Impact of an intense combined sewer overflow event on the microbiological water quality of the Seine River Julien Passerat a , Nouho Koffi Ouattara a , Jean-Marie Mouchel b , Vincent Rocher c , Pierre Servais a, * a Ecologie des Syste `mes Aquatiques, Universite ´ Libre de Bruxelles, Campus de la Plaine, CP 221, Boulevard du Triomphe, B-1050 Bruxelles, Belgium b UMR 7619 Sisyphe, Universite ´ Pierre et Marie Curie, Case 105, 4 place Jussieu, 75005 Paris, France c Syndicat Interde ´partemental pour l’Assainissement de l’Agglome ´ration Parisienne, Direction du De ´veloppement et de la Prospective, 82 avenue Kle ´ber, 92700 Colombes, France article info Article history: Received 5 March 2010 Received in revised form 9 September 2010 Accepted 18 September 2010 Available online 29 September 2010 Keywords: River microbiological contamination Combined sewer overflow Escherichia coli Intestinal enterococci Attached bacteria Sewer sediment abstract For a better understanding of the short and mid-term impacts of a combined sewer overflow (CSO) on the microbiological quality of the receiving river, we studied the composition of a CSO discharge and monitored during several hours the changes in the concentration of fecal indicator bacteria (FIB) in the impacted river water mass. The CSO occurred at the Clichy outfall (Paris agglomeration, France) in summer 2008 as a result of the most intense rainfall of the year. In 6h, 578, 705 m 3 of sewage and 124 t of suspended matter (SM) were discharged into the Seine River. The CSO contained 1.5  10 6 E. coli and 4.0  10 5 intestinal enterococci per 100 mL on average, and 77% of the E. coli were attached to SM. It was estimated that 89% of the CSO discharge was contributed by surface water runoff, and that resuspension of sewer sediment contributed to w75% of the SM, 10e70% of the E. coli and 40e80% of the intestinal enterococci. Directly downstream from the CSO outfall, FIB concentrations in the impacted water mass of the Seine River (2.9  10 5 E. coli and 7.6  10 4 intestinal enterococci per 100 mL) exceeded by two orders of magnitude the usual dry weather concentrations. After 13e14 h of transit, these concentrations had decreased by 66% for E. coli and 79% for intestinal enterococci. This decline was well accounted for by our estimations of dilution, decay resulting from mortality or loss of culturability and sedimentation of the attached fraction of FIB. ª 2010 Elsevier Ltd. All rights reserved. 1. Introduction Many urban centers are drained by a unique sewer network in which wastewater is mixed with urban runoff water in wet weather. When rainfalls are intense, the transport capacity of the sewer system can be insufficient to allow all the water flow to reach the wastewater treatment plant (WWTP) or the treatment capacity of the WWTP can be insufficient to treat all the water flow. In such cases, combined sewer overflows (CSOs) occur, resulting in the discharge without any treatment of a mixture of wastewater and runoff water, loaded with urban surface pollution, into the receiving waters. CSO impacts on aquatic environments are multiple in terms of pollution types and dynamics in time and space, and encompass: (i) oxygen depletion due to the biodegradation of the high load of organic matter brought by the untreated wastewater, (ii) turbidity increase leading to the reduction of photosynthetic primary production, (iii) increase in the concentration of some * Corresponding author. Tel.: þ32 2 650 5995; fax: þ32 2 650 5993. E-mail address: pservais@ulb.ac.be (P. Servais). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 45 (2011) 893 e903 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.09.024