Where precipitation falls mainly as storm events, the majority of mass transport in surface runoff is connected with the start of the storm water runoff. This so-called first flush will mobilise pollutants having accumulated on the pavement surface since the previous storm event (Barbosa & Hvitved-Jacobsen, 1999). Concentrations and masses decrease with time, and the relationship between the mass and the contamination pulse depends on many factors (Sansalone & Cristina, 2004). The amount of pollutant in the storm runoff depends on several conditions before the rain. The consideration of the first-flush phenomenon, inclusive of contaminant fluxes, in stormwater treatment is of much concern among practitioners (Hager, 2001).
The transport of pollutants accumulated during dry weather can be described using the theory of sediment transport with water combined with semi-empirical equations. The wash-off rate of pollutants is directly proportional to the amount of material remaining on the surface. During a storm event, the mass of pollution present on the pavement is decreasing exponentially with time (Hall & Hamilton, 1991). The relation can be described as:
M(t) = M0e-JRpwt (6.13)
where M(t) = pollutant mass on the pavement surface (M/L2) at time t (T); M0 = pollutant mass on the pavement surface at the beginning of the storm hydrograph (M/L2); J = rate coefficient (L2/M); R = runoff (L/T) and pw = density of water (M/L3).
At the beginning of the storm runoff event, various particles from dry deposition are remobilised. As a consequence of the interaction between water and dry sediment during the storm, the concentrations in the diluted phase are also changing with time.
Part of the runoff water is mobilised by the traffic to form splash and aerosols which will be wind-transported away from the road. The vast majority of the pollutants so mobilised will be deposited close to the road but the smaller particles will be wind transported further away from the road, at least some hundred metres (Blomqvist & Johansson, 1999; Folkeson, 2005). To what extent pollutants are transported in the form of splash/spray or in the form of pavement-surface runoff is governed by factors such as traffic characteristics, weather conditions, topography and the type and condition of the pavement surface. For instance, aerial transport is more limited where porous asphalt is used as compared to conventional asphalt (Legret & Colandini, 1999; Legret et al., 1999; Pagotto et al., 2000).
