Hydrogeology and groundwater quality of the Tauranga-Taupo settlement, Lake Taupo

Abstract

The hydrogeology, groundwater quality and land use impacts were investigated in the Tauranga-Taupo lakeshore settlement on the eastern side of Lake Taupo between February and November 2005. The aim of this study was to gain a better understanding of the groundwater dynamics, and the fate and transport of potential contaminants, particularly nitrate nitrogen, available to enter Lake Taupo. The study area is underlain by a heterogeneous, unconfined aquifer system consisting of reworked pumiceous gravels, sands, silts, clays, and peat. The depth of the unconfined aquifer is estimated to be at least 30 metres with localised, discontinuous aquitards 12-18 m below ground. Highly variable hydraulic conductivity values were revealed ranging from 0.4 md-1 to 180.3 md-1. Groundwater flow was determined to be in a north to northwest direction towards Lake Taupo. Water level data show that the aquifer system is hydrologically linked to Lake Taupo and the Tauranga-Taupo River. The average groundwater velocity was calculated at 0.15 md-1. Groundwater was sampled from 11 sites for the analysis of nutrients and major ion chemistry. Water quality in the study area was generally very high. Sodium bicarbonate was found to be the dominant water type, which is typical of rhyolitic aquifers. The mean and median nitrate-N concentrations were 2.92 mg/L and 0.10 mg/L respectively. Concentrations were variable across the study area and showed no obvious seasonal trends. The highest nitrate-N concentration related to agricultural land use. However, fine-grained material beneath the pastoral land is inferred to have promoted more elevated nitrate-N concentrations because of the low dilution capacity of the aquifer media. Elevated chloride, sulphate and calcium levels were also observed at this location indicating land use activities have impacted on groundwater. A significant reduction in nitrate-N was observed in the wetland down gradient of pastoral land. This highlighted that the Tauranga-Taupo wetland was effective in the removal of nutrients from groundwater lowering the potential of contaminants reaching Lake Taupo receiving waters.