Gov. Jerry Brown’s Delta tunnels could harm the quality of Stockton’s drinking water to the extent that water rates would need to be doubled or tripled, a city official testified on Thursday. That’s far from certain and wouldn’t happen for decades. But the tunnels are inching closer to approval, and the state’s voluminous reports on the project are lacking in detail, the city says. “We don’t know exactly how these changes will affect Stockton’s water supply because that information was not included,” Bob Granberg, assistant director of the Municipal Utilities Department, told the State Water Resources Control Board.