What Are the Main Pollutants Found in Water?

Water is an essential resource for all life on Earth. However, human activity pollutes it in many different ways: domestic wastewater, untreated industrial discharge, plastic waste, leaks and accidental spills, and more. All of these sources of pollution threaten both water availability and water quality for current and future generations.

Contaminated drinking water can also cause diarrhoeal diseases, cholera, typhoid and other serious health problems. Fish, plants and other aquatic organisms are direct victims as well. In this article, we explore the main water pollutants and where they come from.

Nitrates and phosphates: pollutants linked to chemical fertilisers

Plants need nutrients to grow, and two of the most important are nitrogen and phosphorus. Nitrogen mainly comes from a complex cycle involving nitrates, nitrites and ammonia.

Phosphorus mainly comes from phosphates. These are naturally found in rocks and minerals, but they are also released through human activity, especially the use of fertilisers and phosphate-based detergents.

Under natural conditions, these nutrients are present in limited amounts, which slows plant growth. However, to increase agricultural productivity, nitrogen fertilisers, livestock waste and other chemical substances are added to soils. Rainwater then carries some of these nutrients into rivers and groundwater, increasing nitrate and phosphate levels.

This process encourages excessive algae growth (eutrophication), which can spread across rivers and lakes, reducing oxygen levels and threatening aquatic life. Dead algae and excess nutrients can also make water unsuitable for drinking and irrigation.

In France, the maximum nitrate concentration permitted in drinking water is set at 50 mg/L. Phosphate levels are not specifically regulated, although recommendations exist to limit their presence in tap water.

Organic matter: aquatic pollutants originating from living organisms

Organic matter is a substance made up of carbon, hydrogen and often oxygen, nitrogen, phosphorus and other elements, originating from living organisms or their remains.

What causes organic matter pollution in water?

Organic pollution naturally comes from the decomposition of living organisms such as:

• dead leaves,
• branches,
• algae,
• animal waste, and more.

Untreated domestic and industrial wastewater from urban sewage systems also contributes to aquatic pollution, especially where wastewater treatment facilities are lacking. Human waste, food scraps, hygiene products and chemicals can all contaminate water and harm aquatic life. Agricultural runoff, pesticides and herbicides are also major contributors.

What happens when organic matter decomposes in water?

Organic matter plays an important role in the carbon cycle by storing carbon and releasing it into the atmosphere as CO2. Its decomposition also releases nutrients such as nitrogen and phosphorus, which are necessary for plant growth.

However, when organic matter is present in excessive amounts, it can create negative effects, including algae blooms. Its decomposition in water also consumes large amounts of oxygen, leading to the death of fish and other aquatic organisms.

Organic matter may also carry bacteria, viruses and harmful chemicals that can affect both human health and aquatic ecosystems.

Micropollutants: harmful substances present even at very low levels

A micropollutant is a chemical substance found in the environment at extremely low concentrations, often compared to a single sugar cube dissolved in an Olympic-sized swimming pool. Despite these tiny quantities, micropollutants can still have significant effects on water systems, ecosystems, drinking water networks and public health.

How do micropollutants end up in water?

Sources of groundwater contamination by micropollutants are varied, although most come from human activity:

• Industry: plastics manufacturing, detergents, solvents, petroleum products and more.
• Agriculture: pesticides, herbicides, fertilisers and similar products.
• Domestic use: cosmetics, paints, household cleaning products and more.
• Hospital activities: medicine residues, contrast agents and hospital discharge.

Today, nearly 100,000 micropollutant molecules have been identified, making it almost impossible to create a complete list. Among them, PFAS and PCBs are particularly concerning because they persist in the environment for long periods.

Although PCBs have been banned in most countries since the 1970s due to their toxicity, they can still be found in older electrical equipment and industrial oils.

What are the health risks associated with micropollutants?

Even at low concentrations, micropollutants can affect living organisms. For example, alkylphenols may disrupt the endocrine system. These substances are commonly found in:

• detergents,
• fuel and lubricant additives,
• perfumes,
• flame-retardant materials,
• tyres,
• coatings and similar products.

Other micropollutants such as parabens, PAHs, PFAS, phthalates and pharmaceutical residues are also known to interfere with hormonal systems, potentially causing reproductive and developmental issues. Some substances, including Volatile Organic Compounds (VOCs), are even classified as carcinogenic.

Heavy metals: pollutants released into the environment

Heavy metals are naturally occurring metallic elements with a density greater than 5 g/cm³. The most common include:

• mercury,
• lead,
• cadmium,
• copper,
• arsenic,
• nickel,
• zinc,
• cobalt,
• manganese.

Where do heavy metals come from?

Heavy metals naturally exist in rocks and soils and may also be released through erosion or volcanic activity.

However, mining and metal production are among the main human sources of heavy metal pollution. Other contributing activities include:

• waste incineration,
• the use of pesticides and fertilisers,
• burning fossil fuels.

Why are traces of heavy metals found in tap water?

Water pollution often occurs through industrial discharge, rainwater runoff and erosion. Even after treatment, traces of metals may remain in tap water. For example, ageing lead pipes, once widely used in urban water systems, can release lead particles into drinking water.

Exposure to heavy metals can affect human health depending on the type of metal, concentration and duration of exposure. Mercury may damage the nervous system, kidneys and lungs, while lead can affect brain development in children and contribute to behavioural and learning difficulties.