Boston has them. Philadelphia has them. Seattle, Baltimore and Atlanta have them.But an effort to bring the popular amphibious vehicles known as "ducks" to Manhattan for tours of the city is being challenged by an environmental group that says they will increase noise and pollution and cause safety problems.The group, Transportation Alternatives Inc., is suing to stop Port Imperial Duck Charterers from building a proposed entry-exit ramp into the Hudson at Pier 78, near West 38th Street.Ducks travel streets like buses, then descend by ramp into water and travel like boats. Some were initially built as military troop carriers known as DUKWs and have been retrofitted for tourism, while others were built new. They each carry about 50 passengers.The Port Imperial company has received the needed permits from the state Department of Environmental Conservation and is awaiting a final one from the Army Corps of Engineers, court papers say.But Transportation Alternatives, which promotes bicycling, skating and walking instead of using cars, says the ducks will pick up "dirt, oil and other contaminants," and then deposit them in the river when the ducks enter the water. The group also points out that the ducks will cross the Greenway, a path used by 10,000 people a day along the Hudson River, every 4.5 to 6 minutes (10 to 14 crossings an hour).Port Imperial spokesman Pat Smith said many other vehicles already cross the Greenway and that the impact on the environment would be negligible.