GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 130 6.2.2 Mataura Stream and wetland Based on the conceptual model (Figure 6.5), the Mataura Stream is considered to form the primary receiving environmental receptor of potential WRS discharges. The stream catchment has a total area of 6.6 km2 with 85% of this being upstream of the proposed portal infrastructure. The catchment can be described as steep and rugged with the majority of its coverage being native vegetation. The Mataura Stream has a number of small tributaries and flows southeast to join the Ohinemuri River. Baseline flow data for the Mataura Stream has been collected during 2020 and 2021. The water levels have been monitored using a pressure transducer at location M1 (Figure 1.2) and converted to flow rates using a combination of methods (modelled and converted using a site-specific rating curve; GHD, 2022a). Based on the flow calculated from the rating curves, the over this period is estimated to range from 1,000 – 100,000 m3/day and median flow in the order of 10,000 m3/day. Flow in the WRS gully tributary (named as Tributary 2 on OGNZL plan in Figure 6.4) has been measured between 1% and 2% of the total Mataura Stream flow during high rainfall season (Table 6.1). Although the Tributary 2 flow was not measured during the March 2021 period, the WSP (2021a) report notes that tributary does not dry out in summer. Although the WRS gully baseflows are expected to be very low, this has not yet been quantified. Table 6.1 Flow gauging measurements (WSP 2021a and 2021b) Location 10-Jul-20 30-Jul-20 17-Aug-20 3-Sep-20 16-Mar-21 21-Sep-21 Mataura Stream (M1) 355 L/s 242 L/s 187 L/s 194 L/s 54 L/s 343 L/s 30,672 m3/day 20,909 m3/day 16,157 m3/day 16,762 m3/day 4,743 m3/day 29,635 m3/day Tributary 2 3 L/s 3.3 L/s 3.6 L/s 4.1 L/s Not measured 4 L/s 259 m3/day 285 m3/day 311 m3/day 354 m3/day - 346 m3/day Trib. 2 % of M1 1% 1% 2% 2% 1% Water quality data has been collected periodically from sampling location Matuara Stream 1 since 2006. The additional sites shown in Figure 6.4 have a smaller data set available from 2020. The water quality from these sites is summarised in the Water Management Report (GHD, 2022a). 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 Mataura Stream. Figure 6.4 Mataura Stream water quality sampling locations
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