Hikers in Hidden Falls Regional Park dodge crews operating a sonic drill boring into the forest floor.

The well being installed is one of nine new groundwater monitoring wells going in along the Mississippi River as Ford Motor Co. and the state move closer to cleaning up one of the region's more colorful toxic waste dumps.

The large waste hill that abuts the St. Paul park is overgrown now, with a parking lot on top that's been used in the past as a State Fair park-and-ride. But from the mid-1940s to the mid-1960s Ford workers dumped paint sludge and solvents from Ford's painting operations there at the base of the river bluff, leaving a toxic legacy that includes lead, arsenic, mercury, cyanide and polychlorinated biphenyls, or PCBs.

Called Area C, the riverfront landfill sits on a 22-acre parcel Ford still owns. The dump has languished, while the main Ford site above was remediated and sold to developer Ryan Cos. for a green new urban village christened "Highland Bridge."

Down below, Area C's fate remains unknown.

Data from the nine new monitoring wells going in and around the site — adding to the 10 already there — are expected to help determine the scale of cleanup Area C requires.

"We'd like to see the entire waste pile removed," said Colleen O'Connor Toberman, director of the River Corridor Program at Friends of the Mississippi River. "We think that would be the right thing to do for the neighborhood and the river."

Area C isn't the worst threat facing the Mississippi River, she said, but it's a unique one in that "there's a very clear responsible party for the contamination, and that responsible party is still around."

Ryan and Xcel Energy are working on plans for a large solar array on the Area C parking lot to supply renewable energy to the Highland Bridge development. It's not clear how a cleanup will affect those plans.

At a minimum, according to the Minnesota Pollution Control Agency (MPCA), the eroding south slope of the waste hill must be excavated to remove crushed barrels and old paint sludge found near the surface. But the worst of the toxic waste lies hidden inside the hill, encapsulated at the bottom like a chamber in an ancient pyramid.

The waste hill has been sampled and investigated since the early 1980s, when someone left an anonymous phone message at the MPCA saying: "During the 1950s near the Ford Plant, waste solvents from the plant's painting operation were dumped over the steep bluff overlooking the river, and waste in barrels was buried."

After Ford stopped dumping the paint sludge at the spot, it basically became a demolition landfill. The hazardous chemicals lie encapsulated under 40 to 50 feet of soil and construction rubble and debris disposed there over the years from rebuilding Ford's Lock & Dam No. 1 and work on Mississippi River Boulevard.

Despite decades of soil and water monitoring, no one has conclusively determined how serious a threat the waste poses. At present, not much of the toxic waste appears to be escaping, according to Amy Hadiaris, a hydrogeologist and supervisor in the MPCA's remediation division. She said she suspects the worst of the damage was decades ago and that most of the pollutants have long since washed down the river.

"It seems unlikely to me that there are intact barrels in that buried waste pile given the setting," Hadiaris said.

So far, only very low levels of some contaminants have been found in the groundwater around the site, according to Hadiaris. They are not at levels that pose a risk to the river, she said.

The state Department of Health has concluded Area C poses no health risk to humans because the groundwater isn't used for drinking, there's little impact on the river's water quality and the contaminants aren't polluting the air.

"The data collected to date does not indicate that public health or the river are at risk from the buried waste," Hadiaris said. "It's not a catastrophic situation."

Not everyone agrees.

"I don't think we know enough about the site to make any assertions about its potential risk," said O'Connor Toberman with Friends of the Mississippi River.

Earlier soil-boring samples from 40 to 60 feet under the parking lot revealed "black smelly soil with glass, metal" and a "solvent odor," the MPCA has said. Trenches dug into the eroding south slope revealed crushed barrels and paint sludge.

In 2018, engineering consultants hired by the local watershed district concluded that the existing research doesn't adequately show how contaminants might be moving in the groundwater around the site. They requested the state require Ford to install more monitoring wells that go deeper, and in different locations to better understand the movement.

That prompted the current round of well drilling by Ford's environmental consultant Arcadis. Arcadis expects to be done by the end of the month. The MPCA agreed to collect groundwater samples over two flooding seasons, with final results in 2022.

Ford said it plans to submit a feasibility study laying out cleanup options by the end of the year. The state won't evaluate it until it finishes analyzing the new samples, Hadiaris said.

Cleanup options will likely range from simply doing more monitoring, to digging out and removing part or all of the waste. Ford remains the party responsible for the remediation. The MPCA said it will probably decide on the cleanup in 2022, after community meetings and public input.

"If conditions change in the future, then Ford has to respond to that," Hadiaris said. "They don't get a free get-out-of-jail card here."

Jennifer Bjorhus • 612-673-4683