1 Tonkin & Taylor Ltd Wharekirauponga Underground Mine – unplanned detonation air quality assessment Oceana Gold New Zealand Limited June 2022 Job No: 1015212.2000 vRev0 1 Introduction 1.1 Overview Oceana Gold (New Zealand) Limited (OGNZL) is seeking resource consent from the Hauraki District Council (HDC) and Waikato Regional Council (WRC) for the activities required to establish a new mining operation in the Wharekirauponga Valley. This new mine, referred to as the ‘Wharekirauponga Underground Mine’ (WUG), is part of the Waihi North Project (WNP) to extend the life of mining activities in Waihi. As part of development of the WUG, OGNZL propose to establish a new underground Class 1 explosives storage facility to supply the mining operation. The new store will contain up to 6,100 kg of Class 1 explosives 380 m below the surface in mine space below the Conservation (Indigenous Forest) Zone under the Hauraki District Plan (HDP). This activity has been assessed in a hazardous substances risk assessment, which was provided by Mitchell Daysh to the HDC as a draft for review in late 2021. On 17 January 2022, the HDC issued letter detailing its initial review comments1, which included a query about the potential for rapid release of explosion gases at the vent shafts in the event of an unintended detonation in the underground store. Reaction gases produced during the detonation of explosives principally consist of carbon dioxide (CO2), nitrogen (N2) and water vapour. However, other gases may also be formed, such as carbon monoxide (CO) and oxides of nitrogen (NOX), the latter including nitrogen dioxide (NO2) which is a respiratory irritant at sufficiently high concentrations. The resultant concentration of these contaminants in ambient air beyond the site can be evaluated using dispersion modelling analysis and comparison with national standards and guidelines for ambient air quality. Tonkin & Taylor Limited (T+T) has been engaged by OGNZL to prepare this air quality assessment to support resource consent applications being prepared by OGNZL. This report has been prepared in accordance with our variation order dated 5 November 2021. 1.2 Purpose and scope This study has been prepared to evaluate the impact of an unplanned detonation in the proposed underground explosives store on ambient air quality in the vicinity of the surface connections. The following reports and their shorthand are referred to throughout: WKP UG Magazine Explosion – Gas Dispersal Study. Entech Limited, February 2022 attached as Appendix A. This report is referred to throughout as the ‘Gas Dispersal Study’. Hazardous substance technical assessment – Waihi North Project –Storage and use of hazardous substances at Willows Road site and the Wharekirauponga Underground Mine. Tonkin & Taylor Limited, June 2022. Referred to as the ‘HSA’. 1.3 Associated reports This report is part of a larger application, and specifically addresses the human health risk of contaminants released from a detonation in the underground store in the WUG. Assessment of other risks associated with the WNP are provided in the following documents: Waihi North Project - Resource Consent Applications and Assessment of Environmental Effects, Mitchell Daysh Limited, June 2022. 1 Resource Consulting Limited. RELEVANT MATTERS IN SUPPORTING DRAFT REPORTS FOR LAND USE CONSENT APPLICATION FOR WAIHI NORTH PROPOSAL WITH REGARD TO HAZARDOUS SUBSTANCE RISKS. 17 January 2022.
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