Annual testing has found that dioxane contamination increased in 20 residential wells in Scio Township near Ann Arbor since last year.
The Washtenaw County Environmental Health Division, in cooperation with the state Department of Environment, Great Lakes and Energy, conducts annual testing of hundreds of drinking water wells to monitor the Gelman plume of dioxane in Ann Arbor’s groundwater.
In June, county officials tested 153 wells in Scio Township. The results showed an increase in dioxane concentrations in 20 wells. Five wells remained the same, while ten showed a decrease in dioxane levels.
Kristin Schweighoefer, director of the county’s Environmental Health Division, said the data can help them track where the plume might be moving.
“It can extrapolate some information about where the 1,4-dioxane is located,” she said. “And in this case, we are seeing it quite a bit further away from known plume boundaries.” The results are not surprising, as they are similar to what the county found in 2023, she said.
Low levels of dioxane in these wells aren’t high enough to significantly harm human health, according to the legal agreement that manages the plume.
Under that consent judgement, Gelman must take action if a residential well tests at 3 micrograms per liter (ug/L) or more. That includes monthly testing and providing bottled water for households affected. None of the residential well detections have reached that threshold.
The state and county are keeping a close eye on wells with the highest hits, Schweighoefer said.
“We want to monitor those homes more closely to make sure that we are abiding by the consent judgment and ultimately making sure that we're protecting public health,” she said.
Dioxane dumped at Gelman’s medical filter manufacturing facility has spread underneath the city since the 1960s. The state first found contamination in residential drinking water wells in 1985. Over 120 residential wells were shut down, leaving households without drinking water while their houses were annexed into the city of Ann Arbor and hooked up to municipal water.
Today, Gelman and the state monitor the plume through a network of monitoring wells. Official maps show the contamination is four miles wide and sits underneath west Ann Arbor.
But residential well testing in 2021 revealed dioxane over a mile north of the plume’s known boundaries. In response, the county’s yearly testing expanded to include more wells that hadn’t been previously tested.
If dioxane is detected in a well where it hadn’t been found before, any residential well within a 1,000 radius must be sampled in the next round of testing. None of the 4 wells with new detections will expand the sampling area, Schweighoefer said.
The state has requested additional sampling of specific wells in August. Some of those are wells that the county wasn’t able to access in June. Others include a well that detected dioxane very close to the 3 ug/L threshold outlined in the consent judgement.
The property where the plume originated is eligible to become an EPA superfund site. The agency is reviewing public comments and will make a decision in the next several months.
If approved, the EPA will conduct additional testing to definitively determine how far the plume has spread.
