This article has been updated on Jan. 22 at 4:00 PM to reflect more recent estimates on the maximum amount of oil that could have spilled in the Yellowstone River.

In eastern Montana, an oil spill under the Yellowstone River over the weekend has tainted the water supply of Glendive, a nearby town of about 6,000 people. The river's thick ice cover, which is two feet in places, is complicating the cleanup efforts.

"My gosh, I know what diesel smells like...and we had a definite diesel smell in our drinking water," said Glendive City Councilman Gerry Reichert. "We sort of have a mess."

Around 10 a.m. on Saturday, Jan. 17, a stretch of the Bridger Pipeline LLC's Poplar Pipeline that crosses the Yellowstone River cracked, for reasons still unknown. Company workers in Wyoming detected a drop in pressure and shut off the pipeline by 11 a.m. During that time, between 300 to 954 barrels (or 12,000 to 40,000 gallons) of oil were released.

Four days after the spill, the Poplar Pipeline, which transports crude oil across Montana, is still shut down. The section of the 50-year old pipeline measures 12 inches in diameter and is a half-inch thick.

According to Bill Salvin, a public relations specialist hired by the pipeline company, the size and the type of the rupture is unknown and it's in a stretch of pipe directly underneath the riverbed. That means most of the oil likely leaked into the water. A 2011 leak from Exxon's Silvertip Pipeline spilled 63,000 gallons of crude oil in the same river—very little of it was recovered.

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Bridger hired SWAT Consulting Inc. to help with the cleanup efforts and provide special equipment to combat spills in cold and icy conditions. Federal, state and local agencies are also involved in the response; they include the Environmental Protection Agency, the Montana Department of Environmental Quality and Dawson County Disaster & Emergency Services.

The spill site is approximately seven miles upstream from Glendive. Where the river passes through town, it's frozen over. Councilman Reichert, who can see the river from his house, told InsideClimate News that no oil is visible.

But around 50 miles downstream, near the city of Sidney, there's open water, and an oily sheen has been detected. Sidney doesn't draw drinking water from the river.

While state and local officials won't say how long they expect the cleanup to last, Glendive Mayor Jerry Jimison predicts it will be a while.

Jimison said the river's ice can linger until as late as mid-March. Until all the ice is gone, he said, "I don't think [the response team] is going to have much success in cleaning up."

Richard Mylott, an Environmental Protection Agency spokesman, told InsideClimate News in an email: "The challenges posed by ice in the river will make collection and recovery more difficult."

Early this week, Montana Gov. Steve Bullock declared a state of emergency in Glendive's county of Dawson and the neighboring county of Richland.

Ice Makes the Going Slow and Unsure

In an oil spill, the goal is to clean it all up, said Dee Bradley, vice president of DOWCAR Environmental Management, Inc., an oil spill response consulting company not involved in this incident.

But that rarely happens, she said, and "when ice is involved, it makes [the cleanup] slower and more difficult."

That's partially because the cleanup situation is riskier for workers in icy conditions. Before oil recovery can begin, the strength of the ice needs to be determined. The thicker and cleaner, or whiter, the ice is, the stronger it is—and the safer it is to walk and work on, said Bradley, who has 23 years of experience of handling such spills. Thinner and dirtier ice is more prone to breaking.

Once the ice is established as safe to move around on, it's possible to start a process called "ice slotting." That involves carving out narrow channels in the ice and scooping up, or skimming, the oil that's floating near the water's surface.

"Crews have begun ice slotting operations in the river between the spill site and Glendive, Montana. Reconnaissance crews are working to identify and access pockets of trapped oil beneath the river in the same segment," according to a Tuesday night report by EPA's on-site coordinator, Paul Peronard.

But this hasn't always been easy, Peronard explains, because "the ice is not structurally sound enough in many locations to conduct response efforts."

To prevent oil from spreading too far, floating barriers to block oil flow called "booms" are set up near the spill's outer edges. But if currents are too strong or there's debris in the water, the boom can get pushed around and even pulled under the ice.

In Montana, a boom was set up 40 miles downstream from Glendive in a patch of open water near the town of Crane. But even there, issues have arisen. According to EPA's Peronard, "further ice formation is creating flows of smaller chunks of ice and bergs that are hampering the ability to boom the river to prevent further migration."

Benzene Shows Up in Town's Water

About 8 a.m. on Sunday, Jan. 18, Mayor Jimison was heading out to church when he received a phone call alerting him of an oil spill nearby. At the time, he was told that the spill from a pipeline break posed no danger to the town or its drinking supply. A test of the city's water supply that morning revealed no contamination.

But later in the day, the mayor received several calls to his home from citizens concerned about strange odors in the drinking water. New tests from Monday morning detected benzene, a known carcinogen found in oil products, in the town's water treatment plant supply. The city's water is now being treated to remove contaminants.

Councilman Reichert finds the presence of benzene disconcerting. The federal Centers for Disease Control says the observed level of benzene isn't bad in the short term, but according to the EPA's website, there's no safe level of benzene exposure in drinking water, he said.

Residents are instructed not to drink or cook with city water and instead use the 10,000 gallons water trucked in daily by the company responsible for the spill. The mayor said Bridger has "really stepped up to the plate" in terms of handling the spill response and the city's needs.

But he is wary of how long it will be before the water supply is deemed safe enough to drink.

For now, Councilman Reichert joked, "I've been drinking beer...and I know how to cook with beer."

On a more serious note, he added: "Will this be an ongoing problem? Will it just clear up? I don't know."

InsideClimate News reporter Elizabeth Douglass contributed to this story.