Polluted groundwater – clean drinking water soon – Knowledge


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In the Swiss Mittelland, the groundwater is contaminated in many places with the decomposition products of a plant protection product. The good news: In a pilot test, these were successfully filtered out of the drinking water. A few years ago that hardly seemed possible.

Roman Wiget has been working with groundwater professionally for many years. But whether the results of those study from the summer of 2019 he was very surprised: “I would not have thought that the degradation products of chlorothalonil would be found in such high concentrations.” Chlorothalonil is a fungicide that has been used in Swiss agriculture for almost 50 years. A pilot study by the Swiss Federal Water Research Institute eawag was able to identify its decomposition products in the groundwater in many places.

Chlorothalonil – “probably carcinogenic to humans”


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Plant protection products with the active ingredient chlorothalonil have been banned in Switzerland since the beginning of 2020. Studies had shown that chlorothalonil can cause cancer in rodents. The substance has since been considered “probably carcinogenic in humans”.

Chlorothalonil itself is not found in Swiss groundwater, but its degradation products. According to the assessment of the Federal Food Safety and Veterinary Office, some of these degradation products are also suspected of being hazardous to health. This means that the strict limit value for pesticides in drinking water applies to them.

The agrochemical group Syngenta, which produces chlorothalonil, has lodged a complaint against this classification of chlorothalonil breakdown products. The decision of the federal court is still pending. Until then, it is unclear which limit values ​​apply to the degradation products of chlorothalonil in drinking water. The measured concentrations are still so low that they do not pose a health risk, says the Federal Office for Food Safety BLV on request.

Roman Wiget is managing director of the Seeländische Wasserversorgung Worben, which extracts drinking water from the groundwater for tens of thousands of people. The water from the catchment near Worben is severely affected by the pollution: the undesirable substances exceeded the limit value for pesticides in drinking water by almost 20 times. The substances are not harmful to health in the measured concentrations. Experts from the federal government and from various cantons agree on this.

Undesirable substances should be removed from the drinking water

Nevertheless, Wiget positioned itself early on with clear words: These degradation products would have to be filtered out of the drinking water. Only: just a few years ago you had for it hardly any solution. “Back then, reverse osmosis was the only known method that managed to remove the breakdown products of chlorothalonil from drinking water,” he says.

It is unfortunate that we now have to filter drinking water that could be used for centuries without any quality risks.

But this process requires a lot of electricity and large amounts of problematic waste water are produced. Although the members of the Seeländische Wasserversorgung approved the construction loan of almost two million francs, Roman Wiget struggled with the disadvantages of the process.

Finest black powder with a huge surface

He and his team therefore started experiments with activated carbon. “The amazing thing about activated carbon is its enormous surface area,” says Wiget. If you rolled out a few grams of it flat, you would get the surface of a football field. “The degradation products of the chlorothalonil stick to this surface,” says Florence Bonvin, scientific director of the company Membratec, which has continued to work on a cleaning process together with Roman Wiget.

Sit out or filter laboriously?


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The study by the Swiss water research institute Eawag in the summer of 2019 was the most comprehensive analysis of Swiss groundwater to date. The researchers found the breakdown products of chlorothalonil and other pesticide active ingredients.

Since then, various drinking water suppliers have tried to reduce the concentrations of undesirable residues by mixing the contaminated groundwater with clean water. The Swiss Gas and Water Association estimates that a few dozen water suppliers are currently considering whether to build a complex filter system.

The plans of the Zeeland water supply are probably the most advanced. In the best case, that system also filters other undesirable substances from the drinking water – possibly including those that may only be found in the future. That is why there is also criticism of the pilot plant of the Seeland water supply: It could weaken the precautionary principle – i.e. the efforts to protect the groundwater as well as possible from the outset.

80 percent of the drinking water in Switzerland comes from groundwater. Until a few years ago, two-thirds of it was considered so clean that it could be drunk directly after simple filtration or without any treatment. Due to a court action by Syngenta, it is currently unclear how much drinking water currently meets the legal requirements.

“But it usually takes too long for the dirt to stick to the activated carbon,” says Bonvin. So she tried to grind the activated charcoal finer and finer. As a result, their surface becomes even larger. In the pilot plant in Worben, the unwanted molecules could be filtered out much more efficiently. The activated carbon had to be replaced less often and the process became economical.

This activated carbon filter should therefore not be more expensive than the originally planned, more problematic reverse osmosis – and could go into operation this year. Roman Wiget is pleased that he has finally found a more environmentally friendly process. At the same time, he says, again in a bitter voice: “It is unpleasant that we now have to filter drinking water that has been used for centuries without any quality risks. This clearly shows that the protection of groundwater was not taken seriously enough.”

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