State officials are considering replacing the 74-year-old Tobin Bridge with a tunnel, prompting mixed reactions from Boston and Chelsea residents.Iris Arroyo said she won’t drive over the bridge. “No, I don’t,” she said. “I’m afraid of it, I go around.” Construction to maintain the aging bridge has been ongoing for years, while state officials look for future alternatives.At a public meeting yesterday, MassDOT project manager Patrick Snyder told the public a tunnel replacing the bridge has not been eliminated from planning.Though it’s unclear where exactly that potential tunnel would go on the stretch of the Mystic River, between Chelsea and Charlestown.”You can’t build it under the existing bridge, and we have a nice community here, so I’m wondering what that would entail,” said Jim Cadden of Chelsea.On the Chelsea side of the bridge, residents are skeptical about the potential project.”You’ve got something that’s proven, put the money into it, you’re …