Just a few days after a massive oil leak in the Keystone pipeline, a key vote is scheduled Monday for the controversial Keystone XL expansion plan.

As CBS News' Roxana Saberi reported Sunday evening, what appears from the air to be a small spot staining South Dakota farmland is actually an estimated 210,000 gallons of oil that gushed from a leak in the existing Keystone pipeline.

Image of Amherst incident taken earlier today by aerial patrol as part of our initial response. For more updates, visit https://t.co/8yWI1Oq2EM pic.twitter.com/uRNtYUdVjL — TransCanada (@TransCanada) November 16, 2017

TransCanada, the Canadian company running the pipeline, says crews shut it down after noticing the leak Thursday morning and brought the spill under control.

"Right now, as it relates to environmental impacts, we don't see any major impacts right now," TransCanada spokesman Mark Cooper said on Friday.

But local landowner Kent Moeckly is worried about contamination.

"I don't want it to happen anywhere, but of course I don't want it to happen on us," he said over the weekend.

The Keystone pipeline delivers oil from Canada to refineries in Illinois and Texas.

Thursday's leak in South Dakota came just four days before regulators in Nebraska were set to announce their decision on whether to allow TransCanada to build a new pipeline through their state.

For months, the proposed Keystone XL pipeline has sparked criticism from Nebraskans, including Art Tanderup.

"What gives a foreign corporation the right to come in and take land away from Nebraska farmers?" Tanderup wants to know.

In March, President Trump paved the way for the project by reversing former President Obama's rejection of it on environmental grounds, and granting TransCanada a federal permit to develop the Keystone XL.

"It's a great day for American jobs," Mr. Trump said, heralding the reversal.

"TransCanada will finally be allowed to complete this long overdue project with efficiency and with speed," the president reaffirmed last week.

Officials in Nebraska insist the spill in South Dakota won't affect their decision on whether the expansion can go ahead -- and by law, they cannot let it; the Nebraska Public Service Commission is prohibited from taking environmental factors into consideration, as that is a federal responsibility.

But critics say the leak is proof of the perils posed by such pipelines.

If the regulators approve the Keystone XL expansion on Monday -- the new pipeline could carry about 830,000 barrels of oil every day from Canada through three U.S. states; Nebraska, Montana and South Dakota, connecting to the existing Keystone pipeline.

The five-member Public Service Commission in Nebraska could also chose to request changes to TransCanada's proposal, and even if it does give its approval, that decision could still be appealed in court.

In the end, the decision today could lead to another long period of legal wrangling over the controversial pipeline plans, while the mess from the leak in South Dakota is cleaned up.