GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) 27 Capping (with NAF material) and rehabilitation. Willows Access Tunnel and WUG Mine Inflow Water Quality The data presented in Table 4 illustrates the considered conservative estimates required to assess both the water treatment requirements and the impact on the receiving environment and has been sourced from the operational water quality monitoring. The Willows Access Tunnel will likely intersect areas of lower mineralisation and portions of the Willows Access Tunnel are expected to be lined with grout / shotcrete to limit groundwater inflow in high inflow zones (i.e., fracture zones). This will likely introduce alkalinity into the discharge water and will, to some degree, provide some buffering capacity to AMD affected waters. This has not been considered within this assessment, however, it is noted that the mine water from the MUG also includes similar influences. Underground dewatering water quality data for Waihi mine operations is assumed to be representative of potential water quality of groundwater inflow to the Willows Access Tunnel due to the similarity in geology and host rock between the sites. This assumption may not hold for the WUG Mine portion; however, it is considered that utilising the 95%ile of the Waihi dewatering data provides sufficient conservatism to account for geochemical differences observed. The relative volume of water from the WUG Mine compared to the Willows Access Tunnel is also small which adds in additional conservatism to the estimates provided. This is considered appropriate given that the Waihi dewatering water is exposed to significant areas of underground workings and backfill areas, resulting in elevated trace element concentrations in water, whereas the oxidation profile of the Willows Access Tunnel walls is expected to be small in comparison. Field Column Derived Inflow Water Quality WUG field column data (Refer Appendix C) has been used to validate the assumptions made for deriving WUG Mine inflow water quality. Antimony, arsenic, selenium, and sulphur have been found to be elevated in the WUG spoil relative to the Martha dataset (Table 3) therefore these elements have been the focus of the field column leachate data. Antimony and selenium were found to not be significantly elevated in the field leachate data (relative to the high sulphate oxidation rates observed), however significantly elevated sulphate and arsenic leachate concentrations were observed (Appendix C). Based on these observations, the field column results have been utilised to predict WUG Mine inflow water quality for arsenic and sulphate to compare to the Waihi Dewatering derived water quality (Table 4). This ensures that the infiltrating groundwater used within the water balance model (GHD, 2022) is appropriate. To reflect the differences in rock mineralogy, the findings of WUG spoil field column leachate data has been used as the basis for predicting sulphate and arsenic concentrations of WUG inflow (Refer Table 4) using the following methodology: 1. Field column sulphate oxidation rates (mg SO4/kg/day) have been calculated for the acidity producing columns (defined as when leachate pH <3); 2. The total exposed area within the WUG tunnel walls has been estimated based on oxidation profile, roughness; 3. The calculcated sulphate oxidation has been factored to account for differences in the mean spoil NAPP and the field spoil NAPP; 4. The total volume of daily oxidation products has been calculated based 1 and 2; 5. The sulphate concentration is calculated based on the daily total expected WUG tunnel inflow water (GWS, 2022) and the daily total oxidation products; 6. The column leachate arsenic / sulphate relationship was determined 7. Based on 5 and 6, the derived WUG tunnel inflow arsenic concentration was calculated. . The methodology as outlined is considered conservative as it assumes all oxidation products are mobilised daily and that the tunnel walls are represented by WUG mine spoil along its entire length. The column leachate sulphate and arsenic relationship is provided in (Figure 20).
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