Lay summary

Pesticides, especially herbicides, are widely used in agricultural catchments. Applied on soils, they are mobilized and transported, through runoff and sub-surface stormflow, to surface waters where they impact non-target organisms. Their occurence in water bodies and their deleterious effects on aquatic ecosystems has been previously demonstrated. Recent legislations therefore propose water quality criteria for pesticides to protect the species living in there. However, these values are mainly defined as long-term protection goals, which do not reflect the pesticide pollution in rivers. Indeed, in this kind of surface waters, the concentrations of pesticides fluctuate greatly in time following rain events. The modelling and the risk assessment of these fluctuating concentrations have poorly been studied until now.

The main goal of our project is to (i) model and propose typical exposure scenarios for pesticides concentration in rivers, and (ii) assess the effects, and thus the risk, of these scenarios for aquatic ecosystems. The project will focus on herbicides as they represent the main part of pesticide contamination in water bodies. The modelling part, conducted by the first PhD, will focus on the development and application of proper tools for quantifying the dynamics of herbicides in soils. His/her specific sub-project will benefit from the data collected for one field site (to be instrumented and partially funded by EPFL) where discharge and herbicide concentration will be jointly measured. This, along with the detailed definition of applied herbicides loads, will lead to the definition and implementation of a proper source zone model as detailed in the research plan. The fate of the contaminant through the catchment and the river will be then modelled through the travel time distribution approach. This model will provide time-varying concentrations of herbicides in the river that will be calibrated against the data collected in the system. To evaluate the effects of the fluctuating concentrations (second Phd), dedicated test systems will be developed. Indeed classical ecotoxicological studies mainly focus on long-term experiment. Different kind of algae and algae communities will be used as test organisms as primary producers are particularly sensitive to herbicides. The results obtained will serve as basis for risk assessment of fluctuating concentrations. Furthermore, specific water quality criteria for this kind of exposure will be proposed.

This research addresses an open question on the risk of fluctuating concentrations of chemical substances in rivers. Indeed, this kind of exposure does not only concern pesticides, but also biocides and other organic compounds reaching surface water with rain event. The results of this project will therefore serve as example to evaluate and manage other environmental pollution.