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 state Health Department, which is monitoring the cleanup, said no water sources were contaminated and no wildlife was hurt from the spill. "The rest is pretty much stranded in the soil," said Eric Haugstad, Tesoro's director of contingency planning and emergency response. University scientists from North Dakota are studying the soil and plan to plant a test crop there next year, state and company officials said.