## Bison Engineering, Inc.

I N T E R 

# MEMO

O F F I C E

To: Michael Ford, Hess Corporation Subject: Tioga Natural Gas Processing Plant, SO 2 modeling notes Date: September 9, 2015
We have conducted dispersion modeling for sulfur di oxide (SO 2 ) 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 model ing results. In general, the purpose of the modeling was to determine whether mo del-predicted SO 2 concentrations at receptors near the facility are consistent with ambient concentrations measured at Hess’s two ambient monitoring sites. The model resu lts, shown below, correspond well with measured concentrations. Monitoring and modeli ng both confirm that concentrations resulting from the plant’s normal op erations are well below the 1-hour average SO 2 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

• SO 2 “special pollutant” option enabled (in order to ca lculate 3-year 99 th percentile)

Emission Sources:

• Hess main stack (SRU)

o Hourly SO 2 data from CEM

o Actual data from September 2014 through June 2015

• Hess flare stack

o Hourly SO 2 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 calendar s 2013, 2014 & 2015

• Williston NWS data used for substitution, cloud cov er, 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 i t 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 relab eled 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 dat a 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 kilomete rs

• 250 meter spacing from 1 kilometers to 3 kilometers

• 500 meter spacing from 3 kilometers to 10 kilometer s

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

Joe Peterson Project Engineer Bison Engineering, Inc. 1400 11 th Avenue Helena, MT 59601 406.442.5768 E-mail: peterson@bison-eng.com Results:

Top 5 Rank 3-year average, 99 th 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 t he 75 ppb threshold of the ambient standard.
Please contact me if you have any questions or wish to discuss the modeling analyses.