The Chesapeake Bay drains a 64,000-square-mile watershed that covers parts of six states. But the air pollution that contributes to the Bay's nutrient problem may originate from hundreds of miles outside the watershed.

In recent years, a Chesapeake Bay "airshed" has been defined using the EPA's Regional Atmospheric Deposition Model. The model simulated nitrogen oxide emissions in the Eastern United States and their movement during various weather conditions throughout the year.

That helped to identify areas where significant portions of emissions result in nitrogen deposition to the Bay and its watershed. The resulting "airshed" is 5.5 times larger than the watershed, or about 350,000 square miles.

Still, sources within the airshed only account for 70 percent of the nitrogen deposition in the watershed. The remaining 30 percent comes from the large number of sources that are even farther away, but which individually contribute only small amounts to the watershed.

The "airshed" boundaries, therefore, reflect the "point of diminishing returns" for control efforts. A true airshed that accounted for all deposition in the watershed would likely include almost all of Eastern North America.

It should be noted that the airshed is actually only a "NOx airshed"- other air pollutants would travel different distances and have their own "airsheds."

Most of the airborne nitrogen lands on the watershed, where much of it is absorbed by forests and other vegetation. Still, about 75 percent of the airborne nitrogen that reaches the Bay comes from the watershed; while about 25 percent lands directly on the Bay or its tidal tributaries.

About 25 percent of what lands within the watershed originates from sources within the watershed. About 40 percent comes from Pennsylvania, Mary- land and Virginia, including portions of those states outside the watershed.

Utility emissions tend to be more responsible for deposition on the watershed, while emissions from vehicles are more responsible for deposition on the Bay and its tidal tributaries.