| Title | Groundwater modelling assessment of shallow and deep geothermal aquifer interactions around Otumuheke Stream, Wairakei-Tauhara geothermal field |
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| Authors | J. Scherberg, J. Williamson, M.J. Sophy |
| Year | 2023 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Groundwater, Geothermal Energy, Tauhara Geothermal Field, Otumuheke Stream, Taupo Hot Springs |
| Abstract | Williamson Water & Land Advisory has collaborated with Contact Energy in a hydrogeological modelling study of the shallow Tauhara Geothermal Field, focused on the Otumuheke Stream area. The stream, fed by geothermally heated groundwater and flowing into the Waikato awa approximately 1 km upstream of Huka Falls, holds significance to local Iwi and is a popular recreation site. Flow and temperature changes have occurred in the Otumuheke stream since the 1960s in response to reservoir pressure trends. Since 2013, reservoir pressure near Otumuheke spring has increased due to nearby reinjection. Contemporaneously, ground subsidence has declined as has flow from the uppermost spring in the valley. The decline in stream flow has coincided with declining groundwater levels within the shallow aquifer that resides within the Tauo Pumice and Oruanui Formations. Groundwater level declines are less evident with increasing distance from geothermal features and subsidence anomalies, indicating that declining groundwater levels and streamflow are not due to climate alone. The hypothesis was that the reinjected geothermal fluid partially submerged or quenched the steam zone at the base of the shallow aquifer, reducing the pressure buffer separating shallow groundwater from the deep geothermal reservoir and allowing increased downward vertical leakage. For this study, a numerical modelling approach was applied to improve understanding of the mechanisms causing the observed changes in the shallow hydrogeological environment. A groundwater model was calibrated with groundwater monitoring data and Otumuheke Stream flows records. Results show that the model is suitable for simulating observed groundwater levels and streamflow, and thereby replicating the interaction between the shallow aquifer and geothermal reservoir. Results indicate that vertical leakage losses currently range from approximately 15,000 to 24,000 m3/day (175 to 277 L/s), which is substantial relative to the median flow of the Otumuheke Stream (less than 100 L/s since 2014), though a small effect relative to the 130 Mm3 estimated for shallow aquifer water storage within the Otumuheke Catchment. The model is suitable for further investigating the shallow aquifer effects of a range of environmental variables and future power plant management options. |