CHARACTERISATION OF GROUNDWATER SYSTEMS AND GEOTHERMAL RESERVOIRS IN THE UPPER WAIKATO AREA, TAUPO VOLCANIC ZONE, NEW ZEALAND

Groundwater systems are important to the economy and ecology of the approximately 4400 km² Upper Waikato River (UWR) catchment in the Taupo Volcanic Zone, New Zealand. Cold groundwater is an important source of water for irrigation and industry and provides baseflow to rivers and therefore significant for recreation. Geothermal reservoirs feed many unique geothermal features in the area. The UWR is also an important source of energy with installed capacity for hydroelectric power and geothermal energy of 1084 MW and 853 MW, respectively.

Models of 3D geology, water budgets and piezometric surfaces were used to understand groundwater circulation systems in the UWR. Together, these models inform water policies in the UWR that include: unified management of surface water and groundwater; and avoidance of a decline in cold groundwater inflow to geothermal systems.

The 3D geological model of the UWR includes greywacke basement and 15 Pleistocene volcanic units and associated sediments. Vertical displacement of greywacke basement of up to 4000 m has been associated with calderas and faults. Important units for groundwater flow include the UWR-wide 300 ka Whakamaru Group ignimbrites; and surficial units such the 27 ka Oruanui Ignimbrite and Tauranga Group sediments.

The water budget shows that: rainfall recharge to groundwater is a large fraction (50%) of rainfall; base flow is a large fraction (94%) of total stream flow; and recharge to geothermal fields is a small portion (2 % to 8%) of rainfall. The piezometric map showed that the surficial units are probably most important for groundwater flow. The extents of the 14 deep geothermal reservoirs were represented in the model with a set of prolate spheroids that were centered on the top surface of the Whakamaru Group, resulting in oval surface expressions that represent the distributions of surface geothermal features.

Topographic catchments were proposed as the recharge zones for geothermal fields. This is because: 1) geothermal fields are generally associated with topographic lows as they are located at the base of large-scale faults, in the middle of basins and calderas and along river channels; and 2) groundwater budgets of these catchments generally provide sufficient recharge for geothermal outflows, such as hot springs, and for cold-water springs and seeps.