Hydrogeologic changes at Tauhara geothermal field

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Allis, R.G. 1983 Hydrogeologic changes at Tauhara geothermal field. [s.l.]: [s.n.]. Report / Geophysics Division 193 46 p

Abstract: Exploitation of Wairakei field has caused a pressure drop of approximately 18 bar within the Waiora pumice breccia aquifer of Tauhara field. Although the initial pressures of Wairakei and Tauhara fields were not significantly different, the deep aquifer of Tauhara field is now about 7 bar higher than that of Wairakei, causing flow from Tauhara to the Warakei production borefield. The extent of drawdown in Tauhara field appears to have been limited by the low permeability of the Huka formation. Deep drawdown has caused a steam zone to form immediately beneath the Huka formation over a large portion of Tauhara field. The steam zone may be around 100 m thick and is at 25-30 bar pressure. Prior to drawdown, upflow of hot chloride water to near-surface aquifers may have occurred beneath Mt. Tauhara, where volcanism has disrupted the Huka formation. Two aquifers appear to be controlling the shallow hydrology of Tauhara field. These aquifers occur in pumice breccia layers; one within, and the other above, the Huka formation. The Huka aquifer is open to both Lake Taupo and the Waikato River, and is probably carrying most of the near-surface liquid of Tauhara field. This aquifer originally contained the chloride water which emerged at Spa Sights, and the dilute chloride water emerging at Waipahihi. Th uppermost (Wairakei) aquifer becomes progressively perched (with respect to the Huka aquifer) along its outflow path. A relatively sharp, west dipping gradient occurs in the water table near the western edge of Tauhara field. This coincides with the wedging-out of the upper Huka mudstone aquiclude, permitting the upper aquifer to drain down to the Huka aquifer. Domestic wells near to this often have downflows, with water level being dependent on the total depth of the well. Many of the changes that have occurred in domestic wells are attributable to the hydrogeologic instability that exists near this western termination of the upper aquifer