Tauhara domestic bores - Updating of geographical and hydrogeological information

Report: TR 2021/16

Author: Jesse Lebe

Shallow domestic geothermal wells have been installed for houses in Taupō for private hot water access and space heating since the 1960s, within the resistivity boundary of the Wairakei-Tauhara geothermal system. In 2020, 524 domestic bores are known to exist, however minimal information updates have been done since 1988. For this study, the geographical information on bore locations required correcting as it was not measured with GPS or satellite technology. 125 bores required no changes, 291 required coordinate revisions, and 90 required coordinate revision and addition on Ourmaps (WRC’s internal mapping software), and 18 are not on the WRC database. Water take and heat exchanger bores are identified as the most common bore types.

Geochemistry data shows that the allocated aquifer from which the bores extract their fluids has a predominant steam-heated water chemistry. Due to minimal data existing on the lithological conditions of the bores, lithostratigraphic data from well THM-18 is used as a reference for interpreting the subsurface stratigraphy of the allocated aquifer area. Elevation and bore depth measurements suggest that the Oruanui Formation is the most common lithological unit from which hot fluids are extracted. The Upper Huka Falls Formation is intersected by fewer bores and records lower temperatures than the overlying Oruanui Formation. Fluids exceeding 160 °C were encountered in the Middle Huka Falls Formation, however, the formation is a less practical drilling target than the Oruanui Formation aquifer for future domestic bore drilling due to its situation at significantly greater depths. The Oruanui Formation is the most productive and most ideal target zone for future fluid extractions due to having a reliable high temperature resource at a relatively shallow depth.

Future monitoring of the bores could create a better understanding of the relationship between the domestic bores and surface geothermal features and streams, which have seen a general decline in discharge in recent years. Reintroducing regular bore inspections, implementing tracer flow tests, and improving on 3D geological modelling would improve on the existing understanding of the geothermal bores and the shallow geothermal hydrogeology of Tauhara.