Modeling of particulate matter fate on urban highway stormwater control systems

Anthropogenic activities, especially vehicular traffic, produce load of pollutants that accumulate on impervious surfaces. In highways, exhaust, automobile parts wear and lubricating parts along with heavy metals and other pollutants accumulated on the pavement surface during dry periods are the sources of pollution. Rainfall-runoff process promotes surface wash-off, contributing to stormwater pollutants’ load. The cumulative load in runoff is normally expressed as an exponential model with a peak concentration at the beginning of the rainfall event known as the first flush, where most part of the load is washed off at the beginning of the event reaching an early peak. This consideration motivates often the use of a first flush storage tank (FFT) to treat stormwater discharge from highways that is then discharged into water courses without any other treatment. It is considered that the FFT would retain the most polluted part of the runoff; however, a weak first flush is observed in some rainfall events, especially for low flow rates. Also, the vehicular traffic occurring during the rainfall event serves as a continuous source. Therefore, the objective of this research is to compare the efficiency on particle removal of an FFT with other methods of stormwater treatment, in this case an infiltration–exfiltration system (IES) consisting of a gravel swale with porous asphalt surface, through hydrological modelling of six rainfall events measured on a highway in Cincinnati (USA). The results showed a compatible removal rate for both the FFT and IES for the six analysed events, consisting of three mass-limit events and three flow-limit events. Particle transport modelling could represent well the behaviour of the events and can be used as a tool to choose between systems, where after setting the particle removal efficiency, other factors can be considered, like cost and system area consumption. This research can be followed up with continuous rainfall simulations and using computational fluid dynamics (CFD) to model IES particle removal.