Modelling of road surface pollution buildup and washoff using rainfall simulator

Abstract

Water quality management of nonpoint source (NPS) pollution is still being confronted with identification and assessment. The extent of pollution due to NPS in tropics is not yet affirmed, and the relative influences of its associated sources were not yet totally understood. This study explored the significance of road as a NPS unit in tropical region of persistent rainfall, and investigated the possible sources of heavy metals in urban areas. To achieve the objectives of this study, the natural rainfall dynamic of the study area was appraised using the flour pellet method. The information was used as a basis for developing a highly efficient Rainfall Simulator (RS) that was used to investigate pollutant washoff process under different rainfall depth and intensities. A total of 30 buildup samples were collected from five chosen roads of varying characteristics, and fractionated into 10 classes of particle sizes each. For quantitative analysis, 60 samples were analysed for dissolved Zn, Fe, Cd, Pb, Cu, Ni, Mn, Al, and Cr concentrations. A multivariate principal component and factor analyses were used to investigate the likely sources of these heavy metals. Three sources were identified, the indigenous, geogenic and scavenge. The natural raindrop sizes were found to vary from less than 1.2 mm to as big as 7.0 mm with median raindrop diameters (D50) of 2.51 mm and a mean of 2.56 mm. These raindrops have an average rain kinetic energy content of 30 J m??2 mm??1. The developed RS can satisfactorily simulate rain intensity similar to natural rainfall, with an average kinetic energy content of 42 J m??2 mm??1 and a D50 between 2.41 and 2.64 mm. An advanced principal component and cluster analysis identified TDS as a surrogate for measuring dissolved metals pollution among eight physicochemical parameters considered, and was therefore used in the modelling of the washoff process. The developed models suggested that the rain intensity plays a more prominent role in the occurrence of first flush, while the rain depth plays a central role in the total washoff event. This research demonstrated that the influence of sediment to retain mass loading did not necessarily translate to higher pollution loading of heavy metals, and the residency of heavy metals in different particle classes cannot be generalised.