Health

What is applied to a field today may reach a stream tomorrow—or appear in groundwater years later. That time lag is what makes pesticides such a difficult water-quality issue: their intended effect is local, but their consequences may travel far beyond the point of application.

In Switzerland, the stakes are particularly high. Around 80 per cent of drinking water comes from groundwater, making its protection essential to a secure long-term water supply. Yet one distinction matters from the outset: detecting a substance in groundwater does not automatically mean that tap water poses a health risk. It is, however, a finding that needs proper assessment rather than casual reassurance.

The water beneath the ground is not necessarily the water in your glass

Groundwater is a natural resource. Drinking water is water that meets the legal requirements and is supplied by your local water provider. Depending on its quality, groundwater may be used directly, blended with water from other sources or treated before it reaches the network.

A result from a groundwater monitoring station therefore cannot simply be transferred to the tap. The assessment depends on the substance, its concentration, its toxicological properties, the duration of exposure and whether—and in what quantity—it actually reaches the drinking water supplied.

Pesticides include more than the original active substance

Pesticides are substances used to control unwanted organisms or influence their development. The term covers, above all, plant protection products and certain biocides. Herbicides target unwanted plants, fungicides fungal diseases and insecticides harmful insects.

For water quality, the original active ingredients are only part of the picture. In soil and water, they may change through chemical or biological processes, producing metabolites—breakdown products with properties of their own. Some are highly mobile, others persistent, and some reach groundwater more readily than the substance from which they formed.

From field to aquifer: the routes of entry

Once applied, plant protection products do not necessarily remain where they were sprayed. Their behaviour depends on the substance’s chemical properties, the soil, weather conditions, the timing and method of application, and local water conditions.

Heavy rain can wash dissolved substances off fields. Eroded soil may carry bound substances into streams, rivers and lakes. Mobile compounds and metabolites can travel downwards with percolating water and reach groundwater; drainage systems can shorten that journey considerably.

Point sources can be significant too. A small mistake while filling or cleaning equipment on a farm can create a substantial local input if plant protection products enter a drain or a drainage system. Agroscope therefore examines erosion, surface runoff, leaching, drainage discharge and farmyard drainage as key pathways. (agroscope.admin.ch)

The national picture is uneven

Switzerland’s National Groundwater Monitoring programme, NAQUA, tracks the condition and long-term development of groundwater at more than 600 monitoring stations. Current figures from the Federal Office for the Environment show a differentiated picture: pesticides exceed 0.1 micrograms per litre at around 1 to 2 per cent of monitoring stations each year. Metabolites are far more widespread. Across the country, pesticide metabolites are found at concentrations above 0.1 micrograms per litre at roughly one in three stations. The intensively farmed Swiss Plateau is particularly affected. (bafu.admin.ch)

Local conditions can be very different. Geology, soil, rainfall, land use, groundwater recharge and the depth of the aquifer all influence which substances occur and at what concentrations. For your drinking water, the relevant figure is therefore not the Swiss average but the situation reported by your local water supplier.

Chlorothalonil: a ban does not erase what is already underground

The fungicide chlorothalonil illustrates how long contamination can persist. It was used from the 1970s onwards and has not been permitted in Switzerland since 2020. Its metabolites nevertheless continue to affect groundwater over wide areas.

For 2024, the Federal Office for the Environment reports seven metabolites at concentrations above 0.1 micrograms per litre. In the Swiss Plateau, the metabolite R471811 exceeds that level at more than half of monitoring stations; nationwide, more than one in four stations is affected. Such concentrations have been recorded in more than half of Switzerland’s cantons. (bafu.admin.ch)

That does not contradict the ban. Groundwater renews slowly, while persistent breakdown products can continue moving through soil and the underground environment for years. A measure can therefore be effective long before its effect becomes visible in a monitoring series.

A limit value is not the same as a poisoning threshold

In Switzerland, a maximum value of 0.1 micrograms per litre generally applies to relevant pesticide active substances and relevant metabolites. This precautionary value protects drinking-water resources and supports the long-term security of drinking-water quality. Groundwater used, or intended for use, as drinking water must be of a quality that allows it to meet food-law requirements after simple treatment processes. (bafu.admin.ch)

The value should not, however, be treated as an individual toxicological threshold for every substance. Exceeding it requires investigation, but does not automatically indicate acute poisoning from drinking the water. Nor should a finding be played down: the precautionary principle is about preventing avoidable inputs wherever possible.

Health risk depends on the whole exposure picture

Whether a substance is relevant to health depends on more than whether it can be detected. Assessment considers the active substance or metabolite, the measured concentration, the frequency and duration of exposure, toxicological properties and possible combinations with other substances. Particularly sensitive groups may also be taken into account.

The Federal Food Safety and Veterinary Office therefore distinguishes between an analytical detection, a precautionary maximum value and a specific health assessment. Approved plant protection products undergo extensive testing and, when used as authorised, are intended not to endanger consumers. (blv.admin.ch)

Rivers and streams face a different test

Pesticides are biologically active—that is precisely why they are used. Outside their intended setting, they can affect sensitive non-target organisms. After rainfall, small streams in intensively farmed areas may be exposed to temporarily higher concentrations.

A concentration that is assessed differently in relation to human drinking water may still matter to aquatic life. Protecting water bodies therefore safeguards not only your future drinking-water supply but also the communities of organisms that depend on healthy streams and rivers.

Prevention starts at the source

Treatment can reduce contamination, but preventing problematic substances from entering soil and water is the more sustainable approach. This includes farming suited to the site, accurate application, appropriate buffer distances from water bodies and—where practical—mechanical, biological or other non-chemical methods.

Where applications cannot be avoided, the details matter. Weather and soil conditions must be considered, while drift and runoff should be reduced through suitable techniques. Equipment should not be filled or cleaned near drains or water bodies. Surplus product and packaging must be disposed of correctly, and plant protection products may only be used in accordance with their authorisation.

Research is providing increasingly precise guidance. Agroscope is, among other things, developing risk maps for around 22,000 small catchments and investigating where runoff, erosion, drainage or point sources are likely to be especially important. (agroscope.admin.ch)

What water suppliers can do when contamination is found

Depending on the circumstances, a water supplier may use a different abstraction point, blend water from several sources or add a treatment stage.

Activated carbon, oxidation processes and membrane technologies can all be considered for organic trace substances. The appropriate method depends on the substance, its concentration and the composition of the water. A process that reliably removes one active ingredient may not retain every metabolite equally well.

Your local water supplier and the relevant cantonal authorities can tell you about the quality of the water supplied in your area. If you want to know what the situation is where you live, they are the most reliable places to start.

Domestic filters: look for evidence, not promises

Broad claims about household filters are of limited use. “Filters pesticides” does not tell you which substance is reduced, or under what conditions.

Look instead for specific before-and-after analyses, independent laboratory results, information about the filter medium, contact time and operating life, and clearly documented maintenance requirements. What matters is demonstrated removal performance for the substance actually found in your water.

No single fix will solve the problem

Improved analytical methods now reveal more metabolites than in the past. At the same time, farming and crop protection are evolving. More precise application technology, forecasting models, resistant varieties, mechanical methods, biological strategies and digital tools can all support more targeted decisions.

The quantity applied is not the only issue. What matters is the risk an application creates for people, soil, water bodies and ecosystems. Effective groundwater protection therefore combines prevention at source, long-term monitoring, scientific risk assessment and treatment where it is genuinely needed.

What enters the soil today may matter for years

Pesticides and their metabolites can be detected in parts of Switzerland’s groundwater. The persistence of breakdown products shows that banning an active substance does not make its legacy disappear from the subsurface overnight. At the same time, a finding in groundwater is not automatically evidence of an acute health risk in drinking water.

The useful question is not simply whether a substance can be detected. It is how it reached the water, how long it will remain there, what effects it may have—and whether it reaches your tap at all.

Keeping those questions separate avoids both complacency and alarm. It provides a sound basis for the practical priority: prevent inputs, monitor water resources and act where there is a clearly identified risk.