Report: TR 2007/26

Author: Uwe Morgenstern (GNS Science)

Abstract

The near pristine water of Lake Taupo has begun to deteriorate, 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. To quantify the delayed arrival of nitrogen from landuse into the lake via the streams, the age of the water was measured in all the larger streams in the northern and western parts of the Lake Taupo catchment. These parts of the catchment contain the largest area of farming. Age distribution of the water allows for identification of future arrival of landuse-impacted water, and for prediction of future nitrogen mass loading to Lake Taupo from current land use practises.

Tritium dating is the only applicable method for determining the water age in streams, but was also the method of choice. It was known from two earlier measurements in Mapara Stream that water ages up to more than 50 years can be expected, which allows unambiguous pre-bomb tritium to be identified. For younger waters, tritium time-series normally can resolve ambiguous results which are caused by the fact that a high tritium concentration could indicate either very young water, or water with several decades residence time containing remaining bomb-tritium.

In the previous study in 2002 and 2004, only short time series tritium data were available to measure the age distribution of the stream waters. While the northern catchment streams had mostly unambiguously old mean ages (>50 years), the western catchment stream waters were clearly younger (<50 years), and the age interpretations of the tritium results were ambiguous. Two mean residence times were possible for the western catchment streams - several decades or less than three years. The tritium time series were too short to resolve the age ambiguity. Therefore, assumptions had to be made on limited hydrogeologic information to exclude one of the two possible ages. The geohydraulic conditions in the northern and western catchment were assumed to be relatively uniform. Therefore, such a large age difference between >50 and <3 years was assumed to be unrealistic and the possibility of a very young age of <3 years was excluded.

The northern part of the catchment is dominated by the relatively thick Taupo ignimbrite, which overlies an older rhyolitic ignimbrite, the Oruanui ignimbrite (c. 26,500 years old). The western part of the catchment is underlain by the much older Whakamaru Group ignimbrites (also rhyolitic: c. 340,000-320,000 years old). These are overlain by the Oruanui ignimbrite. The south-western part of the catchment is underlain by andesitic and basaltic lava, overlain by the Oruanui and Taupo ignimbrites. Historic tritium data from the Kuratau River demonstrated that a significant fraction of the water in the western catchment is very young (<1 year). Therefore, the geohydraulic parameters are not as uniform through the northern and western catchment as previously assumed, with the Taupo and Oruanui ignimbrites having very different water storage capacities compared to the Whakamaru ignimbrite.

With new tritium analyses in 2007, it was hoped the remaining age ambiguity in the western catchment could be resolved by using the new 5-years tritium time-series because the old bomb-tritium shows a decline over that period, while the young cosmogenic tritium should remain constant over that time at constant tritium input. Unfortunately over the last 10 years the concentration of tritium in New Zealand rain has dropped considerably. This drop coincides with the decline in bomb-tritium in old water and has a similar magnitude. Therefore, the young and old age water have identical tritium outputs at the present time and the ambiguity cannot be resolved for some streams on the basis of tritium data only. Despite this ambiguity, the improved hydrogeological information available now strongly indicates that the stream waters in the western catchment have mostly very young waters (<3 years).

Consistent with the initial age interpretation, the streams in the northern catchment contain old water, with the following mean residence times: Whangamata 84 y, Mapara 75 y, Kawakawa 60 y, Otaketake 49 y, and Omoho 42 y. The streams in the western catchment, however, contain significantly younger waters, with the following mean residence times: Waihora 10.5 y, Waihaha and Whanganui 2.5 y, and Whareroa 9 y. Kuratau and Omori on the boundary between the two geologic formations, which have different water storage capacity, contain mostly young water of <1 y from the Andesite/Whakamaru ignimbrite, but also some older fractions from the Oruanui and Taupo ignimbrite, with mean residence times of 30 and 40 years, respectively.

In regard to future total nitrogen (TN) loading from surface streams to Lake Taupo, the estimate for the western catchment is now significantly lower then in the previous estimate as a consequence of the higher fraction of young water. Most of the western streams are already in a steady-state with respect to post-1955 land use water and only a moderate TN load increase of 5 t/y is expected from the western lake catchment areas, mostly from the Kuratau and Omori areas that have significant partial Taupo and Oruanui Ignimbrite cover. A larger increase in TN loading of 16 t/y from surface flows is expected in the northern catchment, in agreement to the previous estimate. This area has thick Taupo and Oruanui ignimbrite cover. The TN load from the surface stream flows in the western and northern catchment are estimated to increase by 21 t/y, from 289 to 310 t/y.

It should be noted that the above estimate of total nitrogen load increase in the northern and western catchment of Lake Taupo covers only the surface stream flows. These, however, form only a fraction of the total discharge from some sub-catchments, particularly in the north. Approximately 80 per cent of the water in the northern catchment discharges to Lake Taupo directly by groundwater seepage via the lake bed, with an equivalent TN load of about 110 t/y. Most of this loading from the northern catchment groundwater seepage is still to come, because the underlying groundwater seepage-flows are likely to be even older than the surface stream flows. The expected large TN load increase from direct groundwater seepage via the lake bed is not included in the TN budget of this report.

Lake Taupo Streams: Water Age Distribution, Fraction of Land Use Impacted Water, and Future Nitrogen Load [PDF, 628 KB]