Chemicals in Every Sample: What 30,000 Schoolchildren Found in Denmark’s Soil

A Nationwide Citizen-Science Effort

In the autumn of 2025, around 30,000 Danish primary- and lower-secondary-school pupils took part in the country’s largest ever soil-sampling campaign in and around urban areas. The initiative - known as the Mass Experiment (Masseeksperimentet) - was coordinated by the national science-education centre Astra and reached classes in 97 of Denmark’s 98 municipalities. Only the small island of Læsø did not take part.

Each class followed the same protocol: pupils dug samples at 10 cm and 40 cm depth in four standardised land-use categories - forest, urban green space, agricultural field and schoolyard. The samples were sent to research teams at Aarhus University, Aalborg University and the Technical University of Denmark (DTU). Of the 923 samples received, around 600 have so far undergone an initial chemical screening of the uppermost soil layer. 

The Danish version of the “Soilblitz” task within SOILTRIBES, a European research project on soil health, joined the activities. By partnering with the existing Mass Experiment initiative, in which Aarhus University was already involved, SOILTRIBES was able to reach a much higher scale of impact than a stand-alone sampling campaign could have achieved.

Chemicals in Every Sample

The early result is striking: every analysed sample contains traces of human-made chemicals. Roughly 80 percent also contain per- and polyfluoroalkyl substances (PFAS), the group of lasting compounds widely known as “forever chemicals”.

Professor Mogens Humlekrog Greve of Aarhus University, who leads the scientific work, describes the result as a confirmation of long-standing concerns about diffuse contamination. “Some of the most widespread substances are flame retardants. They are in electronics and on textiles, and we seem to be finding them everywhere,” he says. He had hoped some sites would be diluted enough for the chemicals to fall below detection limits. “but this is apparently not the case.”

Greve characterises the early findings as “a wake-up call” - they confirm, and in places exceed, what researchers had feared. The team has yet to measure the concentrations of the detected substances, and the deeper 40 cm samples are still to be analysed. Both will be addressed in the years ahead, as the multi-year analytical programme continues.

Persistent Compounds Across the Landscape

Researchers from DTU, who run the chemical analyses, report that soil acts as a long-term archive of persistent and strongly bound compounds. Alongside flame retardants and PFAS, selected pesticides and pharmaceutical residues turn up in a substantial share of the samples.

The picture aligns with independent work by scientists from the University of Copenhagen. In a 2023 campaign, they sampled 72 sites deliberately chosen for their remoteness - central Jutland forests, Møns Klint, and the Bøtø bird reserve on southern Falster. The dominant PFAS congeners, PFOS and PFOA, turned up at every single site. The most likely explanation is long-range atmospheric transport and wet deposition: the chemicals fall with the rain, even far from any local source.

A Health Concern Across Generations

Researchers in environmental medicine from the University of Southern Denmark caution that PFAS exposure has been linked to a range of adverse outcomes - reproductive, immunological, metabolic and oncological - with fetal and early-life exposure of particular concern. They argue that the persistence of these substances raises serious intergenerational ethical questions: future generations will inherit a chemical legacy built up over the past five decades.

The Soil Monitoring Law: A Framework for the Path Forward

The findings emerge at a moment when European soil policy is undergoing a structural shift. On 23 October 2025, the European Parliament adopted the Soil Monitoring Law (SML), the first EU-wide legal instrument dedicated specifically to soil health. The law obliges every Member State to monitor and assess the physical, chemical and biological state of its soils, to support landowners in improving soil quality, to minimise the loss of healthy soil, and to manage contaminated sites.

The overarching goal is healthy European soils by 2050, in line with the EU’s “zero pollution” ambition. The European Commission estimates that around 60 to 70 percent of soils across the Union are currently in an unhealthy state.

What the Law Requires

The SML introduces a harmonised European framework. Monitoring will be carried out using common soil descriptors and an EU-wide sampling methodology, so that data from different Member States can be compared and aggregated. Each Member State must also establish a public register of potentially contaminated sites within ten years of the law entering into force.

Within 18 months of entry into force, the Commission will publish a guidance list of emerging substances of concern. The list is expected to include relevant PFAS compounds and pesticides - exactly the classes of substances that dominate the Mass Experiment’s preliminary findings.

Denmark’s Implementation Timeline

For Denmark, the schedule is concrete. National legislation is to be in place by 2027. The first national soil sampling round is planned for 2028. An initial nationwide assessment of soil health is due by 2031, and all potentially contaminated areas must be mapped by 2035.

Aarhus University has a central role in defining what “healthy soil” means in operational terms. That work means setting threshold values for soil-health indicators across Denmark’s many soil types and cropping systems - a task that requires both data and a clear methodology.

Why the Mass Experiment Matters Now

It is here that the Mass Experiment becomes most useful. Greve calls the dataset a “treasure trove of knowledge” that can be drawn on directly as threshold values are negotiated and the national monitoring network is designed.

“We have to establish a network of measurement points that represents Denmark’s soil types and cropping systems,” he explains. “It is not just the chemical or physical state of the soil that matters - we also have to measure the biology and the microbiology. It is complex.” The pupils’ samples, gathered across forests, fields, urban green spaces and schoolyards, give a baseline of contamination and biological activity from precisely the peri-urban landscapes the official monitoring network will need to cover.

Four Pressures on Danish Soils

Greve identifies four main pressures that the SML framework will need to address: loss of soil organic carbon (especially in eastern Denmark); erosion on cultivated slopes; soil compaction caused by heavy machinery; and biological change driven by pesticides and intensive management.

“The 2050 healthy soils target cannot be met on our current trajectory,” he warns. “There has to be a change.” Article 11 of the directive encourages Member States to support landowners through independent advice, training, research and innovation - an opening, in his view, to promote regenerative cultivation practices, provided that implementation begins quickly.

Outlook

The Mass Experiment is a methodological milestone for Danish environmental science: it pairs large-scale citizen participation with the analytical capacity of three universities, working across soil chemistry, microbiology and ecology. The next phases will quantify contaminant concentrations, look at how chemicals are distributed with depth, and explore how they correlate with the soil microbial communities and earthworm populations measured in parallel.

Together with the Soil Monitoring Law, the project marks a shift away from fragmented, reactive responses to chemical contamination, and toward a systematic, indicator-based stewardship of soil as a shared European resource. The Mass Experiment is supported by the Nordea Foundation and the Villum Foundation.