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Pesticide levels in karst groundwater can spike after heavy rainfall
August 4, 2026 |
Around 18 percent of Switzerland’s groundwater originates from karst groundwater systems, mainly in the Jura and the Alps. Although these regions are generally less intensively farmed due to their topography and soil conditions, some areas are used for intensive crop production. Such agricultural practices, particularly arable farming and specialised crops such as fruit orchards and vineyards, often involve the use of pesticides. Once they enter groundwater, these substances can harm humans and the environment even at low concentrations.
Normally, the soil layer acts as a natural protective filter that can retain pollutants. In karst regions, however, where protective soil layers are often thin and local swallow holes allow direct water infiltration, rainfall can quickly reach groundwater with little filtration through cracks and underground channels.
Strong fluctuations and elevated levels after rainfall
As part of his PhD thesis funded by the Swiss Federal Office for the Environment, Johannes Schorr from the aquatic research institute Eawag detected significant short-term peaks in pesticide concentrations in karst areas. High-frequency measurements using MS2field, the mobile and automated mass spectrometer developed at Eawag, showed that a total of eleven pesticides temporarily exceeded the regulatory limit of 0.1 µg/L at two karst springs in the Swiss Jura. Although these elevated concentrations were only detected after heavy rainfall and persisted for a short period, the findings highlight the particular vulnerability of karst springs. These fluctuations could only be detected thanks to the high sampling frequency: every 20 minutes or every two to three hours. Such concentration peaks often remain undetected in conventional monitoring programmes based on periodic sampling.
To project the results to the entire Jura area, the Swiss Institute of Speleology and Karst Studies conducted calculations based on vulnerability criteria and agricultural land use. The estimates indicate that one to six percent of drinking water sources in the Swiss Jura may face a potential risk of elevated pesticide contamination.
Groundwater monitoring for quality assurance
To safeguard the quality of Switzerland’s groundwater, pesticides and their transformation or degradation products — some of which can be equally problematic — are analysed one to four times a year at around 550 monitoring sites across the country as part of the National Groundwater Monitoring Programme NAQUA. The pilot study “Pesticide residues in karst systems”, funded by the Swiss Federal Office for the Environment, investigated in 2021 and 2022 whether the current monitoring strategy is also suitable for karst regions. The results, now published in the journal Aqua & Gas, show that the current long-term NAQUA monitoring programme may be insufficient for this region and its specific characteristics.
“Our results show that karst springs should be monitored more closely after heavy rainfall. Targeted spot sampling could help identify contamination peaks at karst springs in areas with intensive agricultural use,” says Juliane Hollender, Head of the Department Environmental Chemistry at Eawag.
The MS2field, a mobile and automated mass spectrometer developed at Eawag, is in use throughout Switzerland. (Photos: Eawag [1,2] and Jean Fernex [3])
Cover picture: Swiss karst landscape. Because of the thin soil layer and the characteristics of karst terrain, rainwater can rapidly enter groundwater with little natural filtration. (Photo: Adobe Stock)
Original publication
Funding / Partnerships
- Eawag
- BAFU
