A mining company that extracts sand and gravel on Grey Cloud Island southeast of the Twin Cities is seeking to expand its operations into the Mississippi River itself.

According to project documents, Aggregate Industries, which mines the lower part of the island between Rosemount and Cottage Grove, in the next two decades wants to expand mining operations into 230 acres of the riverbed south of the island. The company says it has about five years left before its current site is exhausted.

The river bottom in the area was dry land before nearby Lock and Dam 2 was built in 1930, which is why it's owned by a partnership that leases the land to the miners. It's just outside the river's navigational channel.

Melissa Collins, a regional environmental assessment ecologist for the Minnesota Department of Natural Resources, wrote in an e-mail that the DNR isn't aware of any other place in the state where a company mines in the waters of the Mississippi.

The mining hole would be 200 feet deep, which the company says it would fill partially with unusable material once it's done with mining.

"It's very unusual that a private company gets to dig a hole in the river," said Colleen O'Connor Toberman, land use and planning program director for the nonprofit Friends of the Mississippi River.

A representative of Aggregate's parent company, Holcim US, said in a written statement that the firm was working closely with regulators and Cottage Grove officials to study potential environmental impacts.

"This area has the potential to provide the Twin Cities market with construction aggregates for an additional 20 years, but until we have completed the environmental assessment, we won't be in a position to finalize any plans," spokeswoman Jocelyn Gerst said.

For now the company is working on an environmental impact statement, which will outline possible effects of the mining. The document is supposed to be completed by March.

Collins said the mine would be dug in an area where several threatened and endangered species have been seen, including three species of fish, the Higgins eye mussel, Blanchard's cricket frog and Blanding's turtle. There also are concerns about what a hole in the riverbed might do to the flow of the river, O'Connor Toberman said.

And then there's the risk that mining might unearth contaminants, like PFAS — "forever chemicals" that were manufactured in a 3M Co. facility nearby in Cottage Grove. Those chemicals have been linked to some cancers and don't break down easily in the environment.

It's not the first time that Aggregate Industries has tried to secure the needed environmental reviews for expanded mining. It started an environmental review process in 2007, but pulled out because of the financial crisis in 2008, Gerst said. O'Connor Toberman said there were major concerns about vulnerable species back then, too.

Rod Hale, one of a handful of residents who live on the lower part of Grey Cloud Island, was part of the group reviewing that proposal 15 years ago. A full environmental statement was never completed, he said.

Hale said he's not against the project. Gravel, he said, is "a resource that does supply valuable product to the metro area for building and highways and so forth, so I understand that part of it."

But residents have concerns, he said, about the location of a conveyor belt that would extend from the new gravel site to Aggregate's processing plant on the island's other side.

At the same time, the proposed mining expansion raises questions about long-running plans to establish a major park on the island. Some land has already been purchased by both Cottage Grove and Washington County for the park.

Cottage Grove Mayor Myron Bailey admitted it's been frustrating to plan for the park. Previously, the city expected that Aggregate would be done with the island in five years. But the dates keep changing, he said.

"There really isn't a lot we can do with putting a park or a development down there until they solidify their plans," Bailey said. "It's going to cost money, and it's going to take years to put all those things together."

Bailey said that he doesn't oppose the project, though he has some concerns about PFAS contamination ending up back on the island if unused material is deposited there.

The full effects of the project might not be clear until the environmental impact statement is completed next year. "I think that will give us a lot more clarity," O'Connor Toberman said.