Riverkeeper, a New York environmental group, has officially warned three alleged Gowanus Canal polluters that they have 60 days to halt their mucky practices or they will be served with a suit. The warning sets a timetable for Sixth Street Iron and Metal, Ferrara Brothers Building Materials and 107 Sixth Street LLC, the companies against which the watchdog group filed intent to sue notices last week. This announcement comes amid fierce debate over whether the Gowanus Canal is eligible for a federally overseen clean up, a plan that both Mayor Michael Bloomberg and Council member Bill DeBlasio openly oppose. But Riverkeeper lawyer Josh Verleun said that the environmental group’s action is a separate matter from the federal “Superfund” designation. “The thing people worry about with Superfund is that anyone who has ever owned property in the area can be held responsible for pollution,” Verleun said. “But these three are each really discrete issues.”