Urban pathways of biocides towards surface waters during dry and wet weathers: Assessment at the Paris conurbation scale

[Display omitted] •18 biocides used in buildings and domestic activities were monitored in urban waters.•Quantification in both dissolved and particulate fractions of WWTP and CSO samples.•Poor WWTP removals were observed for most of the biocides except isothiazolinones.•In CSOs, most of the biocides came from both wastewater and stormwater.•Annual mass loads discharged in the Seine River were higher for WWTPs than CSOs. Eighteen biocides used in building materials and domestic products were monitored in wastewater treatment plants (WWTPs) during dry weather and in combined sewer overflows (CSOs) during wet weather in the Paris conurbation. The aims of this study were to (i) acquire data on biocides in urban waters, which are very scarce up to now, (ii) identify their origins in CSOs with the perspective of reducing these contaminants at source, and (iii) compare and rank biocide pathways to the river (dry vs. wet weather) at the annual and conurbation scales. The results showed the ubiquity of the 18-targeted biocides in WWTP waters and CSOs. High concentrations of methylisothiazolinone, benzisothiazolinone (0.2−0.9 μg/L) and benzalkonium C12 (0.5−6 μg/L) were measured in wastewater. Poor WWTP removals (< 50 %) were observed for most of the biocides. Both wastewater (mainly domestic uses) and stormwater (leaching from building materials) contributed to the CSO contamination. However, benzisothiazolinone mainly came from wastewater whereas diuron, isoproturon, terbutryn, carbendazim, tebuconazole, and mecoprop mainly came from stormwater. Annual mass loads discharged by WWTPs and CSOs into the Seine River were estimated using a stochastic approach (Monte Carlo simulations) at the conurbation scale and showed that WWTP discharges are the major entry pathway.