Contribution of different sources to the pollution of wet weather flows incombined sewers

Experiments performed on "Marais" catchment, in central Paris, aimed to fol low up the duality of wet weather Rows from the entry to the exit of a comb ined sewer network. SS, VSS, GOD, BOD5, Cd, Cu, Pb, Zn concentrations were measured for an important number of rain events in roof, yard, street runof f, as well as in dry and wet weather flows at the catchment outlet. Mass en try-exit totals, at the scale of the catchment, were calculated over 31 rai n events in order to evaluate the contribution of different types of runoff , of sanitary sewage and of sewer sediments to the total wet weather pollut ant loads at the catchment outlet. The erosion of in-sewer pollutant stocks was found to be the main source of particles and of organic matter in wet weather flows, whereas heavy metal loads mainly originated from roof runoff due to the corrosion of metallic roofs. Particles eroded inside the sewer during rain events were found to be quite different from the particles cons tituting the main part of sewer sediments: they are organic and biodegradab le, with rather important settling velocities and seem to accumulate during dry weather periods. A change of the chemical form of heavy metals was not iced during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments. (C) 2000 Elsevier Science Ltd. All rights reserved