Report: TR 2002/18R

Author: Bill Vant, Paul Smith

Abstract

During 2001-02 water samples were collected from near the mouths of 11 streams flowing into Lake Taupo at summer low flow. All 11 streams contain reasonably large areas of pasture in their catchments, and between them they contain about half of the area of pasture in the lake's catchment. Samples were analysed for forms of the plant nutrients nitrogen and phosphorus, and for tritium (a radioactive isotope of hydrogen). The tritium results were used to determine the average age of the water in each stream. They were also used to determine the fraction of the water that is likely to have been affected by the development of the land for pasture that has occurred since about the 1950s.

The stream waters tended to be cool, reflecting their groundwater origins. In most cases, dissolved inorganic forms of nitrogen and phosphorus predominated, and typically comprised about 80% or more of the total nitrogen and phosphorus. Concentrations of dissolved phosphorus tended to be higher in streams where the average age was higher, possibly because greater time underground means there is more chance that phosphorus will be dissolved from the volcanic deposits.

The average age of the baseflow waters ranged from <30 years to 80 years. The oldest water was found in three streams in the northern part of the catchment (Kawakawa, Mapara and Whangamata). In these streams only a small fraction (6-40%) of the water was young enough to have been affected by the pasture development over the past 35-45 years. In the other streams, however, up to 80-95% of the water was young.

Measurements made since the early 1970s show that nitrogen concentrations in the streams containing older water have steadily increased. In two of these (Mapara and Whangamata), concentrations of inorganic nitrogen are now 2-4 times higher than in the 1970s. In streams where the water is younger, the increases have been smaller, as the water is more likely to have already adjusted to the previous land development.

The water age and nitrogen results were used to make approximate estimates of the load of nitrogen that is yet to appear in the streams. These calculations indicate that at steady-state the combined load of nitrogen from areas of pasture in the catchments of the 11 streams will be between about 20% and 80% higher than the current load. Generalising these results to the areas of pasture in the catchment of the lake as a whole, it appears there will be a further moderately-large increase in the load of nitrogen from pasture areas in the future as a result of the land development that has occurred during the past 35-45 years.

Inflows to Lake Taupo— nutrients and water ages [PDF, 204 KB]