| Air Discharges and Associated Mitigation Methods | WNP Wharekirauponga Underground Mine - Assessment of Effects of Discharges to Air | 4397169-66885702-23 | | 30 5.2.2 Factors that influence dust generation The major factors that influence dust generation from surfaces are: ● Wind speed across the surface - the critical wind speed for dust pick-up from surfaces without disturbances such as traffic is 5m/s - above 10m/s the pick-up increases significantly14 ● The percentage of fine particles in the material on the surface ● Moisture content of the material on the surface ● The area of exposed surface ● Disturbances such as traffic, excavation, loading and unloading materials. Vehicles travelling over exposed surfaces tend to pulverise surface particles. Particles are then lifted and dropped from rolling wheels and the road surface is exposed to strong air currents due to turbulence between the wheels and the surface. Dust is also sucked into the turbulent wake created behind moving vehicles. The smaller the particle size on the road or an exposed surface, the more easily the particles are able to be picked up and entrained in the wind. Moisture binds particles together preventing them from being disturbed and mobilised by wind or vehicle movements. Hence, one of the most effective means of minimising dust emissions is by keeping materials and surfaces damp. It is possible to estimate the potential emissions from surface sources, such as will occur at the project site, using emission factors developed primarily by the US EPA and published in a number of publications including the US EPA AP 42 Database15. However, for fugitive dust sources such as those associated with the project, there is a high degree of uncertainty. This assessment of environmental effects has instead been based on comparing the existing and past air quality effects of mining at the Waihi site with the proposed activities, taking into account any differences in the operation and location in relation to sensitive receptors (i.e dwellings). This more qualitative assessment approach provides more certainty for decision making as it compares the actual effects and successful mitigation processes at the larger Waihi operation with the relatively small scale, more remote Willows Road component of the project. The following sections describe the proposed mitigation associated with the dust from surface sources. 5.2.3 Soil disturbance and earthworks The stripping and spreading of topsoil and general earthworks (including construction of the WRS, roads, water collection ponds and noise bunds) has the potential to generate significant quantities of dust if the process is not carefully managed. To control dust from these activities, OGNZL will use the following methods which are described in the appended AQMP and are successfully used at other sites in Waihi: ● Keep exposed areas to a minimum and re-vegetate as soon as practicable ● Use water to dampen surfaces where practicable and necessary ● Plan potentially dusty activities such as stripping and spreading of topsoil for days when low wind conditions are predicted. 14 Air and Waste Management Association (2000) Air Pollution Engineering Manual 2nd Ed edited by Wayne T Davis 15 United States Environmental Protection Agency AP 42 Emission Factor Database Chapter 13.
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