What goes up must come down, and pollution released into the air—by cars, trucks, gas-powered lawn tools, power plants and other sources—will fall back to the earth’s surface, where it can wind up in our waterways. Nitrogen and toxic contaminants are two pollutants that affect both air and water through atmospheric deposition, which is the process through which air pollution settles onto land or water. Atmospheric deposition occurs in several stages:
- First, pollution is emitted into the air, where wind and weather can carry it over long distances.
- Eventually, airborne pollution particles fall onto the land or into the water, sometimes in the form of dry particles and sometimes attached to rain, snow or other precipitation.
- Even pollution that falls onto the land—rather than straight onto the water’s surface—can pollute our water, if it soaks into groundwater or if it is washed off of roofs, streets and sidewalks and into storm drains, rivers and streams.
The area of land over which airborne pollutants can travel to reach the Bay is known as the Chesapeake Bay airshed. The Bay’s airshed is quite large: approximately 570,000 square miles, nine times as large as the watershed itself.
Airborne nitrogen is one of the largest sources of pollution affecting the Bay and its tributaries. Scientists estimate that just over one-third of the nitrogen polluting the Bay comes from the air, most often in the form of nitrogen oxides or ammonia. Nitrogen oxides (or NOx) are produced by machines or processes that are powered by gas, coal or oil, like running a car or heating a building. Nitrogen oxides account for two-thirds of the airborne nitrogen that ends up in the Bay and are a big contributor to ground-level ozone pollution. Ammonia emissions are most often generated by livestock or poultry operations. Ammonia accounts for the remaining one-third of the airborne nitrogen polluting the Bay.
The three most common toxic contaminants polluting the Bay airshed include mercury, polychlorinated biphenyls (or PCBs) and polycyclic aromatic hydrocarbons (or PAHs). Once in the water, these contaminants can bind to sediment and enter bodies of small, bottom-dwelling organisms like worms, clams or crustaceans. Through a process known as bioaccumulation, fish that consume contaminated organisms can accumulate these toxins in their own tissue. Because humans that eat contaminated fish can also be exposed to these chemicals, fish consumption advisories are issued in areas where toxic contaminants are a concern.