Four water samples are described below. Read them, then answer the questions before scrolling on.
A. A crystal-clear stream running off an Alpine pasture, high above any village.
B. A clear, colourless glass of water scooped from the lake 100 metres from the pipe where a city releases its treated sewage.
C. A brown, cloudy river in a forest after heavy rain, with no farms, roads or houses anywhere upstream.
D. A calm, blue, inviting lake in which every scientific sample ever taken has contained tiny pieces of plastic.
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4.4.1 Water pollution has multiple sources and has major impacts on marine and freshwater systems.
Sources include sewage, agricultural run-off, industrial effluent, urban run-off, solid waste disposal and oil spills.
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Before listing the sources, it helps to split them into two types, because the type changes how easy the pollution is to control.
A point source is pollution that enters the water at a single, identifiable place, such as the end of a pipe or a single spill. Point sources are easier to find, to measure and to regulate, because you can stand at the pipe and test what comes out.
A diffuse source (also called a non-point source) is pollution that enters over a wide area from many small places at once, such as fertiliser washing off thousands of fields, or litter blowing in from the whole shoreline. Diffuse pollution is much harder to control, because there is no single tap to turn off.
Many real situations are a mix of the two, but the distinction is one of the most useful ideas in this topic.
The six sources named in the course are below. Each one is illustrated with a real example from the Lake Geneva region, on your doorstep.
https://maps.app.goo.gl/zpQJ6WvL4sHNPEHn8
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Sewage: Vidy Bay, Lausanne (point source)
Sewage is all the waste water from homes, schools and businesses, not just from toilets. The city of Lausanne treats its sewage and releases it into the lake through a pipe that ends about 700 metres offshore. Even after treatment the water still carries dissolved chemicals, and during heavy rain untreated sewage overflows straight in. This has made the Baie de Vidy the most polluted part of the whole lake, with mercury, metals and other pollutants building up in the sediment, just a few kilometres from a drinking-water intake.
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Agricultural run-off: the vineyards and farms around the lake (diffuse source)
When it rains, water runs off farmland and vineyards (including the steep Lavaux terraces that drain straight into the lake) and carries with it nutrients from fertiliser and manure, and pesticides sprayed on crops. The international commission that monitors the lake (CIPEL) lists agriculture as one of the main sources of the phosphorus reaching the water, and regularly finds pesticide residues in the lake, its sediments and even its fish.
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https://maps.app.goo.gl/uccsg98ARV63goW78
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Industrial effluent: Lonza at Visp, Valais (point source)
Effluent is liquid waste released by a factory. Between 1930 and 1976 the Lonza chemical plant at Visp, upstream of the lake on the river Rhône, discharged mercury into a canal as part of its waste water. Around 50 tonnes of this poisonous metal ended up in the canal and the surrounding ground. The problem was only discovered by chance in 2010, and the clean-up is still going on today, which shows how long industrial pollution can last.
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Urban run-off: Geneva Airport (diffuse source)
Urban run-off is rainwater that washes pollutants off hard surfaces such as roads, roofs and car parks. Right next to this classroom, in winter, aircraft are sprayed with de-icing fluid (a type of alcohol called glycol). As it runs off the tarmac the airport collects it in a holding reservoir at the end of the runway before treating it, because glycol uses up oxygen as it breaks down in water and could suffocate fish in the lake. Ordinary city roads do the same thing, carrying metals, oil, tyre dust and winter road salt into the water.
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Solid waste disposal: litter in the lake (diffuse source)
Solid waste means rubbish: bottles, wrappers, cigarette ends and larger items that are dropped, dumped or blown into the water. About 90% of the waste found in Lake Geneva comes from activities on land, and most of it does not rot away. Each year the volunteer clean-up Net'Léman removes between 5 and 10 tonnes of rubbish from the shores and lake bed; one recent weekend brought up over 2.5 tonnes, including hundreds of kilograms of plastic and more than 36,000 cigarette ends.
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Oil spills: fuel on the lake (point source)
Switzerland has no coast, but the lake still suffers regular small spills of oil and fuel, usually from boats or from drains. In 2009 a 25-hectare oily slick spread across the lake off Founex, and in 2024 a spill at Anthy-sur-Léman was traced back to a storm drain. Each spill on its own is small, but the lake protection association warns that, added together, these repeated spills are a permanent stress on life in the lake.
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