GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 79 . A separate tributary TB1E (Figure 4.1), described by OGNZL as a small field drain, has remained dry during all flow gauging events (Appendix F). 4.3.4 Groundwater and surface water quality A summary of the groundwater and surface water chemistry data recorded in the locations presented in Figure 4.1 between 2017 – 2019 and representative piper plots are presented in Appendix E. Groundwater quality The chemistry of the groundwater differs to a small degree between the shallow and deeper monitoring wells, with the latter generally indicating a greater proportion of calcium and bicarbonate and a lower proportion of chloride and sulphate. Most monitoring wells at the NRS site typically record low trace element concentrations, however, elevated concentrations of trace elements recorded in WRS02 (mercury, manganese, iron) and DH05a (zinc, manganese, iron, lead) demonstrate a degree of influence of mineralisation and anaerobic conditions. Influence of existing storage facilities The existing northern storage stockpile located in the south of the proposed NRS site is part of TSF2, which commenced construction in 1987 (Figure 4.7). It is an active stockpile, which was constructed to only receive NAF rock, however it understood that at least some PAF was also stored at this location. The northern stockpile silt pond (NSPSP) is positioned to collect run off from the stockpile and discharge directly to tributary TB1 when the water quality is suitable. Leachate drains are installed beneath the existing northern stockpile to capture leachate generated, and sub-soil and toe drains are installed to intercept potentially impacted groundwater. The toe drains installed at the northern boundary of the northern stockpile demonstrate a rock storage influence with Ca-Mg-SO4 type water indicated by piper diagrams (OGNZL, 2016). GHD has undertaken a high-level review of available toe drain water quality data (OGNZL, 2016). A summary of the rock storage influence recorded in the toe drains is presented below: – Toe drain location T13: This location is installed south of GHD sample location TB1e (Figure 4.1). This drain demonstrates the greatest rock storage influence with the elevated concentrations of sulphate and dissolved iron, and elevated electrical conductivity. This is likely to be either due to T13 capturing a higher proportion of leachate compared to groundwater, or due to toe drain T13 receiving discharge from a larger rock thickness and catchment area than contributes flow to drains T12 and T14. Peak sulphate (3,170 mg/L) and dissolved iron (23 mg/L) concentrations coincided with the minimum recorded pH (3.3) between 1996 and 1999. The concentrations have continued to decline and pH increased through to 2020. This suggests a timeframe of 9 years from construction for the peak PAF rock influence to be realised at this location. – Toe drain locations T12 and T14: Both locations indicate fluctuating trends over the operation of the existing rock storage, with pH (4.3 – 6.8), electrical conductivity (15 – 210 mS/m), sulphate (40 – 3,400 mg/L) and dissolved iron (0.01 – 16 mg/L) confirming collection of rock storage leachate. – Groundwater: Monitoring of wells downgradient of the northern stockpile (OGNZL wells MW12S/D and MW2C13S/D (OGNZL,2020)) indicate very little impact of rock storage leachate to groundwater quality. Oneoff elevated concentrations of electrical conductivity, sulphate and iron have been recorded, however there are no trends that indicate any ongoing influence from tailings or rock storage. The water quality results from WRS02/WRS02a and WRS08/DH08 indicate a rock storage influence, as demonstrated by sulphate (up to 155 mg/L) and dissolved iron (up to 2.6 mg/L), as a result of being situated on the periphery of the existing northern storage area (Figure 4.1). Elevated concentrations of iron and a number of other parameters, also infer a rock storage influence at DH05a, which is installed in the alluvium at the base of the paleo-channel (an inferred preferential groundwater flow pathway) down-hydraulic gradient of the northern storage area. The absence of sulphate in DH05a is likely attributed to a more anaerobic condition within the groundwater recharge zone for the paleo-channel in the centre of the northern storage area. Infiltration of rock storage seepage is likely to be occurring into the up-hydraulic gradient section of the paleo-channel, which is aligned with inferred historical location of Ohinemuri tributary TB1. These results indicate that although complete containment of leachate impacted groundwater is not being achieved, the leachate and drainage systems are reducing the flux of
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