{In Archive {In Archive In Archive} Re:FW:Modeling of Background SO Modeling of Background SO Modeling of Background SO 2Sources for the Sources for the Sources for the 

McElmo Creek Gas Plant Flare 

Scott Bohning Scott Bohning to: thormel 03/24/2011 10:08 AM

Bcc: Andrew Chew

Archive: This message is being viewed in an archive .

Tom -

Thanks, that's good news. My preference would be for a stand-alone document, and for a report, but I 

guess a letter would be OK and would presumably cover much the same material , including e.g. model 

settings, list of sources and their emissions and locations , summary of terrain & meteorology, summary of 

high concentrations and their locations. And yes, I think it would be best if we got the digital files of 

model inputs and outputs.

Thanks for your quick work on this!

- Scott B.

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"Scott Bohning" <bohning.scott@epa.gov>

U.S. EPA Region 9, AIR-7

415/947-4127 fax-3579

"Tom Hormel" Scott, 03/23/2011 05:14:18 PM

From: "Tom Hormel" <thormel@mcvehil-monnett.com>

To: Scott Bohning/R9/USEPA/US@EPA

Cc: <pflynn@rnrc.net>

Date: 03/23/2011 05:14 PM

Subject: FW: Modeling of Background SO2 Sources for the McElmo Creek Gas Plant Flare

Scott,

We completed AERMOD 1-hour SO2 modeling of emissions from the McElmo Creek Gas Plant flare with 

available SO2 emissions and source parameter data provided by the Colorado Air Pollution Control 

Division and the Utah Department of Air Quality. The question we have is on the format of report 

summarizing air modeling approach, emissions and modeled air impacts that you would like to see. Do 

you want an report addendum that would be inserted as an appendix to the 2008 submittal or a 

stand-alone document in letter or report format? I assume that you would also want digital files of the 

model input and output, including the meteorological data used for the modeling.

I look forward to your response.

Thomas Hormel

McVehil-Monnett Associates, Inc.

Vice President & Sr. Project Manager

44 Inverness Drive East, Bldg C.

Englewood, CO 80112

303.790.1332

mcvehil-monnett.com
From: Bohning.Scott@epamail.epa.gov [mailto:Bohning.Scott@epamail.epa.gov]

Sent: Tuesday, March 15, 2011 5:42 PM

To: thormel@mcvehil-monnett.com

Subject: Re: Modeling of Background SO2 Sources for the McElmo Creek Gas Plant Flare

Tom -

I apologize for being hard to get a hold of these past two days... thank you for writing this summary.

It really sounds like there just are not that many SO2 sources around there! Although I doubt it will affect 

the outcome, I would still prefer at least a few sources to be thrown in just in case , and also to forestall any 

possible challenge on the inclusiveness. My suggestion would be to include the sources within 20 km, 

and if those are all quite small and there is a somewhat larger one within , say 30 km, the include that one 

as well.

- Scott B.

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"Scott Bohning" <bohning.scott@epa.gov>

U.S. EPA Region 9, AIR-7

415/947-4127 fax-3579

From: "Tom Hormel" <thormel@mcvehil-monnett.com>

To: Scott Bohning/R9/USEPA/US@EPA

Cc: <pflynn@rnrc.net>

Date: 03/15/2011 03:59 PM

Subject: Modeling of Background SO2 Sources for the McElmo Creek Gas Plant Flare

Scott,

This e-mail presents an update to acquiring and modeling background SO2 emissions data with the 

McElmo Creek Gas Plant flare for comparison to the 1-hour SO2 standard. I contacted the following 

agencies to acquire background SO2

 emissions data:

1. Utah Department of Air Quality (UDAQ)

2. Colorado Air Pollution Control Division (CAPCD)

3. Southern Ute Indian Tribe Air Quality Program

4. Ute Mountain Indian Tribe

I also called the Navajo Nation air quality program phone number and left a message. The following 
responses were received from these agencies:

1. The UDAQ e-mailed background SO2 data from a facility located south of Blanding with a total of 

0.7 tons per year (tpy) average for the last two years and apparently is the only facility within 50 km. of the 

McElmo Creek Gas Plant, approximately 40 km. away. 

2. The CAPCD e-mailed background SO2 data from various facilities with total emissions of 

approximately 34 tpy, located approximately 15 to 50 km. from the McElmo Creek Gas Plant.

3. Responses received from the Tribal agencies indicated that publicly available information on 

background SO2

 emission source data from facilities on Tribal Land are not available

I also did an Internet search for recent NEPA studies performed in the Four Corners Area and found that 

the only available study was the Desert Rock Energy Project Draft EIS. SO2 emission source data in this 

document are from large power plant sources only, located at least 100 km. from the McElmo Creek Gas 

Plant. Therefore, it is not appropriate to include these data in cumulative 1-hour SO2 modeling for the 

flare.

We also performed an inquiry on EPA’s AirExplorer website for monitored 1-hour SO2 data and found that 

the closest monitoring site with available data is in Farmington, New Mexico, located approximately 80 km. 

from the gas plant. Maximum monitored 1-hour SO2 concentrations range from 42 to 79 μg/m3

 for 2008, 

2009 and 2010. Monitored 99 percentile 1-hour SO2 concentrations for 2008, 2009 and 2010 range from 

29 to 66 μg/m3

. The 2008 – 2010 average of the 99 percentile values will be used to add to modeled 

1-hour SO2 concentrations.

Given the limitations on available background SO2 data, we propose to model the flare for 1-hour SO2 with

the background SO2 data provided by the UDAQ and the CAPCD and add the monitored average 99 

percentile 1-hour concentration to compare to the 1-hour SO2

 standard of 196 μg/m3. However, we would

also like you to consider whether background SO2 emission sources from the UDAQ and CAPCD should 

be modeled at all in this case because of the recent EPA guidance memo, dated March 1, 2011, which

applies to modeling of 1-hour NO2 and SO2. On Page 16 of this memo, EPA suggests “that the emphasis 

on determining which nearby sources to include in the modeling analysis should focus on the area within 

about 10 kilometers of the project location in most cases.” The basis of this statement is that modeled 

1-hour NO2

 and SO2 impacts decrease significantly more with distance than when the longer-term 

averaging periods are modeled. Thus, we think it is appropriate to not model the state agency 

background data because the closest emission source is located at least 15 kilometers from the McElmo 

Creek Gas Plant.

 

Please review this e-mail to confirm our proposed approach for cumulative 1-hour SO2 modeling of the 

McElmo Creek Gas Plant flare.

Thanks,

 

Thomas Hormel

McVehil-Monnett Associates, Inc.

Vice President & Sr. Project Manager

44 Inverness Drive East, Bldg C.

Englewood, CO 80112
303.790.1332

mcvehil-monnett.com

 

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