VOA慢速英语：墨西哥湾的“死亡区域”2014-05-31

Killer 'Dead Zone' Grows in the Gulf of Mexico

From VOA Learning English this is As It Is! I'm Anna Matteo in Washington.

Coming up we hear about a dead zone in waters near the United States. This dead zone has nothing to do with burial places, zombies or monsters or that 1983 movie called, "The Dead Zone."

Christopher Cruise tells us what a dead zone is and how some scientists are trying to bring it back to life.

The area known as a dead zone develops every spring in the Gulf of Mexico near the mouth of the Mississippi River. It can be as large as 13,600 square kilometers, extending all the way to the eastern Texas shoreline.

Scientists know what causes this dead zone, and a new study suggests an answer. But the solution might be hard to accept for those who live far away from the coastline.

Bayani Cardenas is a professor of water studies at the University of Texas at Austin.

He wondered why natural cleaning, or filtration, does not remove nitrates from the Mississippi River that create the dead zone in the Gulf of Mexico. He says rivers generally filter out materials like nitrates.

"You can think of it as a spiraling flow back around the bank of the river, where a water molecule goes into the bank and then comes back out into the river at some downstream point, and it does that repeatedly as it travels downstream."

Professor Cardenas says his recent study shows that more than 99 percent of the river's water does pass through the river bank material, or sediments, on its way south. But he adds that the river system is simply overwhelmed by the amount of nitrogen it carries.

This nitrogen-rich water supports the growth of algae. As the algae dies, it sinks to the bottom where it breaks down, or decomposes, taking oxygen from the water. This condition is called hypoxia and is deadly to fish and shrimp.

The Mississippi river system carries water to the gulf from 33 states and two Canadian provinces. Along the way, chemicals like nitrogen that are used in farming enter the river system. Farmers say the chemicals are necessary. But Mr. Cardenas thinks that the only way to fix the dead zone problem is for farmers to use less nitrates.

Aaron Packman is a civil and environmental engineering professor at Northwestern University. He says farmers may be able to better control the amount of nitrate fertilizer they put on fields.

"How much fertilizer do you need to give you good yields and then how much is maybe a marginal gain from adding lots more fertilizer? So there is really a question here: can you maybe back that off ((reduce the amount)) and maybe get close to the same level of yield without having such a negative impact."

Aaron Packman explains how large the Mississippi river system is. He also describes how farming practices upstream affect the areas downstream.

"The Mississippi river system is 40 percent of the surface area of the continental United States. I think it takes some further work in the distributive areas upstream that are the source of a lot of the nutrient."

The water in the Mississippi river system should be able to clean, or filter, itself naturally as it flows downstream. But human-made structures used to improve travel on the river and to control floods have hurt this natural cleaning process.

Bayani Cardenas says filtration works better when the river water travels through twists and turns. These twists and turns force the water to spend more time in the natural materials that clean it.

"A straight channel won't offer this buffering. A very sinuous channel provides a lot of the contact of the river water with the sediment."

In the coming months, storms will mix the gulf water and the dead zone will disappear. But it will return next year. And scientists say it will grow larger in years to come if something is not done to reduce the amount of nitrogen in the Mississippi River.

I'm Christopher Cruise.

Thanks, Christopher. And I'm Anna Matteo in Washington.

Too much nitrogen lead to too much algae which leads to dead fish - proving that too much of a good thing is sometimes not good.

When have you had too much of a good thing? Tell us in our comment section! It's a great place to practice your written English.

词汇解析

generally

难度：3星常用词汇，属常用6000词

英汉解释

adv.一般地;通常;普遍地

参考例句

用作副词 (adv.)

It is generally accepted that smoking is harmful to our health.

吸烟有害健康，这是大家公认的。

Sumptuous feasts are generally forbidden.

丰盛酒席普遍地被禁止。

molecule

难度：3星常用词汇，属常用6000词

英汉解释

n.分子;微粒

参考例句

用作名词 (n.)

A molecule is made up of atoms.

分子由原子组成。

repeatedly

难度：2星扩展词汇，属常用10000词

英汉解释

adv.重复地;再三地

参考例句

用作副词 (adv.)

He drew a long breath and repeatedly chanted the name of Rama, every word showering nectar into the hearts of the devotees.

他深深地吸了一口气，重复地唱颂着罗摩的名字，每一个字都像甘露般地洒落在奉献者的心里。

B. However, in order to restore peace and social order, individual states or the international community has repeatedly used force in history.

然而，历史上为了恢复和平和社会秩序，个别的国家或国际社会再三地使用武力。

oxygen

难度：4星核心词汇，属常用3000词

英汉解释

n.氧;氧气

参考例句

用作名词 (n.)

Water contains hydrogen and oxygen.

水含有氢和氧。

fertilizer

难度：2星扩展词汇，属常用10000词

英汉解释

n.肥料

参考例句

用作名词 (n.)

The stalks will rot and make good fertilizer.

杆儿腐烂后会成为很好的肥料。

The tractor hauled the fertilizer away.

拖拉机把肥料拉走了。

yield

难度：4星核心词汇，属常用3000词

英汉解释

v.生产;获利;屈服;弯下去

n.投资收益;生产量

参考例句

用作动词 (v.)

The combined investment is expected to yield $5,000 in the first year.

第一年该总投资有望获利5000美元。

用作名词 (n.)

So if the share price rises and the dividend does not, then the yield falls.

因此，如果股份上涨而股息不变，收益就下跌。

bureau

难度：4星核心词汇，属常用3000词

英汉解释

n.局;办事处;(带抽屉的)写字台

参考例句

用作名词 (n.)

She put the envelope beneath her underwear in the bureau, a place Richard would never look.

她把信封放在梳妆台抽屉的内衣下面,这里理查德是决不会看的地方。

source

难度：5星基本词汇，属常用1000词

英汉解释

n.来源;发源地;原始资料

v.从...获得

参考例句

用作名词 (n.)

My wages are the principal source of my income.

薪金是我收入的主要来源。

They gathered up from various sources a great amount of firsthand data.

他们从各种来源搜集了大量的第一手资料。

用作动词 (v.)

We source all the meat sold in our stores from British farms.

我们商店里卖的肉均从英国农场购进。