Colloid-Enhanced Contaminant Transport in Stormwater Infiltration Basin BMPs: A Modeling Approach

Rainfall runoff from urban and non-urban stormwater often contains significant quantities of organic chemicals and metal elements. These constituents can adversely affect surface water and groundwater resources. Infiltration basins are best management practices (BMPs) that are designed for passive treatment and are increasingly used to control runoff and reduce stormwater contaminants in surface waters. However, the large amount of percolation in such BMPs over an extended duration can adversely affect groundwater quality by eventual delivery of stormwater contaminants to the water table. Moreover, the existence of naturally occurring colloids and the deposition of air-born colloids in stormwater can enhance the transport of highly sorbing constituents to the groundwater. Due to high surface area of fine colloids, contaminants with high affinity to the solid phase can bind to colloidal particles and therefore the colloids can act as a vehicle for transport of contaminants in the porous media. In this research a colloid-facilitated transport model is developed to predict the effect of such colloids on fate and transport of multiple metal contaminants in infiltration basins. Richard's equation is used to model unsaturated flow and a colloid transport module is developed based on the Langmuir surface jamming approach. The effects of colloid entrapment and release on hydraulic conductivity and the porosity of the porous media are incorporated into the model. The concept of entrapment of colloidal particles onto air-water interface is used to model retardation effects due to unsaturated conditions. Sorption and desorption of contaminants to the colloids and soil matrix are assumed to be kinetically controlled and the effect of competition for sorption sites is taken into account using the Langmuir approach. The model is applied for prediction of transport of several metals in two infiltration basins with different soil types and a sensitivity analysis is done to investigate the effect of various parameters on the fate and transport of metals.