# R ESOLUTE

N ATURAL R ESOURCES

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 80112i 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 DISTRIBU TION 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 DIFFERE NCE BETWEEN SENSITIVE CLASS II AREAS AND MAIN FLARE TABLE 3-1 VISIBILITY LEVEL II SCREEN ING ANALYSIS - HOVENWEEP CAJON UNIT TABLE 3-2 VISIBILITY LEVEL II SCREEN ING ANALYSIS - HOVENWEEP SQUARE TOWER UNITii TABLE 3-3 VISIBILITY LEVEL II SCR EENING 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 CANYONLA NDS 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 FREQUE NCY DISTRIBUTIONS OF WIND SPEED, WIND DIRECTION AND STABILIT Y FOR CANYONLANDS, UT FOR 2005 – 2010 FOR EACH SIX HOUR PERIOD

APPENDIX C CALCULATION OF SIGMA Y AND SIGM A Z FOR EACH HOVENWEEP NATIONAL MONUMENT UNIT

APPENDIX D VISCREEN VISIBILITY IM PACTS FOR EACH SENSITIVE CLASS II AREA WITH LEVEL-II METEORO LOGY FOR EACH SIX HOUR PERIOD1 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 re quest 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 Flar e. Hovenweep National

Monument is a Class II area with units located northea st of the Main Flar e 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). Th is Consent Decree was issued to th e previous owner/ operator of the facility, ExxonMobil Corporation (ExxonMobil) . The PSD permit application document presented a facility process description for exis ting operations, criteria pollutant permit emission limits for the Main Flare, a Best Available C ontrol Technology (BACT) analysis for the Main Flare, air modeling methodology, and air modeling resu lts. 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 in creased emissions due to pro cessing flare gas with a higher BTU content than previously cited.

An additional modeling study for 1-hour SO 2 impacts was submitted on June 1, 2011 in response to a request by EPA Region IX during a meeti ng held on November 30, 2010. This air modeling study was submitted as a supplement to the alrea dy submitted permit application because none of the other non-air modeling information had changed. The SO 2 modeling study was necessitated by the new 1-hour SO 2 ambient air quality standard, eff ective on August 23, 2010. A new 1-hour

NO 2 standard also became effective on April 12 , 2010. However, modeling in the PSD permit application showed maximum 1-hour NO 2 impacts from the Main Flare to be below the significant impact level (SIL), while maximum 1-hour SO 2 impacts from the Main Flare were slightly above the SIL.2 2.0 IMPACT ANALYSIS APPROACH

Visibility can be affected by plume im pairment (heterogeneous ) or regional haze (homogeneous). Plume impairment occurs at loca tions generally less than 50 kilometers from emitting source, while regional haze occurs at lo cations generally greater than 50 kilometers from the emitting source. Therefore, the visibili ty impact analysis presented for Main Flare emissions at the Hovenweep National Monument ar e 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 ev aluating visibility im pacts incorporating techniques presented in the Workbook for Plume Visual Impact Screening and Analysis (EPA454/R-92-021). VISCREEN interna lly estimates visibility imp acts 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 (C p ). Maximum visibility impacts are deemed as being insi gnificantly perceptible if they are below both a  E of

2.00 and a C p of  0.050.

For both a screening (Level I) and a refined (L evel II) analysis, the following default input assumptions are used:

 Particle size distribution

 Background ozone concen tration (0.040 ppm)

A background visual range of 272.5 km. was us ed for Hovenweep National Monument based on an annual average of twelve m onthly average natural condition vi sual ranges from Table 10 of the FLAG Phase I Report – Revised (2010) for Mesa Verde National Pa rk. This background3 visual range was confirmed with Mr. John Notar of the National Park Service du ring a telephone conversation in early September 2012.

VISCREEN estimates visibility impacts based on input short-term (hour ly) emission rates of particulates and oxides of nitrogen (NO x ) only because the flare does not emit primary sulfates. Emissions of nitrogen dioxide (NO 2 ) and secondary particulate (e.g., soot and sulfates) can also be input if appropriate for the source being cons idered. Visibility impacts are based on Gaussian dispersion of the modeled emissions that are assu med to be instantaneously transported to the impact area being evaluated. A Level I analys is does not consider whether wind flow of a specified speed and atmospheric stability is actua lly in a direction in which the impact area is

downwind of the modeled source. A Level II an alysis addresses this limitation by considering site-specific meteorology and using meteorological conditions for only those wind directions transporting source emissions to the impact ar ea. Further discussion of meteorological data processing is presented in Section 2.4.

VISCREEN assumes level terrain between the s ource 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 appr opriate 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 Servi ce, Hovenweep National Monument units located

closest to the McElmo Creek Oil Production Facil ity 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 Flare4 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 ev aluated for visibility impacts using a Level II VISCREEN analysis. The observer was assumed to be at the closest bounda ry 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 Ma in Flare and the evaluated unit was estimated by referencing a line-of-sight from the Main Fl are 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 satell ite 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, ble nded with carbon dioxide (CO 2 ), 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 co mbusts gases that have been bypassed around the plant’s electric-driven compressors or that come through various em ergency 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 an alysis are referenced from an e-mail sent by Mr. Dwight Mallory of Resolute Petroleu m to Mr. Don Sh epherd of the NPS on August 10, 2012. These emission estimat es 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 NO x emission factor of 0.0641 lb/MMBTU is ba sed on a Texas Commission on Environmental Quality derivation for flares, the SO 2 emission factor is based on a maximum process gas content of 700 ppmv H 2 S content and the PM emission factor is based on Section 1.4 of AP-42 for natural gas combustion. Because the flare SO 2 emissions do not contain primary sulfates, no5 primary sulfate emissions were input to VIS CREEN. In summary, th e particulates and NO x as NO 2 emission inputs to VISCREEN were:

Particulates: 3.1 lb/hr = 0.39 gm/sec

NO x : 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, wit hout 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 simila r 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 c onsidering actual wind direction, sp eed and atmospheric stability

class distribution. The actual one-percentile worst-case dispersi on (i.e., maximum air quality impact) meteorological condition transporting a source plume to the nearest boundary of the

sensitive area is then determined. The meteorol ogical data analysis uses one or more years of representative hourly meteorologi cal data values and generates a joint frequency distribution (JFD) of wind direction, wind speed and atmosphe ric 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 sp eed and wind direction pairs that could directly transport a source plume to the near est 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 poten tial, 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 SO 2 impact analysis with AERMOD submitted on June 1, 2011, except for the addition of 2010. Hourly surface meteorol ogical data from the Canyonlands Airport, located near Moab, UT, for the period of 2005 – 2010 were pre-processed w ith concurrent Grand Junction, CO upper air data through AERMET by the Utah Division of Air Quality (UDAQ). The pre-processed meteorological data we re then provided by UDAQ for the 1-hour SO 2 AERMOD modeling. A joint frequency distributi on (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 ba sed on the 2005 – 2009 meteorological data.

Because the pre-processed hourly meteorologi cal 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-processe d 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 re quired. The methodology developed to provide hourly stability class is pr esented in Section 2.4.2.

2.4.2 Determination of Pasqu ill-Gifford Stability Class

P-G stability classification hist orically has been based on eith er cloud cover/time of day/wind speed (P-G method), horizontal wind direction st andard 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 clas sification 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 cla ss, a modified method for using AERMOD-specific meteorological data to de termine P-G stability class is necessary. Instead of using hourly sigma-thet a, cloud cover or temperature grad ient as an initial estimate of hourly P-G stability class, hourly Monin-Obu khov (M-O) lengths (i.e., ratio of mechanical turbulence to buoyancy-induced tu rbulence) readilyavailable in AERMOD meteorological data input are used to provide an ini tial estimate of stability class. Similar to the modified sigma-7 theta method, the initial stability class estimate based on M-O leng th is then mo dified 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 cla ss 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 rele vance 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 da ta 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 Mitc hell-Timbre. Overall P-G stab ility 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, wi nd 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 us ed six wind speed classes (i.e., 0-3, 4-6, 7-10, 11-16, 17-21 and > 21 knots) is pres ented in Appendix A. For the Level II analysis JFDs, wind8 speed classes were defined in one meter per s econd increments up to 10 m/s. Wind speeds above 10 m/s were categorized into one class. Wind di rections 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 fo r 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 meteorologi cal dispersion condition for each HNM unit was determined by the following steps:

1. Identify the set of wind directions in wh ich 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 be tween 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 wher e 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 stab ility class pair, estimate the product of  y (horizontal dispersion coefficient),  z (vertical dispersion coe fficient) 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  z u 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 pe riod 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 th e 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 wi nd direction transporting Main Flare emissions to each HNM unit, the distance between the Ma in 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 cla ss) 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%

W I N D S P E E D ( m / s )

> = 1 1

. 0

8

. 4

- 1 1

. 0

5

. 4

- 8

. 4

3

. 3

- 5

. 4

1

. 8

- 3

. 3

0

. 4

- 1

. 8

C a l m s : 1 1

. 1 2 %

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 FREQUE NCY DISTRIBUTION OF WI ND 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, UTAHStability Class Percent of Occurrence (%)A 7.66B 9.58C 12.59D 24.71E 13.43F 32.03TABLE 2-3. ORIENTATION, DISTANCE AND ELEVATION DIFFERENCE BETWEENSENSITIVE CLASS II AREAS AND MAIN FLAREFlare LocationEasterlyNortherlyElev.(m.)(m.)(m.)659259.67 4125275.62 1459.45HovenweepEasterlyNortherlyElev.Directionfrom FlareWind DirectionElevationDifferenceDistance fromFlareUnit Name(m.)(m.)(m.)(deg.)(deg.)(sector)(m.)(km.)Cajon 660934.65 4129755.00 1581 20.5 200.5 SSW 121.6 4.8Square Tower 670771.15 4139131.40 1594 39.7 219.7 SW 134.6 18.0Holly 673232.73 4141007.15 1632 41.6 221.6 SW 172.6 21.0Horseshoe & Hackberry 674509.80 4141886.90 1682 42.6 222.6 SW 222.6 22.5Cutthroat Castle 678248.68 4145878.15 1767 42.7 222.7 SW 307.6 28.010 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 l ooking directly at the sun, the scattering angle is zero. If th e observer is looking in the oppos ite direction from the sun, the scattering angle is 180 o . The first scattering of 10 o represents forward scatter where the plume would be brightest, while the second scatteri ng angle of 140 o represents backward scatter where the plume would be brightest.

Print-outs of VISCREEN runs for each all fi ve 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-percen tile worst-case wind speed and stability class conditions for each six hour period in the even t that the Main Flare is combusting bypassed process gas during those periods. The worst-ca se 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 s ix-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 D E Plume Contrast D E Plume Contrast D E 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 ( D E) 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 ti me of day with either a sky or terrain background and either looking into the sun or away fr om the sun. If the Main Fl are combusts bypassed gas during Noon to 6 PM, the observer would not see percep tible plume impairment as a worst-case one percentile meteorological would occur during un stable/neutral higher wind speed conditions.

3.2 HOVENWEEP SQUARE TOWER UNIT

Table 3-2 presents an analysis of one-percen tile worst-case wind speed and stability class conditions for each six hour period in the even t that the Main Flare is combusting bypassed process gas during these periods. The worst-ca se 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 s ix-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 D E Plume Contrast D E Plume Contrast D E 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 ( D E) impact exceeds screening criterion of 2.0

If the Main Flare combusts bypass ed process gas during Midnight to 6 AM or 6 PM to Midnight, an observer in the Hovenweep Square Tower un it may see a slightly perceptible plume when looking at a terrain background and al most directly into the sun. Observed plume contrast would not exceed the screening criteri on of 0.05 for any time of day w ith 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 woul d not see perceptible plume impairment.

3.3 HOVENWEEP HOLLY UNIT

Table 3-3 presents an analysis of one-percen tile worst-case wind speed and stability class conditions for each six hour period in the even t that the Main Flare is combusting bypassed process gas during these periods. The worst-ca se 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 s ix-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 D E Plume Contrast D E Plume Contrast D E 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 ( D E) impact exceeds screening criterion of 2.0

If the Main Flare combusts bypass ed process gas during Midnight to 6 AM or 6 PM to Midnight, an observer in the Hovenweep Ho lly unit may see a slightly per ceptible plume when looking at a terrain background and almost dire ctly 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-percen tile worst-case wind speed and stability class conditions for each six hour period in the even t that the Main Flare is combusting bypassed process gas during these periods. The worst-ca se 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 s ix-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 ar e 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 wi nd 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 D E Plume Contrast D E 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 plum e when looking at a terrain background and almost dire ctly 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 th e sun. Therefore, the observer would not see perceptible plume impairment when the Main Fl are combusts bypassed ga s during any time of day.

3.5 HOVENWEEP CUTTHROAT CASTLE UNIT

Table 3-5 presents an analysis of one-percen tile worst-case wind speed and stability class conditions for each six hour period in the even t that the Main Flare is combusting bypassed process gas during these periods. The worst-ca se 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 s ix-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 ar e presented below. The worstcase one percentile meteorological conditions for the 6 AM – Noon and Noon to 6 PM time15 periods consist of unstable/neutral higher wi nd speed conditions and no perceptible plume impairment would be expected.

2.0 m/s; F stability 2.0 m/s; F stability Background Theta D E Plume Contrast D E 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 wh en looking at a terrain background and almost directly in to the sun. Observed plume contrast would not exceed the screening criterion of 0.05 for any ti me of day with either a sky or terrain background and either looking into the sun or away from the sun. Th erefore, the observer wo uld not see perceptible plume impairment when the Main Flare combus ts bypassed gas during any time of day.Source: McElmo Gas Plant FlareSensitive Area:Distance from source to nearest boundary: 4.8 km SSWWind Direction: SSWf cf f cf f cf f cfF,1+1 0.5 6,7556,7552.71.1401.1400.440 0.440 0.000 0.000 0.420 0.420F,2 2 1.5 6,75513,5100.90.790 1.930 0.140 0.580 0.010 0.010 0.470 0.890E,1 1 0.5 15,98115,9812.70.070 2.000 0.280 0.860 0.040 0.050 0.1301.020F,3 3 2.5 6,75520,2660.50.170 2.170 0.030 0.890 0.000 0.050 0.180 1.200D,1 1 0.5 27,72027,7202.70.020 2.190 0.100 0.990 0.030 0.080 0.030 1.230E,2 2 1.5 15,98131,9620.90.030 2.220 0.0901.0800.000 0.080 0.120 1.350E,3 3 2.5 15,98147,9430.50.060 2.280 0.050 1.130 0.000 0.080 0.200 1.550D,2 2 1.5 27,72055,4390.90.000 2.280 0.050 1.180 0.020 0.100 0.070 1.620E,4 4 3.5 15,98163,9240.40.200 2.480 0.060 1.240 0.020 0.120 0.190 1.810E,5 5 4.5 15,98179,9040.30.040 2.520 0.010 1.250 0.050 0.170 0.010 1.820D,3 3 2.5 27,72083,1590.50.000 2.520 0.010 1.260 0.060 0.230 0.040 1.860D,4 4 3.5 27,720110,8790.40.150 2.670 0.040 1.300 0.360 0.590 0.250 2.110D,5 5 4.5 27,720138,5990.30.270 2.940 0.040 1.340 0.070 0.660 0.450 2.560+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods ofthe dayc = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency18 - 240 - 6 6 -12Table 3-1. Visibility Level II Screening AnalysisDispersionCondition(stability, windspeed)Upper BoundCategory WindSpeed (m/s)MeanCategoryWind Speed(m/s)syszatNearestBoundary(m2)syszu at NearestBoundary(m3s-1)TransportTime(hours)Time of DayHovenweep - Cajon Unit12 - 18Source: McElmo Gas Plant FlareSensitive Area: Hovenweep - Square Tower UnitDistance to nearest boundary: 18 kmWind Direction: SWf cf f cf f cf f cfF,1+1 0.5 29,25829,25810.02.3702.3700.630 0.630 0.000 0.000 1.3001.300F,2 2 1.5 29,25858,5163.31.860 4.230 0.220 0.850 0.010 0.010 2.140 3.440E,1 1 0.5 76,77976,77910.00.080 4.310 0.3901.2400.060 0.070 0.910 4.350F,3 3 2.5 29,25887,7752.00.340 4.650 0.050 1.290 0.010 0.080 1.000 5.350E,2 2 1.5 76,779153,5573.30.060 4.710 0.120 1.410 0.010 0.090 0.400 5.750D,1 1 0.5 177,048177,04810.00.000 4.710 0.130 1.540 0.040 0.130 0.050 5.800E,3 3 2.5 76,779230,3362.00.110 4.820 0.080 1.620 0.000 0.130 0.500 6.300E,4 4 3.5 76,779307,1141.40.260 5.080 0.070 1.690 0.020 0.150 0.520 6.820D,2 2 1.5 177,048354,0973.30.000 5.080 0.100 1.790 0.020 0.170 0.120 6.940E,5 5 4.5 76,779383,8931.10.010 5.090 0.010 1.800 0.000 0.170 0.040 6.980D,3 3 2.5 177,048531,1452.00.000 5.090 0.030 1.830 0.050 0.220 0.120 7.100D,4 4 3.5 177,048708,1931.40.230 5.320 0.050 1.880 0.050 0.270 0.540 7.640D,5 5 4.5 177,048885,2421.10.370 5.690 0.080 1.960 0.160 0.430 1.170 8.810+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods ofthe dayc = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequencyTable 3-2. Visibility Level II Screening Analysis12 - 18 18 - 240 - 6DispersionCondition(stability, windspeed)Upper BoundCategory WindSpeed (m/s)MeanCategoryWind Speed(m/s)syszatNearestBoundary(m2)syszu at NearestBoundary(m3s-1)TransportTime(hours)Time of Day6 -12Source: McElmo Gas Plant FlareSensitive Area: Hovenweep - HollyDistance to nearest boundary: 21 kmWind Direction: SWf cf f cf f cf f cfF,1 +1 0.5 32,644 32,64411.72.3702.3700.630 0.630 0.000 0.000 1.3001.300F,2 2 1.5 32,644 65,2873.91.860 4.230 0.220 0.850 0.010 0.010 2.140 3.440E,1 1 0.5 92,072 92,07211.70.080 4.310 0.3901.2400.060 0.070 0.910 4.350F,3 3 2.5 32,644 97,9312.30.340 4.650 0.050 1.290 0.010 0.080 1.000 5.350E,2 2 1.5 92,072 184,1453.90.060 4.710 0.120 1.410 0.010 0.090 0.400 5.750D,1 1 0.5 219,485 219,48511.70.000 4.710 0.130 1.540 0.040 0.130 0.050 5.800E,3 3 2.5 92,072 276,2172.30.110 4.820 0.080 1.620 0.000 0.130 0.500 6.300E,4 4 3.5 92,072 368,2891.70.260 5.080 0.070 1.690 0.020 0.150 0.520 6.820D,2 2 1.5 219,485 438,9703.90.000 5.080 0.100 1.790 0.020 0.170 0.120 6.940E,5 5 4.5 92,072 460,3621.30.010 5.090 0.010 1.800 0.000 0.170 0.040 6.980D,3 3 2.5 219,485 658,4562.30.000 5.090 0.030 1.830 0.050 0.220 0.120 7.100D,4 4 3.5 219,485 877,9411.70.230 5.320 0.050 1.880 0.050 0.270 0.540 7.640D,5 5 4.5 219,485 1,097,4261.30.370 5.690 0.080 1.960 0.160 0.430 1.170 8.810+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods ofthe dayc = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency12 - 18 18 - 24Table 3-3. Visibility Level II Screening AnalysisDispersionCondition(stability, windspeed)Upper BoundCategory WindSpeed (m/s)MeanCategoryWind Speed(m/s)sysz@NearestBoundarysyszu at NearestBoundary(m3s-1)TransportTime(hours)Time of Day0 - 6 6 -12Source: McElmo Gas Plant FlareSensitive Area: Hovenweep - Horseshoe and HackberryDistance to nearest boundary: 22.5 kmWind Direction: SWf cf f cf f cf f cfF,1 1 0.5 34,82434,82412.5* 2.370 0.000 0.630 0.000 0.000 0.000 1.300 0.000F,2 +2 1.5 34,82469,6484.21.8601.8600.220 0.220 0.010 0.010 2.1402.140E,1 1 0.5 95,35795,35712.5* 0.080 1.860 0.390 0.220 0.060 0.010 0.910 2.140F,3 3 2.5 34,824104,4732.50.340 2.200 0.050 0.270 0.010 0.020 1.000 3.140E,2 2 1.5 95,357190,7154.20.060 2.260 0.120 0.390 0.010 0.030 0.400 3.540D,1 1 0.5 237,889237,88912.5* 0.000 2.260 0.130 0.390 0.040 0.030 0.050 3.540E,3 3 2.5 95,357286,0722.50.110 2.370 0.080 0.470 0.000 0.030 0.500 4.040E,4 4 3.5 95,357381,4291.80.260 2.630 0.070 0.540 0.020 0.050 0.520 4.560D,2 2 1.5 237,889475,7794.20.000 2.630 0.100 0.640 0.020 0.070 0.120 4.680E,5 5 4.5 95,357476,7871.40.010 2.640 0.010 0.650 0.000 0.070 0.040 4.720D,3 3 2.5 237,889713,6682.50.000 2.640 0.030 0.680 0.050 0.120 0.120 4.840D,4 4 3.5 237,889951,5571.80.230 2.870 0.050 0.730 0.050 0.170 0.540 5.380D,5 5 4.5 237,8891,189,4471.40.370 3.240 0.080 0.810 0.160 0.330 1.170 6.550+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods ofthe dayc = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequencyTransportTime(hours)Time of DayTable 3-4. Visibility Level II Screening Analysis12 - 18 18 - 240 - 6 6 -12DispersionCondition(stability, windspeed)Upper BoundCategory WindSpeed (m/s)MeanCategoryWind Speed(m/s)syszatNearestBoundary(m2)syszu at NearestBoundary(m3s-1)Source: McElmo Gas Plant FlareSensitive Area: Cutthroat CastleDistance from source to nearest boundary: 28 kmWind Direction: SWf cf f cf f cf f cfF,1 1 0.5 45,33345,33315.6* 2.370 0.000 0.630 0.000 0.000 0.000 1.300 0.000F,2 +2 1.5 45,33390,6675.21.8601.8600.220 0.220 0.010 0.010 2.1402.140E,1 1 0.5 125,473125,47315.6* 0.080 1.860 0.390 0.220 0.060 0.010 0.910 2.140F,3 3 2.5 45,333136,0003.10.340 2.200 0.050 0.270 0.010 0.020 1.000 3.140E,2 2 1.5 125,473250,9465.20.060 2.260 0.120 0.390 0.010 0.030 0.400 3.540D,1 1 0.5 326,253326,25315.6* 0.000 2.260 0.130 0.390 0.040 0.030 0.050 3.540E,3 3 2.5 125,473376,4193.10.110 2.370 0.080 0.470 0.000 0.030 0.500 4.040E,4 4 3.5 125,473501,8922.20.260 2.630 0.070 0.540 0.020 0.050 0.520 4.560E,5 5 4.5 125,473627,3651.70.000 2.630 0.100 0.640 0.020 0.070 0.120 4.680D,2 2 1.5 326,253652,5065.20.010 2.640 0.010 0.650 0.000 0.070 0.040 4.720D,3 3 2.5 326,253978,7593.10.000 2.640 0.030 0.680 0.050 0.120 0.120 4.840D,4 4 3.5 326,2531,305,0112.20.230 2.870 0.050 0.730 0.050 0.170 0.540 5.380D,5 5 4.5 326,2531,631,2641.70.370 3.240 0.080 0.810 0.160 0.330 1.170 6.550+ - Worst-case one-percentile meteorological dispersion conditionNote: Cumulative frequencies in red indicate one-percentile meteorological dispersion condition for other six-hour periods ofthe dayc = frequencycf = cumulative frequency* Plume transport time is greater than 12 hours, frequency for this hour is not included in cumulative frequency12 - 18 18 - 240 - 6 6 -12Table 3-5. Visibility Level II Screening AnalysisDispersionCondition(stability, windspeed)Upper BoundCategory WindSpeed (m/s)MeanCategoryWind Speed(m/s)syszatNearestBoundary(m2)syszu at NearestBoundary(m3s-1)TransportTime(hours)Time of Day16 4.0 CONCLUSIONS

The Hovenweep National Monument units nearest to the Main Flare at the McElmo Creek Unit Oil Production Facility are all se nsitive 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 combus ts bypassed process gas depend upon the specific meteorology coincident with this intermittent and unplanned operation. However, the probability of the Main Flare intermittently co mbusting bypassed process gas coincident with one-percentile worst-case meteorology during a sp ecific 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 woul d not be significant during an y time of day with either a sky or terrain background and either looking into or away from the sun

 Observed plume perceptibility may be si gnificant only when looking at a terrain background and almost directly into the sun during the Midn ight 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 woul d not be significant during an y time of day with either a sky or terrain background and either looking into or away from the sun

 Observed plume perceptibility may be si gnificant 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 period17 Hovenweep Holly Unit (21.0 km.)

 Observed plume contrast woul d not be significant during an y time of day with either a sky or terrain background and either looking into or away from the sun

 Observed plume perceptibility may be si gnificant 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 signifi cant with either a s ky or terrain background and either looking into or away fr om the sun during any time of day

 Observed plume perceptibility would not be si gnificant when looking at a terrain or sky background and looking away or almost direct ly 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 signifi cant with either a s ky or terrain background and either looking into or away fr om the sun during any time of day

 Observed plume perceptibility would not be si gnificant when looking at a terrain or sky background and looking away or almost direct ly into the sun during any time of day

 Therefore, no perceptible plume impairment would occur if the Ma in Flare combusts bypassed process gas during any time of day

In summary, perceptible plume impacts could o ccur during the nighttime hours at the closest three Hovenweep units if one-p ercentile worst-case meteorology is coincident with the Main Flare combusting bypassed proces s gas. At the Cajon Unit on ly, 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 Hove nweep 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 Un ited States District Court, District of

Utah.

Federal Land Managers’ Air Quality Relate d 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 Qual ity Planning and Standards, Technical Support Division, Research Triangle Par k, NC (EPA-454/R92-021).APPENDIX D

VISCREEN VISIBILITY IMPACTS FOR E ACH SENSITIVE CLASS II AREA WITH

LEVEL-II METEOROLOGY FOR EACH SIX HOUR PERIOD