BISMARCK, N.D. (AP) — Officials say a produced water spill at an oil well near Watford City was due to vandalism.

North Dakota Oil and Gas Division officials say valves were opened Sunday, January 23, at the location, leading to a spill of more than 18,000 gallons. Most of the wastewater had been recovered by vacuum trucks.

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

A state inspector has been to the Abraxas Petroleum Corp. site and will monitor cleanup required.