Distribution and fate of pesticides and trace metals in a small stream draining an agricultural watershed. Assessing the effect of hydrological conditions on the transport of contaminants | Distribution et transfert des pesticides et des métaux trace dans un petit cours d'eau viticole. Influence des conditions hydrologiques sur le transport de ces contaminants

[Departement_IRSTEA]MA [TR1_IRSTEA]RIE / PHYLEAU [TR2_IRSTEA]QSA / DYNAQ

English. The intensive use of pesticides to protect culture against diseases leads to the occurrence of these compounds in aquatic ecosystems. In order to better understand the behaviour of these agricultural contaminants in surface waters, the temporal dynamic of selected pesticides and trace metals were studied in a 5 km² agricultural watershed, the Morcille. Water samples were collected during the spreading period (from March to September 2006) in different hydrological conditions. Spatial monitoring along the river was also achieved in order to assess the upstream-downstream variability. Eight pesticides (1 herbicide and its main metabolites and 5 fungicides) as well as trace metals (As, Cu) were analysed in dissolved phase (D) and suspended particles (SP). All the 8 pesticides were detected in the river. Total pesticide concentrations downstream ranged between 0.2 and 5.4 µg/L in baseflow. A large gradient of trace metal and pesticide concentrations was observed along the river. These observations can be attributed to the increasing proportion of vineyard surface in the Morcille River watershed going downstream. Pesticides concentrations follow a seasonal variation, related to their spreading period. Moreover, during storm events, total dissolved concentration of pesticides in surface waters can be multiplied by a factor up to 40 due to the runoff on agricultural surface. This highlights the strong contribution of floods to the overall transport of contaminants in this small river. This preliminary study shows the importance of floods in the transfer of pollutants. Future studies will focus particularly on flood events in order to better characterise the river response to storms in terms of transport of contaminants, and thus to better estimate contaminants load and ecological effect in the river.