B-2 – Area 2 - Assessment of Environmental Effects 66 They require that all sediment laden run-off from an area containing soil disturbance activities is treated by sediment retention structures prior to discharge. They also require that the sediment retention devices must be designed, operated and maintained to achieve a - Discharge turbidity of no greater than 110 NTU; and - Discharge pH of not less than 6.0 and not greater than 9.0 pH units. The sediment retention devices will only discharge to surface water during high rainfall events when the receiving water body contains high flows and elevated suspended sediment levels, and during those events a discharge of this quality will suitably protect instream values. 5.10.3 Site Operation Natural and clean water will be diverted away from areas disturbed by mining activities in order to reduce the volumes of water entering disturbed areas. With respect to the disturbed areas, for the Willows WRS and haul road which may contain PAF rock, surface runoff and seepage will be collected and diverted to the Main Collection Pond before being pumped back to the WTP at Waihi for appropriate treatment. The Main Collection Pond is designed to withstand flood events such that except during extreme rainfall events it will not discharge to the Mataura Stream. This aligns with the existing practice at OGNZLs existing Waihi operations. The technical assessments describe why this an acceptable solution due to the high ambient flows which are present the receiving water bodies during these events. Runoff from other disturbed areas (i.e. those which will not contain PAF rock) will be collected and managed in accordance with Waikato stormwater management guideline (TR2020/07, updated May 2020). This will include diversion of stormwater from within the site to a detention pond for treatment prior to discharge during high flow events. Some seepage from the WRS may over time enter Mataura Stream, however, it will have a very minor effect on water quality, and will not cause an exceedance of the ambient water quality standards which are included in the proposed consent conditions to protect aquatic ecology. The flow from Tributary 2 will be captured as part of the WRS drainage system, which is temporarily expected to reduce the flow of the Mataura Stream a minor amount (up to 2%). Such reductions in stream flow are considered unlikely to be discernible from the background variability in flow. On remediation of the site, it is expected that the natural flow pathways will be restored. The effects on the surrounding groundwater and surface water network are therefore anticipated to be minimal.
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