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To: David Triplett

Company: Entergy Services, Inc.

From: Richard Hamel

File number: 0268066

Date: 17 November 2015

Subject: Analysis of Sierra Club Modeling of Entergy 

Independence Steam Electric Station for 1-hour SO2

Compliance.

ERM was asked by Entergy Services, Inc. (Entergy) to review a modeling 

demonstration submitted by Sierra Club showing modeled concentrations 

associated with emissions from the Independence Steam Electric Station 

(Independence Station) above the 1-hour SO2 NAAQS. The document 

submitted to the U. S. Environmental Protection Agency (USEPA) Region 6 and 

the Arkansas Department of Environmental Quality (ADEQ) by Sierra Club, 

titled “Independence Plant, Newark, Arkansas, Evaluation of Compliance with the 1-

hour NAAQS for SO2“(September 16, 2015), along with the modeling files 

themselves, were received by ERM on October 22, 2015. This memorandum 

describes the findings of that review.

General Summary

Sierra Club’s modeling methodology is basic, with a minimal level of refinement, 

and their analysis is very general. For example, a review of four separate 

modeling reports submitted by Sierra Club related to the 1-hour SO2 NAAQS 

shows the modeling to be identical even to the point of including tables from one 

report to the next that are not relevant for the facility in the report. The 

explanations of their modeling approach are copied from report to report 

regardless of site-specific details. In addition, the discussion of meteorological 

data representativeness does not provide any site-specific reason that the 

meteorological data site chosen is representative, just that it is “close” to the 

project site, and this same reason is given whether the airport is 20 miles away or 

100 miles away from the study site with no discussion of what “close” 

means. The modeling also contains several errors and incorrect assumptions that 

render the modeling inaccurate. Furthermore, the modeling report draws 

conclusions which cannot be supported by the modeling results without further 

refinement and a culpability analysis. 
PAGE 2

Errors in the Sierra Club Modeling

ERM found the following errors in its review of the Sierra Club modeling files 

and report for Independence Station. 

Incorrect Conclusions regarding the 1-hour SO2 Compliance Status of Independence 

Station

In Section 2.2 of the modeling report, Sierra Club states:

“It was determined that based on either current allowable emissions or measured 

actual emissions, the Independence Plant is estimated to create downwind SO2

concentrations which exceed the 1-hour NAAQS.”

Sierra Club then not only fails to show that Independence Station contributes to 

air quality impacts above the NAAQS, but their own Table 1 shows that, in their 

modeling of the Independence units’ 3 most recent years (2012-2014) of actual 

emissions, as allowed by the SO2 NAAQS Designations Modeling Technical 

Assistance Document1, the modeled impacts from Independence Station, when 

added to an ambient background concentration, are well below the NAAQS 

(137.7 μg/m3 compared to the NAAQS of 196.5 μg/m3). Sierra Club’s modeling 

of FutureFuel Chemical Corporation results in impacts above the NAAQS, but 

the modeling is incomplete: Sierra Club did not perform a culpability analysis to 

determine whether Independence Station significantly contributes (that is, has 

modeled impacts greater than the 1-hour SO2 Significant Impact Level (SIL) of 7.9 

μg/m3) to any of the modeled impacts that were predicted to exceed the 

NAAQS. Without such a culpability analysis, Sierra Club has no basis to draw 

any conclusions about Independence Station’s contributions to the modelpredicted NAAQS exceedances caused by FutureFuel.

To determine whether Independence contributed to any exceedances caused by 

emissions from FutureFuel, ERM performed a culpability analysis using Sierra 

Club’s modeling files. No changes of any kind were made to the Sierra Club 

modeling: all inputs, including meteorological data, receptors, emissions, and 

stack parameters remained the same. As a first step, Sierra Club’s modeling was 

rerun and it was confirmed that the model results matched those presented in 

the Sierra Club modeling report.

Next, a review of the results showed that there were a total of 2530 

modeled exceedances of the NAAQS occurring at 371 receptors, spanning 

down to the 35th ranking, and occurring to the north, west, and south of 

FutureFuel. 

 1 http://www3.epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf
PAGE 3

All of the modeled exceedances were then reviewed using the 

MAXDCONT feature of AERMOD to determine if the contribution from 

Independence sources to the modeled exceedances was significant. A 

source significantly contributes to a modeled exceedance if its 

contribution to the model-predicted impacts is greater than the Significant 

Impact Level (SIL), which in the case of 1-hour SO2 is 7.9 μg/m3. Per the 

MAXDCONT output, the maximum contribution from Independence 

sources to any of the model-predicted exceedances was 0.0154 μg/m3.

Sierra Club’s own modeling with the three most recent years (2012-2014) of 

actual emissions data shows that SO2 emissions from Independence Station do 

not cause an exceedance of the 1-hour SO2 NAAQS. Further, as demonstrated 

by ERM’s culpability analysis, Independence Station does not contribute to 

any model-predicted exceedance of the NAAQS.

Assertions regarding Good Engineering Stack Height at Independence Station

In section 2.2 of the Sierra Club modeling report, Sierra Club states:

“An evaluation of facility buildings and structures determined that the current 

stack height of 1,000 feet (305 meters) did not meet the Good Engineering 

Practice (GEP) stack height requirement. Allowable emissions were modeled 

using both this height and the lower GEP stack height of 207 meters.”

Sierra Club’s assertion regarding GEP stack height is incorrect. In accordance 

with USEPA’s Guideline for Determination of Good Engineering Practice Stack Height 

(Technical Support Document for the Stack Height Regulations) (USEPA 1985)2, the 

physical GEP height, (HGEP), is determined from the dimensions of all buildings 

which are within the region of influence using the following equations, 

depending on the construction date of the stack:

(1) For stacks in existence on January 12, 1979 and for which the owner or 

operator had obtained all applicable permits or approvals required, 

HGEP = 2.5H,

provided the owner or operator produces evidence that this equation was 

actually relied on in establishing an emission limitation; 

(2) For all other stacks:

 2 http://www3.epa.gov/scram001/guidance/guide/gep.pdf
PAGE 4

HGEP = H + 1.5L

where:

H = height of the structure within 5L of the stack which maximizes HGEP; 

and

L = lesser dimension (height or projected width) of the structure.

For a squat structure, i.e., height is less than projected width, the formula reduces 

to:

HGEP = 2.5H

In the absence of influencing structures, a “default” GEP stack height is 

creditable up to 65 meters (213 feet). 

The main stack at Independence was in existence and all applicable permits had 

been obtained prior to January 12, 1979. Therefore the GEP formula height is 

HGEP = 2.5H. The controlling structure for the main stack is the Unit 2 cooling 

tower, with a height of 393 feet (119.8 m). Thus, the GEP formula height for the 

main stack is 119.8 x 2.5 = 299.5 meters, not the 207 meters claimed by Sierra 

Club. 

In addition, even if Sierra Club’s GEP stack height calculation were correct, it is 

unnecessary for 1-hour SO2 modeling. According to the SO2 NAAQS 

Designations Modeling Technical Assistance Document3, for the purpose of 

categorizing air quality for the 1-hour SO2 NAAQS, modeling with the full stack 

height, regardless of the GEP formula height, is allowed. Therefore, the 

discussion of GEP stack height is not relevant for the purposes of 1-hour SO2

modeling.

Other Modeling Weaknesses

ERM’s review of Sierra Club’s modeling showed the following additional 

weaknesses, generalizations, and incorrect claims. While some of these are not 

errors in the modeling per se, they demonstrate that Sierra Club failed to perform 

the most basic of refinements commonly applied to modeling studies.

Constant Stack Parameters:

The Sierra Club modeling uses constant exhaust temperatures and exit velocities 

for each of the two coal-fired units based on permitted data, since detailed hourby-hour stack exhaust parameter data are not available from EPA’s Clean Air 

 3 http://www3.epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf
PAGE 5

Markets Database, 4 which is the source of emissions information used in Sierra 

Club’s modeling. Sierra Club’s modeling assumed that the stack parameters for 

the two coal-fired units are identical, which tends to make the impacts from both 

units directly additive, causing higher impacts than would be the case when the 

actual hourly stack parameters are used. Table 1 shows that the average stack 

temperatures of the Units varies by over 20 Kelvin, while the exit velocities differ 

on average by about 1 m/s. 

Table 1: Average hourly stack parameters from CEMS data vs. permitted stack 

parameters.

UNIT CEMS* PERMIT Data Differences

Temp (K) Vel. (m/s) Temp (K) Vel. (m/s) Temp (K) Vel. (m/s)

Unit 1 425.95 24.32 433.71 27.43 7.76 3.11

Unit 2 405.75 25.05 433.71 27.43 27.96 2.38

*Temperature and exit velocity are the average values over three years (2012-2014) of all 

hours with a non-zero emission rate.

Model Version

Sierra Club states that the most recent version of the model was used. The 

modeling was performed in August 2015, at which time v.15181 was publicly 

available. Since the Sierra Club used v.14134, this statement is incorrect.

Ambient Background Data

The modeling uses ambient background data from 2011-2013 in combination 

with emissions and meteorological data from 2012-2014. A single annual ambient 

background value was used: no seasonal-diurnal refinement was performed. 

Section 2.2 of the Sierra Club report claims that the ambient background 

concentration used was “the most readily available design value”. This statement 

is incorrect because the 2014 ambient monitoring data was finalized and was 

readily available to the public from EPA’s database in May 2015, months before 

the modeling was performed.

Flagpole Receptors

The modeling uses flagpole receptors set to 1.5 meters, to represent the 

“inhalation level”. This assumption does not conform to guidance for NAAQS 

modeling but serves to slightly increase modeled concentrations.

 4 http://ampd.epa.gov/ampd/
PAGE 6

Operational Loads

Section 4.2 of the Sierra Club modeling report suggests that because loads of less 

than 100% were not considered, the model under-predicts impacts, citing lower 

flow rate and temperature for the lesser load cases. The report neglects to 

address the fact that emissions are also lower when the units are operating at less 

than 100% load. The modeling performed by ERM using hourly CEMS data, 

including concurrent hourly stack parameters, captures all load cases as they 

happen and shows no particular increase in modeled impacts during those 

periods of lesser loads.

Receptor Grid

The receptor grid in Sierra Club’s modeling covers the entire Independence 

Station property and ignores the fence line of the plant. As a result, the modeling 

includes impacts that are within the ambient air boundary of the facility. Per the 

Guideline on Air Quality Models (GAQM, 40CFR Appendix W) (USEPA 2005)5

, the 

area within a facility’s fence line to which the public is excluded from access is 

not “ambient air” and thus a facility cannot cause or contribute to an ambient air 

quality violation within this boundary. Therefore, receptors should not be placed 

within that boundary.

Meteorological Data Processing

The meteorological data processing assumed default seasonal periods and 

average moisture throughout the 3-year period. ERM’s modeling refined the 

meteorological data by resolving surface roughness and moisture by month.

Additionally, in section 4.5.4 of the modeling report, Sierra Club concludes that 

“Since the Clinton National Airport is located close to Independence Station, this 

meteorological data set was considered appropriate for this modeling analysis”. 

There is no discussion of what “close” means or why the meteorological data are 

representative and Sierra Club provides no maps, drawings or other figures 

showing the relative location of the facility and the meteorological data tower. In 

fact, in the four modeling reports related to SO2 attainment reviewed by ERM 

recently, as well as in past modeling reports by Sierra Club, section 4.5.4 is 

simply repeated in each report with only the name of the airport changed.

 5 http://www3.epa.gov/ttn/scram/guidance/guide/appw_05.pdf
PAGE 7

Allowable Emission Rate Modeling

Sierra Club’s report included modeling of Independence Station using the 

allowable permitted emission rate for each coal-fired unit. Since, per the final 

Data Requirements Rule for the 2010 1-Hour Sulfur Dioxide (SO2) Primary National 

Ambient Air Quality Standard (NAAQS) (USEPA 2015),6 the purpose of the 

modeling is to evaluate the current ambient air quality to determine attainment 

of the 1-hour SO2 NAAQS, the use of continuous operation at maximum 

allowable emissions is irrelevant. Additionally, modeling of allowable emissions 

requires 5 years of meteorological data. Sierra Club only used 3 years of data. 

Conclusion

Sierra Club’s modeling shows that Independence Station, when modeled with 3 

years of actual emissions data, as is allowed in the SO2 NAAQS Designations 

Modeling Technical Assistance Document, combined with an ambient background 

concentration, has modeled impacts that are below the 1-hour SO2 NAAQS. 

While Sierra Club’s modeling predicts that emissions from FutureFuel Chemical 

Corporation exceed the NAAQS, Sierra Club did not perform a culpability 

analysis to determine whether Independence Station contributes to those 

predicted exceedances. Therefore, no conclusions related to the cumulative 

impacts of FutureFuel and Independence can be made from Sierra Club’s 

modeling. The culpability analysis performed by ERM using Sierra Club’s inputs 

shows that Independence Station does not contribute to any modeled NAAQS 

exceedances.

Therefore, Independence Station does not cause or contribute to any modeled 

exceedances of the 1-hour SO2 NAAQS.

 6 http://www.gpo.gov/fdsys/pkg/FR-2015-08-21/pdf/2015-20367.pdf