7-10 

7.4.4 Soils and Vegetation 

Ambient air impacts from the Main Flare presented in Sections 7.1 – 7.3 could have potential 

effects on potentially sensitive soils and vegetation in the region surrounding the Main Flare. 

However, maximum modeled impacts of SO2, NO2 and CO are insignificant and their effects on 

regional soils and vegetation are also insignificant. To demonstrate the insignificance of 

potential effects on regional soils and vegetation from modeled Main Flare emissions impacts, a 

quantitative assessment of these impacts is presented. This assessment is based on guidance 

presented in “A Screening Procedure for the Impacts of Air Pollution Source on Plants, Soils and 

Animals” (U. S. EPA, 1980). Modeled ambient air impacts of NO2, SO2 and CO are compared 

to minimum ambient air concentrations leading to a perceptible effect on sensitive vegetation in 

San Juan County, Utah. Because PM emissions (3.1 lb/hr and 13.8 tpy) from the Main Flare are 

small and metals content in the emitted PM are minimal, effects on soils and vegetation are also 

insignificant. However, the very small increase in soil metal content potentially associated with 

Main Flare combustion PM emissions are estimated per Screening Procedure guidance to 

illustrate their insignificant impact. 

Vegetation in San Juan County include woodlands, dominated by Pinyon Pine and Utah Juniper, 

desert and semi-desert shrubs classified as saltbush because of the prevailing saline soils, 

sagebrush, blackbrush and scattered grassland areas. Specific vegetation designated as sensitive 

for San Juan County are presented in Table 7-8. This list is based on Table 13.1a of the 

“Analysis of Management Situation”, prepared by the Monticello BLM Field Office in January 

2005. 

NO2, SO2 and CO Impacts

A summary of maximum modeled NO2, SO2 and CO impacts from the Main Flare from Sections 

7.1 – 7.3 for various averaging periods corresponding to exposure periods for sensitive 

vegetation are presented below. 

 1-hour 3-hour 8-hour 24-hour Annual 

Pollutant (μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3)

 NO2 2.456 2.210 1.719 0.982 0.15 

 SO2 11.63 10.47 8.14 4.65 0.93 

 CO 21.04 18.94 14.73 8.42 1.68 
7-11 

To assess whether these maximum ambient air impacts are significant when considering 

sensitive vegetation exposure, screening ambient air concentrations from Table 3.1 of the 

Screening Procedure guidance document are presented in Table 7-9. A comparison by pollutant 

of maximum modeled ambient air impacts to screening concentrations for sensitive vegetation 

exposure is presented below. 

NO2

 3-hour 8-hour Annual 

 Max Screening Max Screening Max Screening 

Impact Conc. Impact Conc. Impact Conc. 

(μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3)

 2.210 3,760 1.719 3,760 0.15 94 

SO2

 1-hour 3-hour Annual 

 Max Screening Max Screening Max Screening 

Impact Conc. Impact Conc. Impact Conc. 

(μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3)

 11.63 917 10.47 786 0.93 18 

CO 

 1-week 

 Max Screening 

 Impact Conc. 

 (μg/m3) (μg/m3) 

 8.42* 1,800,000 

* - Maximum 24-hour averaged impact used as conservative estimator of 1-week exposure period 

Conclusions for potential effects of Main Flare NO2, SO2 and CO emissions on sensitive 

vegetation are the following: 

 Sensitive vegetation exposure to maximum NO2 ambient air impacts from the Main Flare 

are nearly three orders of magnitude below respective screening concentrations 

 Sensitive vegetation exposure to maximum SO2 ambient air impacts from the Main Flare 

are approximately 1% of the short-term (1 and 3 hour) exposure screening concentration 

and approximately 5% of the long-term (annual) exposure screening concentration 

 Sensitive vegetation exposure to maximum CO ambient air impacts from the Main Flare 

is at least five orders of magnitude below the one-week exposure screening concentration 
7-12 

Section 3.2.3 of the Screening Procedure guidance document discusses the possible interaction of 

SO2 and NO2 ambient air concentrations on sensitive vegetation and provides coincident 1-hour 

SO2 and NO2 screening concentrations that may present a “synergistic” effect. However, this 

discussion cautions using these screening concentrations as conclusive evidence of an adverse 

synergistic effect on sensitive vegetation. The 1-hour exposure screening concentration 

presented in Table 3.3 of the Screening Procedure guidance document for SO2 and NO2 is 5 

ppmv or 131 and 94 μg/m3, respectively. The maximum 1-hour modeled impacts of SO2 and 

NO2 are 11.63 and 2.456 μg/m3, respectively. These impacts are less than one percent of the 

synergistic screening concentrations. 

In conclusion, there are no adverse effects on regional sensitive vegetation from Main Flare NOx, 

SO2 and CO emissions either individually or in combination. 

PM Impacts

Potentially adverse impacts to soils and vegetation from PM emissions are primarily due to the 

metals content of the emitted PM. Table 3.4 of the Screening Procedure guidance document 

presents screening soil concentrations, in ppmw, that would potentially have adverse impacts on 

sensitive vegetation located in the soil. Equation 5.1 of the Screening Procedure guidance 

document estimates the resultant soil concentration increase from ambient air metals 

concentration as presented below. 

DC (ppmw) = 21.5 (N/d) χ

where: 

DC = deposited concentration (ppmw) 

 N = expected lifetime of source (yr.); assume Main Flare lifetime of 70 yr. 

 d = depth of soil which deposited material is distributed (cm.); 3 cm. is recommended value 

 χ = maximum annual average ambient concentration from source (μg/m3

) 

Maximum ambient air PM impacts from the Main Flare are estimated by applying the ratio of 

estimated hourly emissions of PM and NOx to estimated maximum ambient air NO2 

concentrations. The maximum PM and NOx emissions are 3.1 lb/hr and 27.1 lb/hr, respectively. 

Applying the ratio of 3.1/27.1 to the maximum annual NO2 ambient air impact in Section 7.1 

provides the following estimate of the maximum annual PM ambient air impact: 
7-13 

Maximum Annual PM Impact = (3.1 lb/hr÷27.1 lb/hr) x 0.1965 μg/m3

* = 0.02248 μg/m3

* Annual NO2 impact, uncorrected by the default ozone-limiting ratio of 0.75 

Estimated soil concentration increase for each metal in the Main Flare PM emissions is estimated 

by the following steps: 

1. Metals content of PM emissions from the Main Flare estimated from the ratio of each 

metal emission factor and the total PM emission factor in Tables 1.4-4 and 1.4-2 of AP42 for natural gas combustion, expressed as lb/106

 SCF of natural gas (EPA, 1998). 

2. Maximum annual impact for each metal estimated by the product of the maximum annual 

PM impact and metal/total PM emissions ratio. 

3. Maximum annual metal impact is input to Equation 5.1. 

4. Resultant soil concentration increase is compared to the metal screening soil 

concentration. 

Table 7-10 summarizes comparison of the screening soil concentration to the estimated soil 

concentration increase from emissions of each metal from the Main Flare. The estimated soil 

concentration increase of each metal from Main Flare PM combustion emissions ranges from 

three to six orders of magnitude below the respective screening metal concentration. Therefore, 

Main Flare PM emissions do not affect sensitive regional vegetation because existing soil metal 

concentrations containing these vegetation are not perceptibly increased. 
Table 7-8. Sensitive Plant Species Potentially Occurring in San Juan County, Utah1

Common Name Habitat 

Navajo Sedge Seasaonlly wet, seeps, springs, hanging gardens in Navajo sandstone: 3,770 - 

5,908 ft. elevation 

Chatterly's Onion Moist pinyon-juniper and sagebrush sites 

Culter Milkwood Sand dunes 

Cronquist Milkvetch Cutler formation (Comb Wash), Morrison formation (Aneth) 

Copper Canyon Milkvetch Warm desert shrub, 3,085 ft. elevation, Copper Canyon 

Skull Valley Spring Parsley Deposits of wind-blown sand 

Beck's Spring Parsley Sandy soil of Navajo sandstone origin, crevices and ledges of slickrock, 

higher elevations in Abajo Mountains 

Hole-in-the Rock Prairie 

Clover 

Sandstone bedrock and sand in blackbrush and mixed desert shrub: 4,690 - 

5,000 ft. elevation 

Spineless Hedgehog Cactus Blackbrush, ephedra, sagebrush, pinyon-juniper mountain brush, aspen 

communities: 3,200 - 8,400 ft. elevation 

Nevada Willowherb Talus slopes, crevices 

Kachina Daisy Seasonally wet seeps, hanging gardens on sandstone outcrops 

Redroot Buckwheat Sagebrush and pinyon-juniper: 5,000 ft. elevation 

Cataract Canyon Gilia Mixed warm and cool desert shrub communities: 3,280 - 5,215 ft. elevation 

Alcove Bog Orchid Moist streambanks, seeps, hanging gardens in mixed desert shrub, pinyon-juniper and 

oakbrush: 4,360 - 8,690 ft. elevation 

Canyonlands Lomatium Slot canyons between Entrada sandstone, sandy soil or crevices in sandstone, 

sheltered cool areas of slopes 

Western Hophornbeam Small tree at bases of monoliths, hanging gardens of sandstone: 4,000 - 5,600 

ft. elevation 

Paradox Breadroot Pinyon-juniper and mixed desert shrub: 5,020 ft. elevation 

Alcove Rock Daisy Drier crevices in seasonally wet hanging gardens, alcoves at 4,000 ft. elevation, 

Navajo & Windgate sandstone, Rico Formation 

Howell Scorpionweed Salt and warm desert shrub, pinyon-juniper: 3,690 - 5,000 ft. elevation 

Bluff Phacelia Salt desert shrub: 4,500 ft. elevation 

Mancos Shadscale Salt desert shrub in Morrison Formation 

Jane's Globemallow Sandy soils of weathered white rim and Organ Rock areas of Cutler Formation, 

warm/salt desert shrub: 4,000 - 4,600 ft. elev. 

1

 Table 13.1a (Chapter 13) of “Analysis of the Management Situation”, Monticello BLM Field Office, January 2005. 
Table 7-9. Screening Ambient Air Concentrations for Sensitive Vegetation Exposure1

 1-hour 3-hour 4-hour 8-hour 1-week 1-month 

Pollutant (μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3) (μg/m3) 

SO2 917 786 -- -- -- -- 

NO2 -- -- 3760 3760 -- 564 

CO2 -- -- -- -- 1800000 -- 

1 Based on Table 3.1 of "A Screening Procedure for the Impacts of Air Pollution Sources on Plants, Soils and Animals"2 Reversible decreases in photosynthetic rate have occurred at significantly lower levels, but growth effects have not been demonstrated

Table 7-10. Comparison of Screening Soil Concentration to Estimated Main Flare Concentra

 AP-42 Emission Factor2 Annual Avg. Estimated Soil Scree

 Total PM Pollutant Impact Concentration from Flare3 ConcMetal (lb/106 SCF) (lb/106 SCF) (μg/m3) (ppmw) (pArsenic 2.0E-04 5.9E-07 3.0E-04 Cadmium 1.1E-03 3.3E-06 1.6E-03 Chromium 1.4E-03 4.1E-06 2.1E-03 Copper 8.5E-04 2.5E-06 1.3E-03 Manganese 3.8E-04 1.1E-06 5.6E-04 Mercury 2.6E-04 7.7E-07 3.9E-04 Nickel 2.1E-03 6.2E-06 3.1E-03 Selenium 2.4E-05 7.1E-08 3.6E-05 Vanadium 

7.6 

2.3E-03 6.8E-06 3.4E-03 

1 Screening soil concentrations based on Table 3.4 of "A Screening Procedure for the Impacts of Air Pollution Sources on Plants, Soils and Animals" and are minimum 

vegetation in the soil could be adversely affected 

2 Tables 1.4-2 and 1.4-4 of AP-42, Emission Factors for PM (Total) and Metals from Natural Gas Combustion (7/98)3 Main Flare soil metals concentration based on Eqn. 5-1 of "A Screening Procedure for the Impacts of Air Pollution Sources on Plants, Soils and Animals" with flare l

of metal to PM emissions multiplied by annual PM impact