Since 1976, the Environmental Protection Agency has encouraged companies to meet pollution-control obligations by reducing emissions or acquiring emission-reduction credits.

During the six-year, 102.5-mile Delaware River dredging, about 3,037 tons of nitrogen oxide will be emitted as the channel is deepened from 40 to 45 feet.

The project's local sponsor, the Philadelphia Regional Port Authority, bought $3.3 million worth of credits from five manufacturing or energy companies that shut down operations or reduced emissions below regulatory limits.

The credits, which are registered with the states, are an approved, legal way to offset increases in pollution.

"We went through a broker that specializes in energy credits," port authority special-projects engineer Lisa Magee said. "We said we needed to buy a minimum of 607 tons of nitrogen-oxide credits within a given area. They identified sellers that had credits available. It's like any other transaction: There's a purchase price, and an offer price."

Norfolk Dredging Co. has a $24 million contract to deepen the navigation channel five feet in a 12-mile stretch near Delaware City, Del.; Fort Delaware (Pea Patch Island); and Fort Mott, N.J., Army Corps spokesman Ed Voigt said.

The Philadelphia Regional Port Authority bought the emission credits from five sellers: Exelon Generation Co. L.L.C., Philadelphia; El Paso Merchant Energy North America, Gloucester County; Gerresheimer Glass Inc., Cumberland County; AGC Flat Glass North America, Burlington County; and LaFarge North America Inc., New Castle County, Del.

Under the Clean Air Act, state and local agencies can collect credits from sources that have reduced emissions and bank them so there's room for economic growth, said Bill Harnett, EPA director of air-quality policy.

"It's not unusual, and it is allowable," Harnett said, "as long as they meet certain criteria for being real and quantifiable emission reductions to offset the new emissions coming in."

Philadelphia Inquirer, 2/27/2010