A major problem of increased urbanization is the rise in pollution caused by runoff. A solution to this problem can be found through the use of Sustainable Urban Drainage Systems (SUDS) such as rain gardens. Previous research has focused primarily on hydrologic design including the degree to which groundwater is replenished by these systems and models have been developed to quantify the extent of that recharge. However these models do not simulate the transport or fate of pollutants. In this paper, a preliminary heavy metal retention computer model is proposed that consists of two parts: a water flow module and pollutant retention module. To describe flow behaviour a dual-permeability approach is implemented which facilitates the prediction of both matrix and macropore flow. This subroutine is then combined with the retention module, which utilizes and couples the linear, Langmuir and Freundlich isotherms with the advection-diffusion equation. Both submodels are validated using existing literature data. This validation was assessed using efficiency indexes and showed good results with an R2 > 0.93 for the matrix region, an R2 > 0.89 for the macropore region and an R2 = 0.99 for the pollutant retention module. This results in a valuable tool for the design and implementation of bioretention facilities.