TransCanada estimated on Thursday that about 16,800 gallons of oil leak in a field in South Dakota from the Keystone I pipeline. The spill, discovered by a farmer over the weekend, led the company to close down the pipeline.

The company reported the estimate to the National Response Center and the Pipeline and Hazardous Materials Administration which is equivalent to 400 barrels of oil, according to the Associated Press.

The company began excavating the site on Sunday in a field close the town of Freeman, South Dakota, by turning over topsoil. It said they exposed more than 100 feet of pipe, and the estimate comes from oil being observed in the soil, and the potential area affected.

There are about 100 workers at the site working around the clock to pinpoint the source of the leak in the pipeline. TransCanada spokesman Mark Cooper reported the revised estimate Thursday morning.

“The volume estimate reported this morning to the National Response Centre (NRC) and the Pipeline and Hazardous Materials Administration (PHMSA) was based on the safe excavation of soil to expose more than 100 feet of pipe,” Cooper wrote, according to the Angus Leader. “It takes into account a number of factors, including oil observed in the soil and the potential area impacted.”

Elizabeth Lone Eagle, an official intervener for the state of South Dakota, however is worried about the spill’s close proximity to the James and Missouri rivers and worried that the “groundwater contamination is heading to Yankton, Vermillion, Sioux City… all the way down,” according to CommonDreams.

The leak is the fifth in the state for Keystone I, which was approved by the Public Utilities Commission in 2008. DENR’s spill map shows three releases of petroleum in 2010 and one in 2011, one of which took place at the same pump station in 2010, when less than five gallons were released due to a fitting leak, according to the Argus Leader.

TransCanada’s Keystone I pipeline carries light and heavy crude from Hardisty, Alberta to refineries in Illinois and Oklahoma, while passing through the eastern Dakotas, Nebraska, Kansas and Missouri. The Keystone I pipeline can handle 550,000 barrels, or about 23 million gallons, daily.