GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 105 5.3.5 Surface water-groundwater interactions Groundwater flow direction in both the shallow and deeper aquifers is inferred to be generally to the southwest, along the paleo-gully, toward and along the Ruahorehore Stream (refer to inset on Figure 5.8) and ultimately to the Ohinemuri River. The site has shallow surface water flow paths and farm drains that extend from north to south across the TSF3 area. These drains/channels are within the shallow alluvium/ash surface material, and receive small rates of spring flow from the rhyolite hill areas (refer to spring locations in Figure 5.2), but predominantly convey run-off and receive some shallow groundwater as discussed in Section 5.3.2. Baseflow gauging (Appendix F) results at the proposed TSF3 site indicate that sections along the Ruahorehore Stream vary between losing and gaining conditions, and are predominantly neutral to losing (Figure 5.9). This supports the above inferences that the groundwater catchment is small and is separated from deeper artesian aquifer at this location. Hydro Logic New Zealand Ltd (Hydro Logic) data from 2014 to 2015 indicates strong gaining conditions upstream of RU15, with localised spring-fed conditions occurring at Shed Spring, approximately 1.5 km downstream of TSF3. Conditions return to neutral conditions downstream of Shed Spring (Figure 5.9), suggesting limited groundwater contribution along this stretch of the stream. The Ruddock gauge (Figure 2.1) has measured daily flow on the Ruahorehore Stream since 2000. This gauge is located approximately 850 m downstream of Shed Spring (Figure 2.1). The catchment at this location is approximately 20 km2, with flow ranging from 1,850 to 1,500,000 m3/day, and a median flow of 27,900 m3/day. The total flow at this location is summed from: - Ruahorehore Stream: The catchment size at TSF3 toe is 5.8 km2 (Figure 5.1). Flows gauged during this assessment to typically vary between 540 m3/day and 3,600 m3/day (Appendix F), with a median of 1,340 m3/day. The lower end of the flow range is considered to represent baseflow and is in the order of 1 L/s/km2 (supporting a limited groundwater catchment). In terms of water quality, trace elements within the stream are not notably different from the Ohinemuri River adjacent to the other mine components. However, major ion concentrations (such as calcium, chloride, magnesium, sodium and sulphate) are considerably lower in the Ruahorehore Stream. - Ruahorehore Stream - southern tributary: Approximately 2.8 km2 in size, this catchment is located south of the site as shown on Figure 2.1. Base flow has not been measured but downstream measurements by Hydro Logic suggests that this tributary does not provide meaningful contribution to the Ruahorehore Stream flow (Figure 5.9). - Shed Spring: Located 1.5 km downstream of the TS3 footprint, has flow ranging from 90 to 4,950 m3/day, with a median flow of 430 m3/day (OGNZL 2015 and 2017 daily gauging data). This spring is interpreted from water quality data (Appendix E) and the geological setting to discharge from a fracture system in the rhyolite rock that extends beneath TSF1A (inferred to extend in the general direction of faults to the north/northeast, Figure 2.4). The western rhyolite ridge bounding TSF3 extends beneath TSF1A as shown in Figure 5.10. This ridge is interpreted to form a topographic and structural geological divide between the spring and groundwater of the TSF3 site. The Shed Spring source is therefore interpreted to be within a separate groundwater catchment to the TSF3 site. - Waione Stream: Based on the information above compared against Ruddocks Gauge, it is estimated that the Waione Stream catchment size is less than 12 km2 and the flow contributed during low flow periods is less than 1,300 m3/day. This equates to 1 L/s/km2 from the Waione catchment to the measured Ruddocks flow. This, again, supports limited groundwater contribution to baseflow along this section of the stream. The following conclusions are made regarding groundwater discharge from the TSF3 site, based on this CGM: – The Ruahorehore Stream has only a small groundwater catchment and predominantly provides conveyance of run-off during rain events. – The deeper groundwater in weathered /reworked volcanics is inferred to flow through the subsurface, influenced by paleo-gullies and deep geological structures. The actual discharge location of deeper groundwater is unknown but is expected to be distant, diffuse and difficult to pinpoint. For the purposes of this assessment, all deeper groundwater is inferred to discharge into the Ohinemuri River.
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