Geochemical modelling of heavy metals in urban stormwater biofilters

Abstract

Complex metal compounds from urban wet deposition can be retained in stormwater biofilters and degraded to simple inorganic metal compounds. A mechanistic model that describes the bulk breakdown rate, accumulation, and leaching of copper (Cu), lead (Pb), and zinc (Zn) is presented. The model accounts for aqueous complexation, mineral adsorption, and kinetic methylation of these three key metals found in urban stormwater. The model was tested against experimental pollutographs from inflow and outflow of a stormwater biofiltration system over a period of 100days. Parameter calibration resulted in R2≥98% and residuals lower than 12% against cumulative effluent water and metal mass. The leaching concentration of Cu and Pb was linearly correlated to the hydraulic conductivity as well as equilibrium and kinetic rate constants; Zn leaching concentration was correlated by a power law to these parameters. It was found that ±20% change in these parameters returned changes in Cu, Pb and Zn concentrations within about ±52%, ±45% and ±96%, respectively. Scenarios including inflow events with different frequencies and intensities resulting in the same annual total inflow were simulated. They showed that the maximum annual metal load in the biofilter outflow was obtained for the inflow combination with the lowest frequency and highest intensity. This model can be effectively used to assist in designing biofilters and assessing their long-term performance.