43 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 Table 7.3: Evaluation of GLCs against assessment criteria – ANFO or booster Contaminant Assessment criteria (µg/m3) Maximum GLC (µg/m3) Predicted location Background concentration (µg/m3) Cumulative GLC (µg/m3) CO 30,000 816 (2.7% of criterion) 130 m NE of vent 5,000 (16.7% of criterion) 5,816 (19.4% of criterion) NOX (as NO2) 200 59.9 (30% of criterion) 130 m NE of vent 37 (19% of criterion) 96.9 (48% of criterion) NH3 21,000 1,143 (5.4% of criterion) 130 m NE of vent 0 (0% of criterion) 1,143 (5.4% of criterion) For the purpose of a conservative assessment, all NOX produced in the detonation was assumed to convert instantaneously to NO2. In reality the atmospheric conversion of NO to NO2 may take some time and be limited by the available ozone levels in the atmosphere. The contour plot for NOX is reproduced as Figure 7.3 below, and illustrates the contour pattern for all contaminants. It shows that the highest GLC occurs approximately 130 m northeast of the ventilation shaft (Site 5 – Rise #2). Though it is considered highly unlikely that a person would be located in this area during the worst-case meteorological conditions and a failure of controls leading to a detonation in the Class 1 explosives store, even in this scenario, the level of exposure is well below the assessment criteria for the protection of human health for all identified contaminants. Figure 7.3: Contour plot showing predicted maximum 1-hour average NOX (as NO2) concentrations excluding background. Vent location shown as a light blue cross.
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