TransCanada Corp. says its Keystone pipeline has leaked an estimated 210,000 gallons of oil in South Dakota. The company said that crews shut down the pipeline Thursday morning after a drop in pressure was detected resulting from an oil leak that's under investigation. The section of pipe near the Ludden pump station in Marshall County, South Dakota, has been isolated and the company says emergency response procedures were activated. Brian Walsh, an environmental scientist manager at the South Dakota Department of Environment and Natural Resources, says officials don't believe the leak has affected any surface water bodies or threatened any drinking water systems. TransCanada says it expects the pipeline from Hardisty, Alberta to Cushing, Oklahoma, and to Wood River/Patoka, Illinois, to stay shut down as the company responds to the leak. "Just days before the Nebraska Public Service Commissions decides on whether to approve Keystone XL we get a painful reminder of why no one wants a pipeline over their water supply," said Greenpeace campaigner Mike Hudema. Opponents of Keystone XL say the pipeline would pass through the Sandhills, an ecologically fragile region of grass-covered sand dunes, and would cross the land of farmers and ranchers who don't want it. The Sierra Club also was quick to condemn the spill, urging the commission not to vote for the project. "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 campaign director Kelly Martin. The pipeline would transport oil from Alberta through Montana and South Dakota to Nebraska, where it would connect with existing pipelines that feed Texas Gulf Coast refineries.