Pesticides in water: How do they end up in mains water?

Pesticides are chemicals used to manage insects, weeds, fungi, bacteria, and other pests. They are common in agriculture, forestry, and gardening. Over time, some of these chemicals can reach the mains water we drink every day.

How do pesticides end up in our drinking water? What treatment processes remove them? In this article, we’ll explore these questions and look at ways you can ensure your household water stays clean and safe.

Where do pesticides in water come from?

Agriculture is a major source of pesticide contamination. But urban and suburban areas can also contribute to invisible pollution.

Agriculture: intensive and impactful

Across the UK and beyond, intensive farming is common. This approach can strain soils and disturb local ecosystems.

To protect their crops from pests that appear because of these changes, farmers often use pesticides, sometimes preventively. This makes agriculture one of the main sources of pesticide traces in mains water.

Urban and suburban contamination

Cities and suburbs aren’t exempt. While herbicides like glyphosate are now banned for home gardeners, they are still sold online and can be used without oversight.

Local councils also use insecticides to control stagnant water and mosquitoes. These chemicals can enter wastewater, contributing to overall contamination.

How do pesticides enter water sources?

Pesticides can reach water in several ways:

• Rainwater runoff: Chemicals applied to soil can be carried by rain into rivers, lakes, and groundwater.
• Soil infiltration: On permeable soil or with heavy use, pesticides seep into the ground and contaminate underground water sources.
• Gardens and green spaces: Pesticides used in gardens, parks, and pavements can enter soil and stormwater, eventually reaching rivers and lakes.
• Wastewater: Pesticides in domestic and industrial wastewater may not be fully removed by treatment plants, entering our rivers and reservoirs.

How is mains water treated for pesticides?

Water from rivers, lakes, or underground sources is not ready to drink straight away. Treatment plants work hard to remove impurities and meet strict water quality standards.

Treatment is tailored to the water source and follows UK regulations. Key steps include:

Pre-treatment

Heavily loaded water undergoes pre-sedimentation to remove coarse particles like clay and silt. Pre-oxidation with ozone also helps remove iron, manganese, and certain nitrogen compounds. This improves taste, colour, and odour, and prepares the water for the next treatment stages.

Clarification

This stage makes the water clear by removing:

• Suspended particles like sand, silt, and organic debris
• Colloids such as fine clays and bacteria
• Some dissolved materials, including organic matter and salts

Techniques include:

• Coagulation-flocculation: Adding a coagulant to form larger particles (flocs) that can be removed.
• Sedimentation or flotation: Separating flocs by natural settling or using air bubbles.
• Filtration: Passing water through sand to remove remaining fine particles.

Refinement

Additional treatments improve taste, smell, and clarity. These include:

• Activated carbon adsorption: Removes dissolved organics and micropollutants, including pesticides and some heavy metals.
• Stripping: Removes dissolved gases and volatile compounds by injecting air or oxygen.

Membrane filtration

This advanced step removes extremely fine particles and contaminants:

• Microfiltration: Retains bacteria, parasites, yeasts, and particles causing cloudiness.
• Ultrafiltration: Also removes viruses and colloids.
• Nanofiltration: Reduces calcium and many pesticides and their by-products.
• Reverse osmosis: The most effective method, used for ultra-pure water and desalination.

Disinfection

To protect health, water is disinfected to neutralise viruses and harmful bacteria. Methods include:

• Chlorine, chlorine dioxide, or ozone
• Ultraviolet (UV) treatment
• Some membranes with disinfectant properties

Even after treatment, a tiny amount of chlorine may remain to prevent microbial growth in pipes.

Final treatment

The water’s pH is adjusted to prevent corrosion or limescale, and it may be remineralised if necessary. This ensures the water reaches your tap in top condition.

Monitoring and regulation in the UK

UK drinking water is heavily monitored. Suppliers and regulators ensure water meets strict standards.

Two main players are involved:

• Water companies: Conduct regular lab testing and monitoring, following government regulations.
• UK regulators: Perform independent checks to confirm water safety.

For pesticides, the guideline value is 0.1 micrograms per litre (µg/L). More toxic substances, now banned, have an even lower limit of 0.03 µg/L.

What happens if water exceeds safe pesticide limits?

If tests reveal levels above safe limits, water suppliers must act immediately. They inform local authorities, investigate the source, and implement measures such as mixing water supplies or limiting use for vulnerable groups. In urgent cases, distribution may be temporarily halted.

Reducing pesticides in water

There are several ways to reduce pesticide contamination:

• Sustainable farming: Precision agriculture, agroforestry, and permanent ground cover reduce pesticide use.
• Advanced water treatment: Reverse osmosis, activated carbon, advanced oxidation, and membrane filtration can remove pesticides effectively.
• Community and individual action: Educating the public and monitoring water quality helps identify contamination sources and improve practices.

Home solutions to remove pesticides and bacteria

Even treated mains water may contain trace contaminants, especially in older pipes. Home filtration can remove pesticides and other pollutants, including PFAS, the so-called ‘forever chemicals’.

Gravity-fed activated carbon filters

These simple, effective filters use gravity to purify water naturally. You pour water into the top chamber, and it passes through filter cartridges to remove impurities. Ideal for households of all sizes.

Reverse osmosis systems

These remove pesticides, herbicides, and many other harmful contaminants, including THMs and volatile organic compounds. They require professional installation and a larger initial investment but offer thorough protection.

This guide compares reverse osmosis and gravity water filters and how each system works.

Protect your tap water from pesticides

Pesticides can infiltrate soil and groundwater, impacting health. Using high-quality filtration, such as activated carbon systems, helps remove these contaminants at home. Weeplow systems are designed to remove up to 99.999% of harmful substances. Explore our online store to find the right system for your needs.