First flush assumptions in pollution management for separate stormwater systems: debunking a classical urban (drainage) myth

The concept of First Flush (FF) in pollution management of separate storm sewers has been utilized for over half a century. Initially introduced by Sartor and Boyd (1972), the idea that the greater fraction of pollutants discharged during a rain event is concentrated in the initial part has been investigated by several studies during the last decades (Mamun et al., 2020). Regulations around the world widely adopted the FF concept to mitigate the pollution originating from separate storm systems, by specifically requiring treatment of only the FF fraction, assuming that this will avoid the discharge of the greatest fraction of the stormwater pollutants.
However, the presence of FF in separated stormwater is still debated, with different FF definitions being adopted by various authors (see an overview in Table 1). Similarly, regulations in different countries show a great variety in definitions of the stormwater fraction to be treated, often lacking a strong data evidence to support the enforced criteria (see for example the overview in Jensen, 2022). Our work firstly reviewed the international scientific literature dealing with FF, to establish a common understanding in the ongoing scientific debate. Then, we applied these FF definitions to the events monitored by Métadier and Bertrand-Krajewski (2012) in a French catchment, one of the major sets of high-resolution turbidity data collected so far. A modelled dataset was also included in the analyses for comparative purposes. Our results show an inconsistency in the use of the FF concept when applied to a large dataset of measurements, highlighting that the effect of FF is often overrated and that usage of FF in regulation of stormwater pollution thus poses a risk to the natural water environment.