An oil spill in South Dakota that leaked thousands of gallons of highly polluting oil could damage the environment more than the company has let on.

TransCanada shut down a portion of its highly contested Keystone Pipeline, which transports oil from the Canadian tar sands to refineries in the U.S., at 6 a.m. on Thursday after 210,000 gallons, or around 5,000 barrels, of oil spilled across South Dakota farmland. The type of oil in the pipeline, however, makes pinpointing the size of the spill more difficult than usual, worrying local environmental groups and landowners about its environmental effects.

The spill also comes just days before a crucial moment for the pipeline: Nebraskans will vote on Monday whether to approve the final step in the Keystone XL Pipeline, which would move 830,000 extra barrels of Canadian oil through the Midwest to refineries in Texas and Illinois.

Worries of water contamination

A viscous type of oil called diluted bitumen, or tar sands oil, flows through the Keystone Pipeline. Because it’s so thick, leaks can be difficult to detect. If the oil does spill, it’s often far more detrimental to sensitive water resources. Bitumen is also one of the dirtiest fuels in the world. Unlike conventional crude which can be pumped directly from the ground, water is required to separate the heavy, tar-like substance from the sand it’s found in — a process that depletes and pollutes freshwater resources.

Thursday’s spill happened on a section of pipe along a small road approximately 35 miles south of the Ludden pump station in Marshall County, South Dakota, three miles southeast of the town of Amherst. In a statement, TransCanada said the oil “was completely isolated within 15 minutes and emergency response procedures were activated.” The company did not respond to a request for further comment.

Kent Moeckly, a nearby land owner and member of the Dakota Rural Action Group, told VICE News he’s concerned that the spill could be much larger though, in large part because the computers used to detect oil pressure drops don’t always detect small leaks. “TransCanada thought it was 200,000 gallons. What we found out working with TransCanada, it could very well be 600,000 gallons,” Moeckly said.

His concerns aren’t unfounded. After the last major Keystone oil spill in South Dakota, in April 2016, TransCanada revised its initial estimate of the spill from 187 gallons to 16,800 gallons after the company started digging up the field where the spill occurred. Because diluted bitumen is so dense, it seeps into the soil and river beds rather than rising like conventional, lighter crude, potentially masking the full spill.

In 2010, when over 800,000 gallons of tar-sands oil spilled into the Kalamazoo River in Michigan, the cleanup took nearly three years and cost the company in charge, Enbridge, over $1 billion, making it one of the most expensive clean-ups in history. Enbridge also had to pay over $60 million in civil penalties for violating of the Clean Water Act, which the Trump administration wants to dismantle. That could make spills less costly for oil companies like TransCanada but more expensive for landowners and communities.

Another concern for Moeckly is the proximity of the spill to the Crow Creek drainage ditch, a small tributary to a major water supply for South Dakota, which flows just a few dozen feet from where the spill happened. “What this ditch does is that it takes the snowmelt to the east and escorts it to the James River about 40 miles east of here,” Kent said.

“Any spill of any size presents problems to the soil and to the water,” the Nebraska Sierra Club told VICE News in a statement. “Investing in oil at this time in history is like staking the future on landline phones. In both cases, their days are numbered.”

In a statement to VICE News, however, the South Dakota Department of Environment and Natural Resources said the spill didn’t happen at a site used for drinking water. “There are no nearby municipal drinking-water users in the area of the spill. There is anticipated to be shallow groundwater at this site; however, this groundwater at the site is not being used for drinking water,” a spokesperson said. “The full extent of environmental impacts has not yet been determined as environmental testing will need to be performed.”

Concerns about drinking water contamination aren’t new. Environmentalists feared a major spill from the proposed Keystone XL pipeline could contaminate the Ogallala Aquifer, one of the world’s largest aquifers, which supplies drinking water to more than 2 million people in the Great Plains. A spill from the Keystone XL pipeline over this aquifer, in a worst-case scenario, could contaminate 5 billion gallons of water.

That fear was a key reason for President Barack Obama’s rejection of the Keystone XL pipeline extension, which forced a string of lawsuits from TransCanada. But President Donald Trump put the pipeline back on track in one of his first executive orders earlier this year.

A crucial moment

The spill comes at another critical moment for the future of the Keystone XL pipeline, which has triggered massive protests since its proposal. Diverse groups, such as Native American tribes, landowners, farming associations, and environmental conservation groups, have argued the pipeline unnecessarily threatens the environment.

On Monday, five members of the Nebraska Public Service Commission will vote on whether to go forward with the Keystone XL Pipeline, which would add 1,179 miles to the existing Keystone system. All the extension needs to move forward is approval on a 275-mile portion of the pipeline that cuts through Nebraska.

But a 2011 law prevents the commission from considering Thursday’s spill in their decision. Instead, they’ll rely on oral and written testimonies from over half a million people who have commented so far.

Despite concerns from local landowners and environmental groups, it’s unclear if the spill, regardless of the uncertainty surrounding its effects, will have an impact on the future of the pipeline.