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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules column and benthic sediments, fish consumed by wildlife, and air. Within these four exposure media, we evaluate nine ecological assessment endpoints, which are defined by the ecological entity and its attributes. For PBHAP
other than lead, both community-level and population-level endpoints are included. For acid gases, the ecological assessment evaluated is terrestrial plant communities.
An ecological benchmark represents a concentration of HAP that has been linked to a particular environmental effect level. For each environmental HAP, we identified the available ecological benchmarks for each assessment endpoint. We identified, where possible, ecological benchmarks at the following effect levels: Probable effect levels, lowest-observed-adverseeffect level, and no-observed-adverseeffect level. In cases where multiple effect levels were available for a particular PBHAP and assessment endpoint, we use all of the available effect levels to help us to determine whether ecological risks exist and, if so, whether the risks could be considered significant and widespread.
For further information on how the environmental risk screening assessment was conducted, including a discussion of the risk metrics used, how the environmental HAP were identified, and how the ecological benchmarks were selected, see Appendix 9 of the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk and Technology Review 2020 Proposed Rule, which is available in the docket for this action.

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b. Environmental Risk Screening Methodology For the environmental risk screening assessment, the EPA first determined whether any facilities in the Mercury Cell Chlor-Alkali Plant source category emitted any of the environmental HAP.
For the Mercury Cell Chlor-Alkali Plant source category, we identified emissions of mercury and HCl. Because one or more of the environmental HAP
evaluated are emitted by at least one facility in the source category, we proceeded to the second step of the evaluation.
c. PBHAP Methodology The environmental screening assessment includes six PBHAP, arsenic compounds, cadmium compounds, dioxins/furans, POM, mercury both inorganic mercury and methyl mercury, and lead compounds.
With the exception of lead, the environmental risk screening
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assessment for PBHAP consists of three tiers. The first tier of the environmental risk screening assessment uses the same health-protective conceptual model that is used for the Tier 1 human health screening assessment. TRIM.FaTE
model simulations were used to backcalculate Tier 1 screening threshold emission rates. The screening threshold emission rates represent the emission rate in tons of pollutant per year that results in media concentrations at the facility that equal the relevant ecological benchmark. To assess emissions from each facility in the category, the reported emission rate for each PBHAP
was compared to the Tier 1 screening threshold emission rate for that PBHAP
for each assessment endpoint and effect level. If emissions from a facility do not exceed the Tier 1 screening threshold emission rate, the facility passes the screening assessment, and, therefore, is not evaluated further under the screening approach. If emissions from a facility exceed the Tier 1 screening threshold emission rate, we evaluate the facility further in Tier 2.
In Tier 2 of the environmental screening assessment, the screening threshold emission rates are adjusted to account for local meteorology and the actual location of lakes in the vicinity of facilities that did not pass the Tier 1
screening assessment. For soils, we evaluate the average soil concentration for all soil parcels within a 7.5-km radius for each facility and PBHAP.
For the water, sediment, and fish tissue concentrations, the highest value for each facility for each pollutant is used.
If emission concentrations from a facility do not exceed the Tier 2
screening threshold emission rate, the facility passes the screening assessment and typically is not evaluated further. If emissions from a facility exceed the Tier 2 screening threshold emission rate, we evaluate the facility further in Tier 3.
As in the multipathway human health risk assessment, in Tier 3 of the environmental screening assessment, we examine the suitability of the lakes around the facilities to support life and remove those that are not suitable e.g., lakes that have been filled in or are industrial ponds, adjust emissions for plume-rise, and conduct hour-by-hour time-series assessments. If these Tier 3
adjustments to the screening threshold emission rates still indicate the potential for an adverse environmental effect i.e., facility emission rate exceeds the screening threshold emission rate, we may elect to conduct a more refined assessment using more site-specific information. If, after additional refinement, the facility emission rate
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still exceeds the screening threshold emission rate, the facility may have the potential to cause an adverse environmental effect.
To evaluate the potential for an adverse environmental effect from lead, we compared the average modeled air concentrations from HEM3 of lead around each facility in the source category to the level of the secondary National Ambient Air Quality Standards NAAQS for lead. The secondary lead NAAQS is a reasonable means of evaluating environmental risk because it is set to provide substantial protection against adverse welfare effects which can include effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and hazards to transportation, as well as effects on economic values and on personal comfort and wellbeing.
d. Acid Gas Environmental Risk Methodology The environmental screening assessment for acid gases evaluates the potential phytotoxicity and reduced productivity of plants due to chronic exposure to HF and HCl. The environmental risk screening methodology for acid gases is a singletier screening assessment that compares modeled ambient air concentrations from AERMOD to the ecological benchmarks for each acid gas. To identify a potential adverse environmental effect as defined in section 112a7 of the CAA from emissions of HF and HCl, we evaluate the following metrics: The size of the modeled area around each facility that exceeds the ecological benchmark for each acid gas, in acres and square kilometers; the percentage of the modeled area around each facility that exceeds the ecological benchmark for each acid gas; and the area-weighted average SV around each facility calculated by dividing the areaweighted average concentration over the 50-km modeling domain by the ecological benchmark for each acid gas.
For further information on the environmental screening assessment approach, see Appendix 9 of the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk and Technology Review 2020 Proposed Rule, which is available in the docket for this action.
6. How do we conduct facility-wide assessments?
To put the source category risks in context, we typically examine the risks
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