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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
Model uncertainty concerns whether the model adequately represents the actual processes e.g., movement and accumulation that might occur in the environment. For example, does the model adequately describe the movement of a pollutant through the soil? This type of uncertainty is difficult to quantify. However, based on feedback received from previous EPA SAB
reviews and other reviews, we are confident that the models used in the screening assessments are appropriate and state-of-the-art for the multipathway and environmental screening risk assessments conducted in support of RTRs.
Input uncertainty is concerned with how accurately the models have been configured and parameterized for the assessment at hand. For Tier 1 of the multipathway and environmental screening assessments, we configured the models to avoid underestimating exposure and risk. This was accomplished by selecting upper-end values from nationally representative datasets for the more influential parameters in the environmental model, including selection and spatial configuration of the area of interest, lake location and size, meteorology, surface water, soil characteristics, and structure of the aquatic food web. We also assume an ingestion exposure scenario and values for human exposure factors that represent reasonable maximum exposures.
In Tier 2 of the multipathway and environmental screening assessments, we refine the model inputs to account for meteorological patterns in the vicinity of the facility versus using upper-end national values, and we identify the actual location of lakes near the facility rather than the default lake location that we apply in Tier 1. By refining the screening approach in Tier 2 to account for local geographical and meteorological data, we decrease the likelihood that concentrations in environmental media are overestimated, thereby increasing the usefulness of the screening assessment. In Tier 3 of the screening assessments, we refine the model inputs again to account for hourby-hour plume-rise and the height of the mixing layer. We can also use those hour-by-hour meteorological data in a TRIM.FaTE run using the screening configuration corresponding to the lake location. These refinements produce a more accurate estimate of chemical concentrations in the media of interest, thereby reducing the uncertainty with those estimates. The assumptions and the associated uncertainties regarding the selected ingestion exposure scenario are the same for all three tiers.

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For the environmental screening assessment for acid gases, we employ a single-tiered approach. We use the modeled air concentrations and compare those with ecological benchmarks.
For all tiers of the multipathway and environmental screening assessments, our approach to addressing model input uncertainty is generally cautious. We choose model inputs from the upper end of the range of possible values for the influential parameters used in the models, and we assume that the exposed individual exhibits ingestion behavior that would lead to a high total exposure. This approach reduces the likelihood of not identifying high risks for adverse impacts.
Despite the uncertainties, when individual pollutants or facilities do not exceed screening threshold emission rates i.e., screen out, we are confident that the potential for adverse multipathway impacts on human health is very low. On the other hand, when individual pollutants or facilities do exceed screening threshold emission rates, it does not mean that impacts are significant, only that we cannot rule out that possibility and that a refined assessment for the site might be necessary to obtain a more accurate risk characterization for the source category.
The EPA evaluates the following HAP
in the multipathway and/or environmental risk screening assessments, where applicable: Arsenic, cadmium, dioxins/furans, lead, mercury both inorganic and methyl mercury, POM, HCl, and HF. These HAP
represent pollutants that can cause adverse impacts either through direct exposure to HAP in the air or through exposure to HAP that are deposited from the air onto soils and surface waters and then through the environment into the food web. These HAP represent those HAP for which we can conduct a meaningful multipathway or environmental screening risk assessment. For other HAP not included in our screening assessments, the model has not been parameterized such that it can be used for that purpose. In some cases, depending on the HAP, we may not have appropriate multipathway models that allow us to predict the concentration of that pollutant. The EPA
acknowledges that other HAP beyond these that we are evaluating may have the potential to cause adverse effects and, therefore, the EPA may evaluate other relevant HAP in the future, as modeling science and resources allow.

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IV. Analytical Results and Proposed Decisions A. What actions are we taking pursuant to CAA sections 112d2 and 112d3?
1. MACT standards for Chlorine Emissions In addition to mercury, based on the NEI, the Westlake, West Virginia, mercury cell chlor-alkali facility emits an estimated 0.24 tpy fugitive emissions of chlorine from the mercury cell chloralkali production facility affected source. Chlorine is not emitted from mercury thermal recovery units and furthermore, the facility does not have a mercury thermal recovery unit at the site. In the 2003 final rule, the EPA
made the decision not to regulate chlorine and HCl in the Mercury Cell Chlor-Alkali Plant NESHAP based on the authority under section 112d4 of the CAA. Specifically, the EPA based this decision on the determination that no further control is necessary because chlorine and HCl are health threshold pollutants, and chlorine and HCl levels emitted from chlorine production processes are below their threshold values within an ample margin of safety. 68 FR 70906, December 19, 2003.
However, the EPA has determined that it must now propose standards for all HAP emissions from the source category, including emissions of chlorine, pursuant to CAA section 112d2 and 3.22 As discussed in section III.C.1 above, while there are HCl emissions from the direct synthesis HCl production units at the Westlake, West Virginia, facility, they are not from processes that are part of the mercury cell chlor-alkali plant. Therefore, no emission limitations or work practices are being proposed for HCl since the emissions are not from parts of the site that are within the mercury cell chloralkali plant. As a result, we are only required to propose standards for chlorine emissions pursuant to CAA
section 112d2 and 3.
Fugitive chlorine emissions occur from equipment leaks in the cell room and throughout the other parts of the mercury cell chlor-alkali production facility affected source that handle and process the chlorine gas produced. As stated previously, mercury recovery units are not sources of chlorine emissions.
22 The EPA not only has authority under CAA
section 112d2 and 3 to set MACT standards for previously unregulated HAP emissions at any time, but is required to address any previously unregulated HAP emissions as part of its periodic review of MACT standards under CAA section 112d6. LEAN v. EPA, 955 F.3d at 10911099.

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