If you walk along the Mississippi River in St. Paul, huge chunks of concrete, metal pipes and beams signal you're no longer in Hidden Falls Park.

The park at the river's edge is next to a former dump that was used by Ford Motor Company starting in the 1940s. Now that Ford aims to sell the land where it used to make cars and trucks, state officials are deciding whether to make the automaker clean it up.

Floodwaters easily reach the base of the dump site, and there are still signs that floodwaters came through the area just a few months ago, said Whitney Clark, executive director of Friends of the Mississippi River.

The concern, he said, is less the cement and metal and more the potential hazards that might be buried deep within the mound, which is about six stories high and capped on top by a parking lot the size of nearly three football fields.

Before hazardous waste laws existed, Ford dumped paint waste and other material at the site, which is adjacent to the former assembly plant.

"The chief worry is we don't know how much material is there, we don't know how much of it may have leaked out in the past or how much material may have yet to leak out," Clark said.

In a report filed earlier this year with the Minnesota Pollution Control Agency, Ford said it stopped the dumping in 1966. The company found and removed a few metal drums containing suspected paint waste in 2012, but the report indicates there's likely more of it.

Old paint contains heavy metals like lead and cobalt. The even greater worry is whether drums of liquid solvents used at the plant were also dumped there. Those contaminants can pollute groundwater.

Whitney Clark, executive director of Friends of the Mississippi River, looks up at the former dump site Ford Motor Company used starting in the 1940s. Elizabeth Dunbar / MPR News

Ford declined an interview request, but company officials said in a written statement they are working with the MPCA to address the concerns. The company's report shows there are gaps in what is known about the waste — everything from its size and location to what contaminants are present.

MPCA officials have said there doesn't appear to be an imminent threat to the river or groundwater, but Clark said now is the time to lay any uncertainty to rest. If the debris can be removed safely, he said, the area could someday become parkland.

"It's really important that we find that out [what's there]. Frankly, they should have studied that question long before now," he said. "Ford is a good company. I'm sure they want to do the right thing, and our job is just to encourage them."

Ford did investigate the site in the past. The dump was one of the first sites placed on the state's Superfund list after new environmental laws took effect in the 1980s. Investigators drilled some shallow monitoring wells and also tested river water. They did find contamination, but the results were inconsistent, and by the early 1990s after Ford had done some remediation, state officials decided the site wasn't enough of a threat to remain on the Superfund list.

But things are different now, a decade later. Ford has asked the MPCA for environmental closure, said agency hydrogeologist Amy Hadiaris. She said companies are never free from liability for their pollution, but by taking additional steps to clean up or assess the environmental risks, they can obtain a letter from the MPCA saying they've done what's necessary.

"Something might have been closed out a long time ago, but if you're now looking for closure again in our modern world, we're going to take a fresh look at it, we're going to apply the standards that we apply now, because things change over time," said Hadiaris, who works for the agency's Voluntary Investigation and Cleanup Program.

In Ford's case, the MPCA will require more testing and other work at both the dump site and the larger plant site, where homes and businesses might someday be built.

Metal beams and concrete are a common sight at the base of the former Ford dump on the Mississippi River. Elizabeth Dunbar / MPR News

For the dump site, the MPCA wants Ford to add monitoring wells to check groundwater for contaminants and drill into the waste to better understand its contents. Ford will also have to come up with a range of options and cost estimates — everything from leaving the dump site as is to cleaning it all up.

But Hadiaris said it won't be easy to even assess the site, known as Area C.

"All that concrete on top of the waste, leading off of the waste, greatly complicates the investigation and cleanup. It makes it much more costly to do anything, she said. "This Area C on the floodplain was just a convenient way for other parties to dispose of this huge volume of demolition debris."

The other parties who deposited waste include the U.S. Army Corps of Engineers and a road construction contractor for the city of St. Paul. Ford's report says rubble was dumped during the reconstruction of the lock and dam in 1975 and the repaving of Mississippi River Boulevard in 1981.

What kind of arrangements Ford made with those other parties is unknown. Ford declined to answer that question.

St. Paul city officials said if one of the city's contractors dumped material that long ago, they wouldn't know about it because at that time contractors made those decisions on their own.

Anne Hunt, environmental policy director for St. Paul Mayor Chris Coleman, said more environmental data is needed before any decisions are made.

"We need to let it go through the process," Hunt said. "Ford has entered into this program voluntarily, it's a high profile site and our understanding is they're committed to doing what they need to do."

It's also unclear whether the fate of the main Ford redevelopment site up on the bluff is dependent on resolving the dump issue. Ford said in a written statement that environmental work for both sites is being carried out together and that both sites would be part of a master plan submitted to the city.

But city officials said their own discussions have focused on the larger site.

Both the city and the Friends of the Mississippi River have tapped outside experts to help analyze Ford's studies.

While MPCA officials said they don't believe solvents will be a major problem at the former dump, Jeff Broberg, a geologist that analyzed Ford's report for the Friends said the fact that they were detected at all is a concern. He said past testing at the site wasn't designed to pick up accurate levels of contaminants that can sink deep into groundwater.

"It's really begging for more information and more investigation," he said.