Biocidal substances are ubiquitous in urban areas; they are used as preservatives in daily products, building materials, and as domestic pesticides. Despite these facts, urban sources of biocides, contrary to agricultural inputs, have been overlooked in terms of environmental risk assessment. The overall objective of this communication is to explore the use of a stochastic approach to assess the impact of biocide urban discharges on the river contamination. Biocide contamination in surface waters up-and downstream the Paris conurbation was investigated. The urban origins of these biocides were explored by targeting wastewater treatment plant and combined sewer overflow discharges from comparable watersheds, and by assessing biocide mass loads (discharged and transiting in the river) using a stochastic approach based on Monte-Carlo simulations. The 18targeted biocides were all quantified, and several were found at concentrations that can present high risk for the aquatic ecosystem. An increase in biocide loads between up-and downstream the Paris conurbation for several molecules and the contribution of urban discharges to biocide loads towards the receiving waters were ascertained. The results illustrate the interest of a stochastic approach to quantify the uncertainty on the determined values from a realistic description of the variability of biocide concentrations.