State environmental officials found 1,4-dioxane, a toxic chemical, in six residential water wells in Scio Township during annual state testing.
The dioxane, coming from the Gelman Plume, ranged between 0.33 to 0.86 parts per billion (ppb), well under the Michigan Department of Environment, Great Lakes and Energy’s (EGLE) drinking water limit of 7.2 ppb.
Although the state says the water is safe, some advocates for a more aggressive plume cleanup believe this new detection suggests the plume is moving north. They say it could be dangerous if it reaches Barton Pond, Ann Arbor’s main water source.
“The dioxane plume is expanding mainly northward in the subsurface along low topographic areas directly to the Huron River and then Barton Pond,” Dan Bicknell, president of Global Environment Alliance, said in an email. Bicknell was the first person to discover the plume contamination when he was a graduate student at the University of Michigan in 1984.
“These new detections bring the contamination closer to the Huron River and Barton Pond, which provide drinking water to Ann Arbor,” Congresswoman Debbie Dingell (D-Ann Arbor) said in a press release.
The plume does not meet EGLE’s standards for movement, according to Andrea Munoz-Hernandez, EGLE’s Remediation & Redevelopment Division Jackson District Supervisor.
“The plume is considered to be expanding if dioxane exceeds the 7.2 ppb drinking water criterion in sentinel wells or boundary wells. That has not occurred,” she said in an email.
Kristen Schweighoefer, the environmental health director for the Washtenaw County Health Department, said there is not enough data to confirm that the plume is moving.
“Right now, we have one data point. We don't have a sense of whether the plume has been here for a long or short period of time,” she said.
The plume has been in groundwater underneath Ann Arbor for over 40 years, but officials say it is unclear if it has been moving north towards Scio Township.
“As we continue our testing, we may have a better idea of trends in the future,” Schweighoefer added.
EGLE said they started testing for 1,4-dioxane at 0.2 ppb this year. In previous years, these same wells were tested with a detection level of 0.5 ppb.
Bicknell said some communities have had a “long-standing request” for EGLE to test with lower detection levels.
“Community members expressed concerns that the previous detection limit of 0.5 ppb might miss low concentrations of dioxane in drinking water wells. In response, EGLE's laboratory worked to reduce the reporting limit to 0.2 ppb,” Munoz-Hernandez said.
Schweighoefer said since this is their first data point for detected dioxane, they cannot confirm if the toxin was present at lower levels in past years.
“Five of these detections are due to EGLE’s laboratory lowering the reporting limit from 0.5 ppb to 0.2 ppb,” Munoz-Hernandez said. “This change increases sensitivity and allows us to detect lower concentrations that may have previously gone unnoticed.”
Scio Township started testing its water wells in 2021 due to concerns of dioxane. They tested at lower detection levels than what the state was testing. Their findings led to the state doing more testing in Scio Township.
Schweighoefer said the Washtenaw County Health Department plans to use the data collected this year to inform the 2026 annual testing.
Even though the toxin levels are well below EGLE’s criteria for safety, many advocates believe their dioxane safety standard of 7.2 ppb is too high.
“The EGLE dioxane drinking water criterion of 7.2 (ppb) is not protective of public health,” Bicknell said.
He explained that Michigan has the highest value for dioxane safety compared to all other states with similar regulations.
In her statement, Dingell added that experts have warned that current plume cleanup methods are ineffective.
EGLE continues to assure residents that their water is safe.
“To assess risk on a population basis, scientists develop basic assumptions about age, exposure time, and how people may be exposed to calculate what is considered safe. These assumptions are based on a wide range of scientific studies. EGLE’s drinking water criterion for dioxane is 7.2 ppb. The 2025 detections range from 0.33 to 0.86 ppb, which are well below this criterion,” Munoz-Hernandez said.
Schweighoefer said it is up to the individual to assess the risks.
“One of the benefits of this program is that these individual residents can know what their specific levels in their water are, and then take any actions or precautions that they want to take based on their own risk tolerance,” she said.
As for the safety of Ann Arbor’s drinking water, Schweighoefer is confident the city will continue to pay close attention to the data.
“There's always a risk. However, the city of Ann Arbor is very diligent about making sure they are testing their water intakes,” she said. “I think they're keeping an eye on all of our data. And we're working collaboratively to make sure everybody has all of the information so that they can make good decisions.”
