Chlorothalonil pollution: a challenge for drinking water treatment
Water is a vital resource for any society. However, in a context where environmental pollution is increasing and threatening its quality, it is a limited resource. Pollution from 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 illuminating example.
Benoit Teychene, University of Poitiers and Julie Mendret, University of Montpellier
The metabolite R471811, in particular, has recently crystallized the debate. A large-scale 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 above the authorized limit in more than a third of the samples analyzed from water intended for drinking. In a new opinion published in April 2024, ANSES finally decided to reassess the level of danger posed by this metabolite and downgrade it.
The direct consequence of this was that the concentrations deemed "acceptable" in drinking water were increased tenfold, leading to the reopening of water collection 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 (WICH).
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) has classified it as a Group 1B carcinogen, reserved for substances with a suspected carcinogenic potential. France had granted a grace period until May 2020 before the ban took effect, to allow stocks of the product to be sold off.
Where does the problem lie? After being used in agriculture, pesticides end up in the environment (air, water, soil, etc.), where they can transform into new molecules with different properties: these are called metabolites. In 2019, Swiss researchers published the first study to reveal the presence of chlorothalonil metabolites in Swiss groundwater, casting suspicion on drinking water pollution. The results highlighted the presence of eight chlorothalonil-derived compounds, six of which were identified for the first time.
Chlorothalonil R471811 was detected in 31 Swiss groundwater sources at worrying concentrations of up to 2.7 µg/L. This exceeds the maximum limit set for drinking water by health authorities in both Switzerland and France, requiring additional treatment to make it safe for consumption. In France, if the concentration of a single substance exceeds 2 µg/L (or 5 µg/L for the sum of all substances) in raw water, it is not considered drinkable and the collection point must be closed.
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The regulatory standards in question
To understand the quality thresholds that apply to drinking water, it is first necessary to understand that ANSES distinguishes between two regulatory categories for pesticides and their metabolites:
- "Relevant" metabolites, which may pose a risk to human health, whose concentration must not exceed 0.1 µg/l in tap water (and 0.5 µg/l for their sum), in accordance with the ANSES opinion of January 30, 2019.
- For "non-relevant" metabolites (i.e., those less likely to pose a risk to human health), the limit is set at 0.9 µg/l (vigilance threshold).
Given the toxicity of chlorothalonil, ANSES initially classified the metabolite R471811 of chlorothalonil as "relevant." However,ANSES's opinion of April 2024 re-examined its classification, ultimately considering it to be an "irrelevant" metabolite in water intended for human consumption. The agency based its decision on data from the European assessment report, new information from the registrant (Syngenta), and additional bibliographic research. In the same opinion, the agency classified another chlorothalonil metabolite, R417888, as "relevant."
Result: the alert threshold for R471811 is therefore lowered to 0.9 µg/L, and that for R417888 to 0.1 µg/L. Following this revision of the classification, public drinking water services have a maximum of six years to reduce the concentration of these pollutants below the compliance threshold.
This vigilance threshold value should not be confused with the maximum health value, known as Vmax. This is the limit beyond which exposure to the molecule is considered potentially dangerous to human health. It is constructed from toxicological reference values and is based on the toxicological threshold of concern applicable to metabolites and pesticides in order to protect consumers, taking into account tap water ingested throughout their lifetime.
When the Vmax has not yet been established by ANSES, particularly due to a lack of scientific data, usage restrictions are applied and the transitional health value of 3 µg/L—set by the Department of Health—is applied. This serves as the Vmax until ANSES establishes a definitive Vmax.
Compliance with this transitional health value therefore allows water to be distributed even when it exceeds the limit of 0.1 µg/L. 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 the consumption of tap water.
Drinking water collection points under pressure
In France, drinking water withdrawals are managed by river basin. The fight against pollution (nitrates, phosphates, pesticides, etc.) is one of the key issues addressed by this crucial legislation, which was introduced in 1964 to protect human health and the environment, including 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 type of management increasingly difficult. Water management stakeholders face a complex challenge:
- On the one hand, they need to practice water conservation, which is necessary to preserve the resource (for example, by limiting leaks in distribution networks).
- On the other hand, they need to improve water treatment systems to provide high-quality tap water despite increasing pollution and the considerable investment this requires, which local authorities are struggling to afford.
A striking example of this deterioration is the abandonment of numerous water catchments and facilities, numbering 12,600 between 1980 and 2021. For around 33% of the closed catchments, the main reason for abandonment was the deterioration in the quality of the resource. Of these catchments closed for quality reasons, 40.7% were closed due to excessive nitrate and/or pesticide levels.
Can chlorothalonil 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 activated carbon adsorption techniques and membrane processes (reverse osmosis, nanofiltration) are the most effective technologies. Due to its physicochemical properties, 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 achieve good results in filtering chlorothalonil and its metabolites (90 to 95% retention of metabolite R471811).
However, these processes are costly. Adsorption requires frequent replacement of activated carbon, which has a negative impact on treatment costs. Similarly, the deployment of membrane processes can significantly increase the energy consumption of treatment units and raises the issue of concentrate management.
Towards an increase in water treatment costs
We are therefore moving towards an increase in the operating costs of treatment plants, which will inevitably lead to higher water prices for consumers. One important point is the difficulty of complying with the vigilance value of 0.9 µg/L for metabolite R471811, which, as we have seen above, is the most difficult to eliminate using conventional methods. Furthermore, there is considerable disparity between different local authorities and water utilities. Large urban areas generally have much more efficient treatment processes than rural areas. This situation may exacerbate existing tensions.
If this value is achieved, a general improvement in the quality of the water produced (compared to other pesticides and pollutants that would also be retained) is expected. Provided, of course, that other threats to drinking water quality, such as per- and polyfluoroalkyl substances (PFAS) and other problematic metabolites (such as flufenacet ESA) and their potential cocktail effects, are also under control.
Consumers' increased use of bottled water or point-of-use treatment devices (filter jugs, reverse osmosis systems, charcoal 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! In addition, some solutions, such as reverse osmosis systems and bottled water, have a disastrous environmental impact.
The only viable option therefore remains the unwavering protection of water resources intended for drinking water production. This requires in-depth research to improve knowledge of the state of contamination of resources, the implementation of appropriate and 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
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