Hydrogeology of Lake Taupo catchment - phase 1

Report: TR 2001/01

Author: John Hadfield, Debbie Nicol, Michael Rosen, Colin Wilson, Uwe Morgenstern, Environment Waikato, Institute of Geological and Nuclear Sciences Ltd.

This report describes the geology, groundwater chemistry, flow and age of the northern and western Lake Taupo catchment. These areas have the greatest potential for intensification of agriculture which threatens lake water quality.

Geology is dominated by young (< 0.4 Ma), locally derived, rhyolitic pyroclastics. The sequence in the western catchment is relatively simple with surficial Oruanui ignimbrite overlying a large thickness of Whakamaru ignimbrite. The latter is sufficiently welded in places to have moderate vertical fracture development and to form impressive cliffs along the lake-front. East of Kawakawa Bay faulting is common and there is a more complex sequence of ignimbrites, fall deposits, localised lava extrusions and lacustrine sediments. Of most hydrogeologic relevance are the Oruanui ignimbrite and the underlying grouping of rhyolite pyroclastics. Although not fractured, they are likely to have moderate permeability. Occasional paleosols, which punctuate these formations, are expected to act as localised aquitards and to sometimes induce perching.

Small, non-irrigation, farm groundwater supplies were developed predominantly in the northern catchment. Large farms in the western catchment generally have stream or lesser spring supplies and hence hydrogeologic information in this area is sparse.

Groundwater flow in the area is consistent with topography although more subdued. There is no discrepancy evident between surface water and groundwater catchments. Some divergent flow away from headlands such as Kawakawa Rd area and toward valleys and bays e.g. Kinloch, is evident. A recharge regime generally exists in the catchment indicated by a strong relationship between the measured depths to static groundwater level and well depth. Vertical head gradients vary considerably with some large differentials (> 50 m) being observed. The lake is the sink for groundwater which is recharged from rainfall in the catchment.

Groundwater from a total of 44 wells within, and on the margin of, the catchment were sampled for the analysis of nutrients and major ion chemistry. Relatively uniform sodium bicarbonate dominated groundwater chemistry typical of rhyolitic formation was found. Although water quality was generally high, evidence of land use impacts was found at some sites with elevated nitrate, sulphate and chloride concentrations. Groundwater at one site exceeded the drinking water guideline for nitrate and five further sites were in excess of half the guideline. The mean nitrate-N concentration (2.28 ppm) is substantially higher than lake and surface water concentrations.

Anaerobic or poorly aerobic conditions are indicated at eight sites and manganese concentrations >0.25 ppm were found with similar frequency. These included one site with manganese in excess of the drinking water guideline.

Groundwater was sampled at 11 wells and one spring for age determination using CFCs and tritium. Results show that the mean residence time of groundwater sampled ranges from about 20 to 75 years. The percentage of young water in the samples was found to relate to the measured nitrate concentrations. Nitrate concentrations are expected to increase with the percentage of water recharged since 1965, unless land-use changes.

Groundwater is the primary link for the transport of nutrients derived from land use to the lake. Land-use impacts are clearly evident in groundwater and are expected to increase. The reported investigation is a first phase in the provision of information required as a basis for determining appropriate land management for lake-water quality protection. More detailed investigation into several aspects is planned.