Water pollution: causes, consequences and solutions

Fertiliser and slurry supply nitrogen and phosphorus. Whatever the plants do not absorb seeps into the groundwater with the percolating water. Via drainage and erosion, it reaches streams and rivers. Older figures from the Umweltbundesamt (UBA, German Environment Agency) for 2003 to 2005 show the order of magnitude. Agriculture accounted for over 70% of nitrogen inputs and over 50% of phosphorus inputs into surface waters.

The decline is uneven. Municipal sewage treatment plants cut their nitrogen load by around 70%. Agriculture achieved around 35%, and around 15% for phosphorus. Added to this is a time lag that is often missing from the debate. In the Elbe catchment area, water takes up to 30 years to travel from the soil to the river. Some of today's measures will only take effect in decades.

The second input from agriculture is plant protection products. In drinking water, 14 of 349 active substances and degradation products tested exceeded the limit value of 0.10 micrograms per litre in up to 1% of samples. Details can be found under Pesticides in tap water.

Sewage treatment plants reliably remove solids, nutrients and organic load. When it comes to trace substances, their performance reaches its limit. Pharmaceutical residues, X-ray contrast agents, hormones, sweeteners, corrosion inhibitors and microplastics largely pass through the three classic treatment stages.

Trace substances therefore end up where treated wastewater is discharged. What this means for pharmaceuticals is explained under Pharmaceutical residues in tap water. For plastic particles, see Microplastics in drinking water.

A second route is combined sewer overflows. During heavy rain, the volume of water exceeds the capacity of the sewers. Untreated wastewater then flows directly into the water body together with the rainwater. Sealed surfaces make this worse because rain cannot soak away there.

Industrial plants may discharge wastewater with a permit, subject to limit values for each sector and substance. The more problematic part lies in the past. Contaminated sites from landfills, petrol stations, chemical sites and military areas release substances into the groundwater over decades.

The best-known case of legacy contamination is per- and polyfluoroalkyl substances (PFAS). PFAS practically do not break down in the environment and are therefore regarded as forever chemicals. Since 12 January 2026, the TrinkwV has set limit values for the first time. For the sum of 20 PFAS, the value is 0.000 10 mg/l. For the sum of the four most relevant to health, it is 0.000 020 mg/l. More on this under PFAS in drinking water.

Mining changes water in two ways. Sulphate, iron and heavy metals enter the waters from spoil heaps and mine water. Mine dewatering water and processing wastewater increase the salt load. Elevated salt levels were one of the conditions for the disaster on the Oder in 2022.

Heat inputs from power stations and industry are the silent variant. Warm water contains less oxygen. Cold-water species such as brown trout and grayling come under stress above around 20 degrees Celsius. In hot summers with low water levels, both effects add up.

Erosion, volcanic activity and algal blooms are considered natural causes. With algae, however, the distinction is artificial. Blue-green algae and other mass blooms need two things that humans supply: nutrients and warmth. If a bathing lake is closed in summer, the algae are the trigger and the nutrient load is the precondition.

In water, nutrients act like fertiliser on a field. Algae and aquatic plants grow vigorously and die off. Bacteria break down the biomass, consuming oxygen in the process. The end result is a body of water in which fish and mussels suffocate. The technical term for this chain of nutrients, algal growth and oxygen depletion is eutrophication.

The Oder disaster of 2022 shows how four causes interact. The trigger was the brackish-water alga Prymnesium parvum, also known as golden alga, and the toxins it produces. Its mass development was made possible by high salinity, high water temperatures, low water levels and nutrients. The salt came partly from mining discharges. Over 300 kilometres of river were affected. Almost half of the fish population died, along with mussels and snails. According to studies by the Leibniz-Institut für Gewässerökologie (Leibniz Institute of Freshwater Ecology), a cocktail of micropollutants intensified the effect of the algal toxins.

No single one of the four causes would have done the damage on its own. That is what makes assessing water pollution so difficult.

Worldwide, microbiologically contaminated drinking water is the greatest water-related health hazard. The World Health Organization (WHO) estimates around 505,000 deaths from diarrhoeal diseases per year. In total, 1.4 million people die each year as a result of inadequate water supply, sanitation and hygiene.

In Germany, the risk is different. Here, it is about chemical pollution in low concentrations over long periods. That is why the TrinkwV sets its limit values so strictly, often far below the toxicologically derived threshold. For PFAS, the sum limit value is 100 nanograms per litre.

Polluted water costs money, usually somewhere other than where the pollution was caused. Water suppliers have to use more elaborate treatment, relocate wells or blend water. Fisheries and tourism lose income. Municipalities bear the cost of upgrading wastewater treatment plants.

This is exactly where the new European regulation comes in. It shifts the costs to where the substances come from.

The European Urban Wastewater Treatment Directive entered into force on 1 January 2025. It must be transposed into national law by 31 July 2027. It requires a fourth treatment stage for all wastewater treatment plants with 150,000 population equivalents or more. Smaller plants will follow after a risk assessment.

The fourth stage specifically removes trace substances that pass through the existing three stages. Two methods are permitted: activated carbon filtration and ozonation. To find out how activated carbon does this, see how activated carbon works.

The second part of the directive goes further. In future, at least 80% of the costs of the fourth stage will be borne by the industries whose products introduce the substances, above all pharmaceutical and cosmetics manufacturers. This is the first time the polluter-pays principle has been applied in concrete terms in water management.

For groundwater, the key lever is fertilisation practice. Four instruments are effective: fertilisation tailored to crop needs, catch crops to prevent nitrate leaching in winter, riparian buffer strips to prevent inputs from erosion, and the designation of nitrate-polluted areas with stricter requirements.

With flow times of up to 30 years, patience is part of the task. The slightly falling nitrate levels in the current report are the result of measures that began years ago.

The contribution of individual households is limited, but concrete:

• Dispose of medicines with general household waste, never down the toilet or sink. Whatever goes into the sewer ends up in rivers and lakes.
• Paints, oils, solvents and chemicals should be taken to the recycling centre.
• Wet wipes, hygiene products and food scraps belong in the bin. In the toilet, they cause blockages and overflows.
• Dose detergent according to water hardness rather than out of habit. Your water supplier can tell you the hardness.
• Unseal paved areas where possible. Infiltration relieves the sewer system during heavy rain.