GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 97 5.1.3 Potential influences on groundwater and surface water TSF3 will be positioned within the Ruahorehore Stream catchment (Figure 5.1). The removal of unsuitable materials from the TSF3 footprint will require stream and spring diversion and construction dewatering, whilst the TSF design requires sub-surface drainage to control groundwater pressure. The assessment of potential effects to groundwater and surface water associated with TSF3 has focused on the following: – Construction dewatering required for the excavation of sensitive soils and the potential dewatering effects on the Ruahorehore Stream. – The potential leachate and contaminant discharges from TSF3 and whether engineered underdrainage is likely to mitigate adverse impacts to groundwater and surface water. – The potential influence on groundwater and the Ruahorehore Stream in the event that the proposed underdrainage system does not provide effective interception of contaminants, e.g.. drainage failure. 5.2 Local geology The regional geology is presented in Section 2.3. The local geology for the TSF3 area is presented in Figure 5.8 and Table 5.2 below, which includes the local terminology applied to this TSF3 assessment. Material descriptions not previously used prior to 2018 include the sensitive/redeposited tuff and froth flow within the primary rhyolite unit. These terms have been introduced by EGL in the Geotechnical Factual Report (GFR; EGL, 2018), which provides a detailed description of the local geology and origins (including from GNS (1996)). Cross-sections of the geology beneath the proposed TSF3 are provided in Figure 5.5 (E-W) and Figure 5.7 (NESW). This site is located within a surficial alluvial gully (defined as ‘the paleo-gully’) dipping to the southwest, and surrounded by rhyolite ridges of the Ruahorehore Rhyolite (unit mnr). Both the paleo-gully and rhyolite ridges form significant features at the proposed TSF3 site. Key geological features are summarised below: – Whilst Coromandel Group dacite (unit iu) and regional andesite rock (aw) underlie the site at depth, these units are separated from the Waihi East ore bodies (including Gladstone) by the Golden Valley Fault. – The Golden Valley Fault, which separates TSF3 from TSF1A and TSF2, provided a conduit for the Ruahorehore Rhyolite (a dome of the Homunga Rhyolite), which intruded through the andesite and dacite. The original rhyolite dome is deeply eroded beneath the proposed TSF3 site, and the shallow geology demonstrates an extensive weathering profile (Figure 5.6). The dome remnants form the topographic highs that bound the paleo-gully and will form the northern, eastern and western bounds of TSF3 (Figure 2.4). The rock below the site has been confirmed from sterilisation drilling to have no mineralised veins. – The TSF3 paleo-gully appears to be infilled with remnants of a complex mixture of rhyolite tuffs, pyroclastic flows and gaseous lava flows (Figure 5.7; refer to Figures 12 and 13 of EGL, 2018 for further detail). The surface geology comprises alluvium/colluvium, and/or volcanic ash up to a few metres thick. These materials are unsuitable for use in the TSF3 foundation and are required to be excavated and backfilled, including the redeposited tuff to a depth of 20 m. Figure 5.5 TSF3 geological section (C-C’ of Figure 2.4) (from OGNZL, 2017)
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