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Massive oil pipeline break under N.D. farmer's wheat field

"Protection and care of the environment are fundamental to our core values, and we deeply regret any impact to the landowner," Tesoro CEO Greg Goff said in a statement. "We will continue to work tirelessly to fully remediate the release area."

Wayde Schafer, a North Dakota spokesman for The Sierra Club, said the spill is an example of the lack of oversight in a state that has exploded with oil development in recent years.

"We need more inspectors and more transparency," Schafer said. "Not only is the public not informed, but agencies don't appear to be aware of what's going on and that's not good."

Eric Haugstad, Tesoro's director of contingency planning and emergency response, said the hole in the 20-year-old pipeline was a quarter-inch in diameter. Tesoro officials were investigating what caused the hole in the 6-inch-diameter steel pipeline that runs underground about 35 miles from Tioga to a rail facility outside of Columbus, near the Canadian border.

Roberts said state and federal regulators are monitoring the cleanup, and Tesoro estimated it would cost $4 million.

A natural layer of clay more than 40 feet thick underlies the spill site and has "held the oil up" so that it does not spread to underground water sources, Roberts said.

"It is completely contained and under control," Roberts said Thursday. "They got very lucky."