A pair of unregulated pollutants the U.S. Environmental Protection Agency links to thyroid disorders and childhood developmental issues are being found in two large Michigan public water supplies.

According to several years of local and federal data, the city of Ann Arbor and Plainfield Township near Grand Rapids are the only Michigan utilities detecting levels of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

Maps showing Plainfield Township and Ann Arbor, where public water has tested positive for perfluorinated chemicals.

The toxicants have been found in raw and treated water in both municipalities and officials have not pinned down a source for either, although, there's suspicion a closed landfill may be the culprit in Plainfield Township.

In Ann Arbor, the chemicals are coming through the city's intake pipe under the Barton Pond impoundment on the Huron River. In Plainfield, the chemicals have been traced to the township's backup well field at Versluis Park.

State officials say it's likely, though not proven, the chemicals are entering the Grand River if the PFOS and PFOA are coming from the closed landfill.

"We're still waiting for some data to come in, but that's a natural assumption," said John Bradley, Michigan Department of Environmental Quality site remediation manager with the Superfund section.

Thus far, none of the PFOA or PFOS samples have exceeded a new federal health advisory guideline but a 2013 sample came close in Plainfield Township, where a well field has been shut down out of an "abundance of caution."

In May, the EPA set 70 parts-per-trillion as the non-enforceable exposure threshold for PFOS and PFOA, which are among a large class of compounds called perfluorinated chemicals (PFCs) -- also known as polyfluoroalkyl substances (PFASs) -- that are being studied as "emerging" contaminants.

Michigan has separate surface water thresholds of 11 ppt for PFOS and 42 ppt for PFOA, established to guide fish consumption advisories. Those are lower to account for bioaccumulation, which magnifies chemicals in fish tissue.

In Plainfield Township, samples collected at the plant on June 28, 2013 and Sept. 16, 2013 detected PFOS at 50 ppt and 60 ppt, respectively. This year, PFOS samples collected June 2 and June 28 at the Versluis wells recorded 15 ppt and 17 ppt. At the plant tap on June 28, PFOS tested at 6.6 ppt and PFOA at 2.5 ppt.

In Ann Arbor, a sample collected from the city's finished water reservoir on March 5, 2014 detected PFOS at 43 ppt. This year, PFOS was detected on March 28 at 11 ppt in the reservoir and at 19 ppt at the Barton Pond intake.

"Yes, people are being exposed, but they are exposed to less than the lifetime health advisory number," said Christina Bush, a toxicologist with the Michigan Department of Health and Human Services involved in the Plainfield investigation. "The chemicals are there, but they are less than what we'd be concerned about as far as any potential harm."

Unfortunately, she said, such contamination is the legacy of living in an industrialized society. Once released into the environment, PFOS and PFOA persist and bioaccumulate. In 2012, the EPA added them to the list of 29 unregulated contaminants the agency requires testing for every five years. Since then, the chemicals have been discovered above the minimum reporting level in about 2 percent of all U.S. water systems.

The EPA requires water utilities to report PFOA samples testing above 2 ppt and PFOS samples testing above 4 ppt. Testing has occurred in about half of Michigan counties, with sampling focused on localities near or downstream of urban industrial areas.

Ann Arbor only found PFOA once, at 2 ppt in Barton Pond on March 28.

"It's very, very low," said Brian Steglitz, Ann Arbor water plant manager.

There's no obvious culprit for the pollutants in Ann Arbor. The city hasn't found the chemicals every time they've looked. To Steglitz, that suggests the source is "not something that's there all the time as background."

"It's hard to know without doing a little more investigate work," he said.

In the U.S, PFOS and PFOA have historically been used for water-and stain-proofing carpet, leather, textiles, upholstery and paper packaging. Both chemicals were developed by 3M in the 1940s.

PFOA was later sold to DuPont and formed the chemical backbone of Teflon. The fabric protector Scotchguard relied on PFOS as its key ingredient.

According to the EPA, most domestic PFOS and PFOA manufacturing has been voluntarily phased out as of last year, but the chemicals are still around in older products, discarded items and some imports.

Water contaminated by PFOA and PFOS generally comes from manufacturing and industrial sites, fire and crash training areas, and waste sites.

Based on lab studies on animals and epidemiological studies on exposed human populations, the EPA says PFOA and PFOS exposure to unborn children or breastfed infants over certain levels may cause developmental problems like low birth weight, accelerated puberty and skeletal variations.

The chemicals have also been linked to testicular and kidney cancers, liver damage, thyroid disorders and changes in cholesterol.

In Plainfield Township, the pollutants were traced to Well Field No. 3, a backup field located at Versluis Park on the manmade Versluis Lake. The wells, which are shut down, are about two miles north of an active Superfund site, the closed State Disposal Landfill at 3954 East Beltline Ave NE.

Although it hasn't been conclusively proven the PFOS came from a landfill plume, that's the suspicion. The landfill, owned Waste Management of North America, operated from 1966 to 1976. The DEQ says liquid industrial waste may have been disposed there, but incomplete records make that hard to prove.

Other chemicals detected at elevated concentrations on site include lead, zinc, iron, thallium, trichloroethene, vinyl chloride and benzene.

Bradley, who oversees the landfill cleanup for the DEQ, said groundwater flows north toward the residential and commercial areas along Northland Drive and Plainfield Ave. NE, which are built at a lower elevation. Because the nearby ground doesn't feature a protective layer of clay, the township several years ago passed a wellhead protection ordinance to restrict land use in the area.

The Versluis wells go down about 120 feet, said Rick Solle, Plainfield Township public services director.

When the 2013 samples came back, the township stopped drawing water from the Verlsuis wells, which have been used seasonally during peak demand times. Most of the 11 million gallons per-day the township pumps come from two fields east of Northland Drive near the water plant at 5195 Plainfield Ave NE.

On June 28, one of those wells hit PFOS at 4.9 ppt.

"At no point have we ever exceeded any EPA limits for treated water going out in our system," said Solle.

Cameron Van Wyngarden, township superintendent, expressed "full confidence" in the water system quality.

"I want to assure our water customers that our water is certainly safe to drink," he said. "My family and I drink it every day."

Plainfield serves about 40,000 customers and, with the backup field out of commission, has juggled staffing and hours at the water plant to meet water demand this summer. Ann Arbor serves about 120,000 customers but hasn't taken any action to address the PFOS and PFOA.

Most businesses and homes in the affected area of Plainfield Township are connected to the municipal supply, but officials say there are some "holdouts" in the area who rely on private wells. The Kent County Health Department says it has not sampled for PFCs in local private wells and was unable to quickly estimate how many might be affected.

Bradley said the DEQ wants to see private well tests happen soon. The DEQ also wants Waste Management to speed up scheduled PFC testing at the landfill.

Bush said the DHHS and DEQ "will probably also be looking into whether or not there are any issues with fish in the Grand River."

Both municipalities have disclosed the PFOS results in annual public reports.

"At this point, we're just continuing to monitor and investigate where this might be coming from," said Steglitz. "We're not adjusting the treatment process to address it. We're well below the health advisory levels and the EPA guidance at this point; they are not recommending any action be taken."