Work will start around July 1 and the EPA set a Dec. 31 deadline for completion.

The spill occurred after a break in Enbridge’s 30-in.-dia Line 6B pipeline near Marshall, Mich., during heavy rains that sent the oil almost 40 miles downriver.

Enbridge was hit with a $3.7-million fine. The National Transportation Safety Board, which investigated the spill, cited the company’s slow response to the break and earlier lack of attention to previously-known problems with the pipeline.

Breaks like these fuel fears by safety experts, environmentalists and residents for areas to be crossed by the proposed Keystone XL Pipeline from Alberta, Canada, to the Texas coast.

The dilbit spilled in Michigan is in a “molasses-like state” in the river and the proposed cleanup is the most invasive option, says Steve Hamilton, professor of ecology at Michigan State University who worked on a study of the river after the spill.

“The real challenge (with the bitumen crude) is you can’t see it, you can’t control it, it’s hard to measure and only makes itself known when you disturb the sediment,” he says.

The dredging involves “scooping up the whole bottom habitat,” including clams, mussels and organisms that make up the base of the food chain for fish, he says.

“It is very intrusive, with heavy equipment and a staging area on land,” he says. “It’s about as disruptive as you can imagine, short of draining the place.”

A similar but smaller situation is possible in Arkansas, Hamilton says.

ExxonMobil and Arkansas officials maintain that nearby Lake Conway has not been contaminated, but Hamilton counters that photos and videos of a lake cove show otherwise.

 “They will doggedly go after any oil above the water, and get the floating oil in a matter of weeks,” he says. “The problem will be what’s lurking under the surface.”

Part of Enbridge’s Michigan cleanup cost — the company won’t specify how much — was the purchase of about 150 properties along the river that were damaged during the spill. Most of those were single-family homes.

ExxonMobil has offered to purchase the properties in Mayflower, paying for independent appraisals, closing costs and moving expenses. It is also offering $10,000 per household for “the disruption and inconvenience.”

That has not prevented homeowners from filing a lawsuit over damages resulting from the spill and seeking class-action status.