Fumarolic gas chemistry at Wairakei, New Zealand, 1936–1998

Abstract

The Wairakei geothermal field fumarolic discharges are at their greatest intensity from the Karapiti Thermal Area. This part of the geothermal field contains numerous steam-dominated features in an area of approximately 1 km 2. Since 1952 there have been many changes to the surface features and thermal activity at Karapiti related to the development-induced pressure drawdown over most of the field. A greatly expanded steam cap fed by a large low-pressure steam zone in the Wairakei reservoir has replaced the hot chloride water originally underlying Karapiti. There have been intermittent chemical surveys of the steam vents at Karapiti since 1951, with major chemical surveys undertaken in 1961 and 1990. In 1990, the concentrations of CO 2 were found to be about 200 mmol/100 mol H 2O, double the 1961 values. Of particular interest is the change between 1936 and 1987 in gas chemistry of the main Karapiti feature, the Karapiti Blowhole (F712), which follows the change in heatflow from the Karapiti Thermal Area. Since 1990, gas concentrations appear to be dropping to low, pre-development levels, most likely due to the decreasing pressures in the lower pressure steam zone. Underground processes leading to a high gas content in a large fumarole formed in 1967 and other fumaroles in different parts of the Wairakei field are discussed.