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An early proposal for the Dakota Access Pipeline called for the project to cross the Missouri River north of Bismarck, but one reason that route was rejected was its potential threat to Bismarck’s water supply, documents show.

Now a growing number of protesters are objecting to the oil pipeline’s Missouri River crossing a half-mile north of the Standing Rock Sioux Reservation, which they argue could threaten the water supply for the tribe and other communities downstream.

Early in the planning process, Dakota Access considered but eliminated an alternative that would have crossed the Missouri River about 10 miles north of Bismarck instead of the route currently under construction.

The U.S. Army Corps of Engineers evaluated the Bismarck route and concluded it was not a viable option for many reasons. One reason mentioned in the agency’s environmental assessment is the proximity to wellhead source water protection areas that are avoided to protect municipal water supply wells.

In addition, the Bismarck route would have been 11 miles longer with more road crossings and waterbody and wetland crossings. It also would have been difficult to stay 500 or more feet away from homes, as required by the North Dakota Public Service Commission, the corps states.