About 30 dead zebra mussels were found on barges that will be used to construct footings for a new St. Croix River bridge, but the discovery won’t significantly delay progress on the massive project, two state agencies said Friday.

The fingernail-sized mussels were found this week after the contractor, Edward Kraemer and Sons, hauled at least four of the barges from the Fox River in Wisconsin, said Chris Niskanen, a spokesman for the Minnesota Department of Natural Resources. Mussels were found on six of eight barges, he said.

“They clearly understood they made a mistake. They self-reported it and have been very cooperative with us,” he said.

Zebra mussels, a highly invasive species, threaten waterways because they change ecosystems and cause problems for boaters and swimmers. The contractor sprayed barges with high-pressure, 190-degree water, but didn’t scrape the mussels off the barges, Niskanen said.

A National Park Service study a year ago concluded that zebra mussels largely disappeared from the St. Croix, possibly flushed by high, turbulent water.

Kraemer and Sons was expected to submit a revised decontamination plan on Friday, seeking approval to move the barges off the banks of the Minnesota side of the St. Croix and into the water. Work could resume early next week, Niskanen said.

“We’re only talking about the work on the river,” Niskanen said. “There’s a lot of work elsewhere that’s occurring with contractors and that’s moving forward.”

A spokesman for the Minnesota Department of Transportation, Kevin Gutknecht, said the barges would carry heavy equipment onto the water once the mussel problem is resolved. Engineers and contractors are accustomed to schedule changes, he said, and the overall project will move forward.

“The work is really just getting started,” he said.

The bridge project, estimated at costing as much as $690 million, is a shared undertaking with Wisconsin and involves extensive highway construction and environmental work on either end of the new bridge. Most of the work so far has been engineering and contractor staging in advance of actual construction.

Two laws were violated — transportation of zebra mussels and attempting to put contaminated barges in the water — and a penalty should be decided Monday or Tuesday, Niskanen said.