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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 White 

Bluff 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 White Bluff Steam Electric Station (White 

Bluff) 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 “White Bluff 

Plant, Redfield, Arkansas, Evaluation of Compliance with the 1-hour NAAQS for 

SO2“(August 9, 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 the 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.
PAGE 2

Errors in the Sierra Club Modeling

Sierra Club made several errors and incorrect assumptions in the modeling for 

White Bluff, including the use of incorrect stack exhaust temperatures, 

unnecessary corrections to the stack height, the use of constant stack parameters, 

use of an older model version, failure to consider the facility fence line in 

determining the receptor grid, and the use of allowable emission rates in the 

modeling. The first error alone, regarding the use of the incorrect stack exhaust 

temperatures for White Bluff, singlehandedly invalidates the modeling even in 

the absence of all of the additional problems with Sierra Club’s modeling. Each 

of the errors and incorrect assumptions in Sierra Club’s modeling are identified 

in the paragraphs below.

Erroneous Stack Exit Temperature Units

Sierra Club failed to convert the permitted stack exhaust temperatures in degrees 

Fahrenheit for each of the two coal-fired units to the model input units of Kelvin. 

The exhaust temperature for the two units, according to the Title V operating 

permit for White Bluff, is 321 degrees Fahrenheit, which converts to 433.7 Kelvin, 

the required units for input into the AERMOD model. Sierra Club did not make 

the appropriate conversion, but instead simply input the value 321 as the exhaust 

temperature in the modeling. This error understates the correct exhaust 

temperature by more than 110 Kelvin and significantly alters the modelpredicted impacts. 

ERM corrected this error and reran Sierra Club’s actual emissions modeling with 

no other changes or refinements. The results are shown in Table 1: Sierra Club’s 

modeling, with the proper unit conversion for the exhaust temperature, shows 

that the modeled impacts from White Bluff, when added to an ambient 

background concentration, are below the 1-hour SO2 NAAQS. 

Therefore when the exhaust temperature error is corrected, Sierra Club’s 

modeling with the three most recent years (2012-2014) of actual emissions data 

shows that emissions from White Bluff do not cause or contribute to any 

modeled exceedances of the 1-hour SO2 NAAQS.

The fact that this basic error was not caught and fixed indicates that Sierra Club 

did not undertake even the most rudimentary QA/QC review of the modeling 

before it was submitted to EPA and ADEQ. 
PAGE 3

Table 1: White Bluff Actual Emissions Modeling with Erroneous and 

Corrected Stack Exhaust Temperatures.

S.C modeling with errors S.C modeling Corrected

1-hour SO2 

NAAQS 

(μg/m3)

Exhaust 

Temperature 

(K)*

Modeled 

Design Value 

w/Background 

(μg/m3)

Exhaust 

Temperature (K)

Modeled 

Design Value 

w/Background 

(μg/m3)

321.0 233.6 433.7 156.6 196.5

*Sierra Club erroneously claimed that the temperature was in Kelvin in Table 4 of their modeling 

report.

Assertions regarding Good Engineering Stack Height at White Bluff

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.”

This assertion 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)1, 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:

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.

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

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 White Bluff 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 1 cooling 

tower, with a height of 400 feet. Thus, the GEP formula height for the main stack 

is 400 x 2.5 = 1000 feet, which is the true height of the stack. As a result, Sierra 

Club’s calculated GEP stack height of 207 meters for the White Bluff units should 

be disregarded.

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 Document2, 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, Sierra Club’s 

discussion of GEP stack height is not relevant for 1-hour SO2 modeling.

Other Modeling Weaknesses

ERM’s review of the 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 is not available from EPA’s Clean Air 

Markets Database, 3 which was the source of emissions information used in the 

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

coal-fired units are identical , which tend 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 2 shows that the average stack 

temperatures of the Units varied by almost 16 Kelvin, while the exit velocities 

differ on average by about 1.5 m/s. Note also that the stack temperatures used 

by Sierra Club, based on permitted values, are considerably higher than the 

 2 http://www3.epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf

3 http://ampd.epa.gov/ampd/
PAGE 5

average values in the CEMS data, explaining why the corrected Sierra Club 

modeling had model-predicted impacts that were lower than those performed by 

ERM using the CEMS data with hourly stack temperatures and velocities. Both 

studies showed that emissions from White Bluff, when modeled with the 3 most 

recent years of actual emissions data, do not cause or contribute to any 

exceedance of the 1-hour SO2 NAAQS. 

Table 2: 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 1 395.99 25.50 433.71 27.43 37.72 1.93

Unit 2 380.04 24.09 433.71 27.43 53.67 3.34

*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.

Evaluation of other Sources in the Region

In listing assumptions that Sierra Club claimed would cause their modeling to 

underestimate concentrations, they stated “No consideration of off-site sources. 

These other sources of SO2 will increase the predicted impacts”, with the 

implication that such sources exist. As was fully documented in ERM’s “SO2 Air 

Dispersion Modeling Report for White Bluff Steam Electric Station” (August 2015), 

there are no other major sources of SO2 emissions in the vicinity of White Bluff.

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.
PAGE 6

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.

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 White Bluff 

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. 

According to the Guideline on Air Quality Models (GAQM, 40CFR Appendix W)

(USEPA 2005)4

, 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 White Bluff Plant, 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 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 

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

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.

Allowable Emission Rate Modeling

Sierra Club’s report included modeling of White Bluff 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),5 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

There are numerous deficiencies in the Sierra Club modeling that render it 

incomplete. More important however are the errors to basic inputs in the 

modeling. These errors and deficiencies invalidate the modeling study as 

presented, and as a result, the attainment status of the White Bluff Steam Electric 

Station with respect to the 1-hour SO2 NAAQS cannot accurately be determined 

from Sierra Club’s modeling. If the most significant error in the Sierra Club 

modeling (the incorrect stack gas exhaust temperature) is corrected, then the 

Sierra Club modeling predicts that emissions from the White Bluff Station do not 

cause or contribute to any exceedances of the 1-hour SO2 NAAQS. 

Therefore, White Bluff Station does not cause or contribute to any modeled 

exceedances of the 1-hour SO2 NAAQS.

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