EGL Ref: 9049 23 June 2022 Page 14 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. dewatering pumping system located at RL780m. Water will continue to be piped to the Water Treatment Plant. The MUG dewatering level is at RL665m at the end of 2021 (see Figure 4). The observed response of veins within the Martha groundwater system to dewatering at Martha Mine means that the RL500m water level will also be developed in veins beneath Waihi, including Waihi East (Ref. 35). The existing multi-level piezometers P90 to P95 and P100 to P102 located in the Waihi East, shown in Figure 44, and Figure 45 indicate that the drawdown of the andesite is unlikely to have any notable drawdown effects on the shallower surficial alluvial and younger volcanic groundwater systems. These groundwater levels were recorded in August 2020. The groundwater level of the Martha vein system at that time was RL705m. The deepest piezometers in the boreholes are located in the upper andesite and show significant under drainage, however, the shallow piezometers in the younger volcanics show increasing groundwater pressure with depth indicating that they are maintaining a near hydrostatic profile. Observation of groundwater levels and modelling by GWS (Ref. 35) indicate dewatering the Martha vein system from the current elevation (approximately RL705m) to RL500m is not expected to notably affect groundwater pressures within the younger volcanics. This is consistent with the long-term groundwater monitoring data which shows shallow groundwater pressures have remained relatively unchanged while dewatering has been ongoing. The groundwater pressures in the interconnected veins and other open rock defects connected to the veins are expected to drop nearly immediately in response to lowering of the groundwater level required for the Martha Underground. The groundwater pressures in the andesite rock mass are indicated to respond to dewatering of the veins. The weathering profile depth within andesite rock does vary locally, resulting in a variation in thickness of the low permeability material that perches the shallow groundwater system. However, monitoring and modelling indicates that it is generally thick enough in the areas of works to date to maintain a perched water level in the overlying younger volcanics (Ref. 35). Martha Underground is located in areas of previous andesite watering. Monitoring in the andesite is limited by the depth to which the piezometers have been installed. The lowest piezometers installed in the andesite are P92-156.3m at RL965m (beneath Waihi East) and P106-163m to RL974m (behind Martha Pit North Wall). Project Martha underground mines will be developed down to RL500m. Deeper dewatering of the andesites is interpreted to have very minor effect for structures at the surface. Dewatering levels in the interconnected veins are assumed to match the dewatering for the underground mines. 4.5. Waihi North Project dewatering The groundwater drawdown effects for Waihi North Project have been assessed by GWS (Ref. 36) and GHD (Ref. 37). The following sections summarise the groundwater drawdown situations assumed for this ground settlement assessment for the various project elements.
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