Chlorothalonil in groundwater

Metabolites of the fungicide chlorothalonil pollute the groundwater in large areas with more than 0.1 micrograms per litre. Metabolites are present in these concentrations in more than half of the cantons. The Swiss Plateau, which is used intensively for agriculture, is the main region affected.

Chlorothalonil, an active substance applied in plant protection products, has been used in Swiss agriculture since the 1970s. After the FSVO reassessed this substance with regard to its potential health risk, attention has focused on chlorothalonil metabolites in drinking water. 80% of drinking water in Switzerland is extracted from groundwater.

Metabolites of the pesticide chlorothalonil were detected in groundwater for the first time in 2017 during a pilot study of the NAQUA National Groundwater Monitoring, which is operated by the FOEN in close cooperation with the cantonal authorities. In 2018, the analyses were extended to further monitoring sites. A national data set from the NAQUA monitoring sites has been available for the chlorothalonil metabolite R417888 since 2019, and for the chlorothalonil metabolite R471811 since 2020.

Every third monitoring site affected

Five chlorothalonil metabolites are present in concentrations exceeding 0.1 micrograms per litre in groundwater: chlorothalonil R471811, chlorothalonil R417888, chlorothalonil R419492, chlorothalonil SYN 507900 and chlorothalonil R548580. In particular, the two metabolites R471811 and R417888 contaminate the groundwater in many areas of the Swiss Plateau used for agriculture. Values of over 0.1 micrograms per litre have been detected in the cantons of Aargau, Bern, Fribourg, Geneva, Graubünden, Jura, Lucerne, Neuchâtel, St. Gallen, Schaffhausen, Solothurn, Thurgau, Vaud, Valais, Zug and Zurich.

Ground water is especially affected by the metabolite chlorothalonil R471811, which exceeds 0.1 micrograms per litre at more than 60% of monitoring sites on the Swiss Plateau. Across the country, every third monitoring site is affected.

The metabolite chlorothalonil R417888 exceeds 0.1 micrograms per litre in groundwater of the Swiss Plateau at approximately 15% of the monitoring sites.


As the renewal of groundwater takes a relatively long time and the chlorothalonil metabolites are very persistent, it can be assumed that these contaminants significantly impair groundwater for many years.

Groundwater quality on site

All data gathered as part of the NAQUA national groundwater monitoring are made available to the cantonal authorities. The water suppliers concerned are also informed about the results. The cantonal authorities are responsible for providing detailed information on groundwater quality of a specific region or location. The water suppliers inform their customers about the quality of the local drinking water.

Further Information

Last modification 03.05.2024

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