Chlorothalonil metabolites in groundwater: first nationwide assessment of pollution in Switzerland

12.05.2020 – In December 2019, the Federal Food Safety and Veterinary Office (FSVO) classified all degradation products (metabolites) of chlorothalonil, a plant protection product substance, as relevant for drinking water. A maximum level of 0.1 microgram per litre applies to these substances in drinking water and this limit is also valid for groundwater in this case. The Federal Office for the Environment (FOEN) has now carried out a first nationwide assessment of pollution in groundwater. This reveals that the concentration of several chlorothalonil metabolites exceed this limit in groundwater in large areas of the Swiss Plateau, resulting in significant contamination.

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 network.

Around half of cantons affected

The investigations carried out in 2017 and 2018 enable a first nationwide assessment of groundwater pollution. These results indicate that 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. These results reveal that values of over 0.1 microgram per litre are detected in the cantons of Aargau, Bern, Fribourg, Geneva, Lucerne, Schaffhausen, Solothurn, Thurgau, Ticino, Vaud, Zug and Zurich. Evidence of the metabolites R611968, SYN507900 and SYN548580 is also found at some monitoring sites.

Most data (from 201 monitoring sites) is available on the metabolite chlorothalonil R417888 which 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 200 monitoring sites (2017/2018 pilot study and 2018 long-term monitoring) the map shows widespread diffusion of chlorothalonil R417888 in groundwater of the Swiss Plateau and some large valleys.

The most severe pollution of groundwater is due to the metabolite chlorothalonil R471811. This metabolite has the highest concentration per monitoring site in all cases, but only the results of a smaller number of sites (70) are available to date. The concentration of this metabolite even exceeds 1 microgram per litre at some monitoring sites. Data available for these two metabolites enable to estimate that chlorothalonil R471811 may exceed the limit of 0.1 microgram per litre at over half of all monitoring sites of the Swiss Plateau.

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 70 monitoring sites (2017/2018 pilot study) the map shows extensive diffusion and a high proportion of cases where chlorothalonil R471811 exceeds the limit in groundwater of the Swiss Plateau.

Complete picture of pollution

An almost complete data set on the approximately 550 NAQUA monitoring sites that are part of the 2019 long-term monitoring will be available for the metabolite chlorothalonil R417888 in summer 2020. Further investigations are actually being carried out on the metabolite chlorothalonil R471811 which is very likely to be most widespread and to prevail in the highest concentrations. Cantonal authorities and water suppliers are also currently running their own extensive analysis on drinking water production sites that may potentially be affected.

The data that was gathered as part of the 2017 and 2018 pilot study is – like all NAQUA data – available at the cantonal authorities. The water suppliers concerned have also been 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 12.05.2020

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