Several train cars are immersed in the Yellowstone River after a bridge collapse near Columbus, Mont., on Saturday, June 24, 2023. The bridge collapsed overnight, causing a train that was traveling over it to plunge into the water below. Photo: AP Photo/Matthew Brown

REED POINT, Mont. — Two months after a railroad bridge collapse sent carloads of hazardous oil products plunging into Montana’s Yellowstone River, the cleanup workers are gone and a mess remains.

Thick mats of tarry petroleum asphalt cover portions of sandbars. Oil-speckled rocks and bushes line the shore along with chunks of yellow sulfur, a component of crude. In the middle of the river downstream of the bridge, a tangle of steel juts out of the water from a ruptured railroad tank car that has not been removed.

The railroad, Montana Rail Link, in conjunction with federal and state officials last week halted most cleanup work and stopped actively looking for more contaminated sites. They said dropping river levels that have been exposing more pollution also make it harder to safely operate the large power boats used by cleanup crews.

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Almost half the estimated 48,000 gallons (180,000 liters) of molten petroleum asphalt that spilled has not been recovered, officials said. That includes 450 sites with asphalt in quantities considered too small or too difficult for efficient removal, according to data provided to the Associated Press.

The spill extends more than 125 miles (200 kilometers) along a stretch of the Yellowstone popular among anglers and recreationists and relied on by farmers to irrigate crops. Yellowstone National Park is upstream of the bridge collapse and was not impacted.

The scope of remaining pollution was evident this week when viewed by boat downstream of the collapsed bridge, which has since been repaired. Asphalt could be seen on every river island visited, ranging from globs stuck on riverside vegetation to thick mats of tar oozing across sand bars as summer temperatures heated it into a viscous liquid.

“What we’ve seen out there tells us that there should be a second phase of cleanup. They need to come back and they need to do a better job,” said Wendy Weaver, executive director of Montana Freshwater Partners. The non-profit group focused on water protection has received reports of tar balls and other asphalt at more than 40 sites cleanup workers already passed through.

Elevated levels of a toxic component of oil known as polycyclic aromatic hydrocarbons, or PAHs, have been detected in mountain whitefish downstream of the spill site, prompting an advisory against eating any caught along a 41-mile (66-kilometer) stretch of the Yellowstone. The contamination has not been conclusively linked to the derailment but the asphalt that was spilled contained PAHs, according to documents submitted to federal officials.

Test results on other fish species are pending, said Chrissy Webb with Montana Fish Wildlife and Parks.

Downstream intakes for drinking water and irrigation were temporarily shut down after the spill and have since reopened with no impacts reported.

Asphalt is not as volatile as other oil products such as gasoline or diesel. It emits chemicals toxic to humans and the environment at a slower rate, but also breaks down slowly and can have a more lasting impact, said University of Houston petroleum chemist Ramanan Krishnamoorti.

Asphalt in the water could be especially problematic, he said. It won’t readily set or harden as it typically does after being exposed to air, such as when used in road construction or as roofing material.

“It can be toxic, especially once you let it sit in the environment without it getting really set,” Krishnamoorti said. “It could also get ingested by fish in the water and that could be a massive challenge, because it’s essentially not digestible by most living things so it can sit in the body.”

Federal and state officials cautioned in the days after the derailment that much of the spill would not be recovered and a too-aggressive cleanup risked further harm to the environment. Cleanup crews collected asphalt at 377 sites — along with more than 20 tons of rocks, sand and vegetation that stuck to the asphalt as it began to harden, according to data provided by federal officials.

Montana Rail Link spokesperson Andy Garland said the company was committed to addressing the derailment’s impacts, and decided in coordination with state and federal officials that “a different approach” was needed. He said a local task force will continue to respond to reports about asphalt.

To merit removal the asphalt must cover an area greater than 20 inches (50 centimeters) wide when found in pebbles or rocks, or greater than 6 inches (15 centimeters) in sand.

As recently as Aug. 3, officials anticipated the cleanup work continuing “via boat and land” into fall, according to a planning document approved by the government and railroad. Less than two weeks later, with work slowing as the water dropped, officials said they passed a threshold triggering a wind-down of the cleanup. That threshold was three or fewer sites with contamination deemed extensive enough for removal, over any 10-mile (16-kilometer) stretch of the river.

Crews working downstream reached that point near Custer, Montana, about 136 miles (219 kilometers) from the bridge collapse.

Six tank cars filled with asphalt went in the river during the June 24 derailment along with three cars filled with molten sulfur. Railroad representatives and government officials have declined to say how much sulfur — which is also a petroleum product — was released or how much was cleaned up.

Sulfur is naturally occurring and can give off hazardous gases at high temperatures, but is not considered a threat once it cools and hardens, Krishnamoorti said.

The EPA declined to make anyone available for an interview for this story. Montana Gov. Greg Gianforte’s press secretary, Kaitlin Price, declined to say whether the Republican was satisfied with the cleanup, adding that his priority was protecting public health and the river.

The derailment marked the third large petroleum spill into the Yellowstone in recent years, following ruptures of crude oil pipelines that crossed beneath the river in 2011 near Laurel, Montana, and in 2015 near Glendive. The river has a shifting channel that gets severely scoured during flooding.

The cause of the bridge collapse remains under investigation. It happened following torrential rainfall and at a time when the river was swollen with melting mountain snow.

A follow-up search for asphalt along the river is planned next year and officials said shifting sandbars could reveal more that can be removed. Weaver worries next year’s spring surge of snowmelt could wash the remaining asphalt further downstream or bury it.

“I feel like they’re trying to sweep this under the rug,” she said.