GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 141 baseflow measurements). This is less than the uncertainty of flow gauging devices (2.7%) reported by WSP (2021a and 2021b) and therefore not expected to be a measurable loss. – Wetland: Further discussion of the Mataura Wetland is provided in the GHD (2022e) Mataura Wetland Assessment Technical Memorandum, which should be read in conjunction with this report. 6.5.2 Water quality Influence on groundwater quality Given the large amount of dilution available in the Waihi Basin aquifer, and only 5-10 m3/day of WRS seepage is predicted to discharge to ground, it is not expected that any groundwater users will be impacted. This is due to significant distance to the nearest bore user (1.5 km away, with a deep bore to 200 m), and dilution and attenuation processes. No further assessment is provided. Influence on surface water quality – Mataura Stream The predicted groundwater concentrations are presented in Table 6.4. As noted in Section 6.2, the Mataura Stream is considered to form the primary environmental receptor for WRS seepage to ground. The duration of the WRS operation is expected to be short relative to travel times for contaminants in groundwater to receiving surface water. As such, the steady-state analysis approach is considered to be conservative. The predicted water quality values presented in Table 6.4 are for the WRS at maximum construction height, before it is removed and the catchment remediated. The values presented are based only on mixing and dilution of the WRS seepage. No groundwater dilution or attenuation is allowed for. Seepage is very low relative to flow in the Mataura Stream. The seepage would comprise, at most, 1% of the Mataura stream low flow condition, and 0.1% of the median condition. The dilution in the stream is significant, so that increases predicted (as denoted by “” in Table 6.4) for trace elements are expected to be so minor they are unlikely to be detected within laboratory analytical margins of error. This is particularly the case where further attenuation by geochemical processes is considered. Increases in major ions (such as sulphate) may however, be measurable. This conclusion is consistent with observations in the Ohinemuri catchment near the WTP and existing mine elements. RWQC are not expected to be exceeded. The mass flux leaving the WRS and expected to directly discharge to the Mataura Stream, and ultimately enter the Ohinemuri River, is provided in Table 6.5. These values are presented for inclusion in the Water Management Report for the assessment of cumulative effects to the Ohinemuri River catchment and are not discussed further in this report. As noted in the above assessments, the predicted concentrations of mercury are greater than the RWQC as the laboratory detection limit of this parameter is greater than the criterion. Mercury has not been reported in significant persistent concentrations by OGNZL (2020). The effect of changes to surface water quality to the Mataura Stream resulting directly from WRS groundwater discharges is therefore considered to be limited. The resulting effect to the nearest authorised surface water user (HDC water take AUTH130392.03.01), 5 km downstream on the Ohinemuri River, is similarly negligible, due to further dilution within the river.
RkJQdWJsaXNoZXIy MjE2NDg3