EGL Ref: 9049 23 June 2022 Page 15 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. 4.5.1. Gladstone Open Pit GHD (Ref. 37) has made assessment of the groundwater drawdown at Gladstone Open Pit. There is a shallow groundwater system in the younger volcanics overlying a deeper system in the andesite, similar to the rest of Waihi. However, the younger volcanics (sandy ignimbrite found up to 43.5 mbgl) are of a much more limited depth than Waihi East (up to 140 mbgl). The Favona Underground dewatering is inferred to partially dewater the Gladstone vein system, with deeper piezometers in the andesite near to the vein system (P60 and P61) drawn dry on the north side of the pit. On the south side of the pit, piezometer GLD004 close to the inferred extent of the vein system shows slight vertical upward flows from the andesite to the ignimbrites. Piezometer P79 to the southwest of the pit indicates minor gradual drawdown of the andesites, however groundwater is still present in the monitored levels of the andesites. GLD004 indicates that the veins to the south may not be dewatered. P79 indicates partially dewatering in the andesites with limited hydraulic connection to the vein system is indicated by the lack of drawdown. GHD (Ref. 37) expected that Stage 1 of the Gladstone Open Pit will already be fully dewatered in the andesites (near to the vein). Stage 2 which extends partially into Winner Hill, may only be partially dewatered along the vein, and in the andesites. The pit is expected to have some drawdown effects in the shallow groundwater system around the pit, however these effects diminish away from the pit. The extent of the dewatering in younger volcanics is estimated by GHD (Ref. 37). The extent of dewatering effects in the shallow groundwater system is presented by GHD (Ref. 37) as distance drawdown graphs. In summary the groundwater drawdown at the edge of the pit varies between 3 to 8 m and the drawdown zone of influence extends up to 290 m. The Ohinemuri River is approximately 185m from the pit on its southwest extent. The Ohinemuri River is expected to control the shallow groundwater to the south of the river. GHD (Ref. 37) has assumed that there will be a strong hydraulic connection along the Gladstone vein system which will allow the vein to dewater to the pit floor level of RL1005m. They consider that the drawdown extent will be similar to the extent indicated by P60 (fully drawn down) to GLD04 (not drawn down), however the maximum depth of drawdown will now shift 180 m to the southwest along the vein to the bottom of Gladstone Open Pit (RL1005m). GHD indicated no material reduction on the Ohinemuri River flows, attributed to the separation of the shallow and deep groundwater systems. They have indicated the likely extent of drawdown effects in the deeper andesite in Figure 3.14 of their report (Ref. 37). As there may be some drawdown of the vein system beneath at the Ohinemuri River they have assessed the effect on groundwater from vein dewatering. This is shown in Figure 3.13 of the GHD report (Ref. 37). This drawdown has been considered in the assessment of potential settlement effect in this report (Section 6.1.1).
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