| Potential Effects on the Environment of WNP | Waihi North Project Air Discharge Assessment - Waihi Surface Facilities | 4396626-1331619777-99 | 16/06/2022 | 63 The dwellings located to the west and south-west are located more than 950 m from the Processing Plant. These dwellings would also not be expected to be impacted by plant emissions. None of the complaints received since 2002 have identified emissions from the Processing Plant as a source of the observed adverse air quality effect. Therefore, providing OGNZL continues to operate and maintain the plant as at present, the discharge of particulate matter from the Processing Plant is not expected to have adverse air quality effects. There may be some potential for variation in crystalline silica content in ore mined from the GOP and WUG and therefore its potential to increase the respirable crystalline silica content (RCS) of airborne particulate matter emitted from the Processing Plant. Monitoring work carried out at Yaldhurst near Christchurch47 by Mote Ltd provides useful information on the assessment of the likely effects of RCS from the discharge of dust at Processing Plant. The ambient air concentration of RSC was measured at six locations in the vicinity of seven operational quarries for a period of 4 months. The monitoring site were located between 50 m and 650 m from the quarries. Monitoring also occurred at two background sites. The concentrations of RCS were compared to the OEHHA guideline of 3µg/m3 (annual average). This guideline is recommended for assessing the long-term effects of RCS exposure. The study showed that only two of twenty samples recorded RCS above the level of detection and that the average concentration of these samples was 0.4 µg/m3 (only 13% of the OEHHA guideline value). The two results above the limit of detection occurred at the 50 m downwind site. The Mote study demonstrated that the RCS concentrations downwind of an operating quarry using mechanical aggregate crushing equipment can be elevated above background concentrations but are well below the recognised health guideline value. The study highlighted the need for effective dust control to be carried out, especially when extraction and crushing operations occur within 100m of downwind dwellings. The separation distance between the Processing Plant and nearest dwelling is approximately 600 m. The results of the monitoring study indicate that it is highly unlikely dwellings in the vicinity of the plant would be adverse impacted by RCS, provided appropriate dust mitigation is implemented. 7.7.2 Assessment of Processing Plant discharges The potential air quality impacts of discharges of mercury and other metals from the Processing Plant have been assessed using dispersion modelling methods. The CALPUFF air dispersion model has been used to predict the maximum ground level concentrations of contaminants and the potential impacts have been assessed by comparing the predicted contaminant concentrations against relevant air quality criteria. The assessment methodology and results are summarised in Appendix F. The modelled discharges of mercury from the Processing Plant have based on a worst-case emission scenario assuming an ore processing rate of 2.25 MTPA, of which 0.611 MPTA is GOP ore. The proposed retort oven is assumed to have been installed. A screening approach has been used to assess the discharge of non-mercury metals from the Processing Plant. Screening modelling is used to assess whether emissions to air could theoretically have an adverse effect and whether a more detailed assessment would be required. The approach is conservative and will overestimate the contribution of plant emissions to ambient air contaminant concentrations. 47 Mote Ltd (2018) Yaldhurst Air Quality Monitoring Summary Report 22 December 2017 – 21 April 2018, report prepared for Environment Canterbury
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