An underground wastewater leak that likely started weeks ago has spilled about 20 million gallons of sewage into a Contra Costa County marsh near the bank of the Sacramento–San Joaquin Delta, officials said Tuesday. Staffers with the Delta Diablo Sanitation District had been looking for a leak after noticing reduced inflow into their treatment plant, but they weren’t able to visually identify the spillage until around 3:30 p.m. Monday. The leak, coming from a pipe that carries wastewater from a storage center along its Mouse Trap-like journey to the treatment plant, has deposited nearly 1 million gallons of waste into the nearby marsh between Port Chicago and Pittsburg every day since it started. “At a location such as this one that’s subterranean, that’s in a marshland area, it was difficult to identify on a visual basis,” said Vince De Lange, the general manager of Delta Diablo Sanitation.