2 

## 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 th at updated modeling emi ssion 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

.

Th ose modelin g result s show ed attainment with the 1

- hr SO 2 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 blu ffs 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 facil ity. 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, t he maximum concentration from the facility almost certainly occur s in the closer area of elevated terrain. For this reason the current modeling
domain is sufficient to det ermine 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 a pproximately 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. Si nce 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

a ttainment designat ion s 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 maximum3 impact around the George Neal facilities, the DNR has extended the modeling receptor grid to address EPA’s concerns regarding elevated levels of SO 2 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 SO 2 levels. At the DNR’s discretion, f iner receptor grid s using 50 meter spacing were embedded within areas of eleva ted SO 2 c oncentrations 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, demonstrate s attainment with the 1

- hr SO 2 NAAQS.

Figure 1

. Maximum model ed 1

- hr SO 2 design value ( μ g/m 3 , 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/m 3 ) for the Woodbury Co. (George Neal) analysis

. Model Design
Value Background
Concentration Total
Concentration 1

- Hour SO 2 NAAQS Above NAAQS

163.46 32 195.46 196 No4 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 decree 1 that requires these units to permanently cease burning coal on or before April 1 6 , 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. O nce the air

constr uction 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, t he 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 C argill Inc.

– Sioux City, and concluded that emissions from this source, which are less than 1 ton per year, w ill not aff ect the attainment status of the area

.

The DNR encourage s EPA to designate Woodbury County attainment. If EPA instead designates Woodbury County as unclassifiable because the permanent closure s of George Neal North Units
1 and 2 are n ot yet federally enf orceable, the DNR anticipate s that the State of Iowa will submit

a n attainment redesignation request after their closure is made federally enforceable through
the revocation of their air constr uction permits

.

1 Consent d ecree between Sierra Club and MidAmerican Energy Company , United States District Cour t, Southern
District of Iowa, Case No. 13

- CV

- 21, filed on January 22, 2013.