The found that when highly purified wastewater was stored in an Orange County, Calif., aquifer, the water caused arsenic to escape from clay sediments in a way that naturally infiltrating water did not.

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When it comes to the science of transforming sewage into tap water — or potable reuse — engineers say there’s no question the product is clean enough to drink.

The trouble is, researchers are learning that this drinking water may be too clean to store underground without special treatment.

A study published last week in the journal Environmental Science & Technology found that when highly purified wastewater was stored in an Orange County aquifer, the water caused arsenic to escape from clay sediments in a way that naturally infiltrating water did not.

In some instances, researchers said arsenic concentrations exceeded the drinking-water limit of 10 micrograms per liter, although the increases were temporary and levels eventually returned to normal. None of the affected water entered the public-tap system, officials said.

The root of the problem, according to researchers at Stanford University and the Orange County Water District’s Groundwater Replenishment System, was that the purified, recycled water lacked the minerals that native water acquires as it soaks into the earth or flows along rivers.

“Basically, the water was too pure,” said senior author Scott Fendorf, a Stanford geochemist. “It was devoid of everything other than water molecules.”

The solution, according to the researchers, was to add quicklime or another calcium-rich substance to the purified water before adding it to the aquifer, essentially dirtying it up a bit.

Jason Dadakis, the water district’s director of health and regulatory affairs and a study co-author, said the added calcium appears to be working.

“The initial results look positive,” Dadakis said. “We still have more long-term monitoring we want to do.”

Although scientists have identified several other methods by which arsenic can contaminate groundwater — lack of oxygen can be one — Fendorf said this may be the first time highly purified water was identified as a trigger.

As severe drought continues to strain water resources throughout the West, planners and officials are increasingly considering potable-reuse facilities and aquifer-recharge systems as one answer to the crisis.

At the same time, researchers are becoming increasingly aware of problems that arise when waters of varying chemistries are pumped or filtered underground. Mismatched waters can trigger the release of small solid contaminants that may lead to widespread contamination of an aquifer, they say.

“What you’re seeing in Orange County is something we have to be very careful of across the globe,” Fendorf said.

Arsenic is a natural and ubiquitous component of the Earth’s crust, according to the World Health Organization, and prolonged exposure can cause skin cancer and other serious health problems.

While arsenic has contaminated drinking water everywhere from the United States to East Asia, it wasn’t clear why levels were rising and falling in Orange County’s recharge system.

To find the answer, Fendorf and his colleagues took columns of sediment from beneath the Miraloma Basin, a surface-recharge basin in Anaheim, Calif., and exposed them to a variety of water samples: purified recycled water, water saturated with minerals or salts, and waters with different pH values.

They discovered that a layer of clay beneath the basin contained naturally occurring arsenic. This arsenic was usually held in place by a coating of positively charged calcium and magnesium particles. When natural, mineral-rich water percolated through this clay sediment, the calcium, magnesium and arsenic usually stayed put.

When the purified water soaked through, calcium and magnesium were more likely to leave the clay and hitch a ride with the water, because the water wasn’t already crowded with other minerals. When this happened, the arsenic was set free and essentially “piggybacked” its way into the water, Fendorf said.

The researchers note that this phenomenon may also play a role in future efforts to establish so-called direct potable-reuse facilities. Unlike Orange County’s indirect potable-reuse facility, which stores the water in an aquifer before using it as drinking water, direct potable-reuse systems pump purified recycled water directly in the public-water system.

Historically, the public has been less open to direct potable-reuse projects, which are often called “toilet to tap.”

“This is the benefit of direct potable reuse,” Dadakis said. “You eliminate the potential of environmental degradation.”