Response to Comment on Silt Content Estimate Used as an Emission Factor Input to Estimate Fugitive Emissions of Particulate Matter from Unpaved Road Travel and Implications for NAAQS Demonstration

The USDA – Forest Service, Tonto National Forest (TNF) received comments from U.S. EPA Region 9 (EPA) on the Resolution Copper Draft Environmental Impact Statement (DEIS) pertaining to the analysis used to demonstrate conformity. Specifically, EPA requested that the implications of estimating PM₁₀ emissions from certain activities using mining-sector specific average silt content values from EPA’s AP-42 Chapter 13.2.2 be estimated as an alternative to the use of the Arizona Statewide average for silt content of 3.0% used in the DEIS. While the use of a 3% average silt content is a technically appropriate value, this technical memorandum provides the requested evaluation. The evaluation reaffirms the conclusions of the DEIS that the National Ambient Air Quality Standards (NAAQS) will not be exceeded – including when using the mining-sector specific average silt content values recommended by EPA. Additional confidence in the conclusions of the DEIS comes from the fact that Resolution Copper will have air quality permits issued by Pinal County Air Quality Control District (PCAQCD) and Arizona Department of Environmental Quality (ADEQ) that will include dust control and dust mitigation compliance requirements to ensure that all applicable air quality standards and regulations are complied with during all phases of the Project (construction, operation, and closure).