Reducing air pollution has given an unexpectedly big boost to long-running efforts to clean up the Chesapeake Bay, a new study finds.
Resarchers at the University of Maryland Center for Environmental Science determined that nitrogen pollution in nine mostly forested rivers and streams in the Appalachian reaches of the bay watershed has declined in tandem with government-mandated air pollution reductions for power plants and motor vehicles.
"It was surprising to us," said Keith N. Eshleman, the study's lead author and a professor at UM's Appalachian Environmental Laboratory in Frostburg. The paper was published this week in Environmental Science & Technology.
Clean Air Act legislation adopted by Congress in 1990 ordered reductions in nitrogen oxide emissions from power plants as part of a nationwide effort to combat acid rain. Further nationwide and regional emission cuts were ordered from both power plants and motor vehicles to combat smog and make air healthier to breathe. By one estimate, nitrogen oxide emissions in the eastern United States declined by 32 percent from the late 1990s through 2005.
What surprised Eshleman and his colleagues was the degree to which they saw similar declines in the amount of nitrogen falling on the land and getting into streams. Other studies had not detected such a correlation, he pointed out.
Parsing data collected by a network of acid-rain monitoring stations, the UM scientists found there had been a 34 percent decline over the years in nitrogen deposition on land from rain and snow in the nine watersheds.
Looking at water-quality sampling data for the nine watersheds, the researchers found even greater reductions over the same time period in nitrogen levels.
"Here the Clean Air Act has caused something to happen that ‘s wonderful and good news and completely unanticipated," Eshleman said. "It’s bizzare."
Donald F. Boesch, president of the UM environmental science center, said the study suggests that air pollution controls have been one of the most effective tools employed to date to clean up the bay, surpassed only by upgrading sewage treatment plants.
"It raises the question about how much of the reductions we’ve seen (are due to) reduction of atmospheric deposition and how much due to other management actions," Boesch said, including attempts to curb hard-to-measure farm and suburban runoff.
Boesch recalled that when the bay restoration campaign formally began 30 years ago, federal and state officials considered the impact of air pollution on the Chesapeake to be "uncontrollable."
Attitudes have changed since then. Scientists now estimate that a little more than one-third of the nitrogen polluting the bay comes from the air, according to EPA, and up to half of that comes from outside the bay watershed, carried on winds from as far away as Canada, the Midwest and the Carolinas.
Eshleman noted that the UM study focused only on rivers and streams in the Appalachian Mountains from Pennsylvania through western Maryland into Virginia. The water-quality benefits of cleaner air may be most pronounced in this small portion of the overall bay watershed, he said. The region is heavily forested, with lots of trees there to soak up nitrogen.
But air pollution reductions may help explain nitrogen declines seen in some other rivers as well. Rich Batiuk, associate director in the Environmental Protection Agency's bay program office, said scientists have noted improvements in the Susquehanna River, the Chesapeake's largest tributary, that can't be completely explained by sewage plant upgrades or farm runoff controls.
"We've seen the same thing that Keith has," Batiuk said.
For those hoping cleaner air may relieve bay states from costly sewage plant upgrades or controversial farm and storm-water rules, Batiuk said the "pollution diet" that now governs the bay cleanup has already taken into account the reductions in airborne nitrogen from existing air pollution laws and regulations.
But Eshleman said he still sees room for more progress via regulating air pollution. EPA has moved slowly or even been stymied by court challenges in adopting other clean-air rules that might help the bay, particularly measures aimed at curbing interstate pollution.
"There’s no doubt this is not the lion’s share of nitrogen getting into the bay," Eshleman said of the Appalachian rivers and sterams he studied. But given the cost and controversy around at least some aspects of the bay cleanup, he added, "I think it’s incumbent for us to think what’s working and what isn’t."