A contractor was using a hose and vacuum truck Friday to collect paper fibers that were contained in an accidental discharge of 15,600 gallons of cooling water at the Catalyst Mill in Rumford. The leak of “thermal seal water” went into the Androscoggin River on Thurdsay afternoon. State environmental officials said the leak involved “essentially hot water with paper fiber.” Maine Department of Environmental Protection officials were at the spill site Friday to assess the situation and offer help if needed, the agency said.