Miranda Jones, vice president of Crestwood Midstream Partners Inc., said the cause of the spill appears to involve a separation of the pipe that carries saltwater, a byproduct of the oil drilling process. Crestwood subsidiary Arrow Pipeline LLC owns the pipeline.
Jones said the path of the brine is 8,240 feet long, and the company has estimated around 1 million gallons spilled. Officials have said it damaged trees, brush and grasses in the area.
Crews are carrying equipment down the steep badlands by hand because of the rough topography.
Claryca Mandan, natural resources administrator for Mandan, Hidatsa and Arikara tribes' natural resources department, said the area is "one of the worst places it could have happened" because the pipeline sits atop a bluff, and the saltwater ran down the rugged terrain of the badlands.
The Environmental Protection Agency was assessing the site Thursday to ensure none of the brine affected the lake an American Indian reservation uses for drinking water.
In the first public statement in the two days since the spill was detected, the agency said it had no confirmed reports that the saltwater had reached Bear Den Bay. It leads to Lake Sakakawea, which provides water for the Fort Berthold reservation in the heart of western North Dakota's booming oil patch.
EPA said most of the spill was pooled on the ground, soaked into the soil and held behind beaver dams.
Cleanup at the reservation site continued Thursday and was expected to last for weeks, Jones said.
The leak likely started over the Fourth of July weekend. The pipeline was not equipped with a system that sends an alert when there is a leak, she said, and the spill was only discovered when the company was going through production loss reports.
Although the EPA said additional assessment activity was being conducted, company and tribal officials said the spill had been contained and did not affect the lake.
"We have a berm and a dike around it, around that bay area, to keep it from going into the lake," said Three Affiliated Tribes Chairman Tex Hall.
Saltwater is a naturally occurring, unwanted byproduct of oil and natural gas production that is between 10 and 30 times saltier than sea water. The state considers it an environmental hazard.
The briny byproduct also may contain petroleum and residue from hydraulic fracturing operations.
Karolin Rockvoy, a McKenzie County emergency manager, said it was apparent from looking at vegetation that the spill went undetected for some time.
The number of saltwater spills in North Dakota has grown with the state's soaring oil production. North Dakota produced 25.5 million barrels of brine in 2012, the latest figures available.
Fort Berthold Indian Reservation plays a key role in the state's oil production, the second-highest in the nation. The reservation currently represents more than 300,000 of North Dakota's 1 million barrels of oil produced daily, according to the state's Department of Mineral Resources.