Chlorothalonil pollution, a challenge for drinking water treatment
Water is a vital resource for every society. In a context where environmental pollution is multiplying and threatening its quality, water is a limited resource. Pollution by chlorothalonil (a fungicide used in agriculture) and its metabolites (resulting from its degradation in the environment), which have recently been in the news, provide an enlightening example.
Benoit Teychene, University of Poitiers and Julie Mendret, University of Montpellier
The metabolite R471811, in particular, has recently crystallized the debate. A vast national exploratory campaign conducted between 2020 and 2022 by Anses, which was the subject of a report in March 2023, indicated that it was found in excess of the authorized limit value in over a third of samples analyzed for drinking water. In a new opinion published in April 2024, Anses finally decided to lower the level of danger posed by this metabolite.
As a direct consequence, concentrations deemed "acceptable" in drinking water have increased tenfold, leading to the reopening of water catchment points that had been closed in the meantime. However, this type of pollution poses a very real problem for the treatment of water intended for human consumption (EDCH).
Chlorothalonil pollution in question
As a reminder, chlorothalonil, marketed by Syngenta, has long been used as a fungicide, mainly on cereal crops. It was banned by the European Union in 2019 due to its classification as a probable carcinogen: the European Food Safety Authority (EFSA) gave it a 1B carcinogen classification, reserved for substances with suspected carcinogenic potential. France had granted a grace period until May 2020 before the ban took effect, to allow stocks of the product to run out.
Where does the problem lie? After their agricultural use, pesticides end up in the environment (air, water, soil, etc.) where they can be transformed into new molecules with different properties: these are known as metabolites. In 2019, Swiss researchers published the first study to reveal the presence of chlorothalonil metabolites in Swiss groundwater, raising the suspicion of pollution in drinking water. The results highlighted the presence of eight chlorothalonil-derived compounds, six of which were identified for the first time.
Chlorothalonil R471811 had been detected in 31 Swiss groundwater resources at worrying concentrations, reaching up to 2.7 µg/L. This value exceeds the maximum limit set for drinking water by the health authorities in both Switzerland and France, requiring additional treatment to make the water fit for consumption. In France, above 2 µg/L per substance (and 5 µg/L for their sum) in raw water, it is not considered fit for drinking, and the point of abstraction must be closed.
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Regulatory standards in question
To understand the quality thresholds that apply to drinking water, we first need to understand that Anses distinguishes two regulatory categories for pesticides and their metabolites:
- "Relevant" metabolites, likely to pose a danger to human health, whose concentration must not exceed 0.1 µg/l in water delivered to the tap (and 0.5 µg/l for their sum), in accordance with the Anses opinion of January 30, 2019.
- For "irrelevant" metabolites (i.e. less likely to present a danger to human health), the limit is set at 0.9 µg/l (vigilance threshold).
Given the toxicity of chlorothalonil, Anses initially classified the chlorothalonil metabolite R471811 as "relevant". However, theAnses opinion of April 2024 re-examined its classification, finally considering it to be an "irrelevant" metabolite in water intended for human consumption. In so doing, the agency drew on data from the European assessment report, new information from the notifier (Syngenta) and additional bibliographical research. In the same opinion, the agency classified another chlotothalonil metabolite, R417888, as "relevant".
As a result, the vigilance threshold for R471811 has been raised to 0.9 µg/l, and that for R417888 to 0.1 µg/L. Once this classification has been revised, drinking water utilities have a maximum of six years to reduce the concentration of these pollutants below the compliance threshold.
This threshold value should not be confused with the maximum health value, known as Vmax. This is the limit above which exposure to the molecule is considered potentially dangerous for human health. It is constructed from toxicological reference values and is based on the threshold of toxicological concern applying to metabolites and pesticides, in order to protect consumers by taking into account the tap water ingested over a lifetime.
When the Vmax has not yet been established by Anses, notably for lack of scientific data, restrictions on use are applied and the transitional health value of 3 µg/L - set by the French Ministry of Health - is applied. This acts as a Vmax until the Anses establishes a definitive Vmax.
Compliance with this transitional health value therefore authorizes the distribution of water, even when it exceeds the 0.1 µg/L limit. However, this management value (transitional health value) is only used for a limited period of time, which is why exceeding the Vmax leads to immediate restrictions on tap water consumption.
Drinking water wells under strain
In France, drinking water abstraction is managed on a river basin basis. Pollution control (nitrates, phosphates, pesticides, etc.) is one of the key challenges of this crucial legislation, introduced in 1964 to protect human health and the environment, both flora and fauna. It was at this time that the "polluter pays" principle was introduced.
The challenges have since multiplied, as climate change puts additional pressure on the quantity and quality of available water resources, making this form of management increasingly difficult. Those involved in water management face a complex challenge:
- On the one hand, they need to practice water sobriety, which is necessary to preserve the resource (by limiting leaks in distribution networks, for example),
- On the other hand, they need to improve their water treatment systems to deliver quality water to the tap, despite the increase in pollution and the considerable investment - which local authorities are struggling to afford - that this entails.
A striking example of this deterioration is the abandonment of a large number of water catchments and facilities - some 12,600 - between 1980 and 2021. In the case of around 33% of closed catchments, the main reason for abandonment was deterioration in the quality of the resource. Of these, 40.7% were closed due to excessive nitrate and/or pesticide levels.
Can chorothalonil pollution be eliminated?
Despite the new opinion from Anses, the current situation remains complex. Technologies exist to eliminate these metabolites, but this remains a challenge for water treatment operators.
Initial studies indicate that adsorption on activated carbon and membrane processes(reverse osmosis, nanofiltration) are the most effective technologies. Due to its physico-chemical properties, the metabolite R417888 is easier to remove by adsorption than R471811.
In the Île-de-France region, the Méry-sur-Oise water treatment plant uses membranes that deliver excellent filtration results for chlorothalonil and its metabolites (90-95% retention rate for metabolite R471811).
However, these processes are costly. Adsorption requires frequent renewal of the activated carbon, which has a negative impact on treatment costs. Similarly, the deployment of membrane processes can considerably increase the energy consumption of treatment units, and poses the problem of concentrate management.
Rising water treatment costs
We are therefore tending towards an increase in the operating costs of treatment units, which will inevitably lead to an increase in the price of water for the consumer. An important point is the difficulty of complying with the vigilance value of 0.9 µg/L for the metabolite R471811 which, as we have seen above, is the most complicated to eliminate in conventional systems. Moreover, there is a wide disparity between the various local authorities and water boards. Large conurbations tend to have much more efficient treatment systems than rural areas. This situation can exacerbate existing tensions.
If this value is reached, a general improvement in the quality of produced water (with regard to other pesticides and pollutants which would then also be retained) is expected. Provided, of course, that other threats to drinking water quality, such as per- and polyfluoroalkyl compounds (PFAS) and other problematic metabolites (such as flufenacet ESA) and their potential cocktail effects, are also kept under control.
Consumers' increasing reliance on bottled water or point-of-use treatment devices (filter carafes, osmosis machines, carbon sticks, clay balls, atmospheric water generators, etc.) may seem legitimate. However, they require regular maintenance to prevent bacterial growth: the cure can become worse than the disease! What's more, some solutions such as reverse osmosis or bottled water have a disastrous environmental impact.
The only viable option therefore remains unwavering protection of water resources used to produce drinking water. This requires in-depth research to improve knowledge of the state of contamination of resources, the implementation of appropriate, environmentally-friendly treatment processes, and the promotion of agricultural activities that reduce the intensive use of plant protection products.
Benoit Teychene, Senior Lecturer, University of Poitiers and Julie Mendret, Senior Lecturer, HDR, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.