Volunteers from Tufts FOCUS — an acronym for Freshmen Orientation CommUnity Service — on Tuesday will hit city streets to help mark storm drains. The incoming freshmen will work with staff from the city’s Office of Energy and Environment and volunteers from the Mystic River Watershed Association, which have teamed up on a new effort to educate the public about the threat of storm-water runoff to public waterways. Storm-water runoff carries pollutants, including dirt and oil, that pass through drains emptying into the Mystic and Malden rivers. Students will clean the curbs and affix colorful plastic medallions that read “No Dumping. Drains to River” on roads along the Mystic River, downtown, and West Medford Square.