It could take as long as six months before people in Richmond know what caused a Chevron pipeline to leak hundreds of gallons of a diesel product into San Francisco Bay in February. An independent firm will investigate what caused a quarter-inch hole to open in the line, allowing 500 to 750 gallons of a diesel-water mixture to pollute the bay, Randy Sawyer, deputy director of Contra Costa County Health Services, told Richmond City Council. He said the work will…