CO2 FLUX INVESTIGATION AT WAIRAKEI, A LOW-CO2 GEOTHERMAL SYSTEM IN THE TAUPO VOLCANIC ZONE, NEW ZEALAND

A CO₂ flux survey was undertaken at Te Mihi, Wairakei, a well described geothermal area in New Zealand. The objective was to determine if a wide-spacing (~100m) survey approach could resolve previously mapped faults and the geothermal system boundary defined by geophysical surveying. In addition, previously mapped faults were examined in detail using reduced measurement spacing, both for transects (perpendicular to strike), and along strike. ¹³CO₂ isotope samples were collected from the accumulation chamber for a subset of total measurements. This allowed both the magnitude of CO₂ flux, and the relative proportions of biological and geothermal CO₂ present to be determined. Results from surveying that utilised reduced spacing located anomalies associated with a previously mapped fault, a deep geothermal well, and thermal ground. The wide spacing survey was unable to resolve the system boundary or confirm geothermal CO₂ degassing associated with previously mapped faults. It is likely that the well-developed hydrothermal capping formation at Wairakei (Huka Falls mudstone) prevents widespread diffuse degassing, directing it to areas where the Huka Falls formation is absent (i.e. Karapiti). The lack of a clear surficial, geothermally-sourced CO₂ flux signal at Te Mihi may also be explained by the relatively low CO₂ content of the reservoir. Our results show the combination of CO₂ flux and ¹³C isotope measurements provides a powerful approach to distinguish geothermal from biological CO₂ flux where the magnitude of CO₂flux alone is ambiguous.