Chlorothalonil metabolites in groundwater

17.08.2020 – Metabolites of the fungicide chlorothalonil exceed the threshold value for groundwater in large areas. Groundwater quality is thus significantly affected in more than half of the cantons. The Swiss Plateau, which is used intensively for agriculture, is the main region concerned.  

Sampling of groundwater for trace analysis in the laboratory.
© Damian Poffet

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 in December 2019 and classified its metabolites as relevant, attention has focused on chlorothalonil metabolites in drinking water supply.

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

As part of a pilot study of the National Groundwater Monitoring NAQUA, which is operated by the FOEN in close cooperation with the cantonal authorities, metabolites of chlorothalonil were detected in groundwater for the first time in 2017. 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 and one of the metabolites was partly included in the NAQUA long-term monitoring which carries out analysis on the entire 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 exceed the limit of 0.1 microgram per litre in groundwater. In particular, the three metabolites R471811, R417888 and R419492 are contaminating the groundwater in many areas of the Swiss Plateau used for agriculture. Values of over 0.1 microgram per litre were 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 R611968, SYN507900 and SYN548580 is also found at some monitoring sites.

The metabolite chlorothalonil R417888, for which most data is currently available, exceeds the limit of 0.1 microgram 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 holds the highest concentration per monitoring site. The concentration of this metabolite even exceeds 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 the limit of 0.1 microgram per litre at over half of all NAQUA monitoring sites in the Swiss Plateau. Based on the tests currently being carried out on this metabolite, a complete data set of 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 will significantly impair groundwater quality for many years.

Chlorothalonil R471811 in groundwater
Maximum value per NAQUA monitoring site. LOQ: Analytical limit of quantification. Based on the results of 160 monitoring sites (2017/2018 pilot study and 2019 long-term monitoring) the map shows extensive diffusion and a high proportion of monitoring sites where chlorothalonil R471811 exceeds the threshold in groundwater of 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 the quality of groundwater in specific municipalities. The water suppliers inform the customers about the quality of the local drinking water.

Further Information

Last modification 17.08.2020

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