Efficiency of a constructed wetland in removing contaminants from stormwater

Abstract

As in most large capital cities, urban stormwater discharging into Port Jackson (Sydney) is highly enriched in a wide range of contaminants, which has resulted in degradation of the receiving basin waters and bottom sediments. The objective of the current investigation was to determine the removal efficiency of contaminants in urban stormwater by a wetland constructed in the Sydney catchment. The wetland (700 m 2 ) drains a residential urban catchment of about 480,000 m 2 comprising predominantly houses, streets, gardens, and street parking areas. Samples of stormwater influent and effluent were obtained during rainfall events between April and June 2000. Eight samples were collected at the inlet and outlet to the wetland during each event and analyzed for nutrients, trace metals, total suspended solids (TSS), and organochlorine pes- ticides and polycyclic aromatic hydrocarbons (PAHs). Water quality parameters (temperature, dissolved oxygen, pH, turbidity, conductivity) were measured concurrently. The average removal efficiency of trace metals Cr, Cu, Pb, Ni, and Zn was 64%, 65%, 65%, 22%, and 52%, respectively for the six events measured, whereas for Fe and Mn, removal efficiencies were negative for most events (mean 284% and2294%, re- spectively). The average removal efficiency of NO x and TN was 22% and 16%, respectively. The average removal efficiencies of total Kjeldahl nitrogen (TKN) and total phosphorus (TP) were 9% and 12%, respec- tively. During four high-flow events, the removal efficiency of TSS in stormwater effluent from the wetland was between 9% and 46%; however, substantially higher TSS concentrations were observed in effluent than influent waters during two very high-flow events (removal efficiency 298% and267%). Fecal coliform counts in the stormwater in this catchment are high (maximum: 620,000 cfu/100 ml), but mean removal efficiency was 76% (range 26% to 98%) during the four high-flow events monitored. Nevertheless, most samples from the outflow exceeded the Public Health criterion for secondary contact (e.g., boating) of 1000 cfu/100 ml. Concentrations of organochlorine pesticides and PAHs in stormwater were below analytical detection. Al- though highly variable, lower concentrations of Cr, Cu, Ni, Pb, Zn, NO x, TN, and fecal coliform in the stormwater effluent compared to influent waters indicates that the wetland was moderately efficient in re- moving contaminants from urban stormwater.