FREEMAN, S.D. (AP) — TransCanada estimates that about 16,800 gallons of oil leaked from the Keystone pipeline into a field in South Dakota.

TransCanada says it reported the estimate Thursday to the National Response Centre and the Pipeline and Hazardous Materials Administration. The company says the estimate is based on the excavation of soil to expose more than 100 feet of pipe and takes into account a number of factors including oil observed in the soil and the potential area impacted.

The company says the leak was reported Saturday. TransCanada hasn't said what caused the leak. The pipeline is shut down while officials investigate.

Some 100 workers are at the site, about 4 miles from the Freeman pump station in Hutchinson County.

The pipeline runs from Alberta, Canada, to refineries in Illinois and Oklahoma.

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