Modifications to permeable pavement structure to achieve improved heavy metal attenuation in stormwater runoff

This paper reports a study conducted to investigate the influence of rainfall intensity and subbase materials on attenuation of heavy metals in stormwater runoff through permeable pavements. The porous structure of permeable pavements facilitates the attenuation of particulate heavy metals by filtration. However, the ability of permeable pavements to remove dissolved fractions of heavy metals is still largely at an experimental stage. The porous structure of permeable pavements is a favourable environment to promote adsorption of dissolved heavy metals. However, this capability is heavily dependent on rainfall intensity and duration, which can contaminate groundwater and underlying soil, by leaching heavy metals back to the infiltrate either due to altered flow conditions or after saturation. In this study, a laboratory model of two permeable pavement columns have been tested for heavy metals attenuation with varying rainfall intensity. One column was constructed according to standards and the other one by maintaining a saturated zone, thin sand layer and providing a carbon source to investigate the influence of heavy metal attenuation by biosorption. Results confirmed the ability of the traditional permeable pavements to attenuate the total pollutant loads of Ba, Co, Mn, Ni, Cu and Zn under low and high rainfall conditions. During high rainfall intensity the attenuation of Al and Mo dropped by 50% compared to low rainfall intensity. During heavy rains, Cr can be leached back to the infiltrate. Results also indicated, maintaining a favourable environment for microbial growth improves the heavy metal attenuation even in high rainfall intensities. Findings of this study ebable to modify the permeable pavement subbase by incorporating a sand layer, a compost layer and maintaining a saturated zone in the subbase to obtain an improved heavy metal attenuation depending on the design rainfall condition, predictable pollutants and the structural requirements of the proposed site.