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

The Wairakei geothermal field fumarolic discharges are at their greatest in tensity from the Karapiti Thermal Area. This part of the geothermal field c ontains numerous steam-dominated features in an area of approximately 1 km( 2). Since 1952 there have been many changes to the surface features and the rmal activity at Karapiti related to the development-induced pressure drawd own over most of the held. 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 chemica l surveys of the steam vents at Karapiti since 1951, with major chemical su rveys undertaken in 1961 and 1990. In 1990, the concentrations of CO2 were found to be about 200 mmol/100 mol H2O, double the 1961 values. Of particul ar interest is the change between 1936 and 1987 in gas chemistry of the mai n 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 th e decreasing pressures in the lower pressure steam zone. Underground proces ses leading to a high gas content in a large fumarole formed in 1967 and ot her fumaroles in different parts of the Wairakei field are discussed. (C) 2 001 CNR. Published by Elsevier Science Ltd. All rights reserved.