Late in the evening of April 23, 1988, a tank at the Shell Manufacturing complex in Martinez, filled with hundreds of thousands of gallons of San Joaquin crude oil, began to leak. A hose, designed to drain water from the roof of the tank, failed. Oil began siphoning out into the containment area surrounding the tank. Unfortunately, a storm water release valve had been left open, and the oil continued to drain into a nearby creek, under the freeway, and down into a marsh now called McNabney Marsh. Oil filled the l00-acre marsh to a depth of more than four inches before flowing under the railroad tracks, past the refinery and chemical plant, and finally out into the Carquinez Strait, upstream into Suisun Bay, and, on the next tide, downstream into San Pablo Bay.

Due to darkness, it took a while before anyone noticed the spill and a while longer to figure out where it had come from. Workers at the Shell wharf were the first to recognize and report oil on the water. Before the source of the spill could be located and stopped, about 400,000 gallons of heavy crude oil had leaked out into the environment.

Many federal, state and local agency personnel, oil company representatives, cleanup contractors, scientists and others responded to the spill. In an attempt to recover as much oil from the surface of the water as possible, Clean Bay, an oil company cooperative, dispatched skimmers, and Shell and Coast Guard personnel placed oil boom and sorbant materials. After as much floating oil as possible was removed, cleanup of residues on shorelines began. Pump trucks sucked pooled oil from the McNabney Marsh, and a legion of Shell workers spread and retrieved sorbant boom, pom-pom, and pads, Cleanup of waterfront areas in Martinez and Benicia involved the use of high-pressure water washing to mobilize deposited oil and sorbant pads to recover it. This initially had only limited success, but in the end proved to be quite effective. The McNabney Marsh was ultimately drained, and contaminated vegetation was cut and removed by small crews using hand tools.