Our view of the Chesapeake Bay keeps expanding. First came understanding of its watershed, the six-state drainage basin whose runoff affects water quality downstream in the bay.
Now, it seems, the sky's the limit. Welcome to the "airshed."
Unlike watersheds, defined nicely by the physical slope of the land, airsheds are amorphous beasts, with boundaries fluid as the wind and extending, in the bay's case, to the Ohio River Valley and beyond.
No one wants to embrace such a regulatory tar baby, but there appears little choice, given the unexpected magnitude of the airshed's linkages to water pollution.
The problem is nitrogen, a nutrient and key pollutant in the bay's decline in oxygen and loss of aquatic grasses. An estimated 376 million pounds of it runs into the Chesapeake annually, much of it from sewage, manure and fertilizer. To improve bay oxygen levels even slightly, Maryland, Pennsylvania, Virginia and the District of Columbia must reduce nitrogen pollution by about 75 million pounds, according to federal and state environmental officials.
In just the last couple of years,we have recognized that the Chesapeake's airshed adds to the nitrogen problem -- nearly 400 million pounds, in fact. That's the estimated annual fallout, either directly on the bay (11 million pounds) or on its watershed lands (387 million pounds).
The nitrogen comes primarily from industrial and power-plant smokestacks, automobile exhaust and home heating. A lot is generated within the states of the bay watershed, but a sizable, undetermined quantity comes from farther away. Better definition of the bay's airshed is a current research priority.
Airborne nitrogen seems to fall more heavily across the bay region than anywhere else in the nation, ranging from nearly 10 pounds per acre each year in central Pennsylvania to more than 5 pounds on Virginia's Eastern Shore.
Fortunately, most of this fallout is absorbed or otherwise rendered harmless by the watershed's soils and vegetation. This is another excellent reason to protect all the trees and wetlands we've got left, because they are the best natural defense against airborne nitrogen.
Even so, a lot escapes; perhaps 25 percent of the nitrogen reaching the Chesapeake and causing problems there is falling from the sky -- an impact on the same order as that from human sewage, agricultural manure and fertilizer.
Given this, it may seem odd that federal and state bay managers have elected not to include airborne pollution in the "controllable" sources whose reduction by 40 percent is the official goal of the Chesapeake's restoration.
Achieving those reductions won't be easy or cheap. And, as we agonize over agricultural and sewage cleanups across the watershed, there will be an understandable tendency at times to cast our eyes skyward and ask why the automobile, the power plant and other airborne polluters weren't invited to the reducing session. There is, in fact, considerable new clout to reduce airborne nitrogen through two recently strengthened federal laws -- but only if Maryland and the other bay jurisdictions get serious about exploiting the provisions.
The 1990 re-authorization of the Clean Air Act requires a 60 percent reduction of "NOX," the nitrogen oxides from car exhaust, by the 1998 model year. Nitrogen oxides are the source of most of the airborne nitrogen we're concerned about.
A 60 percent cut seems like a lot, but it's not when set against ever-rising "VMT," or vehicle miles traveled, which are projected to rise as much as 61 percent around the bay in the next decade or so.
Moreover, air-quality officials in Maryland and many other states in the North and East currently are fighting for California-type emission standards, allowing only half the nitrogen of those under the plain vanilla version of the Clean Air Act.
The fight is being waged under the banner of reducing urban smog, but the bay's health would directly benefit, too. To fight acid rain, the Clean Air Act also requires that nitrogen oxides from power plants and other large smokestack sources be cut in half by 1996. Again, this would be impressive in the short run, but growth in energy demand promises to offset it.
Some very preliminary projections from the U.S. Environmental Protection Agency show an overall reduction in airborne nitrogen of only 7 percent for the bay region by the year 2005.
A second new law, the federal Transportation Act of 1991, could substantially magnify the power of the Clean Air Act to cut nitrogen. The Transportation Act contains for the first time both money and mandates to take air quality and land use into account in transportation planning. And the legislation makes it far easier than in the past to divert money from highways to alternative modes of transportation -- and even to greenways and tree planting.
The extent to which this happens will depend, in my opinion, on how aggressively citizens and environmental groups put pressure on public officials -- who surely will be lobbied hard by the road builders to maintain the status quo.
For the short term, EPA and the bay states are right to focus on reducing nitrogen from sewage and agricultural sources. These are well-documented problems, and the solutions are fairly well understood.
Within the next year or two it looks like the airshed will be better defined, and a number of difficult questions about the behavior of airborne pollution should be answered.
I'm convinced that at some point we will need not only to reduce airborne nitrogen, but also to cap it. That's what the new clean-air laws do to sulfur dioxide, a key ingredient in acid rain. And the bay states are committed to putting a cap on nitrogen from sewage and agriculture.
In the meantime, Marylanders can do a lot to protect the bay's airshed:
* Burn less fuel in your own driving; this directly reduces the amount of nitrogen fallout.
* Insist that your legislators back the cleanest-possible automobiles this year, for the bay's health and your own.
* Conserve energy any way you can, so power plants burn less fossil fuel.
* Protect our forests and wetlands. Each acre keeps several pounds a year of airborne nitrogen out of the bay.