Recharging Groundwater Without Contamination

Groundwater in the U.S. is declining as uses exceed the aquifers' abilities to replenish or recharge. In response, coastal communities in California, Florida, and elsewhere are conducting managed aquifer recharge (MAR). Also called water banking, this technique involves topping up water in several ways: by injecting reclaimed or highly processed wastewater water via wells; creating streams or ponds that spill into fast-draining soil; or building catchment systems for rainwater and stormwater to drain into the aquifer.

This action has several positive effects beyond replenishing water. On the coast, recharging can prevent salt water from slipping into open spaces in depleted groundwater. In addition, sinkholes are less likely to form as replenished water fills spaces under the ground that would otherwise lead to soil collapse.

But research published by a team at Washington University, led by Young-Shin Jun and Xuanhao Wu, has established how potable water used to top up an aquifer can become contaminated with dangerous levels of arsenic. Their 2020 research—published in the journal Environmental Science and Technology—provides tips for recharging groundwater in ways that avoid contamination.

The article, titled "Dissolved Organic Matter Affects Arsenic Mobility and Iron(III) (hydr)oxide Formation: Implications for Managed Aquifer Recharge," says that "During managed aquifer recharge (MAR), injected water significantly alters water chemistry in an aquifer, affecting arsenic mobility."

Arsenic in Aquifer Recharge

Jun became interested in the research around 11 years ago while working on managed aquifer recharge with the Environmental Protection Agency (EPA). At that time, the team injected recycled water into an aquifer but later found it was contaminated with arsenic, according to a Washington University press release. The water was bringing oxygen into the aquifer, it turned out.

"By injecting reclaimed water, we are triggering oxidative dissolution of sulfide minerals in the aquifer, which were stable at low oxygen levels," she says in the release. In particular, Li and her team examined arsenopyrite (FeAsS), a mineral that dissolves into iron, sulfur, and arsenic. Normally, before the arsenic reaches troublesome levels, another reaction takes place.

"The iron precipitates into iron oxides or hydroxides," Jun says. The arsenic absorbs, or attaches, to the iron oxides or hydroxides, which are indissolvable. This keeps arsenic out of the water.

There is, however, another important factor. Compounds that contain carbon—dissolved organic matter (DOM)—change the circumstances dramatically. "With high levels of organic compounds, we found the precipitation reaction is suppressed," Jun said. When that happens, arsenic does not absorb properly and remains available in the water.

Recharging Aquifers in Inland and Coastal Locations

Some inland cities and other areas experiencing drought and conditions intensified by climate change are also looking into adding reclaimed and processed water to their aquifers in the same way.

Boise, Idaho, which has a relatively secure water supply, hosted a panel of water experts in late 2021 to discuss the possibility of adding recycled water to its local groundwater. In 2021, researchers considered the potential for MAR in the Upper Midwest's corn belt region.

Avoiding Arsenic in Recharged Groundwater

"We have to take into account the DOMs in injected water to make sure they do not trigger more mobilization of toxins," Jun said. "The roles of DOMs in managed aquifer recharge should be included in the predictive models. Knowing how water chemistry alters the chemical reactions in an aquifer will enable us to fully utilize the water, rather than discarding it as waste."

But if arsenic is detected in groundwater, it can be removed to meet the EPA's safety requirement—10 parts per billion—by using reverse osmosis or other techniques.

As more areas recharge water or find other reasons to check for arsenic, water labs' testing services will be more important than ever. While municipalities deal with the effects of climate change, lab managers will need to stay abreast of new developments in groundwater quality.

There are several resources to stay up to date on testing news, including the importance of well water testing, meeting state regulations, and the importance of testing during a drought.

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Catherine Arnold

An independent writer telling human stories in science, health, and other areas, Catherine Arnold has written articles for the Washington Post, Science Careers, Bicycling, and NBC Health. Follow her on Twitter or Instagram.