In 1988 then-Vice President George Bush aired a campaign advertisement set against the backdrop of the polluted and frothing seawater of Boston Harbor, calling it the "filthiest harbor in America."

Twelve years later, Boston Harbor is cleaner now than it has been in decades. It is a model for large-scale, urban environmental projects both in the United States and overseas.

This month, the completion of a pipeline that transports

But the changes have not come cheap or easy, costing the Massachusetts Water Resources Authority more than $4 billion and coming as a result of several legal battles.

Recen Chhanes

Prior to September 2000, treated wastewater from the Deer Island Treatmen Plant re-entered the harbor through a series of outfall pipes and diffusers located along the President Roads, the main shipping channel connecting the inner harbor with Massachusetts Bay.

But due to the shallow waters of the channel, a plume of wastewater frequently set up off Deer Island that could be seen by any jet aircraft taking off from Logan Airport. Ships passing through the area carried the toxins both away from the harbor and back toward shore. Although 85 percent clean, this plume occasionally fouled dense areas of marine life in large parts of the inner harbor.

Following the opening of the $390 million Massachusetts Bay outfall tunnel, treated Deer Island wastewater now proceeds 9.5 miles under the ocean to a point in the middle of Massachusetts Bay. There it is diffused over almost 1000 feet of pipe into Massachusetts Bay.