Official 53 Set 5

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Two Kinds Of Pollution

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What is the lecture mainly about?
  • A. The difference between point source and nonpoint source pollution

  • B. The source and treatment of a serious type of pollution affecting the Chesapeake Bay

  • C. The causes for the large decrease in algae in the Chesapeake Bay

  • D. The development of farming methods during the past century

显示答案 正确答案: B

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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.

  • 请听一段环境科学课的节选。

    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。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。

    部分原因是影响切萨皮克的污染类型可能是你无法预测的。首先需要说一下污染源主要分两大类。让我们先讲点源污染。

    点源污染有一个明显的来源而且你能发现它特定的地点,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。过去30年都是用这种方式治理的。工厂和污水厂做了一些改变,在污水流入公共水道之前就对其进行了处理。但是也有一类污染,我们称之为非点源污染。

    现在大部分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,而是源自于很大范围内的各个来源。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。

    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。这些营养物比如说磷,特别是氮的流失我们叫做农业径流。当大雨或融雪带来的水将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。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。

    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。

    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。直到60年前,在工业化加剧增长之前,这不是问题。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。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。而且这些农作物和其他的覆盖农作物,在小麦或者玉米丰收后被种植来保持土壤。他们在一年中径流最多的时候存储大量的氮。

    但随着农民面临越来越多的化肥使用压力,为了在同样的土地上增加作物产量,耕作方式发生了变化。但并非越多越好,至少在环境中的化肥方面是这样。而且,在此过程中,农民们从豆类转向更适合集约化、大规模生产的动物饲料。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。正如我们所说,这意味着水中有更多的藻类,鱼类和其他水生生物呼吸的氧气更少。

    所以现在要做什么?有两件事。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。

    第二个策略是在溪流的边上种植缓冲区。这些地区是自然的,种植树木的地方,没有作物。在径流到达溪水之前,这些树木的根能够吸收其中的氮。农民有时反对让树木在原本可能种植庄稼的土地上生长。但有一个政府计划对他们进行补偿,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。这项举措开始显示出成效。

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    题干分析:问这篇文章主要讲什么?

    原文定位:

    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. 

    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. 

    选项分析:文章在开头就引入了C Bay的水污染不容易治理,而这种污染并不是来自有毒物质,而是附近区域使用化肥带来的过营养化造成的污染,然后,文章详细讲了具体原因和一些治理方法,所以选B

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Two Kinds Of Pollution

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