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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
tkelley on DSKBCP9HB2PROD with PROPOSALS

from each source in the source category, the HI for chronic exposures to HAP
with the potential to cause noncancer health effects, and the HQ for acute exposures to HAP with the potential to cause noncancer health effects. The assessment also provides estimates of the distribution of cancer risk within the exposed populations, cancer incidence, and an evaluation of the potential for an adverse environmental effect. The seven sections that follow this paragraph describe how we estimated emissions and conducted the risk assessment. The docket for this rulemaking contains the following document which provides more information on the risk assessment inputs and models: Residual Risk Assessment for the Mercury Cell ChlorAlkali Plant Source Category in Support of the 2020 Risk and Technology Review Proposed Rule. The methods used to assess risk as described in the seven primary steps below are consistent with those described by the EPA in the document reviewed by a panel of the EPAs SAB in 2009; 6 and described in the SAB review report issued in 2010.
They are also consistent with the key recommendations contained in that report.
1. How did we estimate actual emissions and identify the emissions release characteristics?
The HAP emissions from the single mercury cell chlor-alkali plant includes mercury and chlorine. Hydrochloric acid historically had been associated with these facilities, but based on recent reviews of available information and discussions with Westlake Chemical, we conclude that any HCl emissions from the remaining operating facility in West Virginia are due to non-source category emissions sources such as HCl production operations i.e., they are not emitted by an affected source subject to the standards applicable to mercury cell chlor-alkali plants. The mercury emissions are emitted from several emission sources within the mercury cell chlor-alkali facility affected source at the one operating mercury cell chloralkali plant, which, for the purposes of the source category risk assessment, have been categorized into two general emission process groups: 1 Process vents and 2 fugitives from the mercury cell room building. Based on available data, we conclude the chlorine emissions are mostly or entirely emitted 6 U.S. EPA. Risk and Technology Review RTR
Risk Assessment Methodologies: For Review by the EPAs Science Advisory Board with Case Studies MACT I Petroleum Refining Sources and Portland Cement Manufacturing, June 2009. EPA452/R09
006. https www3.epa.gov/airtoxics/rrisk/
rtrpg.html.

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as fugitive emissions associated with the cell room or from pipes or other equipment used to pump the product chlorine to the chlorine storage units or other associated equipment in the mercury cell chlor-alkali facility affected source. The main source of emissions data used in our analyses was the NEI data submitted for calendar year 2017. Data on the numbers, types, dimensions, and locations of the emission points and non-point sources for each facility were obtained from the NEI and Google EarthTM. A description of the data, approach, and rationale used to develop actual HAP emissions estimates is discussed in more detail in the document, Development of the Residual Risk Review Emissions Dataset for the Mercury Cell Chlor-Alkali Plants Source Category, which is available in the docket Docket ID No. EPAHQ
OAR20200560.
2. How did we estimate MACTallowable emissions?
The available emissions data in the RTR emissions dataset include estimates of the mass of HAP emitted during a specified annual time period. These actual emission levels are often lower than the emission levels allowed under the requirements of the current MACT
standards. The emissions allowed under the MACT standards are referred to as the MACT-allowable emissions. We discussed the consideration of both MACT-allowable and actual emissions in the final Coke Oven Batteries RTR 70
FR 19992, 19998 through 19999, April 15, 2005 and in the proposed and final Hazardous Organic NESHAP RTR 71
FR 34421, 34428, June 14, 2006, and 71
FR 76603, 76609, December 21, 2006, respectively. In those actions, we noted that assessing the risk at the MACTallowable level is inherently reasonable since that risk reflects the maximum level facilities could emit and still comply with national emission standards. We also explained that it is reasonable to consider actual emissions, where such data are available, in both steps of the risk analysis, in accordance with the Benzene NESHAP approach.
54 FR 38044
For the Mercury Cell Chlor-Alkali Plants source category, the EPA
assumed actual emissions are equal to allowable emissions. Allowable emissions are the estimated emissions that would occur under normal fullcapacity operating conditions and as allowed under the applicable MACT
standards. There is no available data that suggests the facility is operating at less than full capacity. There is also no evidence that the facility is controlling point source emissions to a degree
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greater than the emission limitations or that they are performing practices in excess of the required work practices for the control of fugitive emissions. This means that they are not reducing emissions beyond the levels required by the MACT standards which would result in actual emissions being less than allowable emissions. In addition, a review of ECHO indicates no enforcement actions for violations of the title V operating limits over the last 5
years, which would result in actual emissions being greater than allowable.
Therefore, we are comfortable with the assumption that actual emissions are equal to the allowable emissions.
3. How do we conduct dispersion modeling, determine inhalation exposures, and estimate individual and population inhalation risk?
Both long-term and short-term inhalation exposure concentrations and health risk from the source category addressed in this proposal were estimated using the Human Exposure Model HEM3.7 The HEM3 performs three primary risk assessment activities:
1 Conducting dispersion modeling to estimate the concentrations of HAP in ambient air, 2 estimating long-term and short-term inhalation exposures to individuals residing within 50
kilometers km of the modeled sources, and 3 estimating individual and population-level inhalation risk using the exposure estimates and quantitative dose-response information.
a. Dispersion Modeling The air dispersion model AERMOD, used by the HEM3 model, is one of the EPAs preferred models for assessing air pollutant concentrations from industrial facilities.8 To perform the dispersion modeling and to develop the preliminary risk estimates, HEM3
draws on three data libraries. The first is a library of meteorological data, which is used for dispersion calculations. This library includes 1
year 2016 of hourly surface and upper air observations from 824
meteorological stations selected to provide coverage of the United States and Puerto Rico. A second library of United States Census Bureau census block 9 internal point locations and 7 For more information about HEM3, go to https www.epa.gov/fera/risk-assessment-andmodeling-human-exposure-model-hem.
8 U.S. EPA. Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose Flat and Complex Terrain Dispersion Model and Other Revisions 70 FR 68218, November 9, 2005.
9 A census block is the smallest geographic area for which census statistics are tabulated.

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