Narrator: Listen to part of a lecture in an environmental science class.

旁白：请听一段环境科学课的节选。

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.

女教授：美国东海岸的切萨皮克湾很大。它是美国最大的河口湾而且对当地经济非常重要。

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.

但是像世界上很多水路一样，切萨皮克正在被污染。而且防治污染的努力并不是十分成功。

And that's partly because of the type of pollution affecting the Chesapeake, which may not be what you might predict.

部分原因是由于影响切萨皮克的污染类型可能是你无法预测的。

Uh, first let's mention that the sources of pollution are of two general types.

首先需要说一下污染源主要分两大类。

And let's begin with what's known as "point source pollution".

让我们从“点源污染”讲起。

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.

点源污染有一个明显的来源而且你能在特定地点找到它，比如说一个地方有一根特定的管子把垃圾倾倒到河湾中。然后我们就在污染来源地处理污水。

And that's what's happened over the past 30 years or so.

过去30年都是用这种方式治理的。

Modifications have been made at factories and sewage treatment plants to treat polluted water before it's released into public waterways.

工厂和污水处理厂做了一些改变，在污水流入公共水道之前就对其进行了处理。

But there's also something we call "non-point source pollution".

但是也有一类污染，我们称之为“非点源污染”。

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.

现在，最严重的污染处理不是来自于任何特定的来源比如说工厂或者污水厂，而是在大范围内的有多种来源。

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.

这种非点源处理方法是一种挑战，因为它不是仅仅通过一根管子进入到河湾。你不能精确地识别污染来自于哪里。

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.

具体一点说，现在切萨皮克湾面临的最大问题不是因为有毒物质而是因为在整个地区农业使用的化肥中的营养物。

These nutrients--like phosphorus and especially nitrogen--wash away in what we call agricultural runoff.

这些营养物比如说磷，特别是氮的流失我们叫做农业径流。

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.

当大雨,或融雪携带这些化学物质带到河流和海湾时，它们就会刺激藻类的爆炸性生长。

And that uses up much of the oxygen in the water, oxygen that fish and other aquatic organisms need to stay alive.

这消耗了水中大量的氧气，这些氧气是鱼和其它水生有机物存活所必需的。

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.

所以，正因为在水进入河湾前没有地方能让你处理这些径流，任何减少这种非点源污染的努力，一般都需要首先将污染排除在河流之外。

But before we go into that, let's look at the role of nitrogen fertilizer in modern farming.

但是在我们讲这个之前，让我们看一下氮化肥在现代农业中的作用。

Until about 60 years ago, before a great increase in industrialization, this wasn't a problem.

直到60年前，在工业化加剧增长之前，这不是问题。

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.

在过去，农民使用自然化肥，而且轮种农作物，所以除了像玉米和小麦这样的经济作物，他们也种像苜蓿和三叶草这样的豆科植物来喂养动物。

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.

但是这些豆科植物通过把空气中的氮转化为硝酸盐也在给土壤补充营养，像小麦或者玉米这样的农作物都可以用硝酸盐作为营养物。

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.

而且这些农作物和其他的覆盖农作物，可以在小麦或者玉米丰收后被种植用来保持土壤。他们在一年中径流最多的时候存储大量的氮。

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.

但当农民面临着在同样的土地上增加作物产量的压力时，他们越来越多地使用化肥，这使耕作方式随之发生了变化。

But more isn't always better, at least in terms of chemical fertilizer in the environment.

但并非越多越好，特别是化肥对于环境来说。

And, along the way, farmers switched from legumes…to animal feeds more suited to intensive, large-scale animal production.

而且，在此过程中，农民们从豆类转向更适合集约化、大规模动物生产的动物饲料。

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.

而过剩的氮，一旦被这些覆盖作物捕获，要么在下一场大雨中被冲走，要么进入地下水，无论哪种方式，最终都流入小溪和海湾。

And that, as we said, means more algae in the water and less oxygen for the fish and other aquatic life to breathe.

正如我们所说，这意味着水中有更多的藻类，供鱼类和其他水生生物呼吸的氧气更少。

So, what's being done? Well, two things.

所以现在要做什么？有两件事。

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.

首先，在主要作物丰收后，更多农民再次开始种植覆盖作物。其他种类--像黑麦和大麦--能够固定氮并防止氮在最有可能发生流失情况的几个月中从土壤中流失。

And the second strategy is to plant "buffer zones" at the edges of streams--not crops, but natural areas…trees.

第二个策略是在溪流的边上种植缓冲区，不种农作物，而是自然植被……和树木。

The roots of these trees can absorb the excess nitrogen in the runoff before it reaches the streams.

在径流到达溪水之前，这些树木的根能够吸收其中的氮。

Farmers sometimes object to letting trees grow on land where they might otherwise be cultivating crops.

农民有时反对在原本能种植庄稼的土地上种树。

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.

但有一个政府计划会补偿他们，为他们在农田和最终流入切萨皮克湾的河流之间建立缓冲区提供资金。

And it's beginning to show some success.

这项举措开始显示出成效。