Sorption and transport behavior of zinc in the soil; Implications for stormwater management

Long-term leaching of accumulated heavy metals from the filter media of stormwater infiltration systems into groundwater, and its subsequent movement into receiving waters, is a concern for stormwater management in urban landscapes. This study provides a critical understanding of the fate of heavy metals, in particular zinc, in the subsurface to improve stormwater management. We studied the sorption behavior of zinc during its transport in soil using batch and column experiments. The use of sorption parameters from batch studies failed to suitably describe the zinc breakthrough curves simulated with the HYDRUS-1D model. While a sharp breakthrough curve front was expected due to the nonlinear nature of the batch sorption isotherms, we observed a more dispersed asymmetrical front in the columns. This demonstrates the existence of chemical non-equilibrium sorption during zinc transport. Conversely, no asymmetry was observed in the conservative tracer (chloride) breakthrough curve, suggesting that there is no physical non-equilibrium component. We conclude that (1) batch tests over-predict the transport behavior of zinc in the soil, (2) column tests better approximate non-equilibrium sorption behavior of zinc during its transport through the soil, and therefore should be conducted to better predict the contaminant transport behavior in the surrounding soils, and that (3) the sorption behavior of zinc transport is predominantly influenced by chemical sorption-related than physical transport-related mechanism. The outcomes of this research suggest that stormwater managers should consider the heterogeneity of surrounding soil between the infiltration systems and receiving waters and its impact on the transport of heavy metals when designing stormwater infiltration systems. Otherwise, they run the risk of not delivering the desired restoration of urban ecosystems.