Report: TR 2007/49

Author: Uwe Morgenstern

Abstract

The near pristine water of Lake Taupo is under threat, mainly as a result of delayed arrival of nitrate from farming. Nitrogen travels from farms to the lake mostly via groundwater, entering the lake via groundwater-fed streams, and directly by groundwater seepage via the lake bed. The age of the water was measured in all the larger streams in the northern and western parts of the Lake Taupo catchment, and in groundwater wells to estimate the time lag between landuse intensification and the arrival of landuse nitrogen in the lake via groundwater and groundwater fed streams. The northern and western sub-catchments contain the largest area of farming. The age distribution of the water can be used to estimate the future arrival of landuse-impacted water from current landuse practises, and predict future nitrogen mass loading to Lake Taupo.

The hydrogeology in the northern part of the catchment is dominated by relatively thick Oruanui ignimbrite (c. 26,500 years old) with areas of overlying Taupo ignimbrite (1,800 years old) overlying much older welded Whakamaru Group ignimbrites (also rhyolitic: c. 340,000-320,000 years old). The western part of the catchment is dominated by the old welded Whakamaru Group ignimbrites overlain by thinner Oruanui ignimbrite. The south-western part of the catchment is dominated by andesitic and basaltic lava, partially overlain by the Oruanui and Taupo ignimbrites.

While tritium data from northern streams clearly shows old (>50 years) water in streams, historic tritium data from the Kuratau River demonstrates that the major fraction of the stream water in the western catchment is very young (<1y). Therefore, the geohydraulic parameters are not uniform through the northern and western catchments. Northern unwelded ignimbrites have greater capacity to store water in the deep groundwater system than western welded Whakamaru Ignimbrites, which more rapidly contribute to surface flow. This is reflected by lower specific yields of northern stream sub-catchments.

Hydrochemistry results are mostly consistent over time. Only a few parameters of some wells vary considerably. About half of the samples have hydrochemistry indicative of anaerobic conditions, some of them up to the stage of methane fermentation. At anaerobic conditions nutrients (nitrate and sulphate) can be reduced, and the gas concentrations in the water can be disturbed causing difficulty in CFC and SF₆ age dating.

About half of the groundwaters have time series tritium data that enables unambiguous age interpretation. Most tritium time series give an excellent match to flow models with unusually high piston flow fraction. The remaining wells, except one, have been accurately dated using additional information.

A number of chemical species show a correlation with age of the water, indicating concentration increase due to progressive leaching from the volcanic aquifer. The best correlation with age in groundwater from Lake Taupo is observed for phosphorus, and to some extend for silica and fluoride, and pH in reduced groundwaters.

The groundwater system in the northern and western Lake Taupo catchment is very complex. The unusually high fraction of piston flow in most areas means that the groundwater system is not well mixed, with preferential flow paths likely to be important. The observed variability in groundwater chemistry, in space and time, reflects this.

Anoxic waters have lower nitrate concentrations by about a factor of two compared to oxic waters with similar fraction of landuse impacted water. This suggests that on average about half of the nitrate is being removed, where groundwaters are anoxic, by denitrification processes.

The groundwaters in the northern and western Lake Taupo catchment are relatively old with mean residence times between 20 and 180 years (only three shallow piezometers have very young water with age of about two years). Groundwater quality in general is not yet in equilibrium with current landuse and further increases in nitrogen load are expected as old pristine groundwater is progressively replaced by landuse impacted water. Nutrients show a clear landuse impact on groundwaters with the data indicating further increase in future for nitrate, sulphate, magnesium and calcium.

A significant increase in total nitrogen load via direct groundwater seepage of about 135 t/y is expected from the northern catchment area, in addition to an increase by 21 t/y increase from streams in the northern and western catchment. The groundwater nitrogen increase is only a rough estimate but it demonstrates that the most significant nitrogen load increase is expected from the northern catchment groundwater seepage.

Lake Taupo Catchment Groundwater Age Distribution and Implications for Future Land-Use Impact [PDF, 1.9 MB]