SANTA PAULA, Calif.—As he slowly pulled his beige van into the driveway following a trip to the hardware store for garden supplies, Ethan Higbee didn’t suspect anything was wrong.

Then he got out of his car.

The unmistakable smell of gas subsumed him.

“I heard rushing, gushing water. What I thought was water. But it wasn’t, it was oil,” Higbee said Monday, exactly six months after rivulets of crude oil ran into a waterway near his home.

The oil—which contaminated at least three-quarters of a mile of a remote tributary of Sisar Creek—came from the top of the hill next to Higbee’s house in Santa Paula. The origin? Oil inside of an above-ground storage tank had breached, state crews later discovered.