Saving water for a dry day
Billions of people tap into groundwater supplies across the world for drinking water—what if they could put back everything they took out?
Underneath the dry scrub deserts of Southern California, in the heart of Orange County, the local public water utility stores treated water underground. Much more water may be taken out of the ground here than goes back in for storage, but researchers and policy makers alike are increasingly looking to replenish aquifers across the U.S. to prevent future water shortages.
Such managed underground storage practices are becoming more common and have great promise, according to a report released by the National Academies in October. In regions such as the southwestern U.S. and Florida, and farther afield in Australia and elsewhere, these projects will become more necessary as growing populations, water scarcity, and climate change make their effects felt.
"As we go forward and we look at trying to meet water demands, this managed underground storage is certainly an important tool for us to save water when it's surplus and use when it's needed," says panel chair Edward Bouwer of Johns Hopkins University. "Having said that, some pilots have been successful, but there are risks."
The potential for contamination plus the monitoring and energy costs for treating water and pumping it below ground remain major drawbacks. The panel "cannot give a blanket endorsement that putting water underground is good. You need good engineering and good policy too," Bouwer says.
The U.S. EPA regulates water injected into the ground by wells but not water allowed to percolate into aquifers over a broad area. States with projects that have moved forward successfully have regulatory frameworks in place that can differ dramatically, tailored to specific regions and geologic settings, notes Denise Fort, a law professor at the University of New Mexico and a panelist on the National Academies report.
For example, Florida water storage and recovery must take into account naturally occurring arsenic, which could be mobilized by artificial groundwater recharge and pulled out once injected water is tapped for drinking. New Mexico allows public utilities, but not private or nongovernmental entities, to set up underground water storage projects. "Ownership of these waters might vary across state lines," Fort says, and could differ depending on whether the water is from aquifers or wastewater effluent.
Local water-quality policy also sets the "degree to which these projects should be required to protect aquifers, or alternatively if the emphasis should be treatment to protect consumers," Fort adds. California is much more stringent than other states about the quality of water injected for underground storage. This may carry over from earlier efforts that triggered public outcry over the idea of using recycled wastewater (or the less appealing "toilet-to-tap" concept). Water districts use various treatment processes, including UV, advanced oxidation, and reverse osmosis, before injecting water into the ground.
Initial data from the U.S. Geological Survey show that underground storage can remove some chemical compounds from water, depending on how long the water is stored. However, some pharmaceuticals and other persistent compounds may remain intact—an issue that requires more study, researchers say. In Las Vegas, Nev., trouble already has cropped up with trihalomethanes and other disinfection byproducts; the contaminants, pumped down in chlorine-treated wastewater, often come back up in higher concentrations that need to be diluted, says Shane Snyder (PDF size: 12 KB) of the Southern Nevada Water Authority.
Still, aquifer storage "saved the city from going dry at one point when we had a problem with our filtration system," says Snyder, who provided data to the National Academies panel. The utility also has a water-banking agreement with Arizona, which pumps water into its aquifers in exchange for Nevada's use of that state's Colorado River allotment. Underground storage in the desert avoids the potential for evaporation in aboveground reservoirs, which can amount to about 6 feet, or 2 meters, per year.
"There's a wealth of information that we have to understand the subsurface," Bouwer emphasizes, referring to modeling and monitoring experience at Superfund sites and older injection wells, such as those in place in Southern California since the 1930s. "But we can't be 100% certain we aren't putting down something that could be harmful."
