The massive Thornton reservoir, a south suburban quarry holding billions of gallons of stormwater and sewage that nearly filled up this past week, will take at least a month before it drains. Thornton is part of the Deep Tunnel flood-control system and resembles a canyon in South Holland. It takes in sewer wastewater during heavy storms that is later pumped out to the Calumet treatment plant at East 130th Street in the Far Southeast Side Riverdale neighborhood. For the first time since opening i