EGL Ref: 9049 23 June 2022 Page 12 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. for MUG, which extend beneath Martha Open Pit. The bores are continuing to lower the groundwater level, which had reached RL665m as of December 2021. 4.3. Piezometer monitoring and interpretation Monitoring wells, referred to as piezometers, have been installed at various locations surrounding the pit and undergrounds to monitor the effect of dewatering on groundwater levels as shown in Figure 33. The piezometers have been installed at different depths to enable the monitoring of groundwater levels or water pressures in the different geological units (alluvium, deeper younger volcanic deposits and andesite rock). The recorded levels are summarised on contour plans in Figure 34, Figure 36, and Figure 39. The contours for the andesite in Figure 39 are indicative of the groundwater levels in the upper surface of the andesite. Groundwater levels in the deeper andesite will be lower. Figure 35, Figure 37, and Figure 40 group and present the recorded groundwater levels over time by geological unit for information. GWS (Ref. 35) provides a detailed review of the site hydrogeology and the effect of pit dewatering on groundwater levels. Generally dewatering has resulted comparatively in much greater changes in groundwater levels in the deeper andesite rock than in the weathered surface of the andesite, younger volcanic deposits and alluvium. This is shown in Figure 42 and Figure 43 which present the recorded groundwater levels at monitoring location P2 and P8. At each location there are multiple devices recording groundwater levels in different geological units. Generally, the groundwater can be drawn down in the andesite without affecting the shallow groundwater table, and only partially depressurising the younger volcanics at depth. In Figure 42 P2-1 and P2-2 tip (measurement) levels are in the andesite rock. They show the progressive drawdown of the andesite following the mine dewatering level. In Figure 43 P8-1 is located in andesite and also follows the mine dewatering. For all three piezometers (P2-1, P2-2 and P8-1) the groundwater levels have been drawn down below the tip and therefore they show a constant level as the mine dewatering continued. Notably the piezometer tips P2-3, P2-4, P8-3, and P8-4 in the alluvium and younger volcanic have not followed the same drawdown trend as the underlying andesite. A shallow groundwater system associated with volcanic ash, alluvium and completely weathered rhyolitic tephra is present at shallow depth. The shallow groundwater system is controlled by rainfall infiltration. It is not affected by underground mine dewatering in the andesite and is only affected by the open excavations close to the pit. Both piezometers P2-4 (tip RL 1101m) and P8-4 (RL1111.8m) in the boulder alluvium do not show any notable drawdown. The younger volcanic deposits are located within a transitional groundwater system. This groundwater system is not influenced significantly by either surface infiltration or underground dewatering of the andesite. It is, however, influenced by drawdown caused by the excavation of the pit walls. Figure 36 shows the contact of the deeper younger volcanics in the pit walls, showing the contact is lowest on the east side of the pit. Monitoring of groundwater levels indicates drainage primarily towards the open pit (see Figure 36). The response of piezometer P2-3 in Figure 42 and P8-3 in Figure 43 shows drawdown in the early stages of pit dewatering before a steady-state situation
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