Viskovic, G.P.D.; Stagpoole, V.M.; Morgenstern, U. 2023 Results of microgravity survey and groundwater sampling, Waiwera, Auckland. Lower Hutt, NZ. GNS Science report 2023/33. 37 p.; doi: 10.21420/0Y8N-K293
Abstract:
The source of the hot water at Waiwera is not well understood because the deep structure of the field has mainly been inferred from the spatial distribution of thermal water from producing bores. Only four boreholes at Waiwera have intercepted Waipapa Group greywacke basement, and none have intersected the inferred fault / fracture zone that is thought to be the source of the upwelling thermal water. In this study, gravity data from 77 new stations were acquired around the greater Waiwera area, modelled and interpreted. Two 2.5D gravity models show a 180-m-high, NNW-trending step that may represent either steep topography or a downthrow to the east fault in the basement. The position and size of the step are well constrained, but the feature is too deep to confidently predict the dip and hence is modelled as almost vertical. This feature may provide a focus for hot water to rise upwards into the Waiwera geothermal system. Radiocarbon analysis of three groundwater samples show that the geothermal water at Waiwera is >20,000 years old and has <0.005% of young water. Geochemical results show possible saltwater intrusion near the coast and the highest observed concentrations of geothermal chemical components near bore 80, close to the modelled fault location (auths)
