Sewage effluent discharge and geothermal input in a natural wetland, Tongariro Delta, New Zealand

Effluent from the oxidation ponds of the town of Turangi, south of Lake Taupo, has been discharged into a natural wetland since the 1960s. This has resulted in elevated concentrations of Na +, K +, Cl − and NH 4 +-N in both ground and surface water. Increased weed invasion and plant growth, and high heavy metal concentrations (e.g. up to 440 ppm Zn) occur in the vicinity of effluent discharge in the wetland. Element and nutrient concentrations decrease with increasing distance from the effluent inflow point, suggesting that the wetland presently acts as a sink for metals and nutrients. Elevated arsenic concentrations (up to 5800 ppm in peat and 11 400 ppm in the ash fraction) in some of the organic-rich sediment suggest a long-term input by geothermal water originating in the Tokaanu–Waihi field. Increased silt input due to more frequent flooding of the Tongariro river over the past 40 years has resulted in a significant change in stratigraphy (from peat to mud) over much of the wetland.