Nearly 40,000 gallons of crude oil have spilled into the Yellowstone River near Glendive, Montana, as a result of a January 17 leak in a pipeline owned by Bridger Pipeline of Casper, Wyoming. So far, only about a quarter of the leaked amount has been recovered.

The 6,000 residents of Glendive were forced to use bottled water for five days until water from the local treatment plant was declared safe. Over that period, benzene levels in the local water supply were found to be 15 parts per billion, three times the maximum safe concentration, resulting in many residents reporting their tap water to smell like diesel.

Benzene is a highly flammable chemical normally found in motor fuels. While the Center for Disease Control has determined that the immediate risks to human health are minimal, there are potential long-term dangers, given that benzene is very soluble in water, can remain embedded in soil for years and is a known carcinogen. So far, no other communities that draw from Yellowstone River have reported contaminated drinking water.

A further advisory has been issued by Montana Fish, Wildlife and Parks and the U.S. Fish and Wildlife Service regarding any fish caught in the Yellowstone River since the spill. It is known that petroleum compounds, such as benzene, can accumulate in fish for 40 or more days after a spill. This can, in turn, impact birds and other creatures that feed on the fish as toxins build up in the food chain. Efforts are ongoing to capture fish and other wildlife, particularly those already tagged, in order to determine the extent of the impact of the spill on local species.

There are also broader environmental concerns. The Yellowstone River is a critical component of the ecologically rich area just north of Yellowstone National Park. Dozens of species live off of and around the river, including northern pike, bigmouth buffalo, black bullhead, black crappie and bluegill. Endangered species include the pallid sturgeon and bald eagle. The river is also depended upon across its entire length for irrigation and drinking water.

Starting from the Bakken oil patch of North Dakota and Montana, the 193-mile-long Poplar Pipeline transports 42,000 barrels (1,323,000 gallons) of crude oil to a facility in Baker, Montana, approximately 55 miles south of Glendive. The various sections of the pipeline were all built in the 1950s.

Bridger Pipeline has a history of minor oil leaks from its 316 miles of pipelines. According to data from the US Department of Transportation Pipeline and Hazardous Materials Safety Administration, from 2006 to 2013, the company has had twice as many incidents per mile of pipeline than the national average. In total, these have resulted in 10,800 gallons of oil spilled and $175,000 in property damage.

Eight of the incidents have been deemed preventable and included corrosion and material, weld or equipment failures.

The cause of the spill is still being investigated, though the aged pipes are a possible factor. A third of the pipes laid down by Bridger Pipeline before 1970 were fused together using low-frequency electric resistance welds, a technique known to be vulnerable to lengthwise cracks and imperfections. The same faulty manufacturing method set the stage for ExxonMobil’s 2013 Pegasus oil spill in Arkansas. Company spokesman Bill Salvin has stated that the broken segment was replaced around 1970, but was unclear whether the pipes were replaced before or after that date. The pipe was last checked in 2011.

In the meantime, cleanup crews have been deployed in an attempt to contain the oil sheens that have appeared. Success has been minimal, as the ice covering the river makes recovering oil extremely difficult. Rather than simply vacuuming oil from the river’s surface, crews are forced to drill into the ice with specialized equipment and attempt to catch the oil piecemeal.

Jeni Garcin of the Montana Department of Environmental Quality told the Associated Press that “We’re not able to get anything substantial, but we’re mopping it up where we find it.”

Further complications arose from the warmer weather that began last Friday. Workers were forced to dismantle oil collection equipment that had been placed 60 miles downstream, and the slowly thinning ice made work hazardous. As oil has already been detected this far from the original spill, there is a concern that even longer stretches of the Yellowstone river will be polluted.

It is also not clear what will happen when the spring thaw begins. Right now, it is suspected that the majority of the oil is trapped beneath the ice covering the river. The increased currents that inevitably come when the snow and ice melt have the potential to push any unrecovered oil much further downstream.

Sister companies of Bridger Pipeline have had similar problems. Over the span of a single month in 2014, lines operated by Wyoming-based Belle Fourche Pipeline had three oil spills totaling more than 100,000 gallons. These spills included an overflow in an oil storage tank, a breach from a corroded section of pipeline and a leak caused when heavy equipment punctured a pipeline.

This is the second major oil spill into the Yellowstone River in four years. In 2011, an ExxonMobil pipeline was breached, releasing 42,000 gallons of crude oil into the environment and forcing the evacuation of 140 people in Laurel, Montana, who were facing toxic fumes and a risk of explosion. That spill left oil remnants 85 miles downstream of the original breach. Despite the environmental dangers this caused, ExxonMobil has so far only been fined $1 million for safety violations. This is less than a slap on the wrist for the company, which made more than $32 billion in 2013.