In lab and in situ assessment of pesticide effects on aquatic organisms: key role of groundwater monitoring of Ariège alluvial plain (France).

International audiencePesticides emitted into the environment may contribute to complex mixtures including the parent molecules, degradates in association with other pollutants. It is of paramount importance to implement suitable monitoring procedures to assess the fate of pesticides, their transfer to groundwater and the effects of water quality on aquatic and terrestrial organisms. The alluvial domain of the Ariège River (about 538 km2) is an unconfined aquifer, the thin thickness of vadose zone making it vulnerable to contamination. The aquifer is in direct hydraulic connection with rivers Ariège and Hers Vif and feeds them during much of the year. As a result, the quality of surface water is highly influenced by groundwater quality. Due to intensive agriculture in this area, related to corn cultivation in particular, groundwater quality is affected by pesticides and some of their metabolites. Monthly monitoring of water quality at 16 water points in the plain was initiated in March 2009 and is still on-going. It includes some 50 organic molecules, major ions such as nitrate and other nonconservative parameters (pH, temperature, redox potential). A strong spatial variability of the contamination was demonstrated, with predominance of degradation products over parent molecules in some locations, and also showed a strong temporal variability. Besides aiming at a better understanding of the pesticides' fate, our study also addresses the effects of altered water quality on various aquatic organisms. Laboratory ecotoxicological tests calibrated with realistic environmental concentrations found in the aquifer and accounting for proportion of the different molecules were performed. In parallel, investigations of in situ exposures were better controlled. Spatial and temporal distribution of the microbial diversity in groundwater is followed to evaluate its relevance as bioindicators. Macro-organisms restricted to groundwater (hypogean organisms such as isopods or amphipods) are investigated in situ at the scale of species, populations and communities while acute and chronic toxicities are determined in laboratory. First results on amphibians, also studied, showed that the contamination of surface water affects populations (richness) in the environment while in most cases no toxicity has been demonstrated in Xenopus larvae. To our knowledge this project may be the first investigation of the effects of pesticides on these three types of organisms using both lab and aquifer studies