B-2 – Area 2 - Assessment of Environmental Effects 32 The WRS will have a total storage capacity of up to 900,000 m³ of rock, to a maximum height of 20 m above natural ground level. Lime will be used to neutralise potentially acid forming contact waters. It may also be used to stockpile ore. The design of the WRS incorporates clean water diversion drains to separate the balance of the catchment water from the WRS contact water. Contact water will be directed to the collection pond (refer Section 3.2.11). The clean water diversion drains will discharge directly to the lower reach of Tributary 2. Captured flows from the WRS underdrains will be collected in a toe seepage pond located directly below the WRS in a narrow gully approximately 200 m upstream from the confluence with Mataura Stream (WRS Toe Seepage Pond). The WRS Toe Seepage Pond will have a maximum stored volume of 500 m³ and will accept a mix of groundwater flow and surface infiltration from the rock stack. As noted in Section 3.7 below, the WRS will be removed at the completion of mining, rehabilitated with topsoil and planted to return the area to its original landform. 3.2.11 Collection Pond A collection pond will be established to capture contact water from the WRS and mine access road. Up to 19,000 m3 in size, the pond will include a pumping station and will be designed to accommodate a 10 year, 24 hour storm event. The pond will be run nearempty in steady state conditions subject to making appropriate provision for fire fighting. Overflows during storm events will be discharged directly to the Mataura Stream. The pond and any associated perimeter drains with a potential to contain potentially acid forming (“PAF”) runoff will be designed to incorporate an appropriate liner. Contact water collected in the collection pond will be pumped back to the WTP for treatment via the Services Trench (Area 4). A forebay and causeway into the main pond will be features of the pond that will aid in maintenance, as is used in the existing collection ponds. The collection pond will be regularly serviced to remove sediment.
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