Chlorothalonil metabolites 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, a plant protection product substance, 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.

As 80% of drinking water in Switzerland is extracted from groundwater and because these substances are extremely difficult to remove during drinking water treatment, groundwater quality with regard to these substances is very important.

Metabolites of the plant protection product 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. The Swiss Federal Institute of Aquatic Science and Technology (Eawag) analysed hundreds of substances in samples taken from 31 specific monitoring sites using a complex, specially developed procedure.

In 2018, these trace-analysis tests were extended to further monitoring sites. A national data set from the NAQUA monitoring sites is now available for one of the chlorothalonil metabolites (R417888) for 2019.

More than half of cantons affected

Several chlorothalonil metabolites are present in concentrations exceeding 0.1 micrograms per litre in groundwater. 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, Basel-Landschaft, Fribourg, Geneva, Jura, Lucerne, Schaffhausen, Solothurn, Thurgau, Ticino, Vaud, Valais, Zug and Zurich. Evidence of the metabolites R419492, R611968, SYN507900 and SYN548580 has also been found at some monitoring sites.

The metabolite chlorothalonil R417888, for which most data is currently available, exceeds 0.1 micrograms per litre in groundwater of the Swiss Plateau at over 20% of the monitoring sites.

Chlorothalonil R417888 in groundwater
Maximum value per NAQUA monitoring site. LOQ: Analytical limit of quantification. Based on the results of approximately 400 monitoring sites (2017/2018 pilot study and 2019 long-term monitoring) the map shows widespread diffusion of chlorothalonil R417888 in groundwater of the Swiss Plateau and some large valleys.

The groundwater is even more severely polluted by the metabolite chlorothalonil R471811, which displays the highest concentration per monitoring site. The concentration of this metabolite is well in excess of 1 microgram per litre at some monitoring sites. Although no nationwide data set currently exists for this metabolite, the level of chlorothalonil R471811 can be estimated based on chlorothalonil R417888: Chlorothalonil R471811 may exceed 0.1 micrograms per litre at over half of all NAQUA monitoring sites on the Swiss Plateau. Based on the tests currently being carried out on this metabolite, a complete data set from the approximately 550 NAQUA monitoring sites will be available in summer 2021.

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.

Chlorothalonil R471811 in groundwater
Maximum value per NAQUA monitoring site. LOQ: Analytical limit of quantification. Based on the results from 160 monitoring sites (2017/2018 pilot study and 2019 long-term monitoring), the map shows extensive diffusion and a high proportion of monitoring sites and a high number of values over 0.1 micrograms per litre of chlorothalonil R471811 in the groundwater on the Swiss Plateau.

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 09.02.2021

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