Improving the knowledge of pesticide transfer into groundwater: constraints by a coupled geochemical and pesticide monitoring approach
Résumé
Enhanced monitoring of groundwater quality over several years has revealed a pesticide contamination of aquifers in North America and Europe, but spatial and temporal fluctuations of pesticides concentrations complicate inv estigation on pesticide fate and transfer into groundwater. Therefore coupling pesticides and geochemical monitoring help deciphering remaining gaps in knowledge. The thin thickness of vadose zone of the alluvial unconfined aquifer of Ariège River (~538 km 2, SW France) makes it vulnerable to contamination due to intensive agriculture. The aquifer is in direct hydraulic connection with rivers Ariège and Hers Vif and probably feeds them during much of the year. Monthly monitoring of groundwater quality at 16 points in the plain was initiated in March 2009 and is still on - going. It includes some 50 pesticides or metabolites, major ions including nitrate and non - conservative parameters (pH, T, redox potential, EC). A strong spatial variability of the contaminati on was demonstrated, with predominance of degradation products over parent molecules in some locations. Metolachlor, a pre or early post emergence herbicide and its ionic metabolites present a high occurrence with sharply variables concentrations for given points. Presented results focus on these molecules. Using the SAM ratio (ratio of metolachlor ESA concentration to the metolachlor concentration) proposed by Philipps et al. (1999), allowed distinguishing several groups according to the SAM values, time - f luctuations and correlation with water level series. Spatially, groundwater geochemical signatures allowed distinguishing the lithology of the different alluvial terraces. Variations of geochemical parameters explain seasonal connection between terraces, l ocal surface - groundwater relations and are also consistent with SAM fluctuations. Potential transfer of waters with different origins and/or according to various mechanisms is thus demonstrated and characterized. The coupled approach of geochemistry and hy drogeology leads to a better understanding of the spatial and temporal fluctuations of the pesticide concentrations in groundwater at the aquifer scale.