EGL Ref: 9049 23 June 2022 Page 30 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. In summary initial ground settlement estimates in the area of WUG could be on the order of 300 to 1000 mm, however, this is not expected to have any material effect on stream flows or the forest environment. EGL recommend that ongoing settlement monitoring is not required at the WUG site. Base survey of reference points at the ventilation shafts and any drill sites will be sufficient. Visual observation of the stream and forest environment as part of monitoring of other environmental effects would be sufficient to check for any unforeseen effects due to settlement. 6.1.8. Additional settlement and groundwater monitoring locations We recommend additional monitoring (settlement points and piezometers located in boreholes). The locations are shown in Figure 66 to Figure 69. They will need to be reviewed and confirmed as part of the updates to the Dewatering and Settlement Monitoring Plan. This will depend on the final arrangement of the WNP elements i.e. confirm decline positions and depths and the final tunnel alignment in the Wharekirauponga Access Tunnel corridor. 7.0 REBOUND Dewatering around the open pits and underground mines will eventually stop once mining in the area has ceased. The area will then be allowed to re-water. The re-watering will occur naturally, but for the areas near-Waihi, may be accelerated by the addition of water pumped from the Ohinemuri River. Re-watering will result in the groundwater levels rising back close to their original levels. Rebound will occur as rising groundwater re-saturates the ground, resulting in a reduction in effective stress within the ground. As already noted, historical underground mining at Martha Open Pit involved dewatering of the ground to about RL540m; approximately 40 m above the targeted RL500m for the Martha Underground. Following the cessation of mining in 1952, groundwater levels rose back to near original levels before modern mining started in 1989. No damage arising from historical re-watering has been recorded. Rebound of the ground to its original level because of rising groundwater levels is unlikely. It is expected that rebound could be in the order of 30 % to 90 % of the settlement that has occurred during modern mining depending on the area of town, and the level of previous dewatering. No damage from rebound is expected as, like settlement, it occurs relatively evenly across the same general areas. However, rebound must be monitored to confirm that there are no unforeseen effects. It is recommended this is continued until groundwater levels have stabilised, any rebound can be demonstrated to be 90 % complete, and there are no known issues unresolved.
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