10 Tonkin & Taylor Ltd Waihi North Project - Technical Review of Air Quality Assessments Oceana Gold (New Zealand) Limited 22 June 2022 Job No: 1017908.0000 4 Processing Plant The Processing Plant report specifically assesses the stack discharges of metal emissions associated with the Processing Plant, considering changes to the plant and the composition of the ore being processed by the plant. The focus of the assessment is on mercury discharges, although the discharge of a range of other metals is considered as well. As discussed later in this review, the assessment uses a standard dispersion modelling approach to assess off site contaminant ground level concentration against relevant human health assessment criteria. The following summarises our review of Processing Plant report where we consider commenting is appropriate. 4.1 Section 2: Processing plant emissions Section 2 of the Processing Plant report seeks to quantify the metal emissions from various discharge points associated with the Processing Plant, both in terms of short term (1-hour) peak and longer term (annual average) emission rates. Particulate emissions from the plant are addressed qualitatively in the Waihi Facilities report. A key consideration for the Processing Plant assessment is the expected higher emission of mercury associated with the processing of ore of the WUG and GOP ores. These ores are expected to have between 2 and 11 times greater mercury content than the Martha Underground mine ore. In this regard, Beca has relied upon mass balance assessments of mercury emission rates that were carried out by Oceana Gold, that take into account the expected processing capacity of the plant. The Processing Plant report states that Oceana Gold’s mass balance calculations were informed by stack emission testing undertaken in 2021 as well as information on the mercury composition of the ore. T+T has not reviewed the detail of Ocean Gold’s mass balance calculation, which was not detailed in the Processing Plant report. However, we note that the results of the dispersion modelling will be significantly influenced by the assumed emission rates. Emission rates for other metals are derived from the stack emission testing of the existing plant carried out in February 2021. Overall, the emission data used in the assessment represents the best available data for the plant, and suitably takes into account the anticipated increases in the mercury content of the ore. However, due to the limited availability of emission testing data there remains some uncertainty. Accordingly, we recommend that a programme of routine stack emission testing be carried out, should consents be granted, to confirm emission rates are within those assumed by the Beca assessment. 4.2 Section 3: Air quality criteria We have reviewed the discussion of air quality criteria and consider that appropriate criteria have been chosen in accordance with MfE guidance7. Of particular note is the discussion in the Processing Plant report regarding the applicability of the 1-hour acute OEHHA REL8 for mercury. Beca notes that the REL for mercury is derived from a single 7 MfE 2016. Good practice guide for assessing discharges to air from industry. Ministry for the Environment. Publication number: ME 1276. 8 OEHHA 2016. Acute, 8-hour and chronic reference exposure levels. Office of Environmental Health Hazard Assessment as of June 2016, California.
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