Effectiveness of Facilitated Wetlands for Treating Agricultural Runoff in South Otago, New Zealand

Facilitated wetlands are promoted as a mitigation tool in agricultural landscapes because of their potential to attenuate contaminant losses while increasing biodiversity. Here, we investigate the contaminant removal efficiency of two facilitated wetlands established on a dairy farm and a sheep and beef farm. Each wetland had a sediment trap installed near the inlet to form a sediment trap–wetland complex. Water flows through the wetland complexes were measured continuously over three calendar years. Water samples were collected manually under low flow conditions and by an automated sampling system during runoff events at high flows. Samples were analysed for sediment, Escherichia coli , phosphorus and nitrogen. Effectiveness of the wetlands was assessed based on contaminant concentrations and annual loads. The wetland on the dairy farm did not significantly reduce any contaminants, and this was attributed to the very small size of the wetland (0.06%) relative to the contributing catchment area. The wetland on the sheep and beef farm (0.2% of catchment area) significantly reduced annual loads of sediment (53%), phosphorus (31%) and E. coli (10%). To maximise effectiveness, wetlands should be sized appropriately for the contributing catchment area, and sediment traps should be cleaned before they are completely full.