.S. and about 40 percent of all of the water used in the U.S. for irrigated agriculture comes from groundwater. For example, in Tucson, Arizona, specifically, about half of drinking water comes from groundwater, which often is overtaxed by lack of rains and mountain runoffs.

"We found that potable groundwater supplies in the U.S. do not go as deep as previously reported, meaning there is less groundwater for human and agricultural uses," said Jennifer McIntosh, a University of Arizona Distinguished Scholar and professor of hydrology and atmospheric sciences.

"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. These potable water supplies are already being used up from the 'bottom up' by oil and gas activities.”

In parts of the western U.S., fresh groundwater extends down an average of 3,400 feet, McIntosh said. The new research found the average depth of transition from freshwater to brackish groundwater in the U.S. overall is about 1,800 feet, contradicting previous studies suggesting that fresh groundwater extends down to 6,500 feet.

In parts of the eastern U.S., the transition from freshwater 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 team wrote.

To find out how deep potable groundwater extends, the scientists analyzed water chemistry data from the U.S. Geological Survey for 28 key sedimentary basins in the U.S. 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, said lead author Grant Ferguson, principal investigator of the University of Saskatchewan-led Global Water Futures project.

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.

This study may be important because groundwater pollution continues. For example, coal ash dump sites across Illinois, including at Dynegy’s E.D. Edwards power plant, have severely contaminated surrounding groundwater supplies, according to a report released in late November by a consortium of environmental groups.

The results are based on data sets made public for the first time earlier this year as part of new federal regulations of coal ash, a toxic byproduct of coal-fired power generation that is commonly stored in unlined ponds or landfills near the plants. The report by the Environmental Integrity Project, Earthjustice, Prairie Rivers Network and the Sierra Club found toxic pollutants emanating from 22 of 24 coal ash dump sites in the state for which the data became available in March.

At the Edwards power plant on the Illinois River south of Peoria, lead levels in groundwater were 18 times the U.S. Environmental Protection Agency’s drinking water standard, the report stated.

Elsewhere, the European Union-funded REGROUND project has developed low-cost nanogeotechnology for the immobilization of toxic contaminants. The project applied the groundwater remediation technology at real scale, with three pilots and two large-scale barriers installed within industrially-polluted sites, observing the reduction in dissolved toxic heavy metal in groundwater.

Prior to the REGROUND project, the team had had a few years’ experience developing technology that injected iron oxide nanoparticles (NPs) into groundwater contaminant plumes. The feasibility of this approach was successfully tested in lab experiments and in the field.

Outlining REGROUND, the project coordinator Dr. Sadjad Mohammadian said, "Our consortium has worked on several projects using nanotechnology for environmental applications. Different aspects of the proposed technology, including distribution of particles in the underground, synthesis of new particles, reactivity and environmental toxicity, have been developed by our members. In REGROUND we consolidated this knowledge to upscale and bring it to the market."

The REGROUND barrier method works by injecting the high-tech iron oxide NPs into sediments, using simple wells in aquifers. The NPs travel pre-determined distances and then precipitate on the aquifer material, without blocking pores.

The contaminated groundwater flows through the NP zone, where the dissolved toxic heavy metals are adsorbed to the NPs, with metal-free water then released downstream. Since it is easy to apply and does not require large-scale infrastructure and soil removal, cleaning costs are significantly reduced.

The approach specifically targets arsenic, barium, cadmium, chromium, copper, lead, mercury and zinc as major groundwater contaminants.

After two pilot applications, REGROUND adopted the innovation for two contaminated aquifers in industrial locations in Spain and Portugal. The project's post-injection monitoring results indicate that heavy metals were successfully removed at the levels targeted in the remediation plan. These results indicate that the technology is market-ready.

However, based on their findings for the U.S., the authors of the university study suggest the amount of fresh groundwater available globally may also be less than previously thought. They note that an estimated more than 5 billion people live in water-scarce areas, many of which rely on groundwater and where, in some cases, significantly more water has been taken out of a groundwater basin than is coming in.