The reduction of heavy metals in a stormwater wetland

Sedimentation has long been recognized as the principal process in the removal of heavy metals from stormwater in natural or constructed wetlands. However, there are a range of other processes including filtration, adsorption, biological assimilation, decomposition, chemical transformation and volatilisation that may also play an important role. To date there have been few attempts to quantify the importance of the various processes. In the current study five sediment trap stations were set up in a long and narrow stormwater wetland and an analysis of total sediment and heavy metal concentration of key elements (Zn, Cu, Pb, Cr and As) was carried out. From the results it has been possible to assign an areal decay rate to the sedimentation process (1300 m per year) and a separate decay rate to the sum of the other processes (biological and chemical) that were acting to reduce heavy metal concentration. It was found that Zn, Pb and Cu concentrations on sediment decreased 57, 71 and 48%, respectively, through the wetland, the concentration of Cr remained relatively constant and that of As actually increased by 150%. It is suggested that the behaviour of the different metals can be explained in terms of their chemical behaviour and the role of organic matter in the wetland. (C) 2002 Elsevier Science. All rights reserved.