RAY, N.D. — A reported 8,400-gallon saltwater leak in northwest North Dakota's oil field actually spilled 1.4 million gallons for nearly a month, according to a state agency.

The leak that Hess Corp. initially reported Aug. 15 in Williams County started July 21, the North Dakota Department of Environmental Quality said Monday, Aug. 22, in a news release. It could take almost a year to clean up, said Karl Rockeman, water division director for the department.

The leak has been stopped, and Hess crews are working to clean the spill, company spokesman Rob Young said in a statement to The Forum.

“Our immediate priorities are cleaning up the release, doing so safely while ensuring we protect the community and the environment,” Young said. “It is not immediately known what caused the release, but Hess is conducting an investigation to identify the issues and incorporate any lessons learned.”

The spill that was discovered by a local farmer happened about 8 miles northwest of Ray, Rockeman said in an interview with The Forum. The small town of roughly 600 people is 90 miles west of Minot.

The leak was about 170 times the amount initially reported by Hess, an oil company headquartered in New York. The company thought only 200 barrels leaked from a 6-inch flexible, steel-reinforced composite pipeline, according to the news release. On Monday, Hess increased the leak amount to 34,000 barrels, citing further investigation, the environmental quality department said.

It’s unclear why the leak was underreported, Rockeman said.

Companies have tools to detect saltwater leaks, such as pressure monitors, Rockeman said.

“While I don’t want to preempt the investigation, what I can say is that we continuously monitor pressure on this pipeline and the system is automated to stop when high pressure is detected in order to avoid over-pressuring the line and causing releases,” Young said. ”In this instance, there was no indication of high pressure, and the system was not configured to take action based on low pressure, so the investigation will need to examine what has occurred.”

Rockeman’s department is investigating what caused the leak and why it was not detected early. The North Dakota Industrial Commission Oil and Gas Division also is looking into the spill.

The spill impacted farmland, though the amount was unknown as of Monday, Rockeman said. Some groundwater was affected by the leak, but the spill did not spoil drinking water, Rockeman said.

The state plans to work with Hess in the event that landowners file requests for compensation, Rockeman said.

Known in the oil and gas industry as produced water, saltwater is a byproduct of oil drilling that contains other chemicals. It can kill vegetation and make groundwater unusable.

Most produced water is deposited into underground wells.

The largest oil field spill in North Dakota unfolded over a five-month period in 2014 and 2015. A pipeline near Williston, N.D., spilled 29 million gallons of produced water.

Summit Midstream Partners initially reported the pipeline only spilled 70,000 barrels of saltwater over a 10-day period. The amount was underestimated by 10 times.

The leak contaminated more than 30 miles of Missouri River tributaries. It is believed to be the largest inland spill in history, according to the U.S. Department of Justice.