New data for a model update of the Waiwera geothermal reservoir in New Zealand

Abstract

Waiwera is a small coastal village located on New Zealand's North Island. The geothermal reservoir below the town consists of compacted and cemented sandstones and siltstones of the Waitemata Group [1]. These are some 400-m thick and have been folded, faulted and fractured by tectonic processes throughout the entire depth. The source of the hot water is not well understood because the deeper structure of the field has mainly been inferred from the spatial distribution of thermal water from producing bores. An inferred fault zone at the base of the reservoir is thought to be the source of the upwelling thermal water. The most recent model of Waiwera integrates the depositional environment, mineralogical composition and geological structures [2]. Numerical simulations indicated that further model revisions are required with support by additional field campaigns to increase the knowledge of the complex reservoir geology [3]. 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 fault downthrow to the east in the basement. The position and size of this step are well constrained, but the feature is too deep to confidently predict the dip, and hence is modelled as being almost vertical. This feature may provide a focus for hot water to rise upwards into the Waiwera geothermal system [4]. Further, radiocarbon analysis of three groundwater samples show that age of the geothermal water at Waiwera is >20,000 years with <0.005% of hydrogeologically young water. Geochemical analyses show possible saltwater intrusion near the coast and the highest observed concentrations of geothermal chemical components near the main production well of the thermal resort, close to the modelled fault location. New models will be implemented with the updated information to investigate a revised conceptual model. We will test if the assumption of hot water accumulating in sedimentary basins away from Waiwera and travelling laterally up-dip near the base of the Waitemata Group sediments and then rising vertically under the Waiwera township is a feasible hypothesis to better represent the system.   [1] Kühn, M., Stöfen, H. (2005): A reactive flow model of the geothermal reservoir Waiwera, New Zealand. - Hydrogeology Journal, 13, 4, 606-626. https://doi.org/10.1007/s10040-004-0377-6 [2] Kühn, M., Präg, M., Becker, I., Hilgers, C., Grafe, A., Kempka, T. (2022): Geographic Information System (GIS) as a basis for the next generation of hydrogeological models to manage the geothermal area Waiwera (New Zealand). - Advances in Geosciences, 58, 31-39. https://doi.org/10.5194/adgeo-58-31-2022 [3] Kempka, T., Kühn, M. (2023): Numerical simulation of spatial temperature and salinity distribution in the Waiwera geothermal reservoir, New Zealand. - Grundwasser, 28, 243-254. https://doi.org/10.1007/s00767-023-00551-8 [4] Präg, M., Becker, I., Hilgers, C., Walter, T. R., Kühn, M. (2020): Thermal UAS survey of reactivated hot spring activity in Waiwera, New Zealand. - Advances in Geosciences, 54, 165-171. https://doi.org/10.5194/adgeo-54-165-202