Groundwater quality along the eastern shores of Lake Taupo, 1975-76

To investigate the extent of spatial and temporal variation in groundwater quality along the eastern shores of Lake Taupo, New Zealand, data were collected from May 1975 to March 1976 from 14 boreholes sited to represent a variety of land use and human population density. Physical parameters investigated were water level, temperature, and electrical conductivity; the chemical constituents monitored were soluble reactive phosphorus, total dissolved phosphorus, nitrate and nitrite nitrogen (combined), ammonium nitrogen, total soluble iron, major cations and anions, and pH. The results show that groundwater quality can vary greatly over time and space. For example, spatial variation in mean soluble reactive phosphorus levels was 1-340 mg.m-3, in mean nitrate and nitrite nitrogen (combined) 3-13 930 mg.m-3, and in ammonium nitrogen from 5-633 mg.m-3. The most extreme variation with time concerned nitrate nitrogen at a site in Taupo township, where the concentration dropped from 16 430 mg.m-3 in early June 1975 to 1170 mg.m-3 in March 1976. Sometimes temporal variation was relatively small: at Motutere nitrate nitrogen stayed within the range 131-202 mg.m-3 during the study period. Generally, a few infrequently monitored boreholes should not be relied on to describe variations in groundwater quality.