BISMARCK, N.D. – After investigating underground, an oil gathering line leak near Williston has been increased from 480 gallons to 16,800 gallons.

The North Dakota Department of Health has been notified of the updated volume, which resulted from a leak in a 4-inch diameter steel gathering line operated by Hiland Crude LLC about 11 miles northwest of Williston.

The spill occurred on May 5 and was reported the next day.

Cleanup in the field where the line leaked has been ongoing since it was discovered.

NDoH official Karl Rockeman said the initial report was a visual look, but after digging holes into the subsurface the spill estimate was increased substantially.

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He said the soil is being excavated and then disposed of in one of North Dakota's 18 specialized landfills for contaminated soils.

Rockeman said the field was isolated and that neither underground nor surface water was affected. A layer of clay below the surface helped the situation, he said.

NDoH employees have been on the site and will continue to monitor the investigation and cleanup, he said.