Contribution by urban and agricultural pesticide uses to water contamination at the scale of the Marne watershed

This study establishes an annual watershed (12,762 km2) budget of pesticide contamination in the Marne River based on detailed enquiries from farmersʹ organizations, public services and residents and pesticide usage. Results showed that urban uses were considerably lower (47 tons/yr) than agricultural ones (4300 tons/yr). However, the proportion of the amounts used transferred to surface water, differs considerably between urban and agricultural environments. Transfer from urban uses was estimated from runoff experiments with different surfaces, including concrete, tarmac, sand and gravel, and grass. Transfer coefficients from agricultural uses were derived from the calibrated value previously obtained from a detailed budget established for atrazine, taking into account the specific adsorption capacity (Koc) and half-life time of each substance used. The calculated annual budget shows a similar contribution by urban pesticides in the Marne River due to runoff over impervious surfaces as compared to agricultural pesticides used on cultivated soils (about 11 tons/yr in both cases). These estimates are consistent with data available from analytical surveys concerning pesticide occurrence in the rivers of the Paris region.