ANN ARBOR, MI - Recent lab tests at Ann Arbor’s water treatment plant indicated 1,4-dioxane in the city’s surface water supply and in the finished drinking water.

Results from samples collected Feb. 6 estimated concentrations of 0.061 parts per billion at the city’s intake in Barton Pond and 0.030 ppb in the treated drinking water.

The city released the results on Friday, March 1, saying it’s important to let city water customers know about “this first-time detection.”

The city maintains the water is safe to drink, with dioxane levels equal to about one drop of water in an Olympic-sized swimming pool.

“With recent advances in lab testing techniques, we are able to detect very low levels of contaminants in water samples,” city spokeswoman Lisa Wondrash said in a statement. “The presence of small amounts of contaminants does not necessarily indicate that water poses a health risk.”

Dioxane is a toxic chemical that’s classified by the U.S. Environmental Protection Agency as likely to be carcinogenic to humans by all routes of exposure. It also can cause kidney and liver damage, and respiratory problems.

Just a few parts per billion in drinking water, with long-term exposure, poses a 1 in 100,000 cancer risk, according to the EPA.

An underground plume of dioxane has been gradually spreading through Ann Arbor’s groundwater for decades, originating at the Gelman Sciences complex off Wagner Road between the 1960s and 1980s. The plume now covers an area more than three miles long and a mile wide, inching toward the Huron River.

Until now, the city has maintained regular testing for years showed there weren’t any detectable levels of dioxane in Barton Pond, an impoundment on the Huron River where the city gets most of its drinking water. Dioxane did contaminate a municipal supply well on the city’s west side several years ago, after which the city shut down the well.

The “virtually undetectable levels” found in Ann Arbor’s drinking water recently were lower than the lab quantification limit and the city is working to investigate the cause, Wondrash said.

The city will test again in March, using two independent water-quality labs to see if any 1,4-dioxane is detectable.

“The number one focus of the city’s drinking water staff is to protect public health; and they take this responsibility very seriously,” Wondrash said.

The city’s water treatment plant can remove some dioxane. Pilot testing in 2006 indicated it could remove up to 70 percent of the contaminant.

It’s possible a new UV disinfection system the city plans to install at the water treatment plant could further treat for dioxane.

Ann Arbor is facing various other threats to its municipal water supply, including PFAS chemical contamination, a diarrhea-causing parasite known as “crypto,” and lingering lead risks.

Ann Arbor News reporter Ryan Stanton contributed to this story.