GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 25 . Table 2.3 Existing consent compliance receiving water quality criteria (WRC, 1999) 2.7 Proposed construction materials Throughout this document, “NAF” (non-acid forming) and “PAF” (potentially acid-forming) materials are discussed. These materials will be used for both backfill of the GOP and construction of tailings embankments. An overview of the geotechnical, structural and geochemical properties of these materials are provided in the EGL geotechnical reports (EGL 2021a and 2021b). Detailed geochemical information of these materials is provided in the AECOM (2021a) report. In summary, PAF material, if exposed to oxygen for a period of time, can oxidise and generate low pH runoff. This can result in the release of heavy metals and poor discharge quality. PAF materials used in construction /backfill are encapsulated in low permeability NAF mine overburden material in specific zones to restrict both oxygen and water entry. AECOM has recommended that high mercury NAF is not used in NAF zones which perform a liner function or are exposed to the surface. Parameter Receiving Water Concentration (2) (g/m3 unless otherwise stated) Hardness 20 g/m3 CaCO3 Hardness 100 g/m3 CaCO3 pH 6.5 to 9.0 6.5 to 9.0 Cyanide (CNWAD) (1) Iron Manganese Copper Nickel Zinc Silver1 Total Ammonia 0.093 1.0 2.0 0.003 0.040 0.027 0.0002 Refer Table 3 0.093 1.0 2.0 0.011 0.160 0.100 0.0024 Refer Table 3 Antimony Arsenic Selenium Mercury Cadmium Chromium (VI) Lead 0.030 0.190 Refer Note (4) 0.000012 0.0003 0.010 0.0004 0.030 0.190 Refer Note (4) 0.000012 0.001 0.010 0.0025 Notes : (1) Site specific derived criteria using US EPA (1985) methodology. (2) Monitoring of metals shall be based on the soluble test method, defined as the concentration of dissolved metals measured in that fraction which passes through a 0.45 um filter except for mercury (Hg) which shall be based on acid soluble concentrations determined on unfiltered samples. (3) Current analytical procedures for mercury have a practical quantification limit (PQL) of 0.0005 ppm. This PQL is acceptable for the purposes of reporting mercury concentrations. The reporting ‘limit’ for mercury concentrations shall be reviewed annually by the consent holder and shall be adjusted in line with improvements in analytical technology. (4) The selenium concentration in the receiving water shall remain below the trigger limits of 0.02 g/m3 97% of the time on an annual basis, and 0.035 g/m3 in any single analysis, based on monitoring undertaken pursuant to condition 16 of consent 971318. In the event that these limits are exceeded, the consent holder shall inform the Waikato Regional Council as soon as practicable and prepare a report, to the satisfaction of the Council, to demonstrate that continued discharges at concentrations exceeding the trigger limits will have no more than minor effects on the Ohinemuri River. This report shall be provided to the Council within two months of the consent holder becoming aware of the trigger exceedence.
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