Arsenic is a problem for many private wells in Michigan. Now a study from the US Geological Survey finds a drought could make things even worse.
Arsenic occurs naturally in many places. Unfortunately, it’s toxic. Too much exposure to the element can damage people’s health in a variety of ways including increasing their cancer risk.
Public water supplies have to limit arsenic levels. But those laws don’t cover private wells. According to the USGS, about 226,000 Michiganders who use them may already get too much arsenic exposure.
In a recent study, the Survey, working together with the Centers for Disease Control and Prevention, found about 94,000 more people in Michigan could be at risk in times of drought – an increase of more than 40 percent.
“The longer the duration of the drought, the greater the increase in the probability of having elevated arsenic” in private wells, New Hampshire-based USGS hydrologist and lead author of the study Melissa Lombard said.
How does arsenic move from the ground into groundwater? “There’s a lot involved chemically,” Lombard explained. “You need to have the correct acidity level and amount of oxygen in a well.”
“There are sweet spots for arsenic to become dissolved in the water from the minerals in the aquifer,” she added.
“When you change the acidity or you change the amount of oxygen, then you could be changing it towards more favorable conditions that result in arsenic dissolving from the minerals into the water.”
The drought factor
To study how dry conditions might affect arsenic in wells, the USGS reconfigured an existing model for arsenic in groundwater. It changed rain and groundwater levels and added data from 2012, when a severe drought hit parts of the Midwest, West and South.
The result: Researchers found drought could favor higher arsenic levels in several ways.
By changing the water level in a well, drought alters where the water touches the ground. “You’re changing where the aquifer materials are exposed to oxygen, where they may not have been before,” Lombard said.
Drought can also change where the water in an aquifer comes from.
During wet periods, "a lot of the water that makes it into that well might be coming from surface,” Lombard said. “But then as that source of water isn’t there during a drought, you may be pulling in older groundwater from deeper in the aquifer, and perhaps that has more arsenic in it.”
Finally, drought tends to lower the amount of water in a well, concentrating any arsenic that was already in it.
Droughts may not be the showiest natural disasters, but they can have a major impact on public health, Lombard said.
“A takeaway from this project is that we should be concerned with the quality of water during droughts, not just the quantity of water,” she said.