| Existing Air Quality Effects | Waihi North Project Air Discharge Assessment - Waihi Surface Facilities | 4396626-1331619777-99 | 16/06/2022 | 52 Respirable silica concentrations did not exceed the OEHHA annual average guideline level of 3 µg/m3 and monitoring was subsequently ceased as the WRC considered that ambient air quality within the Waihi Airshed did not pose any significant potential health risks. The monitoring also indicates that 24-hour average concentrations were also typically less than the annual average guideline level. Continuous monitoring of PM10 concentrations was also carried out by WRC for between 2008 and 201142. The monitoring also occurred at Site 6.61 (Grey Street). During the four years monitoring period, PM10 concentrations did not exceed the NESAQ 24-hour threshold concentration of 50 µg/m3. Observed 24-hour average PM10 concentrations were reported to be less than 33% of NESAQ for approximately 82% of time and less than 66% of the NESAQ for approximately 99.3% of the time. Table 6-7 summarises the results of the WRC monitoring programme. Due to the predominantly rural nature of the proposed WNP Waihi Surface Facilities, background PM10 concentrations near the proposed developments would generally be expected to lower than those measured at the Grey Street monitoring station. Table 6-7. Summary of 24-hour average PM10 concentrations (µg/m3) recorded at the WRC Waihi monitoring station (2008-2011) (Source: WRC) Year Maximum 2nd Highest Average 2008 32 31 12 2009 37 34 12 2010 45 44 12 2011 43 42 13 6.3 PM2.5 As discussed in Section 5.2.3, NIWA estimated the concentration of PM2.5 in the Waihi airshed to be 5.6 µg/m3, of which 2.5 µg/m3 is from natural sources and the remainder from anthropogenic sources. The major sources of anthropogenic PM2.5 in Waihi are expected to be domestic heating sources and vehicle emissions and will include a contribution from OGNZL. However, as noted in Section 3.2.1, dust from mining sources consists predominantly of larger particles that are greater than PM2.5 in size. Consequently, the discharges to air of PM2.5 from WNP are expected to have minimal effects on the concentrations of PM2.5 in the Waihi airshed. The NIWA report estimate a PM2.5/PM10 ratio of 0.48 for the Waihi airshed43. Using this ratio, a maximum 24hour average PM2.5 concentration of 21.6 µg/m3 can be estimated from the maximum 24-hour average PM10 concentrations of 45 µg/m3 observed during the WRC’s 2008-2011 ambient air quality monitoring (refer Table 6-7). The derived maximum 24-hour average PM2.5 concentrations does not exceed the proposed NESAQ criteria limit of 25 µg/m3. Peak 24-hour average PM2.5 concentrations in Waihi are expected to occur during the cooler winter months when the contribution from domestic heating sources to air quality concentration levels would be greatest and the nights more likely to be calm. Maximum downwind concentrations would generally occur during low wind speed and stable atmospheric conditions, typically associated cool calm winter’s evening and early morning periods. In contrast, the critical wind speed for the pick-up of dust from surfaces is 5 m/s. 42 Waikato Regional Council. Air quality monitoring report for Hamilton, Tokoroa, Taupo, Te Kuiti, Matamata, Putaruru, Ngaruawahia, Waihi and Turangi – 2011. Waikato Regional Council Technical report 2012/06 43 It should be noted that PM10 emitted as PM2.5 from mining activities is expected to lower, approximately 0.10% to 0.15% (refer Table 3-1)
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