104 18.0 Northern Rock Stack Freshwater Impacts 18.1 Proposed Activities The Northern Rock Stack is planned to contain up to 7 Million cubic metres of material and is constructed to a nominal height of RL 173 m. Also proposed are topsoil stockpiles, two of which are constructed at the northern extent of OGNZL owned land and adjacent to Golden Valley Road. The proposed layout is shown in Figure 26. The diversion drains include a clean water drain which reports directly to the Ohinemuri River and collects clean run-off and stream flows from upstream of the NRS, and a dirty water drain which collects run-off from the NRS and reports to a collection pond. The drains are separated by a 6 m wide perimeter road. The NRS and diversion drains do not encroach into the T13UP166 Significant Natural Area (SNA) scheduled in the District Plan. The NRS would incorporate similar design features as the existing TSFs to restrict the potential for generation of acid leachate and for leachate to enter surface water or groundwater. These features include a low permeability liner beneath the NRS, subsurface seepage drains to intercept any seepage, leachate collection drains, and capping to minimise oxygen ingress. 18.2 Effects on Freshwater Ecological Values 18.2.1 Stream Diversion An indicative clean water diversion / stream diversion and NRS collection drain will be created as part of the development of the northern rock stack. The diversion drains include a clean water drain which reports directly to the Ohinemuri River and collects clean run-off and stream flows upstream of the NRS, and a dirty water drain which collects run-off from the NRS and reports to a collection pond. The drains are separated by a 6 m wide perimeter road. The NRS and diversion drains do not encroach into the T13UP166 Significant Natural Area (SNA) scheduled in the District Plan. 18.2.2 Groundwater The NRS will incorporate similar design features as the existing TSFs to restrict the potential for generation of acid leachate and for leachate to enter shallow groundwater. These features include a low permeability earth liner beneath the NRS, subsurface seepage drains to intercept any seepage, leachate collection drains above the liner, and capping to minimise oxygen ingress. A perimeter drain directs surface runoff to a collection pond and a seepage and leachate collection system will comprise manholes, pumps and a pipeline to the existing WTP.
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