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# Response **s to**  **EPA Comments on NAAQS**  **Compliance Modeling in Support of the FCPP**

**and Mine EIS** Robert Paine and Mary Kaplan (AECOM) June 20 , 2013

**Introduction** The Arizona Public Service Company (APS) operates the Four Corners Power Plant (FCPP), a coal -fired power plant located on the Navajo Nation, about 25 miles west of Farmington, New Mexico. APS contracted AECOM to provide support in preparation for the env ironmental impact statement (EIS) to address the renewal of the lease and rights - of-wa y for the FCPP, the associated nearby Navajo Mine coal supply, and associated transmission lines , for the 25 -year period of 2016 through 2041. One of the technical stud ies required as part of the National Environmental Policy Act (NEPA) process involves determining t he impact of criteria pollutant emissions from the FCPP and the associated Navajo Mine with respect to the National Ambient Air Quality Standards (NAAQS). A draft NAAQS modeling study was submitted to EPA Region 9 for review in April 2013. This analysis is particularly important because the NAAQS for SO 2 and NO 2 have recently been strengthened. In addition to SO 2 and NO 2 , NAAQS impacts have been assessed fo r PM 2.5 , PM 10 , CO, and lead in this study

. The impacts of emissions from FCPP on the ozone NAAQS is being addressed separately using a photochemical modeling approach, and was not addressed in the recently submitted NAAQS modeling study

. Mr. Scott Bohnin g of EPA Region 9 provided comments via e - mail to Mr. Robert Paine of AECOM on May 31, 2013 after EPA’s review of the NAAQS modeling analysis. These comments are paraphrased below, along with a response from APS/AECOM that addresses each comment. 

### EPA **Com ment #1: AERMOD Modeling**  **Ap proach**  AECOM has selected a version of AERMOD 12345 with EPA- provided options/refinements that are currently being “beta” tested

. EPA Region 9 would appreciate additional justification of this approach.2

**Response:** AERMOD version 12345 with several potential refinements was developed by EPA’s Office of Air Quality Planning and Standards to address concerns submitted at the 10 th Mo deling Conference in March 2012 (and subsequent written comments) related to AERMOD low wind speed over- predictions. Although this version is still being tested , we have good evidence that AERMOD version 12345 with low-wind refinements will produce more realistic results for the FCPP EIS modeling

. Since AERMOD version 12345 with these refin ements is still undergoing evaluation by the AERMOD - user community, use of these improved formulation options is still considered a “beta” approach

. To provide the additional information requested, AECOM has provided EPA with a recentlycompleted 2013 Air & W aste Management Association paper that documents a review and independent evaluation of AERMOD version 12345 with research - grade low- level databases. AECOM has also provid ed to EPA an updated North Dakota SO 2 evaluation study that includes considerati on of AERMOD version 12345 beta options. The North Dakota study features coal -fired power plants with tall stacks, similar to the FCPP application, and it indicates superior performance for the proposed modeling approach. This study was also presented to EPA at the 10 th EPA modeling conference and was also discussed at the 2013 EPA modeling workshop, at which Robert Paine made an invited presentation on AECOM’s review of the low wind speed refinements in AERMOD version 12345. 

### EPA **Comment #2: Modeling t he**  **Au xiliary**  **Boiler Impacts** EPA has noted that the auxiliary boiler was not included in the modeling because its emissions are much lower than those from Units 4 & 5. However, EPA is concerned about a possible near-field impact from the operation of this natural - gas fired source, especially for 1 - h ou r NO 2 impacts. **Response:** The auxiliary boiler would be normally used only for steam assist to the coal -fired units during startup, and therefore it is not expected to be run at the same time as Units 4 and 5 af ter they have achieved normal operations after a startup. However, to address EPA’s concerns, AECOM has rerun the modeling with the very conservative assumption that the auxiliary boiler runs continuously (for short-term NAAQS) along with Units 4 and 5 ru nning continuously at full load. This modeling results in only a 1 μg/m 3 increase in the predicted 1 - hour NO 2 design concentration, so that even with this conservative assumption, attainment with the NAAQS is maintained.

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### EPA **Comment #3: Use of A ctual Emissions**  **for SO**  **2** 

### NAA**QS Modeling**  The EIS modeling for 1 - hour SO 2 NAAQS compliance used three recent years of actual hourly emissions, which is consistent with the draft EPA guidance provided in EPA’s Technical3

Assistance Docu ment for modeling, available at http://epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf . W hile this approach is probably acceptable for the EIS, it sh ould not be used for the PSD application if there are major increases in SO 2 emissions that would require a new modeling compliance demonstration. **Response:** APS does not plan any major increases in SO 2 emissions at FCPP Units 4 and 5, so no new NAAQS compliance modeling for SO 2 is expected to be needed for the PSD permit application. In fact, i t is likely that SO 2 emissions will decrease from Units 4 and 5 because of two factors: 1) some SO 2 will be oxidized to H 2 SO 4 by the SCR catalyst, and 2) if installed, some SO 2 would likely be removed by controls for H 2 SO 4 . These SO 2 emission decreases have not been accounted for in the NAAQS compliance modeling for SO 2 . 

### EPA **Comment #4: Combining PM 2.5**  

### Modeled and Background Concentrations EPA needs clarification as to how PM 2.5 ba ckground concentrations were combined with modeled impacts for compliance with the 24 - hour NAAQS. EPA cites relevant guidance as the March 23, 2010 EPA/Stephen Page memo (add modeled max imum concentrations to 98 th percentile monitored values ). **Response:** T he guidance in the March 23, 2010 EPA/Stephen Page memo was followed; the peak 24 - hour modeled concentration was added to the 98 th percentile background concentration. This procedure has been recently proposed to be replaced by EPA’s draft PM 2.5 modeling procedures posted on March 4, 2013 (available at http://www.epa.gov/ttn/scram/guidance/guide/Draft_Guidance_for_PM25_Permit_Modeling.pdf). In that draft guidance, EPA indicates that the 98 th percentile modeled 24 - hour concentration should be added to the 98 th percentile background concentration when emissions are largely due to fugitive or non - combustion sources . Due to the fact that the peak modeled PM 2.5 concentration for FCPP is due to non - combustion sources (no precursor emissions), the modeling results using the 2010 EPA/Stephen Page memo are very conservative. 

### EPA **Comment #5: Representativeness of Background Monitors** EPA has expressed some general concern over the representativeness of the background monitors

. **Response:** EPA did not provide concerns on specific monitors or pollutants in this comment. The background monitors are generally located in suburban or urban a reas that would tend to provide higher concentration levels than that expected in the rural environment of the FCPP4

and the Navajo Mine. In addition, for most pollutants (except SO 2 and PM 2.5 ), the design concentration is well below the NAAQS, so that eve n factor- of -two uncertainties in the background concentration would not change the overall result. For SO 2 , the 99 th percentile background monitor value represents a conservative Tier 1 treatment at a site near a background source. For PM 2.5 , we are using a monitor that shows higher design concentrations than most in the general area (see Figure 1). Therefore, we contend that the monitored background concentrations are likely to be conservatively high for SO 2 and PM 2.5 . 

### EPA **Comment #6: Sig nificant Concentration Gradient**  **An aly** **sis for San Juan Generating Station**  EPA indicated that its review of the figure showing the relatively flat gradient of the impact of the San Juan Generating Station beyond the location of the Four Corners Power Plant i ndicates support for the position that this background source will not cause a significant concentration gradient for FCPP. However, EPA is not sure that the quantitative criterion of +/- 30% for the variation of impacts as being within the “noise level” is the correct approach. **Response:** APS agrees with EPA that there are no SO 2 sources in the vicinity of FCPP that have a significant concentration gradient and are required to be modeled. Reliance on the quantitative criterion of +/- 30% is apparently not needed in this case

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### EPA **Comment #7: SO 2**  

### and PM **2.5** 

### Modeling Results are Close to the NAA**QS** EPA is concerned that the 1 - h ou r SO 2 and 24 - h ou r PM 2.5 modeling results are close to the NAAQS. **Response:** The reported concentrations are 194.5 μg/m 3 fo r 1 - ho ur SO 2 (the NAAQS is 196.5 μg/m 3 ) and 33.16 μg/m 3 for 24 - hour PM 2.5 (the NAAQS is 35 μg/m 3 ) , which are indeed close to the respective NAAQS. However, t he modeling procedures used for both 1 - h ou r SO 2 and 24 - h ou r PM 2.5 have inherent conservative features, as explained below. For SO 2 , the principle issue is that the highest predicted concentrations are caused by building downwash in high winds due to the influence of the Unit 4 and 5 boiler buildings. However, those b oiler buildings are not solid structures (see Figures 2 and 3), although the modeling treats them as being solid. For a solid building, the modeled (and actual) plume rise would be depressed over that from a porous structure (see example in Figure 4), and the modeling would then result in a plume rise that is too low and a modeled impact that is too high. W e5

note that aside from a narrow area of downwash impacts due to N W winds (see Figure 5), the remaining design 1 - hour SO 2 concentrations are under 150 μ g/m 3 , well below the NAAQS. For PM 2.5 , as explained in the response to Comment #4 above, we followed the guidance in the March 23, 2010 EPA/ Stephen Page memo for adding modeled to background concentrations. This guidance indicates that the peak 24 - hour mo deled concentration should be added to the 98 th percentile background concentration. However, the new EPA draft procedure for PM 2.5 modeling released on March 4, 2013 indicates that the 98 th percentile modeled 24 - hour concentration should be added to the 98 th percentile background concentration in certain situations . This proposed procedure wo uld be applicable in th e FCPP case due to the fact that the peak modeled concentration is due to fugitive emissions (no precursor emissions). Using the recent draft guidance, the 24 - hour PM 2.5 design concentration would be reduced from about 33 μg/m 3 to about 26 μg/m 3

. Therefore, for both SO 2 and PM 2.5 , we believe that the more likely design concentrations are on the order of 75% of the NAAQS, leaving considerable room for uncertainties in background concentrations.6

**Figure 1 A**

**vailable PM 2.5**  **Monitors and 24 -**  **hour Design Concentrations in the A rea of the Four Corners Pow**  **er Plant**7

**Figure 2 View of Units 4 and 5 Boiler Buildings Looking North**  **east**8

**Figure 3 View of Units 4 and 5 Boiler Buildings Looking Southw** **est**9

**Figure 4**  **Wind Tunnel**  **Depiction of**  **Plume Rise**  **From Ac** **tual Porous**  **Structure (Top)**  **and As**  **sumed Solid Structure (Bottom)***

*From Slide #9 at http://www.epa.gov/scram001/10thmodconf/presentations/1 - 11 - 10hModelingConference_A WM A_AB3_Petersen_final.pdf

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**Figure 5 1**  **- Hour SO**  **2 Design Concentrations Predicted by**  **A ERMOD Near FCPP Using**  **Ac tual Hourly**  **Emissions**