Political fight over pesticides in Danish drinking water — what the research shows

Clean drinking water — or the lack of it — is an important issue in the ongoing Danish general election. Danish voters are going to the polls on 24 March.

In party leader debates and showdowns between prime ministerial candidates, voters have been served a barrage of numbers and accusations: Who is responsible for the fact that more than half of all drinking water wells in Denmark contain pesticide residues?

Multiple political parties are calling for a national pesticide spraying ban in agriculture to reverse the trend and save the groundwater.

But what is fact and what is fiction in the debate? What is spin, and what is fact?

The University Post teamed up with two UCPH researchers to bring some science into the election campaign.

Increase in pollution

Bjarne W. Strobel is an associate professor of environmental chemistry at the Department of Plant and Environmental Sciences at the University of Copenhagen.

He explains that almost 100 per cent of Danish drinking water comes from groundwater, which is rainwater that through the course of decades slowly seeps down through the soil.

»It takes many, many years before it all reaches the groundwater,« he says.

Groundwater is pumped up from a depth of around 30-70 metres below the surface. In many other countries, surface water is also used as drinking water, but Danish groundwater is uniquely clean enough to be used as drinking water.

Studies have shown that in recent years there has been a sharp increase in the presence of substances like pesticides in Danish water wells. In more than half of the wells examined last year, pesticide residues were found. In 13.8 per cent of cases, the pesticide residues were above the legal limit.

»A large part of the substances are broken down in the upper layers of soil, but a tiny amount makes its way down into the groundwater. And it is this tiny amount that is the cause of all the trouble,« says Bjarne W. Strobel.

Contaminated drinking water is diluted with clean water, which is why it is not dangerous to drink the tap water in Denmark. But a growing number of pesticide residue findings is the cause of concern about future water quality.

Lisbeth E. Knudsen is a professor in the Section of Environment and Health at the Department of Public Health at the University of Copenhagen. According to her, it is the concern about future generations’ drinking water that is the key point.

»Every time we measure using new methods, we find more substances. So there may very well be more substances in the wells than the ones that we currently test for,« she says.

A national spraying ban?

The residues originate from the agricultural pesticides used on Danish fields. This has led to several political parties to call for a national ban on spraying agents.

»In my view there is only one thing to do now, and that is to introduce a national ban on spraying,« said Prime Minister Mette Frederiksen (Social Democrats) during the first televised debate duel with the Liberal Party’s candidate for prime minister, Troels Lund Poulsen.

But what the prime minister is saying is not the same as a ban on spraying pesticides on all fields across the entire country — even though it may sound like it. The proposal only concerns the 160,000 hectares of vulnerable groundwater-forming areas — that is, roughly six per cent of Danish farming land.

It is the same ban that the Socialist People’s Party, the Red-Greens, the Social Liberal Party, and the Alternatives have proposed in a joint statement.

According to calculations from the Danish Ministry of the Environment, a ban like this would cost the agricultural sector about DKK 360 million a year in terms of economic output.

Troels Lund Poulsen of the Liberal Party believes instead that the problems with contaminated groundwater should be solved as part of the wider Green Tripartite Agreement: This agreement was a political deal reached in 2024 between government, agricultural organisations, and environmental groups to reduce emissions and reshape land use to improve climate, nature, and water quality.

However, the Liberal Party has also indicated it could support a ban on agricultural pesticides in wellhead protection areas.

A wellhead protection area is defined as the area around a well where it takes a given substance one year to reach the groundwater.

Denmark has designated about 4,850 wellhead protection areas, with farming land accounting for around 9,500 hectares of these — that is, less than one per cent of the total farming area.

No matter how many hectares a possible future spraying ban may cover, pesticide residues will still be present in groundwater for decades to come. Just as it takes rainwater 30–60 years to reach the groundwater, so with pesticide residues.

According to Bjarne W. Strobel, this means that substances can still be found in groundwater today even though they have been banned for years.

Harmful health effects

The two researchers agree that a spraying ban covering 160,000 hectares of vulnerable groundwater-forming areas would be best in terms of protecting future Danish drinking water.

»It is absolutely necessary. And the sooner the better. The longer we wait, the greater the risks we run,« says Lisbeth E. Knudsen.

Pesticides are associated with several harmful health effects, according to her.

»This particularly applies to foetuses and children, where we see immunotoxic, neurotoxic, and reproductive effects,« says Lisbeth E. Knudsen.

Researchers have for a long time agreed that the substances have harmful effects, she says. They are at the same time found in higher concentrations in the soil.

And although agriculture is often blamed in the debate over drinking water contamination, Lisbeth E. Knudsen says the picture is not quite that simple.

»We need to assume that farmers only do what they are allowed to do. They have not broken any laws. In that sense, the responsibility lies with the politicians,« she says.

Don’t forget the biocides

Bjarne W. Strobel also believes that it is too simple to blame agriculture as the cause of the problem.

It is not only farmers’ pesticides that end up in drinking water. With biocides, we all contribute to releasing them into the environment.

Chemically, there is no difference between the substances, but they are used in different ways. Pesticides are used on fields to combat weeds, fungi, and insects. Biocides are found in products like medicines and creams — where they treat fungal infections — or in paint and wood preservatives, prevent algae and mould.

The general pesticide limit of 0.1 micrograms per litre is not based on toxicity, but on a precautionary principle and what it is possible to measure

Bjarne W. Strobel, associate professor of environmental chemistry at the Department of Plant and Environmental Sciences

»They are exactly the same substances, and over time they also end up in the groundwater,« says Bjarne W. Strobel. He points out that this is why residues are also found in water wells in urban areas, where there has been no agriculture for decades.

According to Bjarne W. Strobel, how much groundwater contamination comes from agricultural pesticides and how much comes from consumers’ use of biocides depends on where you are in the country.

»In rural areas it is agriculture that accounts for it. But around wells near Copenhagen — where there are also problems with these substances — the use of biocides plays a significant role,« says Bjarne W. Strobel.

This perspective is missing in the current political debate, he says.

»If you only ban the substances used in agriculture, and do not include biocides, you are, broadly speaking, only solving half of the problem,« he says.

The Aalborg nitrate problem

The town of Aalborg features prominently in the political debate. Drinking water from wells in the area contains higher concentrations of nitrate than elsewhere in the country.

Nitrate comes from the use of fertiliser on farms, and the reason Aalborg, in particular, faces a bigger challenge with nitrate in its drinking water than other Danish cities is due to the local geology.

The chalk-rich soil in the surrounding Himmerland region cannot convert nitrate like other soil types, which means that more of the substance ends up in the groundwater.

The higher nitrate content has been linked to more cases of bowel cancer in the northern Jutland town, although this has never been proven.

Nitrate levels in Aalborg remain below the 50 milligrams per litre limit value, but an expert group appointed by the Ministry of the Environment recommended in December 2025 that the limit value should be lowered to six milligrams per litre.

One can fear increased problems with both our reproduction, immune system, and the cancer risk that is being debated

Lisbeth E. Knudsen, professor in the Section of Environment and Health at the Department of Public Health

When it comes to limit values, Bjarne W. Strobel says there is an important point to bear in mind.

While Lisbeth E. Knudsen points out that pesticides may have health effects even at very low concentrations, Bjarne W. Strobel emphasises that the current limit values are deliberately set very low based on a precautionary principle and on what can actually be measured.

»All of these substances will be harmful at a given concentration. But the general pesticide limit of 0.1 micrograms per litre is not based on toxicity, but on a precautionary principle and on what it is possible to measure at all. This means that we may risk introducing a ban on pesticides that significantly changes farming practices, but without achieving a corresponding health benefit,« he says.

Lisbeth E. Knudsen does not agree.

It is far from certain, according to her, that the current limit values give a full picture of the risks.

»We also see an increased cancer risk at low concentrations — and possibly lower than the limit values we have today,« she says. She also points to the so-called cocktail effect, where several different substances amplify each other’s effects.

»So there is reason to be concerned about both our reproduction, our immune system, and the the risk of cancer.«

So the research does not provide a clear answer as to how big the risks really are from pesticide residues in drinking water. While some researchers point to possible health effects even at very low concentrations, others emphasise that the current limit values are already set according to a cautionary principle.

The researchers, like the politicians, disagree.

This article was first written in Danish and published on 11 March 2026. It has been translated into English and post-edited by Mike Young.