BISMARCK, N.D. (AP) — A wastewater spill from a pipeline break last year in the northwestern North Dakota oil patch was 20 times greater than first reported, the state Department of Environmental Quality said Wednesday.

Tallgrass Energy first estimated that nearly 36,000 gallons of produced water was released during the spill. The company now says the volume was actually closer to 714,000 gallons.

Produced water is a mixture of saltwater and oil that can contain drilling chemicals. It’s a byproduct of oil and gas development.

State Environmental Quality officials have inspected this site several times over the past year, the agency said, and continue to work with the company on damages.

The break occurred about 6 1/2 miles south of Watford City.