Heavy metals in a constructed wetland treating industrial wastewater: distribution in the sediment and rhizome tissue

This study assessed copper and zinc distribution in the surface layer of sediment and rhizome tissue within the saturated surface vertical flow constructed wetland of CSBP Ltd, a fertiliser and chemical manufacturer located in Western Australia. Sediment and Schoenoplectus validus rhizome samples were collected at various distances from the inlet pipe while water samples are routinely collected. Water samples were analysed for nutrients and metals, sediments were analysed for total and bioavailable metals and rhizomes were analysed for total metals only. Mean influent copper and zinc concentrations were 0.19 mg/L and 0.24 mg/L respectively. The distribution of bioavailable Cu and Zn in the top sediment layer follows a horizontal profile. Analysis of variance ( ANOVA) showed that the bioavailable fraction of these metals in sediments near the inlet pipe (30.2 mg/kg Cu and 60.4 mg/kg Zn) is significantly higher than in sediments at the farthest location (10.3 mg/kg Cu and 26.1 mg/kg Zn). The average total Cu concentration in the sediment at the 2m location has reached the 65 mg/kg trigger value suggested by the Interim Sediment Quality Guidelines (ANZEEC 2000). Cu and Zn concentrations in the rhizome of S. validus do not vary significantly among different locations. Whether Cu and Zn concentrations at the CSBP wetland may reach toxic levels to plants and bacteria is still unknown and further research is required to address this issue. The surface component of the wetland favours sedimentation and binding of metals to the organic matter on the top of the sediment, furthermore, the sediment which tends to be anoxic with reducing conditions acts as a sink for metals.