TransCanada Corporation had to shut down its Keystone I pipeline Thursday, after the seven-year-old line spilled 5,000 barrels of oil—210,000 gallons, or 795,000 litres—near a pumping station in South Dakota.

The latest in a series of spills came just days before a Nebraska regulatory decision on a final permit that would allow construction of the 830,000-barrel-per-day Keystone XL pipeline between Alberta and the U.S., with National Observer reporting a bad weld as the likely cause—and noting that a whistleblower had anticipated the problem. Regulators “completely disregarded something even though they sent an industry-wide safety notice about the issue” in 2010, said former TransCanada engineer Evan Vokes.

“The pipeline company reported that the spill was discovered after a drop in pressure was detected and that the oil was isolated quickly, within 15 minutes,” InsideClimate News reports. “TransCanada didn’t say how long the pipeline—which carries tar sands oil from Alberta, Canada, to Oklahoma and to Illinois—would be shut down, or what had caused oil to spill.”

“We’ve always said it’s not a question of whether a pipeline will spill, but when, and today TransCanada is making our case for us,” said U.S. Sierra Club Campaign Director Kelly Martin. “This is not the first time TransCanada’s pipeline has spilled toxic tar sands, and it won’t be the last.”

“This spill should be a stark warning for Nebraska’s PSC [Public Service Commission] as it considers TransCanada’s proposed route for Keystone XL through some of the state’s most sensitive farmlands and aquifers,” added Anthony Swift, director of the Natural Resources Defense Council’s Canada project. “Pipelines spill—and TransCanada’s first tar sands pipeline has spilled more than most. Based on the safety record of Keystone I, Nebraska’s PSC should carefully consider the impacts of TransCanada’s much larger Keystone XL pipeline.” [Disclosure: The Energy Mix Curator Mitchell Beer is a consultant with NRDC’s Canada Project.]

Swift adds that the seven-year-old Keystone I “has been hounded by safety issues, with 35 spills in both Canada and the United States in its first year of operation alone. Meanwhile, documents acquired by DeSmog Blog reveal the pipeline has experienced significant levels of corrosion—in one section, 95% of the pipeline had corroded.” South Dakota has seen spills of 21,000 gallons in 2011 and 16,800 gallons last year.

“When TransCanada first proposed Keystone I, the company promised a state-of-the-art pipeline which would ‘meet or exceed world-class safety and environmental standards,’” Swift notes. “In its environmental risk assessment, the company forecast Keystone would leak no more than 1.4 times a decade. The pipeline is not yet 10 years old—and has leaked dozens of times, spilling hundreds of thousands of gallons along its route.”

Keystone XL will rely on similar technology and safety procedures, NRDC states.