DATE: 23 Nov 2016

MEMO: Review of Sierra Club’s 1-hour SO2 SIP modeling evaluation of the NPPD Sheldon 

Power Station in Hallam, Nebraska, and NPPD Gerald Gentleman Power Station in 

Sutherland, NE, conducted by Steven Klafka, P.E., BCEE, Wingra Engineering, S. C., 

Madison WI.

FROM: Lisa M. Alam 

Nebraska Department of Environmental Quality (NDEQ) 

Air Program Planning and Development Team, Air Quality Division

1200 "N" Street, Lincoln, NE 68508-8922

Phone: 402-471-2925

At the request of the Sierra Club, Steven Klafka of Wingra Engineering, in Madison WI completed air 

dispersion modeling of two power plants identified by the US EPA as first round facilities required to do 

air dispersion modeling to support Nebraska area designations of the 1-hour SO2 NAAQS. Those two 

power stations are the NPPD Sheldon Station in Hallam, Nebraska, and the NPPD Gerald Gentleman 

Station in Sutherland, NE.

In reviewing Mr. Klafka’s efforts, I identified four issues that stood out and required comment; emission 

rates, receptors, buildings, and the background concentration.

Emission Rates:

To characterize current 1-hour SO2 air quality attainment, the EPA recommended using actual emissions, 

to reflect the temporally varying emissions. Mr. Klafka modeled actual emissions, but Mr. Klafka also 

chose to model the Potential To Emit (PTE) emissions and present both sets of impacts. PTE emissions 

are produced by the maximum capacity of a stationary source to emit a pollutant. A maximum capacity

assumes that the physical and operational design is operating at 100% of its design capacity, 100% of the 

time. Most consumers know that power plants have peak daily and peak seasonal loads, when the plant is 

not operating at 100% of its design capacity. The modeling of PTE emissions does not reflect the 

temporal impact of the two power plant facilities, and the addition of this emission rate to this modeling 

demonstration only causes confusion. 

Receptors

There were no fenceline receptors in either the Sheldon or the Gerald Gentleman models, and nonambient air receptors inside the fenceline were included in both air dispersion models. Additionally, there 

were receptors covering a large body of water in the Gerald Gentleman model. These receptors should 

have been excluded, as per EPA’s 1-hour SO2 SIP modeling guidance.

Buildings

There were no buildings in either model. Mr. Klafka claims the lack of buildings is more conservative. 

Proof of this assertion would be helpful. In my experience, buildings create cavitation and therefore 

higher impacts.

Background Concentrations

A background concentration was calculated by Mr. Klafka to be 88.9 μg/m3. The monitor used to arrive 

at this value was identified only as “2011-13 design value for Douglas County, Nebraska.” There are 

only two SO2 monitors in the entire State of Nebraska, and both monitors are located in Douglas County, 

in Omaha, NE, pictured below.
Both NPPD Gerald Gentleman Power Station and the NPPD Sheldon Power Station in Hallam, NE are 

located in sparsely inhabited rural areas, see figures below. Using a background concentration of SO2

from an urban area like Omaha, NE would overestimate the background concentration of SO2 normally 

found in rural areas like Hallam, NE. In air dispersion modeling, background concentrations are added to 

the predicted modeled impact to account for natural sources, nearby sources not directly included in the 

modeling effort, and distant unidentified sources. Using a background concentration from an urban area 

like Omaha would overestimate these background contributions which is why the background 

concentration from the rural monitor located at Cedar Bluff Reservoir in Trego County, KS. This monitor 

is far from any nearby large SO2 sources, so is representative of background concentrations in rural areas 

of Nebraska exclusive of nearby major source impacts. 

To check Mr. Klafka’s background concentration calculations, I calculated the concentrations from both 

available monitors in Omaha, NE, and came up with the results in the table below. I calculated a three 

year average for 2011 through 2013, and 2012 through 2014. I don’t know how Mr. Klafka was able to 

arrive at 88.9 μg/m3. 

2014-

12

2011-

13

99th Percentile

3-yr 

ave

3-yr 

ave

CBSA Address Site ID 2014 2013 2012 2011 ppb ppb

Omaha-Council Bluffs, NE-IA Douglas County Hosp 42nd & Woolworth 310550019 21.0 27.0 38.0 37.0 28.7 34.0

Omaha-Council Bluffs, NE-IA 1616 Whitmore 310550053 54.0 56.0 73.0 66.0 61.0 65.0

http://www3.epa.gov/airdata/ad_rep_mon.html

2014-

12

2011-

13

99th Percentile

3-yr 

ave

3-yr 

ave

CBSA Address Site ID 2014 2013 2012 2011 μg/m3 μg/m3

Omaha-Council Bluffs, NE-IA Douglas County Hosp 42nd & Woolworth 310550019 55.0 70.7 99.5 69.6 75.1 79.9

Omaha-Council Bluffs, NE-IA 1616 Whitmore 310550053 141.4 146.7 191.2 124.1 159.7 154.0