Almost two thirds of the population without safe drinking water! No quick solutions in sight! Costly new infrastructure needed! This news sparked spiteful online comments: "Who are these well poisoners?" asked one concerned reader, for example. However, Solothurn's concerns about the quality of drinking water are not an isolated case. The Seeland water association, for example, from which the Bernese cities of Biel and Lyss, among others, get drinking water, had to close four out of five groundwater catchments at the end of 2019 due to pesticide residues. "We have lost 70 percent of our capacity and therefore have a serious problem," explains Roman Wiget, board member of this water association, which supplies around 100,000 people.

Problematic degradation products

The unusual drinking water problems were not caused by the use of a new pesticide, but because the authorities took a closer look at a substance that has been in use for decades. Chlorothalonil has been used by farmers since the 1970s to protect vegetables and fruit from fungal infestation. But it is only since 2019 that a targeted search for degradation products of this substance has been carried out in groundwater. Based on new findings, the Federal Food Safety and Veterinary Office (FSVO) has classified chlorothalonil residues as "relevant" for the quality of drinking water. Stricter limits for the degradation products of the pesticide have now come into force, and chlorothalonil itself has been banned.

"Due to the new limit values, various groundwater sources are now considered contaminated," explains Michael Schärer, head of the Water Protection Section at the FOEN. "According to initial estimates, the groundwater catchments affected supply around 1 million residents with drinking water." Many of these catchments will have to be shut down for the coming years. This means a "severe reduction in supply security" in the affected regions and endangers the decentralized organization of the Swiss drinking water supply.