GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 43 . in GLD02 and GLD03, typically demonstrates aerobic conditions, with the presence of nitrate, low trace element concentrations and low dissolved solids content. Moving deeper through the shallow profile and closer to the mineralised breccia in the south and eastern extents of the proposed GOP, groundwater demonstrates elevated sulphate and trace element concentrations, characteristic of sulphide mineral oxidation. This is most evident in GLD02a, and to a lesser extent GLD01a, which demonstrate elevated arsenic, antimony, nickel, sulphate, and zinc relative to shallower monitoring wells. Towards the Ohinemuri River, distant from the breccia, shallow groundwater remains relatively low in dissolved solids and indicators of mineralisation influence, as represented by water quality results from GLD04s. Deep groundwater within the andesite to the north and southwest of the proposed GOP, as represented by monitoring wells P79d and GLD04d, respectively, typically reports higher iron, manganese and sulphate concentrations than the shallow groundwater system, but notably less influence of mineralisation than in monitoring wells interpreted to be influenced by breccia. 3.3.5 Surface water-groundwater interactions Tributaries of the Ohinemuri River The results of flow gauging of the Ohinemuri River and its tributaries, including the intermittent watercourses present south and north of the proposed GOP (TB4 and TB5), are presented in Appendix F. The monitoring locations are presented in Figure 3.1. The intermittent watercourses south of Gladstone Hill that flow towards the Gladstone Wetland are typically recorded as dry during summer, with the downgradient tributary at monitoring location TB4 found to have very shallow ponded and stagnant water during these summer conditions (21 March 2019; Opus, 2019). However, following rainfall and during periods of higher groundwater levels, the headwaters of the TB4 tributary is inferred to receive groundwater discharge. A culvert passing beneath the road at this location is expected to provide water level and discharge control on the wetland. The following observations recorded during a site visit by GHD in June 2017 suggest that the watercourses become losing downstream of the groundwater discharge, before gaining again prior to the confluence with the Ohinemuri River: – Flowing water (approximately 50 to 100 mL/s) in the upper headwaters. – Boggy conditions with no flowing water 200 m downstream of the headwaters, and downward vertical hydraulic gradient measured in the wells at GLD03. – Flow upstream of the Ohinemuri confluence measured at approximately 5 L/s. – Changing water chemistry as the springs flow to the wetland (Appendix E). Further discussion of the Gladstone Wetland is provided in the GHD (2022d) Gladstone Wetland Groundwater Assessment Summary Technical Memorandum, which should be read in conjunction with this report. The Ohinemuri tributary TB5 does not appear to receive groundwater baseflow, and only flows after significant rainfall events. With the exception of the September 2019 gauging event, it has been recorded as dry during all flow gauging events in 2019 and 2020 (Appendix F). A rainfall measurement of 132 mm was recorded over the two weeks prior to the September 2019 flow gauging event, suggesting that the flow in the TB5 tributary at this time was derived from stormwater runoff and interflow. Ohinemuri River Flow in the Ohinemuri River during baseflow conditions ranges between 0.2 and 0.5 m3/sec between the Martha Process area and Gladstone Hill (Appendix F). Flow gauging recorded losing conditions between monitoring locations OH11 and OH6 adjacent to the proposed Gladstone Pit in June 2019 and January 2020 (Appendix F). As discussed in Section 3.3.3, upwards vertical hydraulic gradients are recorded between the deep and shallow groundwater systems adjacent to the river (GLD04 well series). This provides further evidence of the separation of the shallow and deep groundwater systems in the vicinity of the Ohinemuri River.
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