Comparison of groundwater quality from forested (Waimarino River), urban (Turangi), and natural wetland (South Taupo Wetlands) subcatchments at the southern end of Lake Taupo

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Rosen, M.R.; Reeves, R.R.; Eser, P.; Chague-Goff, C.; Coshell, L. 1998 Comparison of groundwater quality from forested (Waimarino River), urban (Turangi), and natural wetland (South Taupo Wetlands). subcatchments at the southern end of Lake Taupo. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 98/01 59 p.

Abstract: Comparison of groundwater quality of three different land uses, (1) exotic pine plantation ready for harvest (Waimarino River Catchment), (2) an urban area characterised by a land treatment facility for sewage effluent from Turangi (Turangi oxidation ponds), and (3) a natural wetland (South Taupo Wetlands) demonstrates that groundwater quality in the southern region of the Lake Taupo catchment is controlled by both natural and human influences in the area. Comparative water quality issues can be summarised as follows. (1) Naturally high concentrations of reduced iron and manganese are present in all three study areas, with the highest concentrations found in the natural wetland area and around the Turangi land treatment facility. (2) Concentrations of sodium, chloride, potassium, and ammonium in the groundwater down-gradient of the Turangi oxidation ponds are elevated relative to the other two study areas. Stable isotopic signatures also show that the groundwater has been influenced by surface water from the oxidation ponds, mostly due to additional evaporation caused by the relatively long residence time of the water (125 days) in the oxidation ponds. Elevated concentrations of ammonium also occur in deep groundwater under the forest areas of the Waimarino River catchment. (3) The water at all three sites is generally unsuitable for drinking supplies due to naturally elevated concentrations of reduced iron and manganese in the groundwater and elevated concentrations of ammonium at many monitoring sites, particularly around the Turangi land treatment site and the Waimarino deep aquifer monitoring sites. Aeration followed by settling or filtration of the groundwater could significantly reduce the concentrations of iron and manganese. (4) Elevated concentrations of reduced iron and manganese are unlikely to affect the water quality of Lake Taupo as all reduced iron and manganese will be oxidised once the water reaches the lake and precipitate as oxyhydroxide minerals onto the lake sediments. (5) At present, the extent of the ammonium-rich groundwater is unknown. However, should it reach Lake Taupo, oxidation of the ammonium to nitrate would occur at the groundwater-lakewater interface and increase the nutrient concentrations in the lake. (6) Preliminary results from groundwater quality measurements during harvesting in the Waimarino catchment do not show any deterioration that could be linked to harvesting practices. Long-term monitoring will be needed to confirm this. (auth)