Minnesota environmental engineers have proposed digging 27 wells to contain a plume of toxic PFAS chemicals threatening the water supplies of fast-growing east metro communities.

The system created by the Minnesota Pollution Control Agency and its consultant AECOM would consist of 18 wells to suck up tainted water; pumps and pipelines to send it to a treatment station; and nine wells to inject clean water back underground. It's a strategy that's currently protecting the aquifers in Los Angeles from seawater, but never used at this scale in Minnesota.

The plan, which will be refined and detailed in June, is "the best thing we have right now" to contain the spread, said Matt Simcik, a University of Minnesota professor of environmental chemistry who has extensively studied PFAS but did not help craft the state's proposal.

The MPCA isn't yet saying how much the work would cost, or how it would be funded. It could draw from an $850 million settlement the state struck with 3M six years ago, but the cost of cleaning up that company's pollution will likely exceed that amount.

The legacy of 3M chemical waste from former dumps, the underground PFAS plume has tainted drinking water supplies for years. Over the next 50 years, it is expected to reach new areas in Oakdale and Woodbury, which are already grappling with the chemicals. It has already flowed toward the northern reaches of West Lakeland Township, where everyone relies on private wells.

Minnesota's ability to manage this PFAS may dictate how clean-up of the chemicals is handled elsewhere in the state. Eighteen communities now have PFAS levels in their drinking water higher than a new federal limit (the Minnesota Department of Health previously estimated it was 22 communities, but have revised those numbers since). All of them rely on groundwater, the MDH said.

While the Twin Cities has seen many cases of tainted groundwater in the past — from creosote, solvents and other chemicals — PFAS is unique. It's nearly indestructible, builds up in people's bodies and moves easily underground.

Building a moat

In the middle of the last century, Maplewood-based 3M helped pioneer PFAS, or per and polyfluoroalkyl substances. The chemicals repel water, stains and grease, and quickly became a key part of Scotchgard, nonstick pans and many other products. But research has shown some of these chemicals are linked with certain cancers, immune issues and developmental problems.

3M used PFAS for decades at a plant in Cottage Grove, and in the early 2000s, the chemicals were discovered in water outside the plant. Chemical waste had been dumped at four sites across the eastern Twin Cities, where it seeped into the Prairie du Chien and Jordan aquifers. The plume the state wants to lock in place stems from the two northern disposal sites, and it's flowing to the south, east and slightly northeast, including toward the St. Croix River.

Under the plan, a ring of extraction wells on top of the plume, and the injection wells outside the areas of highest contamination, would create a moat of underground pressure. Engineers conducted tests already to ensure the interventions won't dry up the aquifer, and will check the results after each well is installed, said Rebecca Higgins, a hydrogeologist at the MPCA.

This method has been used before in Minnesota. Xcel Energy has used a series of wells to trap radioactive water that leaked out of a nuclear plant in Monticello and to try to keep it from reaching the Mississippi River. 3M is pumping water from a different area of PFAS contamination and treating it inside its facility in Cottage Grove, spokesman Sean Lynch said.

Higgins said the best comparison for what the MPCA wants to do is in Los Angeles. There, 159 wells push 9,000 gallons a minute into the ground to stop salty groundwater under the ocean from infiltrating freshwater supplies, according to a report in LAist.

The east metro system would likely entail a smaller network of wells, pumps, pipelines, special filtration machines and some still-developing technology to destroy the PFAS. All this equipment would run for decades, at a minimum.

Project planners are talking about how to respond to another potential challenge. Some proposed wells could be affected by restrictions on pumping to protect nearby White Bear Lake. Homeowners there successfully sued the Department of Natural Resources for allowing the lake to shrink from excessive withdrawals of groundwater.

Traveling chemicals

While PFAS has been slowly moving underground, it's also spreading through surface water.

A regional flood control project installed in the east metro in the 1980s allowed the chemicals to move through streams, lakes, retention ponds and stormwater pipes. In some places, the PFAS seeped back into the ground and created new areas of underground pollution, the MPCA and consultant AECOM said in a December presentation.

In one of those areas, the aquifer contamination was traced to seepage from several small lakes and ponds along the drainage path, including Horseshoe Lake, just to the southeast of the Royal Golf Club in Lake Elmo, AECOM engineer Hanna Temme said.

That's why some private wells are polluted in West Lakeland, which is about 7 miles from the Oakdale disposal site where the contamination originated. Town Board Chair Philip Moosbrugger said the state has helped install granular activated carbon filters in each affected household, all paid for by the settlement.

As the plume moves northeast, more wells in the area could be affected. Moosbrugger said he generally feels protected by the charcoal filters, but he welcomed the state's plan to stop the plume "If this engineering can be effective at scale."

The drainage system that helped spread the chemicals empties into the St. Croix River. Higgins said the state has tried to take some samples at the end of the final pipe and found low levels of PFAS — but it's hard to say how much is being deposited into the river, because the pipe is underwater and its discharge is immediately diluted.

Funding filtering

While the state refines its long-term plan for the plume, communities are each working on their own plans to filter the water that comes out of the ground.

The situation in Woodbury is so severe that the city asked legislators this year to fund a $19 million, 2 million-gallon water tower to help the city manage until 2028, when it will open a permanent, $192 million water treatment plant to serve the entire city.

Nine of Woodbury's 20 wells today have interim treatment systems, said assistant public works director Jim Westerman. "We believe we're well positioned to provide water of the highest quality between now and when we construct new water quality treatment to come on line," he said.

But if a period of high demand comes along before the final plant is completed — picture a hot day when residents are watering their lawns — the city may have to blend water from a contaminated well into the system, Westerman said. That's why the additional water tower is needed.

This is just one of the many places grappling with the expense of the plume — and most of the communities are relying on the 3M settlement money. State projections released last September also showed that some of those projects are more expensive than originally estimated.

The state hasn't determined a date when the settlement funds could be exhausted, said Hannah Sabroski, an MPCA spokeswoman. But according to a fall presentation on costs, a 2007 consent order with 3M would require the company to pay additional drinking water expenses.

3M did not directly answer a question about whether it was responsible for paying for drinking water into the future, even if the settlement is depleted. Spokesman Lynch simply responded that the $850 million settlement "was agreed upon with the state of Minnesota to enhance the quality, quantity and sustainability of the drinking water in the east metro area."

Star Tribune reporter Matt McKinney contributed to this story.