GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 128 Figure 6.1 Willows Rock Stack and Holding Pond (in foreground) (Mitchell Daysh, 2022) 6.1.3 Potential influences on groundwater and surface water This assessment of effects has focused on the potential for the proposed WRS to influence: – Groundwater levels and baseflow to the Mataura Stream. – Long-term groundwater and river water quality changes resulting from leachate seepage from the rock stack. Potential effects resulting from other components of WUG have been assessed by GWS (2022a; 2022b) and are not included in this assessment. 6.2 Conceptual groundwater model The CGM for the WRS and WUG site is sourced from GWS (2022a). A summary of the key conceptual understanding is provided below. 6.2.1 Local geology The regional geology is presented in Section 2.3. The local geology for the WRS gully is presented in Figure 6.2. A detailed description of the geology between the Wharekirauponga portal and mine is provided in the WKP Conceptual Geological Model Data Report (GHD, 2020). Soil distribution over the site is also described in the GWS (2022a) report. Whiritoa Andesite lies beneath the proposed WRS site, which is comprised of highly weathered regolith soils at the surface, some weathered tuff, and slightly to completely weathered andesite at depth. The residual cover (including tuff) is present at varying thicknesses in the WRS gully. In the centre of the WRS gully, there is no residual soil cover, with the andesite rock exposed at the base of Tributary 2 (Figure 6.3). Residual soils were encountered at thicknesses ranging from 2.5 m to 14.5 m in test pits and boreholes beneath the proposed WRS (refer to Golder/WSP 2022a for logs). Laboratory particle size distribution (PSD) testing indicates that the residual soils predominantly comprise sandy clayey silts with high plasticity. Further down towards the Mataura Stream, downgradient of the WRS, where topography is relatively gentle, surface alluvium terrace deposits are present at up to 7 m depth.
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