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　　NARRATOR:Listen to part of a lecture in an environmental science class.
　　FEMALE PROFESSOR: The Chesapeake Bay, on the east coast of the United States, is huge—the largest estuary in the U.S.—and it’s very important to local economies.But, like many of the world’s waterways, the Chesapeake is being polluted; and efforts to stop that from happening have not been entirely successful.
　　And that’s partly because of the type of pollution affecting the Chesapeake… which may not be what you might predict.Uh, first let’s mention that the sources of pollution are of two general types.And let’s begin with what’s known as “point source pollution” .
　　Point source pollution has an identifiable source, and you can find the specific point where, say, one particular pipe is dumping pollutants into the bay—and then treat the water right there where the pollution’s coming from.And that’s what’s happened over the past 30 years or so.Modifications have been made at factories and sewage treatment plants to treat polluted water before it’s released into public waterways.But there’s also something we call “non-point source pollution.”
　　Nowadays, the most serious pollution threat doesn’t come from any particular source, like a factory or sewage treatment plant, but originates from many sources over a large area.And this non-point source pollution is a challenge to deal with, because it doesn’t just enter the bay through one pipe—you can’t identify precisely where it’s coming from.
　　And to be specific, the biggest problem now facing the Chesapeake Bay is due not to toxins, but to nutrients contained in chemical fertilizers used on farms all over the region.These nutrients—like phosphorus and especially nitrogen—wash away in what we call agricultural runoff.That’s when water from a hard rain or from melting snow carries these chemicals down to streams and into the bay, and there they stimulate the explosive growth of algae.And that uses up much of the oxygen in the water, oxygen that fish and other aquatic organisms need to stay alive.
　　So, since there’s no single place you can treat the runoff before it reaches the bay, any efforts to reduce this non-point source pollution generally need to be aimed at keeping pollution out of the streams in the first place.
　　But before we go into that, let’s look at the role of nitrogen fertilizer in modern farming.Until about 60 years ago, before a great increase in industrialization, this wasn’t a problem.In the past, farmers used natural fertilizers and rotated crops so that, in addition to commercial food crops, like corn and wheat, they might plant legumes, like alfalfa and clover, for animal feed.But these legumes also enriched the soil—by converting nitrogen in the atmosphere into nitrates—a form of nitrogen the crops like wheat or corn could use as a nutrient.And, these and other “cover crops,” planted to hold the soil after the wheat or corn was harvested—they stored much of the surplus nitrogen during the time of the year when the runoff tended to be greatest.
　　But farming practices changed as farmers came under pressure to use more and more chemical fertilizer in order to increase crop production on the same amount of land.But more isn’t always better, at least in terms of chemical fertilizer in the environment.And, along the way, farmers switched from legumes…to animal feeds more suited to intensive, large-scale animal production.And the excess nitrogen, once trapped by these cover crops, either washed away in the next big rain or went down into the groundwater and, either way, eventually ended up in the streams and the bay.And that, as we said, means more algae in the water and less oxygen for the fish and other aquatic life to breathe.
　　So what’s being done? Well, two things.First, after the main crops are harvested, more farmers are planting cover crops again—other kinds, like rye and barley—that hold the nitrogen and keep it from washing out of the soil during the months when that’s most likely to occur.
　　And the second strategy is to plant “buffer zones” at the edges of streams—not crops, but natural areas…trees.The roots of these trees can absorb the excess nitrogen in the runoff before it reaches the streams.Farmers sometimes object to letting trees grow on land where they might otherwise be cultivating crops.But there’s a government program that compensates them, that pays them for creating these buffer zones between their fields and the streams that eventually feed into bays like the Chesapeake.And it’s beginning to show some success.