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12.27: Water Pollution

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    Is the ocean a good dumping ground?

    Unfortunately some people think so. A lot of garbage ends up washing ashore, and some garbage stays floating out in the ocean. Animals can be strangled by floating trash or mistake inedible trash for food. Not only is the pollution of our oceans a problem, but also our precious freshwater resources are often polluted.

    Sources of Water Pollution

    While to many people clean water may seem limitless and everywhere, to many others this is not so. Water pollution is a serious issue facing hundreds of millions of people world-wide, having harmful effects on the lives of those people. Water is not in unlimited supply and cannot just be made fresh when it is wanted. Water is actually a limited resource, and for many people, fresh, unpolluted water is hard to find. A limited resource is one that we use faster than we can remake it. It is a resource that can be used up.

    Water pollution happens when contaminants enter water bodies. Contaminants are any substances that harm the health of the environment or humans. Most contaminants enter the water because of humans. Surface water (river or lake) can be exposed to and contaminated by acid rain, storm water runoff, pesticide runoff, and industrial waste. This water is cleaned somewhat by exposure to sunlight, aeration, and microorganisms in the water. Groundwater (private wells and some public water supplies) generally takes longer to become contaminated, but the natural cleaning process also may take much longer. Groundwater can be contaminated by disease-producing pathogens, careless disposal of hazardous household chemical-containing products, agricultural chemicals, and leaking underground storage tanks.

    Water pollution causes detrimental effects to both ecology and human health
    Figure \(\PageIndex{1}\): Water pollution can cause harmful effects to ecology and human health. Shown is the pollution in Jakarta, Indonesia.

    Natural events, like storms, volcanic eruptions and earthquakes can cause major changes in water quality. But human-caused contaminants have a much greater impact on the quality of the water supply. Water is considered polluted either when it does not support a human use, like clean drinking water, or a use for other animals and plants. The overgrowth of algae, known as an algal bloom, can result from the runoff of fertilizer into bodies of water. This excess of nutrients allows the algae to grow beyond control, bring harm to the rest of the ecosystem.

    The main sources of water pollution can be grouped into two categories:

    • Point source pollution results from the contaminants that enter a waterway or water body through a single site. Examples of this include untreated sewage, wastewater from a sewage treatment plant, and leaking underground tanks.
    • Nonpoint source pollution is contamination that does not come from a single point source. Instead, it happens when there is a buildup of small amounts of contaminants that collect from a large area. Examples of this include fertilizer runoff from many farms flowing into groundwater or streams.

    Effects of Water Pollution

    Water pollutants can have an effect on both the ecology of ecosystems and on humans. As a result of water pollution, humans may not be able to use a waterway for recreation and fishing. Drinking water can also be affected if a toxin enters the groundwater.


    In a marine ecosystem, algae are the producers. Through photosynthesis, they provide glucose for the ecosystem. So, can too much algae be a bad thing? Eutrophication is an over-enrichment of chemical nutrients in a body of water. Usually these nutrients are the nitrogen and phosphorus found in fertilizers. Run-off from lawns or farms can wash fertilizers into rivers or coastal waters.

    Plants are not the only things that grow more quickly with added fertilizers. Algae like the excess nutrients in fertilizers too. When there are high levels of nutrients in the water, algae populations will grow large very quickly. This leads to overgrowths of algae called algal blooms. However, these algae do not live very long. They die and begin to decompose. This process uses oxygen, removing the oxygen from the water. Without oxygen, fish and shellfish cannot live, and this results in the death of these organisms (Figure below).

    Certain types of algal blooms can also create toxins. These toxins can enter shellfish. If humans eat these shellfish, then they can get very sick. These toxins cause neurological problems in humans.

    Figure \(\PageIndex{2}\): Lake Valencia, Venezuela, showing green algal blooms. How did the algal bloom form? What will it do to the lake over time?

    Ocean Acidification

    Ocean acidification occurs when excess carbon dioxide in the atmosphere causes the oceans to become acidic. Burning fossil fuels has led to an increase in carbon dioxide in the atmosphere. This carbon dioxide is then absorbed by the oceans, which lowers the pH of the water. Ocean acidification can kill corals and shellfish. It may also cause marine organisms to reproduce less, which could harm other organisms in the food chain. As a result, there also may be fewer marine organisms for humans to consume.

    Aquatic Debris

    Aquatic debris is trash that gets into fresh- and saltwater waterways. It comes from shipping accidents, landfill erosion, or the direct dumping of trash. Debris can be very dangerous to aquatic wildlife. Some animals may swallow plastic bags, mistaking them for food. Other animals can be strangled by floating trash like plastic six-pack rings. Wildlife can easily get tangled in nets (Figure below).

    Figure \(\PageIndex{3}\): Marine trash can harm different types of aquatic life. Pictured here is a marine turtle entangled in a net. How can you keep this from happening?

    Waterborne Diseases

    Unsafe water supplies have drastic effects on human health. Waterborne diseases are diseases due to microscopic pathogens in fresh water. These diseases can be caused by protozoa, viruses, bacteria, and intestinal parasites. In many parts of the world there are no water treatment plants. If sewage or animal manure gets into a river, then people downstream will get sick when they drink the water. According to the World Health Organization (WHO), diarrheal disease is responsible for the deaths of 1.8 million people every year. It was estimated that 88% of the cases of diarrheal disease are caused by unsafe water supplies.

    Science Friday: Poop and Paddle: An Eco-Friendly Floating Toilet

    How do wetlands filter water? In this video by Science Friday, inventor Adam Katzman describes how his toilet-boat converts human waste into cattails and clean water.



    • Water is a limited resource, but it is often polluted by humans.
    • Sources of water pollution can be grouped as point source pollution (large amounts entering through a single site) or nonpoint source pollution (small amounts entering from many sites.)
    • High levels of nutrients, called eutrophication, can cause conditions that deprive fish of oxygen.
    • Ocean acidification occurs when excess carbon dioxide in the atmosphere causes the oceans to become acidic, which harms corals and shellfish.
    • Debris can be dangerous to aquatic wildlife.

    Explore More

    Use the resource below to answer the questions that follow.

    1. How can human sewage throw ecosystems out of balance? What nutrient cycle(s) are involved?
    2. How does agriculture run-off effect ecosystems? How does this change move through the food web? What can the result be?
    3. How can drugs excreted by humans affect aquatic organisms? How does this affect the ecosystem?
    4. What is heat pollution? What affect can this have on aquatic ecosystems? Explain your answer as fully as possible.


    1. Why is fresh water a limited resource?
    2. What is water pollution?
    3. What are two main sources of pollution of surface water?
    4. What are two main sources of groundwater pollution?
    5. What's the difference between a point source and nonpoint source of water pollution?
    6. What is eutrophication?
    7. Explain what causes eutrophication and how it affects the ecosystem.
    8. Explain what causes ocean acidification and how it affects the ecosystem.
    9. What are waterborne diseases? What causes waterborne diseases?

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