RESOLUTE NATURAL RESOURCES

 for Resolute Natural Resources Company 

 1675 Broadway, Suite 1950 

 Denver, CO 80202

McElmo Creek Unit Oil Production Facility - 

Main Flare, Gas Compression Facility 

Level-II VISCREEN Visibility Analysis 

Resolute Natural Resources Company 

January 2013 

MMA Project Number 2440-10 

 

by McVehil-Monnett Associates, Inc. 

 44 Inverness Drive East, Building C 

 Englewood, CO 80112 
i 

TABLE OF CONTENTS

Page No.

1.0 INTRODUCTION ....................................................................... 1 

2.0 IMPACT ANALYSIS APPROACH .................................................... 2 

 2.1 MODEL DESCRIPTION ....................................................... 2 

 2.2 EVALUATED SENSITIVE CLASS II AREAS ............................. 3 

 2.3 MAIN GAS FLARE OPERATION ............................................ 4 

 2.4 METEOROLOGICAL DATA PROCESSING ............................... 5 

 2.4.1 Meteorological Data Set ................................................. 6 

 2.4.2 Determination of Pasquill-Gifford Stability Class ..................... 6 

 2.4.3 Meteorological Data Joint Frequency Distribution .................... 7

 2.4.4 Level-II Meteorological Data Analysis ................................. 8 

3.0 LEVEL II VISIBILITY ANALYSIS RESULTS ....................................... 10 

 3.1 HOVENWEEP CAJON UNIT .................................................... 10 

 3.2 HOVENWEEP SQUARE TOWER UNIT .................................... 11 

 3.3 HOVENWEEP HOLLY UNIT .................................................. 12 

 3.4 HOVENWEEP HORSESHOE AND HACKBERRY UNIT ................ 13 

 3.5 HOVENWEEP CUTTHROAT CASTLE UNIT .............................. 14 

4.0 CONCLUSIONS ............................................................................. 16 

5.0 REFERENCES ............................................................................... 18 

LIST OF TABLES

TABLE 2-1 JOINT FREQUENCY DISTRIBUTION OF WIND SPEED AND DIRECTION 

 CANYONLANDS AIRPORT, UTAH 

TABLE 2-2 ANNUAL STABILITY CLASS DISTRIBUTION AT CANYONLANDS 

 AIRPORT, UT 

TABLE 2-3 ORIENTATION, DISTANCE AND ELEVATION DIFFERENCE BETWEEN 

 SENSITIVE CLASS II AREAS AND MAIN FLARE 

TABLE 3-1 VISIBILITY LEVEL II SCREENING ANALYSIS - HOVENWEEP CAJON 

UNIT 

TABLE 3-2 VISIBILITY LEVEL II SCREENING ANALYSIS - HOVENWEEP SQUARE 

TOWER UNIT 
ii 

TABLE 3-3 VISIBILITY LEVEL II SCREENING ANALYSIS - HOVENWEEP HOLLY 

UNIT 

TABLE 3-4 VISIBILITY LEVEL II SCREENING ANALYSIS - HOVENWEEP 

HORSESHOE AND HACKBERRY UNIT 

TABLE 3-5 VISIBILITY LEVEL II SCREENING ANALYSIS – HOVENWEEP 

CUTTHROAT CASTLE UNIT 

LIST OF FIGURES

FIGURE 2-1 LOCATION OF McELMO CREEK GAS PLANT AND SENSITIVE CLASS II 

AREAS EVALUATED FOR VISIBILITY IMPACTS 

FIGURE 2-2 WIND ROSE OF CANYONLANDS AIRPORT, UTAH METEOROLOGICAL 

DATA 

LIST OF APPENDICES 

APPENDIX A JOINT FREQUENCY OF WIND SPEED, WIND DIRECTION AND 

PASQUILL-GIFFORD STABILITY CLASS FOR CANYONLANDS, 

UT FOR 2005 – 2010 

APPENDIX B LEVEL-II JOINT FREQUENCY DISTRIBUTIONS OF WIND SPEED, 

WIND DIRECTION AND STABILITY FOR CANYONLANDS, UT 

FOR 2005 – 2010 FOR EACH SIX HOUR PERIOD 

APPENDIX C CALCULATION OF SIGMA Y AND SIGMA Z FOR EACH 

HOVENWEEP NATIONAL MONUMENT UNIT 

APPENDIX D VISCREEN VISIBILITY IMPACTS FOR EACH SENSITIVE CLASS II 

AREA WITH LEVEL-II METEOROLOGY FOR EACH SIX HOUR 

PERIOD 
1 

1.0 INTRODUCTION 

Resolute Natural Resources Company (Resolute) is submitting this supplemental visibility 

screening modeling analysis for potential impacts from the Main Flare at the McElmo Creek Oil 

Production Facility at Hovenweep National Monument, at the request of the National Park 

Service (NPS) in an e-mail of August 6, 2012. This evaluation is provided as an “Additional 

Impact Analysis” to the PSD analyses already done for the Main Flare. Hovenweep National 

Monument is a Class II area with units located northeast of the Main Flare as close as six 

kilometers away. 

A Prevention of Significant Deterioration (PSD) permit application for its Main Flare at the 

McElmo Creek Oil Production Facility, located on the Navajo Nation near Aneth, Utah was 

submitted on March 22, 2007 to meet the terms of a Consent Decree (DOJ Case No. 90-5-2-1-

2237, filed June 8, 2005). This Consent Decree was issued to the previous owner/operator of the 

facility, ExxonMobil Corporation (ExxonMobil). The PSD permit application document 

presented a facility process description for existing operations, criteria pollutant permit emission 

limits for the Main Flare, a Best Available Control Technology (BACT) analysis for the Main 

Flare, air modeling methodology, and air modeling results. Impacts were evaluated in both Class 

I and II areas. A revised permit application was submitted in April 2008 that responded to EPA 

Region IX comments concerning increased emissions due to processing flare gas with a higher 

BTU content than previously cited. 

An additional modeling study for 1-hour SO2 impacts was submitted on June 1, 2011 in response 

to a request by EPA Region IX during a meeting held on November 30, 2010. This air modeling 

study was submitted as a supplement to the already submitted permit application because none of 

the other non-air modeling information had changed. The SO2 modeling study was necessitated 

by the new 1-hour SO2 ambient air quality standard, effective on August 23, 2010. A new 1-hour 

NO2 standard also became effective on April 12, 2010. However, modeling in the PSD permit 

application showed maximum 1-hour NO2 impacts from the Main Flare to be below the 

significant impact level (SIL), while maximum 1-hour SO2 impacts from the Main Flare were 

slightly above the SIL. 
2 

2.0 IMPACT ANALYSIS APPROACH 

Visibility can be affected by plume impairment (heterogeneous) or regional haze 

(homogeneous). Plume impairment occurs at locations generally less than 50 kilometers from 

emitting source, while regional haze occurs at locations generally greater than 50 kilometers 

from the emitting source. Therefore, the visibility impact analysis presented for Main Flare 

emissions at the Hovenweep National Monument are for plume impairment since all evaluated 

units are less than 50 kilometers (km.) from the emissions source. 

2.1 MODEL DESCRIPTION 

VISCREEN is an EPA-approved model for evaluating visibility impacts incorporating 

techniques presented in the Workbook for Plume Visual Impact Screening and Analysis (EPA454/R-92-021). VISCREEN internally estimates visibility impacts at a variety of observer 

locations both inside and outside the impact area being considered against sky and terrain 

backgrounds at various sun angles (i.e., various times of the day with the observer facing the sun 

and looking in the opposite direction from the sun). Maximum visibility impacts are identified 

by VISCREEN from these internal estimations for an observer both inside and outside the impact 

area viewing against either a sky or terrain background. Visibility impacts are quantified by two 

different parameters consisting of plume perceptibility (E) and plume contrast (Cp). Maximum 

visibility impacts are deemed as being insignificantly perceptible if they are below both a E of 

2.00 and a Cp of  0.050. 

For both a screening (Level I) and a refined (Level II) analysis, the following default input 

assumptions are used: 

 Particle size distribution 

 Background ozone concentration (0.040 ppm) 

A background visual range of 272.5 km. was used for Hovenweep National Monument based on 

an annual average of twelve monthly average natural condition visual ranges from Table 10 of 

the FLAG Phase I Report – Revised (2010) for Mesa Verde National Park. This background 
3 

visual range was confirmed with Mr. John Notar of the National Park Service during a telephone 

conversation in early September 2012. 

VISCREEN estimates visibility impacts based on input short-term (hourly) emission rates of 

particulates and oxides of nitrogen (NOx) only because the flare does not emit primary sulfates. 

Emissions of nitrogen dioxide (NO2) and secondary particulate (e.g., soot and sulfates) can also 

be input if appropriate for the source being considered. Visibility impacts are based on Gaussian 

dispersion of the modeled emissions that are assumed to be instantaneously transported to the 

impact area being evaluated. A Level I analysis does not consider whether wind flow of a 

specified speed and atmospheric stability is actually in a direction in which the impact area is 

downwind of the modeled source. A Level II analysis addresses this limitation by considering 

site-specific meteorology and using meteorological conditions for only those wind directions 

transporting source emissions to the impact area. Further discussion of meteorological data 

processing is presented in Section 2.4. 

VISCREEN assumes level terrain between the source and the impact area and that the source 

emits at ground-level with no plume rise due to buoyancy and plume momentum. VISCREEN 

considers emissions from one source location at a time. The most appropriate application of 

VISCREEN results is for an averaging period of 12 hours or longer with the input hourly 

emissions assumed to occur for the duration of the period. 

2.2 EVALUATED SENSITIVE CLASS II AREAS 

At the request of the National Park Service, Hovenweep National Monument units located 

closest to the McElmo Creek Oil Production Facility were evaluated for visibility impacts from 

the Main Flare. Specific Hovenweep National Monument (HNM) units evaluated for visibility 

impact were: 

 Cajon Unit - closest boundary 4.8 km. north-northeast from the Main Flare 

 Square Tower - closest boundary 18.0 km. northeast from the Main Flare 

 Holly - closest boundary 21.0 km. northeast from the Main Flare 

 Horseshoe & Hackberry - closest boundary 22.5 km, northeast from the Main Flare 

 Cutthroat Castle - closest boundary 28.0 km. northeast from the Main Flare 
4 

Figure 2-1 presents a map with the location of the McElmo Creek Oil Production Facility and the 

five units of the Hovenweep National Monument evaluated for visibility impacts using a Level II 

VISCREEN analysis. The observer was assumed to be at the closest boundary to the Main Flare 

for each HNM unit to provide a conservative Level II analysis estimate of visibility impacts at 

that unit. The maximum distance between the Main Flare and the evaluated unit was estimated 

by referencing a line-of-sight from the Main Flare to the closest boundary and determining where 

it intersects the unit outer boundary. 

2.3 MAIN GAS FLARE OPERATION AND EMISSIONS 

 

Currently, produced fluids at the oil production facility are processed, sold, and re-injected 

through a series of satellite facilities, main tank batteries, and injection plants. Produced fluids 

are gathered and separated at one of eleven satellite facilities located throughout the field. Gases 

from the satellite facilities are directed to the gas compression plant, and liquids are pumped to 

one of four central tank batteries. Gases sent to the gas compression plant are compressed, dried 

through a glycol dehydrator, blended with carbon dioxide (CO2), and re-injected into the field. 

Liquids are further separated at the central tank batteries, with oil being sold and transported 

through a pipeline system. Water from the central tank batteries is pumped to the water flood 

injection plant. 

The Main Flare at the gas compression plant combusts gases that have been bypassed around the 

plant’s electric-driven compressors or that come through various emergency relief valves in the 

compression plant. Additionally, the vent stream from the plant’s glycol dehydrator is directed 

to the Main Flare. 

 Estimated emissions for the Main Flare used for the VISCREEN Level II analysis are referenced 

from an e-mail sent by Mr. Dwight Mallory of Resolute Petroleum to Mr. Don Shepherd of the 

NPS on August 10, 2012. These emission estimates are based on a maximum process gas 

volumetric flow rate of 618,104 SCF/hr with a heat content of 682.4 BTU/SCF. The NOx

emission factor of 0.0641 lb/MMBTU is based on a Texas Commission on Environmental 

Quality derivation for flares, the SO2 emission factor is based on a maximum process gas content 

of 700 ppmv H2S content and the PM emission factor is based on Section 1.4 of AP-42 for 

natural gas combustion. Because the flare SO2 emissions do not contain primary sulfates, no 
5 

primary sulfate emissions were input to VISCREEN. In summary, the particulates and NOx as 

NO2 emission inputs to VISCREEN were: 

Particulates: 3.1 lb/hr = 0.39 gm/sec 

NOx: 27.1 lb/hr = 3.41 gm/sec 

2.4 METEOROLOGICAL DATA PROCESSING 

A VISCREEN Level I analysis only uses one set of wind speed and atmospheric stability class 

values to determine plume perceptibility and contrast impacts. A Level I analysis is considered 

very conservative because: 

1. The source plume is assumed to instantaneously transport in a straight-line trajectory to 

the nearest sensitive area boundary, without regard to actual wind direction 

2. The source plume is dispersed under a very low wind speed and very stable atmospheric 

conditions. 

A VISCREEN Level II analysis uses inputs similar to the Level I analysis, except for one key 

difference. The selected wind speed and atmospheric stability class are based on a detailed 

meteorological data analysis considering actual wind direction, speed and atmospheric stability 

class distribution. The actual one-percentile worst-case dispersion (i.e., maximum air quality 

impact) meteorological condition transporting a source plume to the nearest boundary of the 

sensitive area is then determined. The meteorological data analysis uses one or more years of 

representative hourly meteorological data values and generates a joint frequency distribution 

(JFD) of wind direction, wind speed and atmospheric stability class for four six-hour periods 

(i.e., Midnight to 6 AM, 6 AM to Noon, Noon to 6 PM and 6 PM to midnight). The onepercentile worst-case dispersion analysis only considers those wind speed and wind direction 

pairs that could directly transport a source plume to the nearest sensitive area boundary within 12 

hours. The one-percentile analysis ranks wind speed and atmospheric stability class pairs by 

dispersion potential in ascending order (i.e., the wind speed and stability class pair with the 

potential for maximum air quality impact, or lowest dispersion potential, is placed first). This 

technique is in accordance with the Workbook for Plume Visual Impact Screening and Analysis. 
6 

2.4.1 Meteorological Data Set 

The meteorological data used to perform a Level II visibility analysis were the same data 

modeled for the 1-hour SO2 impact analysis with AERMOD submitted on June 1, 2011, except 

for the addition of 2010. Hourly surface meteorological data from the Canyonlands Airport, 

located near Moab, UT, for the period of 2005 – 2010 were pre-processed with concurrent Grand 

Junction, CO upper air data through AERMET by the Utah Division of Air Quality (UDAQ). 

The pre-processed meteorological data were then provided by UDAQ for the 1-hour SO2

AERMOD modeling. A joint frequency distribution (JFD) of wind speed and direction for the 

five-year data period of 2005 – 2009 is presented in Table 2-1. Figure 2-2 presents a five-year 

wind rose for the Canyonlands Airport based on the 2005 – 2009 meteorological data. 

Because the pre-processed hourly meteorological data are compatible with the input 

requirements of AERMOD, the determination of hourly Pasquill-Gifford (P-G) atmospheric 

stability class is not as straightforward as from using pre-processed data for earlier EPA 

approved programs such as ISC3. Thus, a means of determining hourly P-G stability class from 

meteorological data output by AERMET was required. The methodology developed to provide 

hourly stability class is presented in Section 2.4.2. 

2.4.2 Determination of Pasquill-Gifford Stability Class 

P-G stability classification historically has been based on either cloud cover/time of day/wind 

speed (P-G method), horizontal wind direction standard deviation (sigma-theta method) or the 

sigma-theta method modified by wind speed and time of day (modified sigma-theta developed 

by Mitchell-Timbre). These P-G stability classification methods were readily useable with 

meteorological data designed to allow this classification for previously approved EPA models 

such as ISC3. Because the dispersion algorithm for AERMOD uses turbulence-based 

parameters, rather than dispersion coefficients based on P-G stability class, a modified method 

for using AERMOD-specific meteorological data to determine P-G stability class is necessary. 

Instead of using hourly sigma-theta, cloud cover or temperature gradient as an initial estimate of 

hourly P-G stability class, hourly Monin-Obukhov (M-O) lengths (i.e., ratio of mechanical 

turbulence to buoyancy-induced turbulence) readily available in AERMOD meteorological data 

input are used to provide an initial estimate of stability class. Similar to the modified sigma-
7 

theta method, the initial stability class estimate based on M-O length is then modified by the 

wind speed and time of day criteria developed by Mitchell-Timbre. 

P-G stability classification based on M-O length has been widely documented with varied 

relationships shown between stability class and M-O length, depending upon surface roughness 

length. A presentation by Roger W. Brode at the EPA Modelers Workshop on May 16, 2006 

entitled, “AERMOD Technical Forum”, showed a relationship between M-O length and P-G 

stability class for surface roughness lengths of 0.1 m. (10 cm.) and 0.5 m. (50 cm.). Because of 

the recent date of this presentation and its relevance to the current use of AERMOD as an EPAapproved model, these relationships between M-O length and P-G stability class are referenced 

for this analysis. The specific relationship for a surface roughness length of 0.1 m. is presented 

below because the calculated five-year hourly average of surface roughness length from the 

Canyonlands Airport meteorological data set is approximately 0.1 m. 

 Stability Monin-Obukhov Length (L) 

 Class Range (m.) 

 A -12.5 < L < 0 

 B -25 < L < -12.5 

 C -65 < L < -25 

 D │L│ > 65 

 E 30 < L < 65 

 F 0 < L < 30 

The initial estimate of hourly P-G stability class based on this scheme was then modified by the 

time of day and wind speed per Mitchell-Timbre. Overall P-G stability classification distribution 

for the Canyonlands Airport meteorological data set for 2005 – 2010 using the above 

methodology is presented in Table 2-2. 

 

2.4.3 Meteorological Data Joint Frequency Distribution 

The six-year data set was processed through an MMA computer program to generate joint 

frequency distributions (JFD) of wind speed, wind direction and atmospheric stability class for 

all hours of the day and the four six-hour periods required for the Level II analysis. The tabular 

JFD for all hours of the day with the typically used six wind speed classes (i.e., 0-3, 4-6, 7-10, 

11-16, 17-21 and > 21 knots) is presented in Appendix A. For the Level II analysis JFDs, wind 
8 

speed classes were defined in one meter per second increments up to 10 m/s. Wind speeds above 

10 m/s were categorized into one class. Wind directions were defined in sixteen 22.5 degree 

sectors and atmospheric stability distinguished by the six Pasquill stability classes (e.g., A – F). 

Tabular summaries of the JFD analysis for each six-hour period (e.g., 0000 – 0600, 0601 – 1200, 

1201 –1800 and 1801 – 2400) are presented in Appendix B. 

2.4.4 Level-II Meteorological Data Analysis 

The one-percentile worst-case meteorological dispersion condition for each HNM unit was 

determined by the following steps: 

1. Identify the set of wind directions in which emissions from the Main Flare would be 

transported to any part of the nearest boundary of the HNM unit area 

2. Divide the closest distance between the HNM unit area and the Main Flare by the midpoint 

value of each wind speed class (e.g., 0.5 is the midpoint of 0.0 – 1.0, 1.5 is the midpoint of 

1.0 – 2.0, etc.) to identify wind speed classes where the transport time from the Main Flare to 

the nearest HNM unit area boundary is less than 12 hours. 

3. For each wind speed class and atmospheric stability class pair, estimate the product of y

(horizontal dispersion coefficient), z (vertical dispersion coefficient) and the upper bound 

value of the wind speed class (e.g., 0.0 – 1.0 would be 1.0). y and z are dependent upon 

distance between the Main Flare and the HNM unit area and the atmospheric stability class. 

4. Arrange the products of yzu in ascending order using the following wind speed and 

atmospheric stability class pairs: F,1 – F,3; E,1 – E,5; D,1 – D,5. 

5. For each wind direction analyzed, tabulate the frequency of occurrence for each wind speed 

and atmospheric stability class pair for each six-hour period; calculate a cumulative 

frequency based on the individual frequency of occurrences for the wind speed and stability 

class pairs in which the transport time is greater than 12 hours. 
9 

6. After the analysis in Step No. 5 has been completed for each wind direction, identify the 

wind speed and stability class pair in which the cumulative frequency first exceeds one 

percent based on individual six-hour period summations for each wind direction. 

7. If intervening terrain between the Main Flare and the HNM unit area boundary exceeds 500 

meters above the base elevation of the Main Flare, then account for the turbulence-inducing 

effect of this terrain on atmospheric stability by adjusting the stability class to one category 

less stable (e.g., F,2 would become E,2). 

The worst-case one percentile meteorological condition (i.e., upper bound wind speed class value 

and stability class) identified by the above procedure is then input to the VISCREEN model to 

provide a refined estimate of potential visibility impacts for the specific HNM unit area 

analyzed. Calculations of sigma y and sigma z for each HNM unit are presented in Appendix C. 

Table 2-3 summarizes the determination of the wind direction transporting Main Flare emissions 

to each HNM unit, the distance between the Main Flare and each HNM unit and the elevation 

difference between the Main Flare and each HNM unit. Worst-case one-percentile 

meteorological conditions (i.e., wind speed and stability class) input to VISCREEN for each 

HNM unit are presented in Sections 3.1 – 3.5. 

WRPLOT View - Lakes Environmental Software

WIND ROSE PLOT

Canyonlands Airport Station

Meteorological Data

DATE:

4/22/2011

PROJECT NUMBER:

2440-10

Figure 4-1

NORTH

SOUTH

WEST EAST

2%

4%

6%

8%

10%

WIND SPEED

(m/s)

>= 11.0

8.4 - 11.0

5.4 - 8.4

3.3 - 5.4

1.8 - 3.3

0.4 - 1.8

Calms: 11.12%

TOTAL COUNT:

43205 hrs.

CALM WINDS:

11.12%

DATA PERIOD:

2005-2009 

Jan 1 - Dec 31

00:00 - 23:00

AVG. WIND SPEED:

2.64 m/s

DISPLAY:

 Wind Speed

Direction (blowing from)
TABLE 2-1. JOINT FREQUENCY DISTRIBUTION OF WIND SPEED AND DIRECTION 

 

 McELMO GAS PLANT

 CANYONLANDS AIRPORT, UT 

 JOINT FREQUENCY DISTRIBUTION 

 (WIND SPEED AND DIRECTION) 

 STABILITY INDEX ALL 

 FROM JANUARY 1 2005 THROUGH DECEMBER 31 2010 

 +------WIND SPEED CLASSES (KNOTS)-------+ AVERAGE 

 WIND 

 DIRECTION 0-3 4-6 7-10 11-16 17-21 >21 TOTAL SPEED 

 N 0.02053 0.01218 0.00343 0.00157 0.00025 0.00004 0.03800 4.43 

 NNE 0.04385 0.02492 0.01456 0.00820 0.00204 0.00038 0.09394 5.52 

 NE 0.01518 0.00755 0.00349 0.00336 0.00085 0.00007 0.03050 5.50 

 ENE 0.01805 0.00732 0.00551 0.00441 0.00090 0.00007 0.03625 5.60 

 E 0.01968 0.01384 0.00782 0.00163 0.00004 0.00002 0.04304 4.82 

 ESE 0.02118 0.02378 0.00764 0.00045 0.00002 0.00000 0.05307 4.65 

 SE 0.02329 0.03117 0.01543 0.00325 0.00018 0.00000 0.07331 5.44 

 SSE 0.01861 0.01400 0.00846 0.00423 0.00040 0.00002 0.04573 5.58 

 S 0.01426 0.00649 0.00558 0.00571 0.00078 0.00007 0.03290 6.31 

 SSW 0.01458 0.00591 0.00770 0.01167 0.00450 0.00067 0.04503 8.74 

 SW 0.02774 0.01055 0.01431 0.01429 0.00520 0.00096 0.07305 7.66 

 WSW 0.07213 0.01572 0.01035 0.00690 0.00233 0.00056 0.10798 4.52 

 W 0.05231 0.02091 0.01825 0.00802 0.00193 0.00058 0.10200 5.37 

 WNW 0.02658 0.01921 0.02091 0.01355 0.00354 0.00078 0.08458 7.30 

 NW 0.02766 0.03229 0.01630 0.00712 0.00264 0.00054 0.08655 6.26 

 NNW 0.01852 0.02452 0.00887 0.00184 0.00027 0.00007 0.05408 5.24 

 TOTAL 0.43415 0.27037 0.16860 0.09618 0.02586 0.00484 1.00000 

 AVG SPD 2.33531 5.27592 8.61656 13.43270 19.02510 26.53308 5.81 

 NUMBER OF CALM HOURS - 0 

 FREQUENCY OF CALMS - 0.00000 

 NUMBER OF MISSING HOURS - 7927 

NUMBER OF HOURS OF DATA - 44657
Table 2-2. ANNUAL STABILITY CLASS DISTRIBUTION AT CANYONLANDS AIRPORT, U

Stability Class Percent of Occurrence (%) 

A 7.66 

B 9.58 C 12.59 

D 24.71 

E 13.43 

F 32.03 

TABLE 2-3. ORIENTATION, DISTANCE AND ELEVATION DIFFERENCE BETWEEN 

SENSITIVE CLASS II AREAS AND MAIN FLARE 

Flare Location 

Easterly Northerly Elev. 

(m.) (m.) (m.) 

 659259.67 4125275.62 1459.45 Hovenweep Easterly Northerly Elev. 

Direction 

from Flare Wind Direction 

Elevation 

Difference 

Unit Name (m.) (m.) (m.) (deg.) (deg.) (sector) (m.) Cajon 660934.65 4129755.00 1581 20.5 200.5 SSW 121.6 Square Tower 670771.15 4139131.40 1594 39.7 219.7 SW 134.6 Holly 673232.73 4141007.15 1632 41.6 221.6 SW 172.6 Horseshoe & Hackberry 674509.80 4141886.90 1682 42.6 222.6 SW 222.6 Cutthroat Castle 678248.68 4145878.15 1767 42.7 222.7 SW 307.6 
10 

3.0 LEVEL II VISIBILITY ANALYSIS RESULTS 

VISCREEN cites two scattering angles (θ) to estimate potential visibility impacts. Θ is the angle 

between the sun and the light of sight. If the observer is looking directly at the sun, the 

scattering angle is zero. If the observer is looking in the opposite direction from the sun, the 

scattering angle is 180o. The first scattering of 10o represents forward scatter where the plume 

would be brightest, while the second scattering angle of 140o

 represents backward scatter where 

the plume would be brightest. 

Print-outs of VISCREEN runs for each all five HNM units are presented in Appendix D. 

Analyses of visibility impacts from VISCREEN with the one-percentile worst-case wind speed 

and stability class conditions for each HNM unit are presented below. Visibility impacts are 

referenced to an observer in the sensitive Class II area. 

3.1 HOVENWEEP CAJON UNIT 

Table 3-1 presents an analysis of one-percentile worst-case wind speed and stability class 

conditions for each six hour period in the event that the Main Flare is combusting bypassed 

process gas during those periods. The worst-case one percentile meteorological condition for 

each six-hour period is summarized below. 

Time Period Stability Class Wind Speed (m/s)

Midnight – 6 AM F 1.0 

6 AM – Noon E 2.0 

Noon – 6 PM D+ 5.0+ 

6 PM – Midnight E 1.0 

+ - Conservative estimate of one-percentile worst-case meteorology; visibility impacts would be much lower than during other six-hour periods 

VISCREEN estimates of visibility impacts when the Main Flare combusts bypassed process gas 

during the Midnight – 6 AM, 6 AM - Noon and 6 PM – Midnight time periods are presented 

below. 
11 

1.0 m/s; F stability 2.0 m/s; E stability 1.0 m/s; E stability 

Background Theta DE Plume 

Contrast DE Plume Contrast DE Plume Contrast 

Sky 10 1.434 0.018 0.437 0.005 0.833 0.010 

Sky 140 1.281 -0.011 0.389 -0.003 0.744 -0.007 

Terrain 10 10.012* 0.025 3.367* 0.009 6.189* 0.017 

Terrain 140 0.301 0.001 0.090 0.000 0.176 0.001 

* - Plume perceptibility (DE) impact exceeds screening criterion of 2.0 

If the Main Flare combusts bypassed process gas during Midnight to Noon or 6 PM to Midnight, 

an observer in the Hovenweep Cajon unit may see a perceptible plume when looking at a terrain 

background and almost directly into the sun. Observed plume contrast would not exceed the 

screening criterion of 0.05 for any time of day with either a sky or terrain background and either 

looking into the sun or away from the sun. If the Main Flare combusts bypassed gas during 

Noon to 6 PM, the observer would not see perceptible plume impairment as a worst-case one 

percentile meteorological would occur during unstable/neutral higher wind speed conditions. 

 

3.2 HOVENWEEP SQUARE TOWER UNIT 

Table 3-2 presents an analysis of one-percentile worst-case wind speed and stability class 

conditions for each six hour period in the event that the Main Flare is combusting bypassed 

process gas during these periods. The worst-case one percentile meteorological condition for 

each six-hour period is summarized below. 

Time Period Stability Class Wind Speed (m/s)

Midnight – 6 AM F 1.0 

6 AM – Noon E 1.0 

Noon – 6 PM D+ 5.0+ 

6 PM – Midnight F 1.0 

+ - Conservative estimate of one-percentile worst-case meteorology; visibility impacts would be much lower than during other six-hour periods 

VISCREEN estimates of visibility impacts when the Main Flare combusts bypassed process gas 

during the Midnight – 6 AM, 6 AM - Noon and 6 PM – Midnight time periods are presented 

below. 
12 

1.0 m/s; F stability 1.0 m/s; E stability 1.0 m/s; F stability 

Background Theta DE Plume 

Contrast DE Plume Contrast DE Plume Contrast 

Sky 10 0.792 0.010 0.443 0.005 0.792 0.010 

Sky 140 0.663 -0.006 0.370 -0.003 0.663 -0.006 

Terrain 10 2.808* 0.013 1.617 0.007 2.808* 0.013 

Terrain 140 0.153 0.001 0.086 0.000 0.153 0.001 

* - Plume perceptibility (DE) impact exceeds screening criterion of 2.0 

If the Main Flare combusts bypassed process gas during Midnight to 6 AM or 6 PM to Midnight, 

an observer in the Hovenweep Square Tower unit may see a slightly perceptible plume when 

looking at a terrain background and almost directly into the sun. Observed plume contrast would 

not exceed the screening criterion of 0.05 for any time of day with either a sky or terrain 

background and either looking into the sun or away from the sun. If the Main Flare combusts 

bypassed gas during 6 AM to 6 PM, the observer would not see perceptible plume impairment. 

3.3 HOVENWEEP HOLLY UNIT 

Table 3-3 presents an analysis of one-percentile worst-case wind speed and stability class 

conditions for each six hour period in the event that the Main Flare is combusting bypassed 

process gas during these periods. The worst-case one percentile meteorological condition for 

each six-hour period is summarized below. 

Time Period Stability Class Wind Speed (m/s)

Midnight – 6 AM F 1.0 

6 AM – Noon E 1.0 

Noon – 6 PM D+ 5.0+ 

6 PM – Midnight F 1.0 

+ - Conservative estimate of one-percentile worst-case meteorology; visibility impacts would be much lower than during other six-hour periods 

VISCREEN estimates of visibility impacts when the Main Flare combusts bypassed process gas 

during the Midnight – 6 AM, 6 AM - Noon and 6 PM – Midnight time periods are presented 

below. 
13 

1.0 m/s; F stability 1.0 m/s; E stability 1.0 m/s; F stability 

Background Theta DE Plume 

Contrast DE Plume Contrast DE Plume Contrast 

Sky 10 0.671 0.008 0.372 0.005 0.671 0.008 

Sky 140 0.560 -0.005 0.311 -0.003 0.560 -0.005 

Terrain 10 2.397* 0.012 1.360 0.007 2.397* 0.012 

Terrain 140 0.147 0.001 0.081 0.000 0.147 0.001 

* - Plume perceptibility (DE) impact exceeds screening criterion of 2.0 

If the Main Flare combusts bypassed process gas during Midnight to 6 AM or 6 PM to Midnight, 

an observer in the Hovenweep Holly unit may see a slightly perceptible plume when looking at a 

terrain background and almost directly into the sun. Observed plume contrast would not exceed 

the screening criterion of 0.05 for any time of day with either a sky or terrain background and 

either looking into the sun or away from the sun. If the Main Flare combusts bypassed gas 

during 6 AM to 6 PM, the observer would not see perceptible plume impairment. 

3.4 HOVENWEEP HORSESHOE AND HACKBERRY UNIT 

Table 3-4 presents an analysis of one-percentile worst-case wind speed and stability class 

conditions for each six hour period in the event that the Main Flare is combusting bypassed 

process gas during these periods. The worst-case one percentile meteorological condition for 

each six-hour period is summarized below. 

Time Period Stability Class Wind Speed (m/s)

Midnight – 6 AM F 2.0 

6 AM – Noon D+ 5.0+ 

Noon – 6 PM D+ 5.0+ 

6 PM – Midnight F 2.0 

+ - Conservative estimate of one-percentile worst-case meteorology; visibility impacts would be much lower than during other six-hour periods 

VISCREEN estimates of visibility impacts when the Main Flare combusts bypassed process gas 

during the Midnight – 6 AM and 6 PM – Midnight time periods are presented below. The worstcase one percentile meteorological conditions for the 6 AM – Noon and Noon to 6 PM time 

periods consist of unstable/neutral higher wind speed conditions and no perceptible plume 

impairment would be expected. 
14 

2.0 m/s; F stability 2.0 m/s; F stability 

Background Theta DE Plume 

Contrast DE Plume Contrast 

Sky 10 0.328 0.004 0.328 0.004 

Sky 140 0.272 -0.003 0.272 -0.003 

Terrain 10 1.142 0.006 1.142 0.006 

Terrain 140 0.072 0.000 0.072 0.000 

 

If the Main Flare combusts bypassed process gas during any time of day, an observer in the 

Hovenweep Horseshoe and Hackberry unit would not see a perceptible plume when looking at a 

terrain background and almost directly into the sun. Observed plume contrast would not exceed 

the screening criterion of 0.05 for any time of day with either a sky or terrain background and 

either looking into the sun or away from the sun. Therefore, the observer would not see 

perceptible plume impairment when the Main Flare combusts bypassed gas during any time of 

day. 

3.5 HOVENWEEP CUTTHROAT CASTLE UNIT 

Table 3-5 presents an analysis of one-percentile worst-case wind speed and stability class 

conditions for each six hour period in the event that the Main Flare is combusting bypassed 

process gas during these periods. The worst-case one percentile meteorological condition for 

each six-hour period is summarized below. 

Time Period Stability Class Wind Speed (m/s)

Midnight – 6 AM F 2.0 

6 AM – Noon D+ 5.0+ 

Noon – 6 PM D+ 5.0+ 

6 PM – Midnight F 2.0 

+ - Conservative estimate of one-percentile worst-case meteorology; visibility impacts would be much lower than during other six-hour periods 

VISCREEN estimates of visibility impacts when the Main Flare combusts bypassed process gas 

during the Midnight – 6 AM and 6 PM – Midnight time periods are presented below. The worstcase one percentile meteorological conditions for the 6 AM – Noon and Noon to 6 PM time 
15 

periods consist of unstable/neutral higher wind speed conditions and no perceptible plume 

impairment would be expected. 

2.0 m/s; F stability 2.0 m/s; F stability 

Background Theta DE Plume 

Contrast DE Plume Contrast 

Sky 10 0.296 0.004 0.296 0.004 

Sky 140 0.241 -0.002 0.241 -0.002 

Terrain 10 0.904 0.005 0.904 0.005 

Terrain 140 0.067 0.000 0.067 0.000 

If the Main Flare combusts bypassed process gas during any time of day, an observer in the 

Hovenweep Cutthroat Castle unit would not see a perceptible plume when looking at a terrain 

background and almost directly into the sun. Observed plume contrast would not exceed the 

screening criterion of 0.05 for any time of day with either a sky or terrain background and either 

looking into the sun or away from the sun. Therefore, the observer would not see perceptible 

plume impairment when the Main Flare combusts bypassed gas during any time of day. 
Source: McElmo Gas Plant Flare

Sensitive Area:Distance from source to nearest boundary: 4.8 km SSWWind Direction: SSW

f cf f cf f 

F,1+ 1 0.5 6,755 6,755 2.7 1.140 1.140 0.440 0.440 0.00

F,2 2 1.5 6,755 13,510 0.9 0.790 1.930 0.140 0.580 0.01

E,1 1 0.5 15,981 15,981 2.7 0.070 2.000 0.280 0.860 0.04

F,3 3 2.5 6,755 20,266 0.5 0.170 2.170 0.030 0.890 0.00

D,1 1 0.5 27,720 27,720 2.7 0.020 2.190 0.100 0.990 0.03

E,2 2 1.5 15,981 31,962 0.9 0.030 2.220 0.090 1.080 0.00

E,3 3 2.5 15,981 47,943 0.5 0.060 2.280 0.050 1.130 0.00

D,2 2 1.5 27,720 55,439 0.9 0.000 2.280 0.050 1.180 0.02

E,4 4 3.5 15,981 63,924 0.4 0.200 2.480 0.060 1.240 0.02

E,5 5 4.5 15,981 79,904 0.3 0.040 2.520 0.010 1.250 0.05

D,3 3 2.5 27,720 83,159 0.5 0.000 2.520 0.010 1.260 0.06

D,4 4 3.5 27,720 110,879 0.4 0.150 2.670 0.040 1.300 0.36

D,5 5 4.5 27,720 138,599 0.3 0.270 2.940 0.040 1.340 0.07

+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods of the day c = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency

0 - 6 6 -12 

Table 3-1. Visibility Level II Screening Analysis

Dispersion 

Condition 

(stability, wind 

speed)

Upper Bound 

Category Wind 

Speed (m/s)

Mean 

Category 

Wind Speed 

(m/s)

sysz at 

Nearest 

Boundary 

(m2)

syszu at Nearest 

Boundary 

(m3s-1)

Transport 

Time 

(hours)

Time of Day

Hovenweep - Cajon Unit

1
Source: McElmo Gas Plant Flare

Sensitive Area: Hovenweep - Square Tower UnitDistance to nearest boundary: 18 kmWind Direction: SW

f cf f cf f 

F,1+ 1 0.5 29,258 29,258 10.0 2.370 2.370 0.630 0.630 0.00

F,2 2 1.5 29,258 58,516 3.3 1.860 4.230 0.220 0.850 0.01

E,1 1 0.5 76,779 76,779 10.0 0.080 4.310 0.390 1.240 0.06

F,3 3 2.5 29,258 87,775 2.0 0.340 4.650 0.050 1.290 0.01

E,2 2 1.5 76,779 153,557 3.3 0.060 4.710 0.120 1.410 0.01

D,1 1 0.5 177,048 177,048 10.0 0.000 4.710 0.130 1.540 0.04

E,3 3 2.5 76,779 230,336 2.0 0.110 4.820 0.080 1.620 0.00

E,4 4 3.5 76,779 307,114 1.4 0.260 5.080 0.070 1.690 0.02

D,2 2 1.5 177,048 354,097 3.3 0.000 5.080 0.100 1.790 0.02

E,5 5 4.5 76,779 383,893 1.1 0.010 5.090 0.010 1.800 0.00

D,3 3 2.5 177,048 531,145 2.0 0.000 5.090 0.030 1.830 0.05

D,4 4 3.5 177,048 708,193 1.4 0.230 5.320 0.050 1.880 0.05

D,5 5 4.5 177,048 885,242 1.1 0.370 5.690 0.080 1.960 0.16

+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods of the day c = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency

Table 3-2. Visibility Level II Screening Analysis

0 - 6 1

Dispersion 

Condition 

(stability, wind 

speed)

Upper Bound 

Category Wind 

Speed (m/s)

Mean 

Category 

Wind Speed 

(m/s)

sysz at 

Nearest 

Boundary 

(m2)

syszu at Nearest 

Boundary 

(m3s-1)

Transport 

Time 

(hours)

Time of Day

6 -12
Source: McElmo Gas Plant Flare

Sensitive Area: Hovenweep - HollyDistance to nearest boundary: 21 kmWind Direction: SW

f cf f cf f 

F,1 + 1 0.5 32,644 32,644 11.7 2.370 2.370 0.630 0.630 0.00

F,2 2 1.5 32,644 65,287 3.9 1.860 4.230 0.220 0.850 0.01

E,1 1 0.5 92,072 92,072 11.7 0.080 4.310 0.390 1.240 0.06

F,3 3 2.5 32,644 97,931 2.3 0.340 4.650 0.050 1.290 0.01

E,2 2 1.5 92,072 184,145 3.9 0.060 4.710 0.120 1.410 0.01

D,1 1 0.5 219,485 219,485 11.7 0.000 4.710 0.130 1.540 0.04

E,3 3 2.5 92,072 276,217 2.3 0.110 4.820 0.080 1.620 0.00

E,4 4 3.5 92,072 368,289 1.7 0.260 5.080 0.070 1.690 0.02

D,2 2 1.5 219,485 438,970 3.9 0.000 5.080 0.100 1.790 0.02

E,5 5 4.5 92,072 460,362 1.3 0.010 5.090 0.010 1.800 0.00

D,3 3 2.5 219,485 658,456 2.3 0.000 5.090 0.030 1.830 0.05

D,4 4 3.5 219,485 877,941 1.7 0.230 5.320 0.050 1.880 0.05

D,5 5 4.5 219,485 1,097,426 1.3 0.370 5.690 0.080 1.960 0.16

+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods of the day c = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency

1

Table 3-3. Visibility Level II Screening Analysis

Dispersion 

Condition 

(stability, wind 

speed)

Upper Bound 

Category Wind 

Speed (m/s)

Mean 

Category 

Wind Speed 

(m/s)

sysz @ 

Nearest 

Boundary

syszu at Nearest 

Boundary 

(m3s-1)

Transport 

Time 

(hours)

Time of Day

0 - 6 6 -12
Source: McElmo Gas Plant Flare

Sensitive Area: Hovenweep - Horseshoe and HackberryDistance to nearest boundary: 22.5 km Wind Direction: SW

f cf f cf f 

F,1 1 0.5 34,824 34,824 12.5 * 2.370 0.000 0.630 0.000 0.00

F,2 + 2 1.5 34,824 69,648 4.2 1.860 1.860 0.220 0.220 0.01

E,1 1 0.5 95,357 95,357 12.5 * 0.080 1.860 0.390 0.220 0.06

F,3 3 2.5 34,824 104,473 2.5 0.340 2.200 0.050 0.270 0.01

E,2 2 1.5 95,357 190,715 4.2 0.060 2.260 0.120 0.390 0.01

D,1 1 0.5 237,889 237,889 12.5 * 0.000 2.260 0.130 0.390 0.04

E,3 3 2.5 95,357 286,072 2.5 0.110 2.370 0.080 0.470 0.00

E,4 4 3.5 95,357 381,429 1.8 0.260 2.630 0.070 0.540 0.02

D,2 2 1.5 237,889 475,779 4.2 0.000 2.630 0.100 0.640 0.02

E,5 5 4.5 95,357 476,787 1.4 0.010 2.640 0.010 0.650 0.00

D,3 3 2.5 237,889 713,668 2.5 0.000 2.640 0.030 0.680 0.05

D,4 4 3.5 237,889 951,557 1.8 0.230 2.870 0.050 0.730 0.05

D,5 5 4.5 237,889 1,189,447 1.4 0.370 3.240 0.080 0.810 0.16

+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods of the day c = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency

Transport 

Time 

(hours)

Time of Day

Table 3-4. Visibility Level II Screening Analysis

0 - 6 6 -12 1

Dispersion 

Condition 

(stability, wind 

speed)

Upper Bound 

Category Wind 

Speed (m/s)

Mean 

Category 

Wind Speed 

(m/s)

sysz at 

Nearest 

Boundary 

(m2)

syszu at Nearest 

Boundary 

(m3s-1)
Source: McElmo Gas Plant Flare

Sensitive Area: Cutthroat CastleDistance from source to nearest boundary: 28 kmWind Direction: SW

f cf f cf f 

F,1 1 0.5 45,333 45,333 15.6 * 2.370 0.000 0.630 0.000 0.00

F,2 + 2 1.5 45,333 90,667 5.2 1.860 1.860 0.220 0.220 0.01

E,1 1 0.5 125,473 125,473 15.6 * 0.080 1.860 0.390 0.220 0.06

F,3 3 2.5 45,333 136,000 3.1 0.340 2.200 0.050 0.270 0.01

E,2 2 1.5 125,473 250,946 5.2 0.060 2.260 0.120 0.390 0.01

D,1 1 0.5 326,253 326,253 15.6 * 0.000 2.260 0.130 0.390 0.04

E,3 3 2.5 125,473 376,419 3.1 0.110 2.370 0.080 0.470 0.00

E,4 4 3.5 125,473 501,892 2.2 0.260 2.630 0.070 0.540 0.02

E,5 5 4.5 125,473 627,365 1.7 0.000 2.630 0.100 0.640 0.02

D,2 2 1.5 326,253 652,506 5.2 0.010 2.640 0.010 0.650 0.00

D,3 3 2.5 326,253 978,759 3.1 0.000 2.640 0.030 0.680 0.05

D,4 4 3.5 326,253 1,305,011 2.2 0.230 2.870 0.050 0.730 0.05

D,5 5 4.5 326,253 1,631,264 1.7 0.370 3.240 0.080 0.810 0.16

+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods of the day c = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency

0 - 6 6 -12 1

Table 3-5. Visibility Level II Screening Analysis

Dispersion 

Condition 

(stability, wind 

speed)

Upper Bound 

Category Wind 

Speed (m/s)

Mean 

Category 

Wind Speed 

(m/s)

sysz at 

Nearest 

Boundary 

(m2)

syszu at Nearest 

Boundary 

(m3s-1)

Transport 

Time 

(hours)

Time of Day
16 

4.0 CONCLUSIONS 

The Hovenweep National Monument units nearest to the Main Flare at the McElmo Creek Unit 

Oil Production Facility are all sensitive Class II areas, ranging in distance from only 4.8 km. to 

28.0 km. away. Potential visibility impacts on these sensitive Class II areas during those 

infrequent times when the Main Flare combusts bypassed process gas depend upon the specific 

meteorology coincident with this intermittent and unplanned operation. However, the 

probability of the Main Flare intermittently combusting bypassed process gas coincident with 

one-percentile worst-case meteorology during a specific time period of the day would be 

extremely low. Nevertheless, conclusions presented below for visibility impacts assume that this 

intermittent Main Flare operation is coincident with one-percentile worst-case meteorology 

occurring during each time period of the day. 

Hovenweep Cajon Unit (4.8 km.)

 Observed plume contrast would not be significant during any time of day with either a 

sky or terrain background and either looking into or away from the sun 

 Observed plume perceptibility may be significant only when looking at a terrain 

background and almost directly into the sun during the Midnight to Noon and 6 PM to 

Midnight time periods 

 Observed plume perceptibility and contrast would not be significant if the Main Flare 

combusts bypassed process gas during the Noon to 6 PM time period 

Hovenweep Square Tower Unit (18.0 km.)

 Observed plume contrast would not be significant during any time of day with either a 

sky or terrain background and either looking into or away from the sun 

 Observed plume perceptibility may be significant only when looking at a terrain 

background and almost directly into the sun during the Midnight to 6 AM and 6 PM to 

Midnight time periods 

 Observed plume perceptibility and contrast would not be significant if the Main Flare 

combusts bypassed process gas during the 6 AM to 6 PM time period 
17 

Hovenweep Holly Unit (21.0 km.)

 Observed plume contrast would not be significant during any time of day with either a 

sky or terrain background and either looking into or away from the sun 

 Observed plume perceptibility may be significant only when looking at a terrain 

background and almost directly into the sun during the Midnight to 6 AM and 6 PM to 

Midnight time periods 

 Observed plume perceptibility and contrast would not be significant if the Main Flare 

combusts bypassed process gas during the 6 AM to 6 PM time period 

Hovenweep Horseshoe and Hackberry Unit (22.5 km.)

 Observed plume contrast would not be significant with either a sky or terrain background 

and either looking into or away from the sun during any time of day 

 Observed plume perceptibility would not be significant when looking at a terrain or sky 

background and looking away or almost directly into the sun during any time of day 

 Therefore, no perceptible plume impairment would occur if the Main Flare combusts 

bypassed process gas during any time of day 

Hovenweep Cutthroat Castle Unit (28.0 km.)

 Observed plume contrast would not be significant with either a sky or terrain background 

and either looking into or away from the sun during any time of day 

 Observed plume perceptibility would not be significant when looking at a terrain or sky 

background and looking away or almost directly into the sun during any time of day 

 Therefore, no perceptible plume impairment would occur if the Main Flare combusts 

bypassed process gas during any time of day 

In summary, perceptible plume impacts could occur during the nighttime hours at the closest 

three Hovenweep units if one-percentile worst-case meteorology is coincident with the Main 

Flare combusting bypassed process gas. At the Cajon Unit only, perceptible plume impacts 

could also occur during the 6 AM to Noon time period. No perceptible plume impacts would 

occur at the two more distant Hovenweep units during any time of day. 
18 

5.0 REFERENCES 

Consent Decree, United States of America v. Mobil Exploration & Producing, U.S., Inc., DOJ 

Case No. 90-5-2-1-2237, filed June 8, 2005 in United States District Court, District of 

Utah. 

Federal Land Managers’ Air Quality Related Values Workgroup (FLAG), Phase I Report, 

 December 2000; Revised Draft October 2010. 

U. S. Environmental Protection Agency, 1992. “Workbook for Visual Impact Screening and 

Analysis”, Office of Air Quality Planning and Standards, Technical Support Division, 

Research Triangle Park, NC (EPA-454/R92-021). 











































APPENDIX D 

VISCREEN VISIBILITY IMPACTS FOR EACH SENSITIVE CLASS II AREA WITH 

LEVEL-II METEOROLOGY FOR EACH SIX HOUR PERIOD