EGL Ref: 9215 23 June 2022 Page 41 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. • Performance monitoring and mitigation • Maintenance 6.7. Rehabilitation 6.7.1. Rehabilitation of slopes Land use at closure is affected by the rehabilitation strategy proposed. At the time of original consenting the restoration of the previous pastureland was considered to be best option. Consequently, the rehabilitated surface of the TSFs has been pasture dominated. A rehabilitation strategy that enhances indigenous biodiversity by planting and vegetating more areas in native species is now considered a better approach. A rehabilitation strategy that enhances biodiversity would include greater areas of indigenous vegetation and with a wide variety of species including planting of noble trees and ferns and development of different habitat for a wide variety of invertebrates and native skinks. This could include areas of rock outcrops and placement of wood (felled trees or slabs cut from felled trees). Small areas of Storage 2 are vegetated, but species are limited to shallow rooting plants to avoid any adverse impact on the integrity of the outer sealing layer (Zone G) which functions to limit ingress of oxygen and water to prevent oxidation and generation of acid leachate in the rock forming the embankments. Placement of vegetation on embankments is normally not considered acceptable for water storage dams as it hinders visual surveillance. Grassed surfaces are preferred so that seepage, instability, or erosion are easily detected. However, tailings embankment dams are different from water storage dams where the risks of seepage, instability and internal erosion are much greater. This is because over time the retained contents (i.e., tailings) consolidate to have the properties of a soil with strength that varies from soft at shallow depth to stiff at depth and with low permeability. We consider that slopes of a TSF formed using a dam should be initially in pasture. After a period where monitoring indicates the slopes are in a stable condition vegetation could be planted and areas established to increase biodiversity by placement of large rocks and wood in strategic areas. Monitoring of the existing TSFs indicate that tailings are near fully consolidated after approximately 5 years following deposition and that deformations of the embankments reduce significantly after approximately 5 years. Consequently, subject to appropriate review, we consider that the opportunity exists for planting of vegetation and development of different habitats after approximately 5 years following completion of construction of the embankments forming the TSFs. 6.7.2. Tailings pond surface Options for closure of the surface of TSFs include dry capping, inundation with water or a combination. The current closure concepts comprise a dry cover against the embankment crest with a wetland zone and a small pond of water. A spillway is proposed to control long-term water levels in the pond. This option provides opportunity for biodiversity, will maintain the tailings in
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