BISMARCK, N.D. (AP) — One year after a pipeline rupture flooded a wheat field in northwestern North Dakota with more than 20,000 barrels of crude, Tesoro Corp. is still working around the clock cleaning up the oil spill — one of the largest to happen onshore in U.S. history.

Cleanup costs have soared from the company’s original estimate of $4 million to a forecast of more than $20 million, and it may be at least another year before work is completed, the company and state officials said. The oil-sopped parcel of land, about the size of seven football fields, is no longer usable for planting at present.