The influence of groundwater hydrology and stratigraphy on the hydrochemistry of Stump Bay, South Taupo Wetland, New Zealand

The Stump Bay wetland forms part of the South Taupo wetland, which is situated on the southern shore of Lake Taupo and belongs to the largest continuous wetlands in the North Island of New Zealand. The wetland's extent has increased since the beginning of Lake Taupo's artificial lake level regulation in 1941. Hydrological and hydrochemical data collected seasonally from nine piezometers installed in the Stump Bay wetland over a duration of two years show the influence of the stratigraphy and groundwater flow on the hydrochemistry of the wetland. The hydrology is characterised by through-flowing or discharging groundwater, with a main flow direction towards the lake. The piezometric contour map shows a groundwater divide running from south-west to north-east, creating an eastern and western discharge system. Higher dissolved oxygen concentrations (0.3-6.1 g/m(3)) and slightly higher temperatures (8-18.9 degrees C) in the western part of Stump Bay suggest the presence of Tongariro River seepage water in this area. The eastern part of the wetland is likely to be fed by discharging water originating from the Turangi plain. The main chemical characteristic of Stump Bay's groundwater is the high concentration of total dissolved iron (up to 145 g/m(3)). High iron concentrations are found to be associated with the presence of a clay layer within the top metre of peat/sediment. The reduction of iron controls the groundwater's conductivity (0.07-0.6 mS/cm), alkalinity (18-423 g/m(3)), and pH (6-7.2). The nutrient status of the wetland's groundwater is generally low. Increased microbial activity at higher temperatures during summer is likely to cause seasonal fluctuations in pH, alkalinity, concentrations of calcium, magnesium, and for some sites dissolved iron. All of these parameters have higher values in summer and lower values in winter. (C) 1999 Elsevier Science B.V. All rights reserved.