Bison Engineering, Inc.

I N T E R

O F F I C E

MEMO

To: Michael Ford, Hess Corporation 

Subject: Tioga Natural Gas Processing Plant, SO2 modeling notes 

Date: September 9, 2015 

We have conducted dispersion modeling for sulfur dioxide (SO2) in the vicinity of Hess’s 

Tioga natural gas processing plant which itself is located near Tioga, ND. This memo 

summarizes the model input parameters and the modeling results. In general, the 

purpose of the modeling was to determine whether model-predicted SO2 concentrations 

at receptors near the facility are consistent with ambient concentrations measured at 

Hess’s two ambient monitoring sites. The model results, shown below, correspond well 

with measured concentrations. Monitoring and modeling both confirm that 

concentrations resulting from the plant’s normal operations are well below the 1-hour 

average SO2 ambient air quality standard of 75 ppb. (40 CFR 50.17). 

The following is a short synopsis of the model and the results: 

Model: 

• AERMOD (Version 15181) 

• AERMET (Version 15181) 

• Regulatory default enabled 

• SO2 “special pollutant” option enabled (in order to calculate 3-year 99th percentile) 

Emission Sources: 

• Hess main stack (SRU) 

o Hourly SO2 data from CEM 

o Actual data from September 2014 through June 2015 

• Hess flare stack 

o Hourly SO2 data from CEM 

o Actual data from September 2014 through June 2015 

Surface Winds Data 

• Actual data from on-site ambient monitoring station
• Hourly data: 3 years (June 2012 through June 2015)

• Data formatted to appear (to the model) as calendars 2013, 2014 & 2015 

• Williston NWS data used for substitution, cloud cover, etc. 

• Supplemented available 1-minute NWS data (2013 data were unavailable) 

Upper Air Data 

• Glasgow, MT 

Data Preparation: 

• Extrapolated emissions data to create full year of data to represent normal operations 

o Plant improvements completed September 2014 

o Available normal operations emissions data for 9/14/ 2014 through 6/15/2015 

o Copied 4/16/2015 through 6/14/2015 data, appended it to the normal emissions 

data, and relabeled as 6/16/2015 through 8/13/2015 to create a full 12 month data 

set indicative of normal operations 

o Replicated full-year data set three times and relabeled with the appropriate year to 

match the corresponding meteorological data set (i.e., 2013, 2014 and 2015) 

• Because only first-half of 2015 meteorological data were available, they were combined 

with second-half 2012 data to create a 12 month data set for model year 2015 

Receptor Grid. 

• Cartesian grid. 

• 4,554 total receptors 

• 100 meter fence-line receptors 

• 100 meter spacing from plant boundary to 1 kilometers 

• 250 meter spacing from 1 kilometers to 3 kilometers

• 500 meter spacing from 3 kilometers to 10 kilometers 

• 1,000 meter spacing from 10 kilometers to 20 kilometers 
From the desk of... 

Joe Peterson 

Project Engineer 

Bison Engineering, Inc. 

1400 11th Avenue 

Helena, MT 59601 

406.442.5768 

E-mail: peterson@bison-eng.com

Results: 

Top 5 Rank 

3-year average, 

99th percentile of the daily 1-hour 

maximums 

(PPB) 

1 50 

2 49 

3 48 

4 48 

5 48 

As you will observe the values are all well below the 75 ppb threshold of the ambient 

standard. 

Please contact me if you have any questions or wish to discuss the modeling analyses.