A "big tiramisu” of waste is seriously threatening the quality of the air and water in the towns of Southbridge, Charlton and Sturbridge, according to a local resident and MassPIRG representative. Kirstie L. Pecci, a Sturbridge resident and staff attorney for MassPIRG, told selectmen Monday night that Casella Waste Systems has filed a proposal with the Massachusetts Environmental Policy Act Office of the Executive Office of Energy and Environmental Affairs to expand the Southbridge Landfill.