Halt pollution from a smokestack and our air becomes cleaner within hours.
Stop the flow of sewage into a river and it begins to rebound within weeks.
Even toxic disasters, as when tons of the pesticide Kepone were dumped into Virginia's James River, fade after a few years, because fresh sediments bury the contamination on the river bottom.
Then there is ground water, whose pollution looms as perhaps the longest-term obstacle to meeting restoration goals for Chesapeake Bay.
The "dark, invisible sea," Rachel Carson called it in "Silent Spring."
She wrote: "In the entire water pollution problem, there is probably nothing more disturbing then the threat of ground water's widespread contamination."
Ground water is not easy to define, even for geologists. It's not the underground rivers of popular conception, nor is it, as far as the bay is concerned, the deep aquifers several hundred feet or more underground from which many towns draw their drinking water.
Think of it as a thick slurry formed as rainwater perfuses every nook and pore of the soils extending from just beneath the land's surface to depths of several dozen feet.
It does not flow so much as it soaks like a stain through a blotter, traveling as little as inches per day, often taking up to several decades to finally ooze into the bay or its tributaries.
Visualize ground water by looking at your local stream or non-tidal river when there is no rain. All the flow comes from one source -- the slow, steady ooze of ground water. Bay-bound seepage amounts to a vast, invisible tributary whose flow roughly equals that of the James River, the Chesapeake's third-largest visible source of fresh water after the Susquehanna and Potomac rivers.
And this underground ocean is widely and badly polluted; not with the pesticides Rachel Carson helped to ban but with compounds of nitrogen, which are linked to critical declines of both oxygen and underwater plants in the bay and its tributaries.
Substances other than nitrogen cause problems. But there will be no higher priority in the foreseeable future than reducing it, and little likelihood of a healthy bay if we fail.
Nitrogen accounts for so many forms of pollution that it symbolizes the Chesapeake's plight -- and serves as a measurement of progress.
Sewage from the bay watershed's 14 million humans is a major source of nitrogen; so are the nitrogen oxides emitted by autos and electric generating plants. So also is the runoff of farm fertilizers and manure. Of these sources of nitrogen, fertilizers and manure are most responsible for overfertilizing the ground water tributary. Nitrogen, unlike other components of farm runoff (mainly soil and phosphorus), dissolves easily in rainwater.
Seeping quickly below ground, nitrogen escapes traditional pollution-control techniques that mostly aim at stopping surface runoff. In fact, strategies that attempt to slow surface runoff to control other pollutants, or drain farm soils for more productivity, even hasten nitrogen's passage into the ground water.
And there it stays with little decomposition, oozing bayward, a vast reservoir reflecting the tremendous buildup in our use of farm fertilizers in pursuit of ever-higher crop yields.
The implications are sobering, researchers say. Because it moves so slowly, the ground water now entering tributary streams is "old" water whose nitrogen load does not yet reflect peak levels of fertilization that occurred during the late 1970s and early 1980s.
"The full strength [of the pollution] has yet to be reached in the ground water moving into streams in areas where we've looked," says Robert J. Shedlock of the U.S. Geological Survey's office in Towson.
His agency has completed a survey showing widespread nitrate (nitrogen) contamination of ground water across the Delmarva Peninsula, an area where nearly half of all land is farmed and where manure from hundreds of millions of chickens creates a major disposal problem.
That the worst is yet to come, as "younger" and more contaminated ground water emerges, is also the view of University of Maryland researchers tracking the movements of nitrogen from crop fields.
This also means that even if we could instantly, today, turn off the tap -- abolish or drastically reduce all nitrogen contamination on land -- we would see results in the bay only years or decades later.
On one Pennsylvania farm draining to the Chesapeake, state-of-the-art pollution controls were put into effect several years ago. But levels of nitrogen in the farm's ground water have not declined.
So what's to be done? In the short term, you have to be realistic. We're not likely to meet the federal-state commitment to a 40 percent reduction in nitrogen from land-based sources by the year 2000.
Political leaders will face intense pressure to meet the goal anyhow, even if the accounting methods gloss over the problem of nitrogen buildup in slow-moving ground water.