DRAFT 3/7/11

Dispersion Modeling of Carbonyl Sulfide Emissions from a Natural Gas Compressor Station near Lake Arlington

Melanie Sattler, Ph.D., P.E.

for Fort Worth League of Neighborhoods

March 2011

1

DRAFT 3/7/11

INTRODUCTION

The objective of this project was to estimate concentrations of carbonyl sulfide surrounding a natural gas compressor station located near Lake Arlington (see Fig. 1), to determine whether the potential exists for adverse impacts on human health. Although 2 1-hour ambient air samples were collected by Titan Engineering in June 2010 to assess the potential for health impacts, these 2 samples did not represent a worst case in terms of human exposure.

Concentrations of pollutants to which humans are exposed are functions of:1) Emission rate,2) Meteorology.

If meteorological conditions are right for diluting pollutant concentrations, then humans will be exposed to low levels of pollutants. Such diluting conditions include high wind speeds and significant vertical mixing/turbulence. On the other hand, if meteorological conditions are right for concentrating pollutants, then humans will be exposed to high levels of pollutants. Such concentrating conditions include low wind speeds and stable atmospheric conditions. Sampling for 1-hour is not enough to assess the worst-case concentrations that humans might be exposed to: unless the sampler is very lucky, the meteorological conditions during sampling will not be worst case. The Titan Engineering report states that conditions during the 1-hour of sampling were normal (not worst-case). In addition, sampling at 2 locations is not enough to assess the worst-case concentrations that humans might be exposed to: unless the sampler is very lucky, he/she will not happen to be sampling at the location where the highest concentration occurs.

In this project, 9480 hours of meteorological data (from June 1, 2009 to June 30, 2010) were processed through air quality dispersion modeling software to determine what carbonyl sulfide concentrations would be under a variety of meteorological conditions. In addition, pollutant concentrations were estimated at 2220 receptor locations surrounding the compressor station to provide a more complete picture of how concentrations would vary as a function of location. The computer modeling results show that under meteorological conditions that concentrate pollutants, carbonyl sulfide concentrations surrounding the Lake Arlington exceed health impact levels, for distances as great as one mile.

METHODOLOGY - MEASUREMENTS

Titan Engineering, Inc., in a project conducted for the Barnett Shale Energy Education Council, collected 2 1-hour ambient air samples near the Lake Arlington compressor station for sulfur analysis. Although additional samples were collected for analysis of volatile organic compounds and formaldehyde, only these 2 samples at the Lake Arlington site (COFW 5) were analyzed for sulfur compounds. The 2 samples were collected in summa canisters on June 11, 2010 between 2:15 and 3:15 p.m., at the following locations:

“Upwind Sources @ Boundary (Center)” (UTM coordinates 667,605 E and 3,622,549 N), termed Upwind Location in this report, and

2

DRAFT 3/7/11

“Downwind Sources @ Boundary (Center) (UTM coordinates 667,624 E and 3,622,695 N), termed Downwind Location.

The summa canister samples were analyzed by TestAmerica, Austin, TX, for 20 sulfur compounds. Results of the testing are available on-line at www.bseec.org in “Appendix G Lab Reports: Part 2 of 2.” Of the 20 sulfur compounds tested for, carbonyl sulfide and carbon disulfide were detected in the canister samples. Carbonyl sulfide concentrations were 130 and 330 µg/m3 at Locations 1 and 2, respectively. The 1-hour Texas Commission on Environmental Quality (TCEQ) Effects Screening Level (ESL) for carbonyl sulfide is 135 µg/m3; one of the sample concentrations exceeded this level.

METHODOLOGY - MODELING

Gaussian dispersion modeling is a widely used computer simulation method of estimating atmospheric concentrations of air pollutants, given source emission rate information, meteorological data, and terrain data. Dispersion modeling is required by the Texas Commission on Environmental Quality as part of its permitting process for certain sources. Dispersion modeling can supplement field measurements of concentrations in order to provide a more complete picture of how concentrations vary as functions of time and space.

Software. The Gaussian dispersion model AERMOD was used to estimate pollutant concentrations and impact areas. Modeling was conducted using AERMOD View (Lakes Environmental Version 6.4.0).

Source Data. Compressor stack heights, diameters, temperatures, and flow rates were taken from manufacturer information for the compressor engine models at the Lake Arlington site (4 Caterpillar G3608 LE 2370 hp engines and 2 Waukesha 7044GSI 1680 hp engines).

Dispersion models also require source emission rate information in mass/time. The Titan study did not measure emissions directly from the compressor station stacks. Manufacturer emission rate information was obtained for the compressor engine models at the Lake Arlington site from Caterpillar and Waukesha. Unfortunately, the manufacturers do not test for sulfide compounds. In addition, the manufacturer emission rates are based on engines burning pipeline quality natural gas, which may have a different composition from gas burned by the Lake Arlington compressors.

Due to the lack of available emission rate information, a preliminary model run was used to determine the emission rate from the compressor station (in g/sec) that replicated the ambient 1-hour concentrations measured on June 11, 2010, using the meteorological data for the 2:00 and 3:00 p.m. hours, since sampling was conducted from 2:15-3:15. The preliminary model run assumed that all of the carbonyl sulfide measured in the canister came from the compressor station site. Given the close proximity between the compressor station site and the sampling site, this is not an unreasonable assumption. If direct measurements of carbonyl sulfide emissions from the compressor station engines become available as a result of the Fort Worth study, modeling could be updated using a direct emission rate.

3

DRAFT 3/7/11

Using the emission rate determined from the preliminary model run, the model was then re-run with all of the hourly meteorological data from June 1, 2009 through June 30, 2010 (9480 hours).

Meteorological Data. Hourly meteorological data for Lake Arlington compressor station site (Latitude 32.7286 N, Longitude 97.2110 W) was obtained from Lakes Environmental for the period from June 1, 2009 through June 30, 2010 (9480 hours). Lakes Environmental processed surface and upper air meteorological data for the sampling location from MM5 data. The data included hourly values of temperature, wind speed, wind direction, and cloud cover. AERMET was used to preprocess the meteorological data for AERMOD.

Terrain Elevations. Terrain/receptor elevations were obtained from 30 m digital elevation model [DEM] data from the Texas Natural Resource Information System (http://www.tnris.state.tx.us/) . Terrain/receptor elevations were processed using the terrain preprocessor AERMAP. The elevated terrain option was selected.

Receptor Grid. A 50 m x 50 m Cartesian grid receptor network was used to cover a 2950 m x 1800 m area surrounding the compressor station. Concentrations were thus estimated at 2220 receptor locations.

Averaging Time. A 1-hour averaging time was used, to facilitate comparison with TCEQ short-term Effects Screening Levels (ESLs), which are based on a 1-hour averaging time.

Other Model Options. Regulatory default mode was used. No significant removal was assumed due to wet deposition, dry deposition, chemical reaction (exponential decay), or gravitational setting.

Output. Dispersion model outputs included the highest 1-hour averages of pollutant concentrations for each receptor location. Concentrations exceeding TCEQ short-term ESL (135 µg/m3 for carbonyl sulfide) were plotted.

RESULTS

Figure 1 below shows the maximum 1-hour carbonyl sulfide concentration at each receptor over the modeling time period. The lowest concentration shown on the scale, 135 µg/m3, is the 1-hour ESL for carbonyl sulfide. In other words, anywhere a color is shown on the map, the 1-hour ESL for carbonyl sulfide is exceeded. The top tick mark on the scale is 500 m; the bottom tick mark on the scale is 1000 feet. The exceedances of the ESL (purple cover) thus extended at least a mile from the compressor station site.

4

DRAFT 3/7/11

Figure 1. 1-hour maximum modeled carbonyl sulfide concentrations surrounding the Lake Arlington compressor station

5