SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 16 Figure 11 Mataura Stream Catchment Area and Willows Farm 4.1.3 Soils and Geology The majority of the site soils are indicated to be primarily residual soils as shown on Figure 12, with a weathered regolith overlying volcanic rock. Given indicated surface slopes, down slope movement would be expected to maintain reduced soil cover on the steeper slopes with an increased thickness of the soil profile on the lower slopes. On the flatter parts of the site near the Mataura Stream terrace deposits of alluvial material are measured to a depth of 7 m, with two levels of terraces apparent. Figure 13 shows the distribution of the soil types at the site. The primary soil mapped at the portal and infrastructure sites is Otorahanga orthic allophanic loam (well drained, moderate permeability), while at the proposed vent shaft site, Figure 8 shows Moehau 2 acidic orthic brown loam soils (well drained, moderate permeability). The geology of the site is included in the ground model prepared by GHD (August, 2020) and this has been complemented by an investigation program that has included test pits, boreholes and geotechnical testing. The data from the investigations relevant to this assessment are included in Attachment C. In general terms, the site is noted to consist of a depth of primary weathered rock and/or pyroclastic deposits that are weathered to form clay and silt soils. These materials are a few metres thick on the steeper slopes (Figure 8) and thicken in the topographic lows to some 7 to 15 m thick. Beneath these soils either lies relatively fresh andesite rock in the northern part of the site (Waipupu Andesite) or completely weathered tuff (Whiritoa Andesite). In the low-lying areas adjacent to the Mataura Stream alluvial terrace deposits exist consisting of silty gravel sands. These materials directly overly the completely weathered tuff.
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