2015-12-16_GENERAL DOCUMENTS - C1981035 (2)

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A number of scenarios were analyzed to generate results from different traffic levels, model options, and dust control factors. All modeled scenarios show the 24-hour NAAQS for PM10 will be exceeded at the five resident/structure locations closest to the unpaved section of County Road 120. The results indicate possible adverse health effects to these residents, as EPA's primary PMIo NAAQS is designed to protect public health. An EPA publication explaining the health effects from particulate pollution and an EPA fact sheet containing information on the PMIo NAAQS are attached to this document. The analysis was performed using EPA's regulatory air dispersion model called AERMOD. As further explained below, this model uses meteorological and emission data to predict pollutant concentrations at selected locations (receptors) for comparison with the NAAQS. Impacts from this analysis may be considered underestimated (i.e., other reasonable assumptions or methods may show higher concentrations) due to a number of factors. These factors include: • The modeling did not include any background (i.e., local) traffic counts, but only the two levels of GCC traffic provided in the Roadrunner TIA. Concentrations will increase with additional background counts. • Average traffic levels were assumed to occur daily throughout the year. On days with higher than average traffic, model results may show increased concentrations. • While we might propose to use the adjust u* model option (described later in the report) if performing regulatory modeling for an industrial client, there is no guarantee that this option would be approved by the regulating agency. The adjust u* option generally predicts lower concentrations, and was a model setting included in this analysis. Improvements to the analysis may be possible with more site-specific input data, such as more precise local traffic counts and surface material parameters from both the paved and unpaved sections of the road. Air Quality Dispersion Modeling Dispersion modeling uses mathematical formulations to characterize the atmospheric processes that disperse a pollutant emitted by a source (or sources). Using emission data and meteorological inputs, a dispersion model can be used to predict pollutant concentrations at selected downwind receptor locations. Air quality models are used to determine compliance with the NAAQS.' Model System Description The regulatory default modeling system for near -field (within 50 km of the source) dispersion modeling is AERMOD (current version 15181). The AERMOD model is specified for this purpose in the Guideline on Air Quality Models4. The AERMOD system contains two main pre- processing steps (AERMAP and AERMET) and the AERMOD dispersion model itself. These components are briefly discussed below. 3 http://www3.epa.gov/ttn/scram/dispersionindex.htm 4 40 CFR Part 51, Appendix W 9250 East Costilla Avenue 1 Suite 630 1 Greenwood Village, Colorado 80112 2 303/790-1332 1 Fax 303/790-7820 www.mevehil-monnett.com