Variability of herbicides (triazines, phenylureas) and tentative mass balance as a function of stream order, in the river Marne basin (France) - Triazine and phenylurea flux and stream order

The magnitude of the contamination of the river Marne by various herbicides of agricultural origin (triazines and phenylureas), led the CGE (Compagnie Generale des Eaux) and the IHC/LGA (Institut d'Hydrologie et de Climatologie/Laboratoire de Geologie Appliquee) to investigate the spatial and temporal distribution of these contaminants. The field work was carried out from 1991 through 1994 on the river Marne, particularly in the sub-basin of the Grand Morin and its tributaries, direct (Orgeval) or secondary (Melarchez). The chronic contamination of the streams by atrazine and simazine with a background concentration around 100 ng l(-1), results from contamination of groundwaters in the Calcaire de Champigny and Calcaire de Brie layers (CGE, 1993). The maximum atrazine and simazine concentrations (> 500 ng l(-1)) were usually found from April to June. During this period the atrazine concentration peaks might reach 2000 ng l(-1) in the agricultural brooks as well as in the river Marne. Isoproturon was also detected throughout the year, except during the summer low water period. The maximum concentrations of isoproturon were found between February and April: ranging from 500 ng l(-1) to 1200 ng l(-1), depending on the rainfalls. The annual balances for 1991 and 1992, estimated from the exportation fluxes at the elementary catchments basin outlets, showed that the exportation levels of atrazine, simazine and isoproturon did not exceed 0.3% of the mass applied agriculturally in the Melarchez basin and 1.5% of the mass of atrazine applied in the Grand Morin river basin. The evolution of herbicide concentrations, from the elementary brook in a rural area to the river Marne outlet in an urban area, showed that there was no longitudinal decrease of contamination. In fact, the waters of the river Marne lower course, and of its minor tributaries in the urban area, might display herbicide concentrations similar or even higher than those of rivers in the agricultural area.