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 situa ted on the southern shore of Lake Taupo and belongs to the largest continuo us wetlands in the North Island of New Zealand. The wetland's extent has in creased since the beginning of Lake Taupo's artificial lake level regulatio n in 1941. Hydrological and hydrochemical data collected seasonally from ni ne piezometers installed in the Stump Bay wetland over a duration of two ye ars show the influence of the stratigraphy and groundwater flow on the hydr ochemistry of the wetland. The hydrology is characterised by through-flowing or discharging groundwate r, 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 concentra tions (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 se epage water in this area. The eastern part of the wetland is likely to be f ed by discharging water originating from the Turangi plain. The main chemical characteristic of Stump Bay's groundwater is the high con centration of total dissolved iron (up to 145 g/m(3)). High iron concentrat ions are found to be associated with the presence of a clay layer within th e top metre of peat/sediment. The reduction of iron controls the groundwate r'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. Incr eased microbial activity at higher temperatures during summer is likely to cause seasonal fluctuations in pH, alkalinity, concentrations of calcium, m agnesium, and for some sites dissolved iron. All of these parameters have h igher values in summer and lower values in winter. (C) 1999 Elsevier Scienc e B.V. All rights reserved.