Study： Mississippi River Overwhelmed by Agricultural Chemicals

From VOA Learning English, this is the Agriculture Report.

An area known as a dead zone develops every spring in the Gulf of Mexico near the mouth of the Mississippi River. It can spread as much as 13,600 square kilometers, extending all the way to the eastern Texas coast. Scientists know what causes the dead zone -- too much nitrogen, but the solution might be hard to accept.

Bayani Cardenas is a professor of water studies at the University of Texas at Austin. He says the movement of rivers usually removes material like nitrates. So he wondered why that natural filtration process does not remove nitrates from the Mississippi River.

Professor Cardenas says his recent study shows that more than 99 percent of the river's water does pass through the river's sediment which is on the shore or bottom of the river. But he says the study found that there is so much nitrogen in the river system that is simply can not filter all of it.

The water that has nitrogen still in it supports the growth of algae. As the algae dies, it sinks to the bottom of the river where it breaks down or decomposes. It then takes oxygen from the water. This condition is called hypoxia and it is deadly to fish and shrimp.

Where does all this nitrogen come from? The answer is farms. The Mississippi River system carries water from 33 American states and part of Canada to the gulf of Mexico. Along the way nitrogen and other chemicals used in farming enter the system. Farmers say these chemicals must be used to produce enough food for a growing world population.

Aaron Packman is a professor of Civil and Environmental Engineering 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? There is really a question here: can you maybe [reduce the amount] and get close to the same level of yield without having such a negative impact?" said Packman.

The water in the Mississippi River system should be able to clean itself naturally as it flows down stream. But flood controls and other human-made structures have hurt this filtering process.

Storms in the next few month 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.

And that's the VOA Learning English Agriculture Report. For more agriculture stories, go to our website tingvoa.com. I'm Caty Weaver.

词汇解析

pressure

难度：5星基本词汇，属常用1000词

英汉解释

n.压强;压力;压迫

v.施压

参考例句

用作名词 (n.)

The tooth became sore to pressure and there is a dull gnawing pain.

该齿对于压迫作痛，且有钝麻如咬的疼痛。

用作动词 (v.)

Don't be pressured into making any rash decisions.

不要在施压下做出轻率的决定。

severe

难度：4星核心词汇，属常用3000词

英汉解释

adj.严厉的;严重的;剧烈的;严格的;严峻的

参考例句

用作形容词 (adj.)

The premier came under severe criticism.

这位总理遭到严厉的抨击。

spread

难度：5星基本词汇，属常用1000词

英汉解释

v.传播;展开;散布;铺开;伸展;涂撒

n.范围;传播;差额;幅度;盛宴

参考例句

用作动词 (v.)

The radio spread the news as soon as the accident happened .

事故一发生,收音机就把这条消息传播出去了。

用作名词 (n.)

Tv has become a more powerful vehicle for the spread of news.

电视已成为影响力更大的传播消息的媒介。

normal

难度：5星基本词汇，属常用1000词

英汉解释

adj.正常的;正规的;精神健全的

n.常态;标准

参考例句

用作形容词 (adj.)

Everything is absolutely normal.

一切完全正常。

用作名词 (n.)

It was several days before the floodwater sank and life returned to normal.

过了几天洪水才退,生活恢复了正常。

mixture

难度：3星常用词汇，属常用6000词

英汉解释

n.混合;混合物

参考例句

用作名词 (n.)

You need a spoon to beat the mixture smooth.

你需要用一把调羹来搅均混合液。

The bottom layer is a mixture of sand and clay.

最下面的一层是沙和泥的混合物。

greatly

英汉解释

adv.很;非常;大大地

参考例句

用作副词 (adv.)

His writing has improved greatly in this semester.

这学期他的写作有了很大进步。

rescue

难度：4星核心词汇，属常用3000词

英汉解释

vt.营救;援救

n.营救;救援

参考例句

用作及物动词 (vt.)

The government has rescued the firm from bankruptcy by giving them a grant.

政府拨款给这家公司使其免于破产。

用作名词 (n.)

Rescue workers rushed to the site of the plane crash.

抢救人员冲向飞机坠毁的现常

recognize

难度：4星核心词汇，属常用3000词

英汉解释

vt.承认;认出;意识到;表示感激;识别

参考例句

用作及物动词 (vt.)

His thorough knowledge and competence were recognized.

他的渊博的学识和工作能力得到了承认。

The firm recognized Tom's outstanding work by giving him an extra bonus.

公司发给汤姆一笔额外奖金以表彰他出色的工作。