Water pollution in the UK: the causes and effects

27th October 2025

Water pollution is a huge challenge for freshwater in the UK, impacting our rivers, streams, lakes, ponds and the wildlife that call them home.

In September 2025, over 6,000 people measured the water quality of their local freshwater bodies as part of the Great UK WaterBlitz. The results showed that 60% of freshwater sites in the UK had poor ecological health.

But what is causing the increase in water pollution and what are the impacts? Our previous Director of Science & Policy, Dr Sasha Woods, explains.

What are nitrates and phosphates and why are they a problem?

Nitrates and phosphates are nutrients that occur naturally in the environment. Plants need them to grow. Healthy freshwater ecosystems usually have small amounts of these nutrients helping aquatic plants to survive.

However, human activity on land can cause unnaturally high levels of nutrients – and other pollutants – to enter our rivers, streams, lakes and ponds, resulting in nutrient pollution.

What are the causes of water pollution?

Agriculture

The modernisation and intensification of agriculture during and after World War II resulted in too much nitrate entering our environment. The manufacture and application of artificial fertilisers since the 1940s has increased the amount of surplus nitrate.

Artificial fertilisers dissolve easily in water, and unlike natural fertilisers, add no organic content to the soil. When artificial fertilisers are used on land, rainfall washes them into ditches and rivers. They also drain from the soil into groundwater (underground water in the cracks and spaces in soil, sand and rock).

This makes agricultural areas a historical source of diffuse water pollution. Diffuse pollution is where small amounts of contaminants build up from multiple sources across a large area. This is in contrast to pollutants that enter watercourses from a specific point, such as a pipe or outflow.

Although the amounts vary depending how local land is used, the Environment Agency estimates that agriculture is responsible for 50-60% of nitrate entering waterways. With phosphates, agriculture contributes 20% of the total phosphorous entering our watercourses.

Sewage and wastewater

The majority of phosphate pollution (around 80%) comes from urban wastewater. As for nitrates, sewage effluent (liquid waste) contributes about 25-30% nationally.

Urban wastewater, which is generally called sewage, is a mixture of domestic wastewater from baths, sinks, washing machines and toilets.

It also includes wastewater from industry, and rainwater washing off roads and other surfaced areas (called surface run-off). Over 300 different chemicals are released from car tyres wearing down on roads.

Every day, 347,000 kilometres of sewers collect over 11 billion litres of wastewater. This goes through sewage treatment works before being released back to inland waters, estuaries and the sea. The treatment cleans and recycles the water by removing organic substances.

Solid matter is filtered out (primary treatment) and then bacteria is used to ‘digest’ and break down the organic substances (secondary treatment). Finally, nitrates and phosphates are removed (tertiary treatment) with various methods including sand filtration, activated carbon filtration, and chemical oxidation.

During heavy rainfall, the sewers can fill up and overflow.  This can result in the inundation of sewage works and the potential for dirty water to back up and flood peoples’ homes, roads and open spaces.

To prevent this, combined sewer overflows were developed with overflow valves to reduce the risk of sewage backing up during heavy rainfall. However, this results in untreated sewage being sent to our rivers and other freshwater bodies instead, causing water pollution.

What are the effects of water pollution?

High levels of nutrients trigger a process called eutrophication. This is when excessive plant and algal growth (caused by the extra nutrients) leads to high levels of bacteria. The bacteria decrease oxygen levels in the water, killing plants and animals.

Lots of algae on the water’s surface can also choke waterways preventing wildlife from feeding, and block sunlight causing further problems for aquatic plants.

Our rivers are being put under pressure by historical and current intensive farming. Sewage discharge is making things even worse. The high nitrate levels are driving the poor ecological state of freshwater across the UK, with phosphate levels adding to the dire mix.

We’re fighting for healthy fresh water

Our FreshWater Watch programme is a global citizen science initiative where community groups and individuals are trained to monitor and protect their local rivers, streams, lakes, ponds and other freshwater habitats.

Citizen scientists use our FreshWater Watch testing kit to measure nutrient pollution, and record basic visual observations of their chosen local waterbody. These include surrounding land use, pollution sources, and presence of algae.

How can you help?

Our freshwater habitats are in trouble and we need urgent action to make the invisible, visible. Take part in the next Great UK WaterBlitz between 24-27 April 2026 to join the fight for healthy freshwater! Help us build a national picture of water quality across the UK. Every datapoint counts.