Part 5  Water Pollution 水污染

    Comprising over 70% of the Earth's surface, water is undoubtedly the most precious natural resource that exists on our planet. Without the seemingly unvaluable compound comprised of hydrogen and oxygen, life on Earth would be non-existent: it is essential for everything on our planet to grow and prosper. Although we as humans recognize this fact, we disregard it by polluting our rivers, lakes, and oceans. Subsequently, we are slowly but surely harming our planet to the point where organisms are dying at a very alarming rate.

    Water pollution occurs when a body of water is adversely affected due to the addition of large amounts of materials to the water. The sources of water pollution are categorized as being a point source or a non-source point of pollution. Point sources of pollution occur when the polluting substance is emitted directly into the waterway. A pipe spewing toxic chemicals directly into a river is an example. A non-point source occurs when there is runoff of pollutants into a waterway, for instance when fertilizer from a field is carried into a stream by surface runoff.

    There are several types of water pollution. A toxic substance is a chemical pollutant that is not a naturally occurring substance in aquatic ecosystems. The greatest contributors to toxic pollution are herbicides, pesticides and industrial compounds.

    Organic pollution occurs when an excess of organic matter, such as manure or sewage, enters the water. When organic matter increases in a pond, the number of decomposers will increase. These decomposers grow rapidly and use a great deal of oxygen during their growth. This leads to a depletion of oxygen as the decomposition process occurs. A lack of oxygen can kill aquatic organisms. As the aquatic organisms die, they are broken down by decomposers which leads to further depletion of the oxygen levels.

    Thermal pollution can occur when water is used as a coolant near a power or industrial plant and then is returned to the aquatic environment at a higher temperature than it was originally. Thermal pollution can lead to a decrease in the dissolved oxygen level in the water while also increasing the biological demand of aquatic organisms for oxygen.

    Ecological pollution takes place when chemical pollution, organic pollution or thermal pollution are caused by nature rather than by human activity. An example of ecological pollution would be an increased rate of siltation of a waterway after a landslide which would increase the amount of sediments in runoff water. Another example would be when a large animal, such as a deer, drowns in a flood and a large amount of organic material is added to the water as a result. Major geological events such as a volcano eruption might also be sources of ecological pollution. And this is just one of the biggest problems facing the countries now which were hit by tsunami in December last year.

    There are many sources of water pollutants.Farms often use large amounts of herbicides and pesticides, both of which are toxic pollutants. These substances are particularly dangerous to life in rivers, streams and lakes, where toxic substances can build up over a period of time. Farms also frequently use large amounts of chemical fertilizers that are washed into the waterways and damage the water supply and the life within it.

    Waste and sewage generated by industry can get into the water supply, introducing large organic pollutants into the ecosystem. Many industrial and power plants use rivers, streams and lakes to despose of waste heat. The resulting hot water can cause thermal pollution. Thermal pollution can have a disasterous effect on life in an aquatic ecosystem as temperature increaces decreace the amount of oxygen in the water, thereby reducing the number of animals that can survive there.

    Water can become contaminated with toxic or radioactive materials from industry, mine sites and abandoned hazardous waste sites. Acid precipitation is caused when the burning of fossil fuels emits sulfur dioxide into the atmosphere. The sulfur dioxide reacts with the water in the atmosphere, creating rainfall which contains sulfuric acid. As acid precipitation falls into lakes, streams and ponds it can lower the overall pH of the waterway, killing vital plant life, thereby affecting the whole food chain.

    Sewage generated by houses or runoff from septic tanks into nearby waterways, introduce organic pollutants that can cause eutrophication. Fertilizers, herbicides and pesticides used for lawn care can runoff and contaminate the waterway. As with agriculteral fertilizers, home fertilizers can lead to the eutrophication of lakes and rivers.

    Improper disposal of hazardous chemicals down the drain itroduce toxic materials into to the ecosystem, contaminating the water supplies in a way that can harm aquatic organisms. Leaks of oil and antifreeze from a car on a driveway can be washed off by the rain into nearby waterways, polluting it.

    Clearly, the problems associated with water pollution have the capabilities to disrupt life on our planet to a great extent. In order to combat water pollution, we must understand the problems and becomepart of the solution.

    As we head into the 21st century, awareness and education will most assuredly continue to be the two most important ways to prevent water pollution. If these measures are not taken and water pollution continues, life on earth will suffer severely.

    水体因某种物质的介入，而导致其化学、物理、生物或者放射性等方面特征的改变，从而影响水的有效利用，危害人体健康或者破坏生态环境，造成水质恶化的现象称为水污染。

    水污染物源于很多的人类活动。工业污染物可从工厂中的排泄管流出，或者可从管道和地下储存罐中外泄。受污染的水也可能是从矿山流出，而流经矿山的水通过含矿物的岩石的沥滤，或已受到加工矿石的化学制品的污染。

    城市和其他居民区的水污染大多源自下水道中混有的微量家庭化学制品。有时工厂将污染物排泄到城市的污水系统中，使城区各种污染物质增多。来自于像农田。牧草场、饲育场和大农场等农业源头的污染物有动物粪便、农业化学制品以及腐蚀过程中产生的沉淀物。

    人类的活动会使大量的工业、农业和生活废弃物排入水中，使水受到污染。目前，全世界每年约有4200多亿立方米的污水排入江河湖海，污染了5.5万亿立方米的淡水，这相当于全球径流总量的14%以上。

    水污染有两类：一类是自然污染；另一类是人为污染。当前对水体危害较大的是人为污染。水污染可根据污染杂质的不同而主要分为化学性污染、物理性污染和生物性污染三大类

    在发展工业，建设城市的同时，就要注意到水资源的保护。因为一旦水资源受到污染，将严重的制约工业、农业的发展以及人类的生存。

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