Three truck drivers who hauled barges to the St. Croix River with zebra mussels attached will be issued civil citations of $500 each, a Minnesota Department of Natural Resources spokesman said Tuesday.

Meanwhile, eight barges where the invasive species was found have been cleared for use in construction of a new four-lane bridge after the contractor, Edward Kraemer and Sons, presented a revised decontamination plan to the Minnesota Department of Transportation (MnDOT).

About 30 dead fingernail-size zebra mussels were found about two weeks ago after at least four of the barges were hauled from the Fox River in Wisconsin to a staging area in Oak Park Heights. Minnesota law prohibits transporting zebra mussels, which threaten waterways because they change ecosystems and cause problems for boaters and swimmers.

Two trucking firms and three drivers transported the mussels, said Chris Niskanen, a DNR spokesman. The department hasn't yet determined identities of the drivers, he said.

"We looked at this carefully and really the law that was violated here was transporting zebra mussels or an invasive species over the road," he said. "It seems to us the people who were responsible for that, who should have known about it, were the truck drivers."

Kevin Gutknecht, a MnDOT spokesman, confirmed that Kraemer and Sons had resolved the barge problem and said it never delayed the massive two-state project.

Workers and boaters alike must pay strict attention to laws that protect waterways, Niskanen said.

"Minnesotans are really concerned about keeping their lakes free of invasive species,'' he said. "We can't have DNR people at every single boat ramp and every body of water checking every piece of equipment."