Distribution of pollutant mass vs volume in stormwater discharges and the first flush phenomenon

The dimensionless M(V) curves indicating the distribution of pollutant mass vs volume in stormwater discharges are used to compare pollutant discharges from different rainfall events and catchments. The M(V) curves are very variable and dispersed for each pollutant and for a series of rainfall events in a given catchment. From the analysis of 197 rainfall events in 12 separate and combined sewer systems, the following results can be extracted: - in separate sewer systems, 80% of the total pollutant mass is transported in the first 74% of the total volume for 50% of the rainfall events; - in combined sewer systems, 80% of the total pollutant mass is transported in the first 79% of the total volume for 50% of the rainfall events. The characteristics of the M(V) curves depend on the pollutant, the site, the rainfall event and the functioning of the sewer system. No general multi-regression relationships can be established to explain their shape and their variability because of the complexity of the phenomena involved and the multiplicity of influencing factors.