Trained volunteers will be available by appointment at the Watertown Senior Center beginning Monday to help the town’s senior citizens file their income tax returns, according to the center. The volunteers are trained in an IRS-certified program sponsored by AARP. Each appointment takes about one hour, officials said. The tax sessions are free, but officials said participants could make a donation to the senior center in exchange for the assistance. For more information, call the center at 617-972-6490.