INFOGRAPHICS. What we know about pesticide residues in the tap water of thousands of municipalities (and uncertainties about their toxicity)

The authorities repeat it, rightly. Tap water is one of the most monitored and controlled foods in France. Certain molecules are attracting more and more attention: pesticides. Once spread on crops, they seep into the soil and stay there for years. Tap water, often drawn from groundwater, retains traces of it at sometimes high levels.

“The long-term health effects of exposure to low doses of pesticides are difficult to assess”, notes the Ministry of Health. And this uncertainty weighs even more heavily on one type of substance in particular: metabolites. These are molecules resulting from the degradation of pesticides once spread. Glyphosate turns into Ampa; chloridazone is transformed into desphenyl chloridazone… From how many micrograms (μg) of metabolite per liter does tap water become toxic? For some molecules, we still don’t know, and the precautionary principle is not always applied.

On the occasion of the broadcast of a “Complément d’Enquête” survey, Thursday September 22 on France 2, franceinfo took an interest in tap water analysis data, between January 2021 and July 2022. The compilation of this data together with an article from the World reveal exceedances of the environmental quality thresholds set by law.

From Ajaccio to Roubaix, we are not looking for the same molecules in water

There are hundreds of pesticide molecules and therefore metabolites. Are they all tracked down in tap water? Far from it. Out of more than 1,000 existing pesticide molecules, the health authorities searched for an average of 170 per sample between January 2021 and July 2022. But the number and list of substances monitored differ from one region to another, and even from one levy to another. With, sometimes, huge differences. In Corsica, for example, an average of 30 molecules are tested per sample, compared to 386 in Ile-de-France.

The detail by department reveals even greater disparities: 24 molecules per sample for Haute-Corse against 477 in Hauts-de-Seine. How to explain these differences? The list of molecules to be tested is defined locally. It depends in particular on the population density on the territory of the water distribution network, the capacity of the laboratories to detect such and such a new molecule and the financial resources of the ARS to carry out these costly operations.

But this depends above all on the phytosanitary products most used in the territory. In a region where mainly beets are grown, such as in the north of France, research will, for example, target the molecules of chloridazone, a herbicide specific to this crop. Where corn is mainly grown, as in the West, we will mainly look for molecules derived from S-metolachlor.

Quality overruns measured in nearly 9,000 municipalities since January 2021

What do these analyzes show? In nearly 80% of cases, nothing to report: the samples did not reveal any exceedances of pesticide molecules or their metabolites. But in just over 20% of the samples, the health authorities noted that the quantity of one or more molecules of pesticides or their metabolites exceeded the regulatory threshold. A water quality limit, mostly located at 0.1 micrograms per liter (μg/L). This represents 12,281 exceedances between January 2021 and July 2022, affecting 8,959 municipalities.

French territories are not all concerned in the same way by this phenomenon. The map below, which represents the number of overruns by municipality, reveals that the north and west of France are particularly affected. The gray areas are the municipalities where, when we looked for a molecule of pesticide, no excess was found. The white areas correspond to the places where no molecule of pesticide has been sought (read our methodology at the end of the article).

But in the detail of the overruns, we do not find the same molecules. The most common is ESA-metolachlor, a metabolite of S-metolachlor, a herbicide used in the cultivation of corn, sunflower or sorghum. Between January 2021 and July 2022, the health authorities noted 6,550 exceedances of the quality threshold, evaluated at 0.1 μg/L. In second and third position are two metabolites of chloridazone, for which values ​​above the quality limit were observed 4,125 times and 2,539 times respectively.

Due to the uses of phytosanitary products which differ from one region to another, the map of overruns of the most concerned products is revealing. Chloridazone and its metabolites exceed the limit, especially in the distribution networks of Hauts-de-France, Marne and sometimes in Normandy. While S-metolachlor and its metabolites are found in western France, the Rhone Valley and, to a lesser extent, the Southwest.

For most of the molecules concerned, the health authorities explain that these overruns do not mean that drinking tap water is toxic. It is still declared “non-compliant with quality requirements”, and the manager is required to implement measures to stem the phenomenon. The water therefore continues to be distributed as long as it does not exceed a second threshold, called “maximum sanitary value”. For ESA-metolachlor, for example, this “Vmax” is set at 510 μg/L, a threshold never exceeded over the period we studied. But for some molecules, this second threshold does not exist.

For some molecules, overruns but no maximum health value

According to ANSES data, there are 23 pesticides or metabolites for which the authorities have not defined a maximum health value. Due to a lack of scientific knowledge and knowledge of toxicological limits, “Anses may not be able to provide Vmax”explains the organization.

As the journalists of “Complément d’Enquête” note, the authorities have oscillated in recent years on the way forward in the face of this lack. In an instruction of December 2020, the Directorate General for Health (DGS) deemed it preferable to apply a precautionary principle as soon as the quality threshold of 0.1 μg/L was exceeded. “It is recommended to restrict the uses of water as soon as the exceeding of the quality limit is confirmed”, she wrote.

This is for example the position adopted by the city of Chemilly-sur-Serein (Yonne), where “Complement of the investigation” has placed its cameras. The samples revealed excesses of desethyl-terbumeton, a metabolite which has no Vmax. And the distribution of water has been banned for eight years. “As we do not know [si c’est dangereux]as a precaution, we have taken restrictions of use”, explains Pascale Charbois-Buffaut, head of the ARS in the Yonne department.

This is far from being the case for all the municipalities concerned. The recommendation of the DGS to apply the precautionary principle was issued before massive testing of desphenyl-chloridazone, and the discovery of numerous excesses. In Merlieux-et-Fouquerolles (Aisne), for example, “Complement of the investigation” notes that unprecedented measurements (for 2021) of desphenyl-chloridazone (up to 12.46 μg / L) have been observed. In fact, since January 2021, the measurements of this molecule in this village have never fallen below 7 μg/L, as shown in the graph below.

If the precautionary principle were applied in all cities like Merlieux-et-Fouquerolles, this would amount to restricting the use of tap water for millions of inhabitants. Result: the health authorities have recently set up a new threshold, called “transient health value”. It is set at 3 μg/L, i.e. thirty times more than the quality threshold (0.1 μg/L). This new limit still does not provide, for the moment, the automatic prohibition of the consumption of tap water, but reinforces monitoring. ARS Hauts-de-France announced on September 15 the placement of 105 municipalities under enhanced surveillance, “before any restrictive measures”notes the agency.

It must be noted that, compared to its five homologs with a transitional value, desphenyl-chloridazone is an exception: in the samples taken from January 2021 to July 2022, it is the molecule for which we count the no more exceedances above this new threshold. This type of overrun has been observed 134 times. For its cousin, methyl-desphenyl-chloridazone, it happened only ten times. And for another metabolite, Flufenacet ESA, it happened only once.

Traces of pesticides twenty years after their ban

Analysis of data on the quality of tap water highlights another phenomenon: in 2022, we still measure sometimes high levels of pesticide molecules (or their metabolites), which have been banned for years. This is the case of chloridazone, the use of which has been prohibited since January 1, 2021 and whose metabolites are still found in water intended for consumption.

But this is also true for substances that have been banned for much longer. Atrazine is a herbicide widely applied for forty years, especially on corn crops, before its ban decided in 2001. But more than twenty years later, we still find some of its molecules, or its metabolites, in the water coming out of some taps. Over the single period from January 2021 to July 2022, values ​​above the limit of 0.1 μg/L were observed in more than 2,300 samples taken in 878 municipalities. Exceedances which however remain below the maximum health values ​​of these molecules.

In the same way, we find traces of substances whose agricultural use has been banned for a long time: paraquat in 2016 in the Médoc, diuron regularly measured in Seine-et-Marne or Normandy… it that spreading was carried out after the prohibition? It’s hard to tell. The infiltration of chemical substances in basements is a complex science. What is certain is that the environment can preserve these molecules for many years.


To carry out this work, franceinfo.fr worked with the “Complément d’Enquête” teams and the datajournalist Alexandre Lechenet. From an initial data compilation work carried out by the latter, we carried out our own analysis. It is based on data from SISE-Eaux, a national tool for monitoring the quality of water intended for human consumption. This tool monitors dozens of parameters, concerning micro-organisms (bacteria, viruses…), nitrates, radioactivity, lead… Pesticides being only part of these parameters, it was necessary to filter the SISE-Eaux database using a list of pesticide molecules defined upstream.

To define this initial list, with the aim of approaching completeness, we have compiled the pesticide databases established by ANSES (here or here), the Ministry of Ecological Transition and Sandre, also supplemented by our cares. We finally collected results for 1,137 molecules of pesticides or their metabolites. We were then able to analyze the results of the samples by compiling the data by molecule, distribution network and municipality. As some municipalities are crossed by several distribution networks, we have calculated average overruns at the scale of the municipality to present results by city.

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