Surface water / groundwater interaction and catchment influence on waters entering Lake Taupo, New Zealand

Abstract

Lake Taupo is a national icon. Although water quality in the lake is currently excellent, some degradation has been observed in recent years. Further information on the interaction of surface water and groundwater and catchment characteristics is needed to improve our understanding of land-use effects on the quality of water entering the lake. Water chemistry and flow was investigated at 39 surface water sites during the summer of 2001/2002. Of the surface water sites, 17 were subsequently monitored during winter 2002 and summer 2002. Water chemistry was monitored at 29 groundwater sites in nine catchments to investigate the influence of land-use on water quality. Comparison between northern and western catchment nitrate-nitrogen concentrations show that spatially, the two areas are generally similar. Surface water quality is good but nitrate-nitrogen concentrations are increasing in many catchments as indicated by a comparison of data from this study and research by Schouten et al. (1981) in the 1970s. Concentrations in streams ranged from <0.05 - 2.18 g m-3. A comparison between the northern and western catchments shows that the two areas have similar nitrate-nitrogen concentrations in streams. Nutrient contribution and flow distribution within catchments is variable. It is estimated that about two thirds of the flow and ~90 % of nitrate-nitrogen is derived from the bottom two kilometres of the Mapara Stream catchment. By contrast nearly all of the nitrate-nitrogen and much of the flow in the neighbouring Whangamata Stream catchment is generated from springs in the upper reaches. Nitrate-nitrogen concentrations decrease along some lower stream reaches e.g. Whangamata, Otaketake and Tutaeuaua streams indicating significant in-stream nitrate-nitrogen reduction processes. Mass nitrate-nitrogen export from catchments is variable, with significantly higher export from western catchments due to precipitation and pasture development. There is a significant relationship between nitrate-nitrogen and percentage pasture at a Lake Taupo catchment and smaller sub-catchment scale. Tritium dating indicates uniform mean residence times within catchments. The Mapara and Whangamata streams have the oldest waters with mean residence times estimated to range from 65 years to 87 years. Other catchments showed mean residence times from 35 years to 40 vears. The interaction between surface water and groundwater has been found to occur predominantly between streams and deep groundwater in seven of the nine catchments. Chemical, hydrological and age analyses suggest interaction predominantly with deep groundwater, shallow groundwater is interacting with stream water in the Waihora and Omori stream catchments. At times not all of the datasets imply the same story. Interpretation of the chemical, hydrological and age datasets together results in a fuller picture of the linkages between surface water and groundwater. In summary, the influence of catchment land use and the interaction of surface water and groundwater is important in nitrate-nitrogen transport to Lake Taupo. This emphasizes the need for initiatives to appropriately manage the Lake Taupo catchment for the protection of the values which the community hold important, while ensuring the enjoyment of future generations.