42 Tonkin & Taylor Ltd Hazardous Substances Technical Assessment – Waihi North Project - Storage and use of hazardous substances at Willows Road site and the Wharekirauponga Underground Mine Oceana Gold (New Zealand) Limited June 2022 Job No: 1015212.1000 vRev0 A person would need to be located less than 5 m from a surface connection shaft above the WUG in the Coromandel Forest Park to experience an overpressure level of more than 2 kPa. The ventilation shafts are located in remote areas and securely fenced to prevent public access. With consideration of the level of impact at the maximum overpressure level, the consequence to human health and property of a detonation at either the surface or WUG Class 1 explosives stores proposed is considered minor. 7.3.2.2 Discharges to air from the WUG Release of the products of combustion from the WUG ventilation shafts from a detonation in the underground Class 1 explosives store has been assessed in the Air Quality Assessment (AQA) in Appendix I. The assessment finds that in the event of a detonation, the gaseous products of carbon monoxide (CO), oxides of nitrogen (NOX) including nitrogen dioxide (NO2) and small quantity of ammonia (NH3) will be generated within the WUG. The mine gas dispersal study prepared by Entech (Appendix A to the AQA inAppendix I) found that the maximum emission scenario would occur with full mechanical ventilation to the mine, resulting in all the gases venting from the return air rise vent described as Site 5 in this report (and Site 4 in the GSS report attached as Appendix B, which did not include the Waihi Portal as Site 1). In the fully ventilated scenario, Entech determined that all contaminants would be exhausted from the mine in 15 minutes and 40 seconds. The average emission rate calculated for this period was modelled in atmospheric dispersion modelling as the continuous emission rate over 1 hour in order to compare the predicted results with criteria for human health. This will conservatively overstate the quantity of contaminant emitted. The relevant assessment criteria for emissions of CO, NO2 and NH3 are summarised inTable 7.2 below. Table 7.2: Assessment criteria for ambient air quality Contaminant Averaging period Concentration (µg/m3) Reference CO 1-hour 30,000 NZ AAQG NO2 1-hour 200 NESAQ / NZ AAQG NH3 1-hour 21,000 US EPA AEGL (Level 1) NESAQ = National Environmental Standards for Air Quality NZ AAQG = New Zealand Ambient Air Quality Guidelines USEPA AEGL = United States Environmental Protection Agency Acute Exposure Guideline Levels The 99.9th percentile value for the modelled ground level concentration (GLC) is compared with the assessment criteria as the maximum GLC in accordance with the Ministry for the Environment’s Good Practice Guide for Atmospheric Dispersion Modelling (MfE, 2004) recommendations for short term averaging periods.
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