B-7 – Area 7 - Assessment of Environmental Effects 133 Table 6: Consideration of Alternative Methods of Discharge. Discharge Receiving Environment Alternative Methods of Discharge Discharge of water to land within the sediment retention ponds and to surface water via spillways. Refer to Sections 2.13, 2.14 and 2.16 The proposed use of sediment retention ponds is considered best practice construction methodology to reduce the potential for sediment runoff to land and into waterbodies. Sediment retention ponds are designed to allow spill into adjacent waterbodies when flows are sufficiently elevated to provide effective dilution of contaminants. Consideration was given to designing larger capacity sediment retention and collection ponds. The current size was considered appropriate as it can accommodate a 1% AEP and one in ten year, 72 hour rain event respectively. During events of this magnitude, overflow from the sediment retention and collection ponds can spill flows into the adjacent waterbodies with limited adverse environmental effect due to the higher flows and mixing capacity of the waterbodies. Larger ponds would therefore provide limited additional environmental benefit. To discharge water to surface water from Collection Ponds (S6 and S7) via an emergency spillway. Collection Ponds are commonly used to capture discharges from perimeter drains surrounding mining sites. This technique is currently used by OGNZL at its existing mining facilities at Waihi. Collection ponds are both designed to allow spill into adjacent waterbodies when flows are sufficiently elevated to provide effective dilution of contaminants. Consideration was given to designing larger capacity collection ponds. The current size was considered appropriate as it can accommodate a one in ten year, 24 hour rain event. During events of this magnitude, overflow from the collection ponds can spill flows into the adjacent waterbodies with limited adverse environmental effect due to the higher flows and mixing capacity of the waterbodies. Larger
RkJQdWJsaXNoZXIy MjE2NDg3