The role of groundwater monitoring in the sustainable management and operation of the Ng? Tamariki geothermal field

Abstract

The extensive network of monitoring wells at the Ng? Tamariki Geothermal Field provides valuable information guiding the sustainable management and operation of the resource. In total, 34 shallow and intermediate wells are monitored for chemistry, pressure, water level, temperature, which give a detailed overview of a complex groundwater system above the deep geothermal reservoir.
The reservoir is overlain by three distinct hydrogeological units, including: a confined intermediate aquifer hosted within rhyolite lavas and breccias, an aquitard complex predominantly hosted within the Huka Falls Formation, and a shallow unconfined aquifer hosted in the Oruanui Formation and Taup? Pumice Alluvium. Chemistry and temperature monitoring indicate an upwelling of reservoir fluid into the northern part of the intermediate aquifer through a leak in the underlying clay-cap. Monitoring wells in the south contain cold sodium-bicarbonate waters, commonly encountered in Taup? and Middle Waikato catchments.
The objective of this paper is to provide a brief overview of the information obtained from the Ng? Tamariki groundwater monitoring program starting with the baseline sampling in 2009-2011, followed by the startup of the Ng? Tamariki binary plant in 2013, and continuous operation through to the present. We provide description of sampling and downhole monitoring techniques and share lessons learned associated with the ongoing maintenance of the network. Ultimately, the groundwater monitoring program provides comprehensive understanding of the regional groundwater system and supports the safe and sustainable operation of the field, with capability for early detection of responses to development.