Water Pollution: Sources and Effects

Introduction

Rivers, lakes and seas have always received waste, and in small amounts nature can break much of it down. The trouble starts when the load is too large or the waste is too toxic for natural processes to cope with. Then fish suffocate, drinking water supplies are poisoned and beaches are coated in oil. Building on how water dissolves many substances, this page looks at three major kinds of water pollution — sewage, industrial waste and oil — and at how each one harms living things.

Core explanation

Sources of water pollution. Pollution may come from a point source , such as a factory outflow pipe or a sewage works, or from a diffuse source , such as rainwater running off farmland, roads and cities over a wide area. Point sources are easier to find and control.

Sewage. Untreated or poorly treated sewage contains human waste, food scraps, detergents and huge numbers of microorganisms. It causes two main problems:

- Disease. Sewage can carry bacteria and viruses that cause cholera, typhoid and dysentery. Drinking contaminated water is still a major cause of illness in parts of the world without proper sanitation. - Oxygen depletion. Bacteria in the water feed on the organic matter in sewage. They use aerobic respiration, which consumes the dissolved oxygen in the water. Water holds only a small amount of oxygen — about 9 mg per litre at 20 °C — so a large load of sewage can use it up quickly. Fish and other animals then suffocate. Scientists measure this effect as the biochemical oxygen demand (BOD) : the more organic waste, the higher the BOD.

Industrial waste. Factories and mines can release acids, alkalis, toxic organic chemicals and heavy metal ions such as mercury, lead and cadmium. Heavy metals do not break down. They are taken up by small organisms and pass along food chains, becoming more concentrated at each step. Predators at the top, including large fish, birds and humans, can receive harmful doses. Warm water from power station cooling systems is also a pollutant, because warm water holds less dissolved oxygen.

Oil spills. Crude oil is less dense than water and does not mix with it, so it spreads as a thin film over a huge area.

Effect of oil Result --- --- Coats birds' feathers Feathers lose waterproofing and insulation; birds cannot fly or keep warm Coats fur of sea mammals Animals lose body heat Film on the surface Reduces oxygen entering the water and light reaching plants Toxic compounds Poison fish, shellfish and seabed life Washes ashore Damages beaches, fisheries and tourism

Control. Sewage treatment works, laws limiting what factories may discharge, double-hulled oil tankers and regular testing of rivers all reduce pollution.

Step-by-step reasoning

To explain why sewage kills fish:

1. Sewage adds organic matter to the river. 2. Bacteria multiply by feeding on it. 3. The bacteria respire aerobically, using dissolved oxygen. 4. The dissolved oxygen concentration falls. 5. Fish and other animals cannot get enough oxygen and die.

Visual explanation

Imagine a graph of dissolved oxygen along a river. The line is high upstream, drops sharply just below a sewage outfall where bacteria are busiest, and then slowly recovers downstream as the waste is used up and oxygen dissolves back in from the air.

Real-world analogy

Adding sewage to a river is like inviting a huge crowd into a small, sealed room. The room had enough air for a few people, but the crowd breathes it all, and soon nobody has enough oxygen. The bacteria are the crowd; the fish are the original occupants.

Real-world example

In the 1950s, factory waste containing mercury was released into Minamata Bay in Japan. The mercury built up in fish and shellfish, and people who ate them suffered severe nerve damage. The disaster led to an international treaty, the Minamata Convention, to control mercury pollution.

Why?

Why does warm water discharged from a power station harm fish even if it is perfectly clean? Gases are less soluble in warm water than in cold water, so the warmed river holds less dissolved oxygen, while fish need more oxygen because their metabolism speeds up.

Common misconception

"Clear water is clean water." Many harmful pollutants, such as dissolved heavy metal ions, nitrates and some microbes, are invisible. Only chemical and biological testing can show whether water is safe.

Worked example

Question: A river contains 9.0 mg/L of dissolved oxygen above a sewage outfall and 2.5 mg/L just below it. Calculate the percentage decrease and suggest its effect.

Reasoning: Decrease = 9.0 − 2.5 = 6.5 mg/L. Percentage decrease = (6.5 ÷ 9.0) × 100 ≈ 72%.

Answer: About 72% decrease; the water no longer holds enough oxygen for most fish, so many will die or leave.

Quick check

1. Why do heavy metals become more concentrated in animals at the top of food chains? Answer: They are not broken down or excreted, so they accumulate at each feeding step.

Exam focus

When explaining sewage pollution, always mention bacteria respiring and using up dissolved oxygen; "the sewage uses up the oxygen" loses marks. For oil spills, link each effect to a property of oil, such as being less dense than water and insoluble in it.

Advanced insight

Some pollutants change form in the environment. Bacteria in sediments can convert inorganic mercury into methylmercury, which dissolves in fats and is absorbed far more easily by living things. This is why mercury in fish is much more dangerous than the same mass of mercury locked in sediment.

Summary

Water pollution comes from point sources such as factory pipes and diffuse sources such as run-off. Sewage spreads disease and, as bacteria break it down, uses up dissolved oxygen so fish suffocate. Industrial waste can contain toxic heavy metals that accumulate along food chains, and warm water holds less oxygen. Oil spills float on water, coat wildlife and poison marine life.

Practice questions

1. Give two harmful effects of untreated sewage entering a river. Answer: It spreads diseases such as cholera, and bacteria decomposing it use up dissolved oxygen so fish die. 2. Explain why oil spreads over the surface of the sea. Answer: Oil is less dense than water and insoluble in it, so it floats and spreads as a thin film. 3. Why are seabirds often killed by oil spills? Answer: Oil coats their feathers, destroying waterproofing and insulation, so they cannot fly, float or keep warm; they may also swallow toxic oil. 4. What is the difference between a point source and a diffuse source of pollution? Answer: A point source is one identifiable place such as a pipe; a diffuse source is spread over a wide area, such as run-off from farmland.