Ground water quality of the Wanaka and Wakatipu basins, Central Otago, New Zealand

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Rosen, M.R.; Reeves, R.R.; Stewart, M.K.; Taylor, C.B. 1997 Ground water quality of the Wanaka and Wakatipu basins, Central Otago, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 97/01 67 p.

Abstract: Ground water in alluvial aquifers of the Wakatipu and Wanaka basins, Central Otago, New Zealand, has a composition dominated by calcium and bicarbonate. The composition can be expressed in equivalent units as Ca2+ Mg2+ approx =Na+ > K+ for cations, and HCO3- >> SO42- > NO3- approx =Cl- for anions. Ca2+ and HCO3- occur on a 1:1 equivalent basis and account for >80 percent of the ions in solution. However, some ground water has increased proportions of Na+ and SO42- which is interpreted as reflecting a different source for that water. Based on chemical and biological data collected during 1995, almost all water quality problems in both basins are the result of 1) faecal coliform contamination or, 2) high iron and manganese concentrations. In the Wakatipu Basin, 59 percent of analyses of ground water were below the maximum acceptable values (MAV) for all parameters measured, 24 percent were above the MAV for faecal coliforms, 20 percent were above the MAV for iron or manganese, one analysis was above the MAV for nitrate, and another one analysis was above the MAV for boron. Three percent of the bores were above the MAV for both faecal coliforms and iron. In the Wanaka Basin, 86 percent of the analyses of ground water wells were below the MAV for all parameters measured, 6 percent were above the MAV for faecal coliform, 7 percent were above the MAV for iron, and one was above the MAV for nitrate. The high iron and manganese concentrations are considered to be due to natural processes in the ground water and most of these occurrences are in the Wakatipu basin. High faecal coliforms are mostly due to stock grazing in areas associated with a high ground water table, poor well-head protection, and poor well construction in the Wakatipu Basin. Nutrient levels are generally low compared to many rural areas of New Zealand; of most concern are the high concentrations of nitrate. Although nitrate concentrations are for the most part below the drinking water MAV for New Zealand, the trend for increasing nitrate to be accompanied by increasing chloride suggests an anthropogenic contribution of these ions and indicates that the situation should be monitored. Ground water levels appear to have been relatively constant over the measurement period (May 1995 - March 1996), with little change to the piezometric surface of either basin during this period. This suggests that any pumping of ground water during the study period did not greatly affect ground water levels. Ground water is shallow in the Wakatipu Basin generally less than 5 m below the surface, whereas it is relatively deep (greater than 20 m) in Wanaka, except near the Cardrona River. This indicates that Wakatipu Basin ground water will be more susceptible to land-use impacts than Wanaka Basin ground water . The rock mater ial of the alluvial aquifers of both basins is derived from the erosion and weathering of metamorphic Otago Schist rocks (grey and green schists). Calcite is an accessory mineral in both the grey and green schists at less than 5 percent of the rock Geologic mapping of both basins indicates that dissolution of calcite from the schist is the only likely mechanism for producing ground water with such a constant composition dominated by Ca2+ and HCO3- on a 1: 1 equivalent basis. Ground water with higher proportions of Na+ and SO42- occurs near areas where the schist crops out at the surface, and represents deeper and possibly older water derived from aquifers within the basement rock. Anomalously high K+ in the Wakatipu basin and high NO3- concentrations in the Wanaka basin cannot be accounted for by interaction with basement lithologies and probably represent the influence of anthropogenic sources on the ground water composition. The age of the ground water is relatively young, probably less than 20 years old, based on tritium analyses, but the existence of older water ( more than 40 years) can be seen in both basins. Stable isotope data indicates that recharge to the Wanaka Basin occurs mainly from the Cardrona River. Recharge in the Wakatipu Basin is more complex, but is probably derived from the mountains north of the basin. A plume of isotopically-light water can be seen pushing to the south from the north-central part of the basin. (auth)