Assessment of the contribution of surfactants to mixture toxicity in French surface waters

Surfactants are widely used 'down-the-drain' chemicals with the potential to occur at high concentrations in local water bodies and to be part of unintentional environmental mixtures. Recently, increased regulatory focus has been placed on the impacts of complex mixtures in aquatic environments and the substances that are likely to drive mixture risk. This study assessed the contribution of surfactants to the total mixture pressure in freshwater ecosystems. Environmental concentrations, collated from existing French monitoring data, were combined with estimated ecotoxicological thresholds to calculate hazard quotients (HQ) for each substance, and hazard indices (HI) for each mixture. Two scenarios were investigated to correct for concentrations below the limit of quan-tification (LOQ) in the dataset. The first (best-case) scenario assumed all values 0, hence unquantified chemicals were not accounted for in the mixture. The second (worst-case) scenario considered values 0), the HIs of the surfactants within the mixture were below 1 for almost all (233/234) mixtures. The median HI for the surfactants was 0.03, whereas the median HI for non-surfactants was 0.5. In the second scenario (1 in all mixtures. Detailed evaluation of the results demonstrated that these exceedances were primarily driven by laurylpyridinium, while other surfactants displayed HIs between 0 and 0.5 and non-surfactants displayed a median HI of 243. Across both scenarios, this study found that the relative contribution of the investigated surfactants to the total mixture pressure in French surface waters was measurable but low compared to non-surfactant substances in the mixture. This indicates that mixture risks of surfactants are of minor importance in these aquatic environments.