June 2022 G-01483.84-017-R-Rev0 Because the 29 7.0 WATER MANAGEMENT 7.1 Approach Overview WRS water management elements include a network of diversion and stormwater collection drains, culverts and collection ponds. Drainage elements are incorporated within the WRS design to direct contaminated water runoff and seepage flows for ultimate pumped transfer to the Waihi water treatment plant. The water management elements pertaining to the WRS development include: ▪ To reduce contaminated flows, clean water diversions: ▪ around the WRS footprint, split into a north and south catchment ▪ above the portal and access roads ▪ To collect contaminated surface runoff and seepage flows: ▪ WRS under drainage ▪ WRS toe perimeter drain ▪ WRS graded benches and contour drains ▪ WRS haulroad, WUG Portal access road and lower access road side drains ▪ Willows Collection Pond (WCP) and WRS Toe Seepage Pond (TSP) Based on current investigations the in situ highly weathered volcanic rock should be suitable to contain the tunnel spoil material unlined (GHD 2022). Testing of the subgrade soils during detailed design will be required to confirm that the in-situ materials meet the permeability requirements. A rockfill underdrain, formed in the main gully, connects to a seepage containment pond at the toe of the WRS. The rockfill drain will intercept seasonal seepage from the underlying volcanic rock as well as any water percolating down from infiltrated rainfall. Zones of high permeability, crushed, non-acid generating rock that has been selectively won from either the tunnel spoil or imported specification material will be used. The drains have not yet been sized and this will be completed during detailed design when the available gradation of rockfill has been assessed. From the WRS TSP water can gravity flow through a pipeline along the access road and collected, along with other contaminated surface runoff, in the WCP. It is currently assumed that all foundation seepage into the WRS will be directed to the water treatment plant via the collection ponds. Consideration has been given to reducing the volume in foundation seepage by incorporating depressurization wells below the WRS. This could be achieved by drilling low angle boreholes from below the toe elevation of the WRS to intercept the water bearing units within the volcanic rock beneath the WRS. Clean water flows from the drains could be directed into the clean water surface drains. However, these flows would need to be monitored to ensure that acid seepage is not inadvertently captured. A risk assessment should be undertaken during detailed design to assess whether depressurization wells should be incorporated in the final WRS design.
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