Monitoring drinking water quality and protecting human health: ANSES proposes a method for identifying relevant pesticide metabolites

Pesticides disperse into the natural environment where they can transform into one or more other compounds called "metabolites". These chemical metabolites can potentially contaminate water resources and even water supplied to consumers. Water quality controls are increasingly detecting situations where regulatory quality limits are exceeded, meaning that management measures are needed before distribution. In order to address these challenges, the Directorate General for Health asked ANSES to define a methodology for identifying the relevant metabolites regulated by the European Drinking Water Directive. ANSES therefore presented the management authorities with a method for identifying which pesticide metabolites warrant priority attention with regard to the health issues associated with drinking water consumption.

When pesticides come into contact with the different environmental media in which they disperse, they can degrade into metabolites, depending on their characteristics and the physico-chemical conditions they encounter. These metabolites can accumulate in different environmental compartments: soil, surface water and groundwater, sediments, plants and the atmosphere, and can potentially contaminate water resources and even drinking water.

Regulatory framework for pesticide metabolites in drinking water

The presence of pesticide residues and their metabolites in drinking water is regulated by European Directive 98/83/EC, transposed into French law, which includes permanent monitoring of drinking water quality. This European directive sets quality limits (which do not correspond to health thresholds) for pesticides and their relevant metabolites: 0.1 µg/L per individual substance and 0.5 µg/L for the sum of these compounds. However, it does not define what is a relevant metabolite (1). As a result, the French health authorities have so far considered that all pesticide metabolites detected in drinking water must comply with the quality limit of 0.1 µg/L.

To protect human health, the drinking water quality monitoring carried out by the Regional Health Agencies (ARSs) has evolved in terms of the number of substances screened for and the efficacy of the analytical methods used. In France, for example, nearly 700 substances and about 40 metabolites (2) are subject to periodic checks in drinking water. More than three million analytical results are produced each year.

During these checks, pesticide residues or metabolites above the quality limits are detected locally in a variety of situations. When the quality limit is exceeded, the regulations require management measures of varying costs and complexity (interconnection, dilution, enhanced treatment before distribution, greater protection of the resource, etc.) to restore the water's compliance with the quality limit (i.e. 0.1 µg/L for each relevant metabolite).

A methodology for identifying metabolites relevant to consumer health risks in drinking water

In order to focus management measures on priority situations, the Directorate General for Health asked ANSES to develop a methodology for identifying relevant metabolites in drinking water.

The approach is based on a definition of relevance to protecting health. Assessing the relevance of a metabolite in drinking water involves several steps to examine the potential health effects (genotoxicity, reproductive toxicity, carcinogenicity, endocrine disruption), as well as the potential for a pesticide metabolite to transform during the treatment process into a product that is harmful to human health.

This methodology can be applied to any quantifiable metabolite in drinking water. It is intended to be implemented as part of ANSES’s collective scientific expert appraisal, using the available data (approval dossiers, scientific literature, etc.).

Application of the methodology

Eight metabolites were assessed in order to test the proposed methodology. These eight compounds were herbicide metabolites from four active substances (two of which are no longer authorised). Of the eight compounds, five metabolites (3)  were classified as "not relevant to drinking water" and three metabolites (4) were classified as "relevant to drinking water".

For the metabolites assessed as "relevant", the current quality limits (0.1 µg/L and 0.5 µg/L) will continue to apply. For the metabolites assessed as "not relevant to drinking water", the expert appraisal proposed an adapted threshold value (0.9 µg/L), based on the threshold of toxicological concern (TTC, EFSA/WHO 2016).

Lastly, ANSES indicates that the classification of relevance to drinking water is likely to evolve along with the available scientific knowledge (re-assessment of active substances, new available data, etc.). Similarly, advances in knowledge regarding new hazards, and/or new developments in assessment methodologies are likely to lead to a revision of the proposed method.

(1) Only the European Regulation (EC) No 1107/2009 establishing the conditions for placing plant protection products (PPP) on the market defines the concept of relevant metabolites. This relevance is established only for metabolites found in groundwater, according to criteria relating to the "pesticide" activity and toxicological criteria, and is defined in comparison with the properties of the parent substance.

(2) DGS data from quality control in 2015.

(3) alachlor ESA, acetochlor ESA, acetochlor OXA, metazachlor ESA and metazachlor OXA

(4) alachlor OXA, metolachlor ESA and metolachlor OXA