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Attachment

Des Moines and Wapello Counties

On December 23, 2015, the DNR submitted revised dispersion modeling results for Burlington 

and Ottumwa generating stations, located in Des Moines and Wapello Counties, respectively. 

In that updated modeling emission rates were defined using either existing federally 

enforceable maximum allowable emissions rates or three years of recent historical actual 

emissions data. The use of proposed maximum allowable emission rates was eliminated. 

Those modeling results showed attainment with the 1-hr SO2 NAAQS in both Wapello and Des 

Moines Counties. Additionally, the DNR is confident that the receptor grids used in those 

modeling analyses capture the areas of maximum impact.

• The Burlington Generating Station is situated in the Mississippi River valley. The area around 

the generating station consists of the flood plain and surrounding bluffs. The change in 

elevation between the flood plain and the highest points along the bluffs is on the order of 150-

200 feet, which could have an impact on predicted concentrations. The nearest area of 

elevated terrain is the bluff along the western edge of the flood plain, which is located only 

approximately 1 km north-northwest of the facility. This area of elevated terrain is included in 

the current modeling domain provided to EPA. 

The next nearest area of elevated terrain is the bluff along the eastern edge of the flood plain in 

Illinois. This area is approximately 8 km away. Based on proximity, meteorological conditions 

in the area, and dispersion characteristics, the maximum concentration from the facility almost 

certainly occurs in the closer area of elevated terrain. For this reason the current modeling 

domain is sufficient to determine the magnitude of maximum concentration from this facility.

• The Ottumwa Generating Station is situated along the edge of the Des Moines River valley. The 

area around the generating station consists of the narrow Des Moines River valley and multiple

small valley tributaries, along with the areas of higher elevation above these valleys. The 

elevations in the area range from approximately 640 feet above sea level in the valley to 890 

feet above sea level, with the main boiler stack base elevation at approximately 680 feet above 

sea level. The current modeling domain provided to EPA encompasses the entire range of 

elevated terrain in this area. As such, there is no need to expand the receptor grid in order to 

encompass additional terrain features. Since the modeled concentrations are decreasing at the 

edge of the existing receptor grid, the current modeling domain is sufficient to determine the 

magnitude of maximum concentration from this facility.

This information, in combination with the revised modeling results, demonstrates that 

attainment designations are appropriate for Des Moines and Wapello counties.

Woodbury County

The DNR also continues to recommend an attainment designation for Woodbury County.

Although the original modeling results submitted to EPA captured the areas of maximum 
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impact around the George Neal facilities, the DNR has extended the modeling receptor grid to 

address EPA’s concerns regarding elevated levels of SO2 near the edge of the grid. The radius of 

the grid was doubled, from 5 km to 10 km, and also extended further to the northwest, in the 

vicinity of the elevated SO2 levels. At the DNR’s discretion, finer receptor grids using 50 meter 

spacing were embedded within areas of elevated SO2 concentrations to enhance spatial 

resolution. The modeled concentrations are now clearly decreasing at the edges of the grid, as 

shown in Figure 1, and the maximum total concentration, as show in Table 1, demonstrates

attainment with the 1-hr SO2 NAAQS.

Figure 1. Maximum modeled 1-hr SO2 design value (μg/m3, without background) for the 

Woodbury County (George Neal) analysis. The receptor points are the faint dots shown behind 

the isopleths and the location of maximum concentration is labeled.

Table 1. Model predicted concentration (μg/m3) for the Woodbury Co. (George Neal) analysis.

Model Design

Value

Background

Concentration

Total

Concentration

1-Hour SO2

NAAQS Above NAAQS

163.46 32 195.46 196 No
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These modeling results conservatively assume that George Neal North Units 1 and 2 will 

combust natural gas. However, George Neal North Units 1 and 2 will permanently shut down

on or before April 16, 2016. Their retirement satisfies conditions of the consent decree1 that 

requires these units to permanently cease burning coal on or before April 16, 2016. The 

process to revoke their air construction permits, which will make their shutdown federally 

enforceable, cannot be completed until after a 60-day public comment period. Once the air 

construction permits are revoked for those two sources, administrative rules federally 

approved in Iowa’s State Implementation Plan, specifically Iowa Administrative Code (IAC) 567 -

22.1(1), will also prevent those sources from resuming operation. The DNR expects to 

complete the permit revocation process near EPA’s July 2, 2016, designations deadline.

Concerning other sources in Woodbury County, the DNR notes a correction needed to the Draft 

Technical Support Document that accompanied EPA’s February 16, 2016, letter. EPA states that 

the DNR did not mention Cargill Inc. – Sioux City. The DNR’s technical support document did

discuss Cargill Inc. – Sioux City, and concluded that emissions from this source, which are less 

than 1 ton per year, will not affect the attainment status of the area.

The DNR encourages EPA to designate Woodbury County attainment. If EPA instead designates 

Woodbury County as unclassifiable because the permanent closures of George Neal North Units 

1 and 2 are not yet federally enforceable, the DNR anticipates that the State of Iowa will submit 

an attainment redesignation request after their closure is made federally enforceable through 

the revocation of their air construction permits.

 1 Consent decree between Sierra Club and MidAmerican Energy Company, United States District Court, Southern 

District of Iowa, Case No. 13-CV-21, filed on January 22, 2013.