White PA, Seward AM, Kora T, Bradshaw D. 2021. Te Ahi Tipua ki Tahorakuri A1 Section 30: science review of the Ohaaki Geothermal Field to inform a hapū economic plan in degraded geothermal environments. Lower Hutt (NZ): GNS Science. 43 p. (GNS Science report; 2021/23). doi:10.21420/9EAP-9P20.
Abstract:
Ohaaki Geothermal Field is one of the geothermal fields within the Taupō Volcanic Zone that has been used by Ngāti Tahu for centuries providing social, cultural, environmental and economic value to iwi. Ohaki is one of the principal settlements of the Ngāti Tahu iwi who have lived in close association with the Waikato River in permanent and temporary kāinga from Huka Falls west to Orākei Korakō and Pohaturoa, with the river having an important part in transportation. The Tahorakuri A130 Ahu Whenua Trust administers the 55 ha A130 land block located near Ohaki Marae. The Trust aims to continue to regrow, rebuild and reconnect whānau and hapū back to this land and to each other and unlock the value of their key assets-base: land, people, resources. To do this, the Trust aim to establish an income-generating eco-papakāinga on the A130 land block. Use of the Ohaaki Geothermal Field for power generation beginning in 1989, has seen significant adverse effects from the extraction of geothermal fluids. These effects have included the loss of many natural surface geothermal features at Ohaki, and subsidence of the area around the Ohaki Marae including the bed of the adjacent Waikato River. Widening of the Waikato River channel has resulted in the recent construction of a bund to protect the Ohaki Marae. Potential effects of Ohaaki Geothermal Field operation on the A130 land block, in regards of the land’s suitability for eco-papakāinga development, have been assessed. Historic subsidence has affected land surface level to the southeast of the A130, but generally has been of minor influence on land levels in the majority of the block as it is located north of the Ohaki subsidence bowl. Generally, the A130 land is suitable for use as an eco-papakāinga in most of the area other than the land at risk of inundation from shallow groundwater tables. A small area in the south-eastern margin of the A130 land block (i.e., near Ohaki Marae and abutting the Waikato River) is at risk from shallow groundwater tables, and possibly land inundation. This is a risk because the groundwater table elevation has been increasing in some wells located south of the block and the general land subsidence in the area, resulting in the groundwater levels being shallower. This elevation rise of groundwater levels is possibly related to Waikato River controls, e.g., the westward movement of the Waikato River river-edge position, and to increased rainfall recharge. Field work is recommended to assess the risk from shallow groundwater tables in the A130 land that includes measurement of land levels and groundwater table elevation. (auth)
