Appendix L 

Risk Assessment 
APPENDIX L 

Environmental Fate and Effects of Natural Gas Liquid Releases 

When natural gas is removed from the ground, it is compositionally different than what is transported 

through natural gas transmission systems and ultimately used as an energy source for end users for home 

heating and cooking, and industrial energy. When removed from the ground, the mixture is predominately 

methane, but also includes heavier hydrocarbons and inert gases. Although the mixture can vary greatly, a 

typical natural gas stream may include 85 percent methane, 10 percent heavier hydrocarbons called 

natural gas liquids (NGLs), and 5 percent inert gases. NGLs usually are removed from natural gas during 

processing. Lighter ends, including propane and butane may be removed, stored and shipped separately. 

Alternatively, the operator may simply ship raw NGLs to a NGL plant for separation. 

NGLs exist in fields as constituents of natural gas but are recovered separately as liquids. NGL should not 

be confused with natural gas, which is composed primarily of methane. NGL molecules liquefy more 

readily than methane. Overland Pass NGLs are composed of 53 percent ethane, 25 percent propane, 

14 percent butane (normal and iso-), with smaller amounts (4 percent) of (iso) pentane, and pentane plus 

hexane and heptane (sometimes referred to as natural gasoline or plant condensate) (Table L-1). NGL is 

denser than natural gas and becomes combustible at different concentrations than methane. 

. 

Table L-1 Composition of Overland Pass Pipeline NGLs 

Component Liquid Volume Percent 

Methane 0.36 

Ethane 53.16 

Propane 25.75 

Butanes 13.50 

Pentanes 4.42 

Hexanes + 2.61 

BTEX 0.12 

Carbon dioxide 0.11 

Note: Specific gravity is 0.446 

NGLs have a variety of different uses, including enhancing oil recovery in oil wells, providing raw materials 

for oil refineries or petrochemical plants, and as sources of energy. They can also be used in many 

applications, such as: 

• Ethane is primarily used for the production of plastics. 

• Propane is typically used for heating purposes, gas grills, lanterns etc, but can also be used in the 

production of plastics. 

• Butanes and natural gasoline are primarily used for motor gasoline blending 

Properties and Hazards 

Ethane (C2H6) exists as a colorless, odorless gas at ambient pressure and temperature. It is heavier than 

air and may concentrate in low-lying areas. Although ethane is not considered highly toxic it can replace 

oxygen molecules and cause asphyxiation. 

Propane (C3H8) also exists as a gas at ambient pressure and temperature, but is heavier than air with 

1.52 specific gravity as a vapor at 1 atmosphere and 60o F. (air = 1.0). Liquid Propane is lighter than 

water. It is about one-half as heavy as an equal volume of water. 

Butane (C4H10) exists in both gaseous and liquid forms at ambient pressure and temperature, being less 

stable than propane in a gaseous form. Butane is heavier than air and heavier than propane. Its main use 

L-1 
these days is industrial as a chemical feedstock (mostly after conversion to iso-butane), and is otherwise 

more waste than product. 

Heavier NGLs (C5H12+), such as pentanes, hexanes, heptanes exist as liquids at ambient pressure and 

temperature and are shipped in liquid form. Heavier NGLs often are used as a diluent for shipping crude 

oil via pipeline; it reduces the viscosity of heavier crude oils. 

Once released into the environment, NGLs are volatilized and quickly evaporate. NGLs are not generally 

toxic; however, as the gas takes the place of air, it becomes a simple asphyxiant and can cause 

suffocation from lack of oxygen. Hydrocarbon liquids shipped via pipelines generally have vapors that are 

heavier than air and that will collect in low or enclosed areas. These vapors are simple asphyxiants in that 

they will displace oxygen and create an oxygen-deficient atmosphere in those areas. NGLs are relatively 

non-toxic below the lower explosive limits (e.g., 30,000 ppm for ethane). Direct exposure to NGLs can 

result in frostbite to the skin and headache or nausea if inhaled. In addition, an accidental release of NGL, 

while in transport, may result in the following hazards: 

• POOL FIRES - Upon release, heavier NGLs are flammable and, therefore, pose the hazard of 

thermal radiation. 

• FIREBALLS - A large, pressurized release of a liquefied hydrocarbon such as propane or butane 

may burn in the form of a fireball, which grows larger and also moves upwards. Thermal radiation 

is the related hazard. 

• VAPOR CLOUD FIRES - Upon release, propane or butane can form a vapor cloud that spreads 

horizontally. If little or no wind is present and atmospheric conditions are very stable, the 

spreading cloud mixes slowly with oxygen. It can burst into flames if ignited and flash back to the 

source of the release. Thermal radiation is the hazard. 

• VAPOR CLOUD EXPLOSIONS - NGL vapor clouds are potentially explosive both in unconfined 

and confined situations. The hazard of such an explosion generally results from building damage 

or breaking windows. 

Table L-2 provides a summary of the general chemical characteristics of NGLs. 

Table L-2 Summary of Properties and Hazards

 Ethane n-Butane Propane Pentane+ 

Formula C2H6 C4H10 C3H8 C5H12 

Family Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon 

Appearance liquid Colorless Colorless colorless colorless 

Appearance vapor Foggy in Foggy in Foggy in Foggy in 

concentration concentration concentration concentration 

Odor Odorless Virtually odorless Odorless Odorless 

Main hazard Flammable Flammable Flammable Flammable 

BP@ATP -88.6°C -0.5°C -42.1°C °C 

Flash point -135°C -60°0C -105°C °C 

Flammable limits 3.0-12.5% 1.9-9.0% 2.1-9.5% % 

(LEL – UEL) 

Auto-ignition 515°C 405°C 450°C °C 

TLVa No TLV 800 ppm critical effect 2500 ppm critical 600 ppm critical 

narcosis effect simple effects irritation and 

asphyxiant narcosis 

Odor threshold Odorless 5000 ppm Odorless Odorless 

Reaction-air No reaction No reaction No reaction No reaction 

Reaction - water No dangerous No dangerous No dangerous No dangerous 

reaction reaction reaction* reaction* 

a

ACGIH 

*may form solid hydrates, insoluble. 

L-2 
Environmental Fate 

Accidental releases of NGLs can occur during pipeline transport. Media distribution calculations according 

to Mackay Level 1 (1992) show that in the event of an accidental release of NGLs all of the material will 

end up in the air compartment due to the volatility of the hydrocarbons (API 2000). 

Aerobic biodegradation of ethane is greater than 65% after 35 days (ZoBell 1963). API Robust Summary 

of Information on Petroleum Gases (2000) notes that ethane, propane and butane can be used by bacteria 

as carbon sources. When released into water, most of the NGLs will float to the surface where they will 

evaporate. 

Evaporation will be the primary mechanism of loss for NGLs, rapidly reducing exposure and toxicity. 

Dissolution of NGLs in water is not a significant process (i.e., NGS are negligibly water soluble). 

Photodegradation of NGLs increases with greater solar intensity. It can be a significant controlling factor, 

especially of lighter products and constituents; but it will be less important during cloudy days and winter 

months. 

Overall, the environmental fate of released NGLs is controlled by many confounding factors. Major factors 

affecting the environmental fate include spill volume, type of product, dispersal rate of the product, terrain, 

and weather. In the event of a leak, NGLs released from the pipeline would begin to vaporize, the gases 

would percolate through the soil and sediments, and then dissipate into the atmosphere. Only that 

component of the NGL stream that does not readily volatilize at atmospheric pressure (approximately 2 to 

4 percent of the NGL) would remain to potentially migrate through the overlying surface materials and 

enter the groundwater. Once released, the physical environment largely dictates the environmental 

persistence of the spilled material. Generally, NGLs evaporate quickly and releases would be expected to 

rapidly dissipate. 

Environmental Effects 

If released into the environment, NGLs pose a range of risks to the environment and to human 

populations. NGLs shipped via pipelines generally have vapors that are heavier than air and that will 

collect in low or enclosed areas. These vapors are simple asphyxiants in that they will displace oxygen 

and create an oxygen-deficient atmosphere in those areas. 

Hydrocarbon vapors are flammable or combustible, although different hydrocarbon liquids have different 

flash points (the temperatures at which sufficient flammable vapors are emitted) and flammable ranges 

(the range of vapor concentration in air expressed as a percentage by volume). This potential for 

flammability is likely the most serious threat to human health and the environment. 

NGLs released to the environment may cause adverse biological effects on birds and mammals via 

inhalation exposure from highly volatile compounds. Acute toxic effects include narcotic effects, and 

possible death. While releases of NGLs may have an immediate and direct effect on wildlife populations, 

the potential for physical and toxicological effects attenuates quickly as the volume of material diminishes. 

In mammals, high molecular weight alkanes are considered virtually nontoxic. Overall, NGLs have rapid 

rates of evaporation and low environmental persistence, thus the primary hazard from NGLs is considered 

to be acute, not chronic, airborne toxicity. Short-term impacts to vegetation in the area of a release may be 

possible. Aquatic toxicity of petroleum gases is not applicable. 

Rats exposed to various concentrations of propane in air showed signs of central nervous system 

depression, intoxication and death. Recovery from a non-lethal exposure was rapid. The calculated effect 

concentration (EC50) was 280,000 ppm (504,996 mg/m3

 (Clark & Tinson 1962). A similar study with 

isobutene resulted in an EC50 of 200,000 ppm (475,444 mg/m3). Hydrocarbons that volatilize into the 

atmosphere are broken down into smaller compounds by photodegradation and photo-oxidation 

L-3 
The behavioral responses of terrestrial wildlife may help reduce potential adverse effects. Many birds and 

mammals are mobile and will avoid leak areas. The greatest environmental impact of an NGL spill is the 

potential for fire and the resulting short-term ecological devastation. 

Additional information regarding the fate of NGL and specific NGL components if accidentally released into 

the environment, including groundwater, can also be found in sections 4.3.4.1 and 4.3.4.2 of the 

Environmental Assessment for the Mid-America Pipeline Company, LLC (MAPL) Western Expansion 

Project. This Environmental Assessment was prepared by the BLM in 2005. The BLM’s Environmental 

Assessment concluded that “a release from an NGL pipeline would result in the evaporation of most, if not 

all, of the liquids on the surface of the ground or in the vadose zone above the water table. Under certain 

conditions it would be possible for a very small portion of the release to reach the water table. Because of 

their slight solubility in water, contamination from NGLs would be limited to a few parts-per-million. These 

concentrations would be further reduced by diffusion and natural attenuation further reducing the risk to 

potential receptors.” This Environmental Assessment is available at 

http://www.nm.blm.gov/nmso/mid_am_pipeline/mid_am_pipeline.htm. 

Assessment of Worst Case Scenario 

As noted, in the event of a pipeline rupture NGLS will mostly volatilize and quickly dissipate, thereby 

minimizing the potential impacts to both humans and ecological receptors. The product contains 0.12% 

benzene, which, if enough material is spilled, could impact aquatic organisms or humans ingesting 

drinking water from the contaminated source. The potential for impacts was investigated by evaluating a 

conservative scenario whereby a complete pipeline rupture is assumed to dump the entire spilled product 

directly into the water. Mixing is assumed to be immediate and complete and the aquatic organisms and 

humans are assumed to live in and drink water containing the estimated benzene concentration from that 

location. Acute and chronic exposure of organisms and ingestion of water by humans are examined. 

The pipeline throughput is assumed to be 150,000 bpd. Two stream flows were evaluated; moderate 

(100 cfs) and high (1000 cfs). In addition, two occurrences were evaluated; 

• Complete rupture of the pipeline, and 

• 1% leak. 

Benzene concentrations were estimated assuming: 

• All of the spilled product from the leak or rupture goes into the water 

• Benzene is completely solubilized in the water column 

Acute Chronic Drinking Water 

Rupture Time: 1-hour 1-hour 1-hour 

Averaging Time: 0-hours 168-hours (7-day) 24-hours 

Toxicity Value: 7.4 mg/L 1.4 mg/L 0.005 mg/L 

If 6,250 barrels of product are spilled to a moderate flow stream the potential benzene concentrations at 

that location will be at a level seven times the acute toxicity threshold. Concentrations would rapidly drop 

as the plume moves downstream. The occurrence interval for this scenario is greater than every 

50,000 years. All other predicted water concentrations would be below the acute toxicity threshold in water 

(Table L-3). No long-term or chronic impacts would be expected (Table L-4); however, if the spill occurred 

directly into a drinking water source, the potential benzene concentration could exceed the corresponding 

MCL, if it is assumed that at least 62.5 barrels are spilled into the water (Table L-5). Based on the 

occurrence interval, the likelihood of an impact to drinking water is very low. 

L-4 
Table L-3 Comparison of the Acute Toxicity Threshold for Aquatic Life with Estimated 

Benzene Concentrations following Pipeline Rupture or Leak to Streams 

Throughput – 

150,000 bpd 

Stream 

Flow Rate 

(cfs) 

Benzene 

Acute 

Toxicity 

Threshold 

(mg/L) 

Scenarioa Projected 

Occurrence 

Intervalc 

(# yrs between spills) 

Complete rupture 

(6250 barrels) 

1% Leak 

(62.5 barrels) 

(mg/L) (mg/L) 

Complete 

Rupture 

1% 

Leak 

Moderate Flow 

Stream 

100 7.4 52b 0.5 51,496 23,303 

High Flow 

Stream 

1000 7.4 5 0.1 9,363 4,237 

a Estimated proportion of benzene in the NGL is 0.12 percent, and is assumed to be entirely water solubilized in the event of a spill. 

The resulting concentration was calculated by multiplying 0.12 percent of the total amount of petroleum product released in 1 hour 

divided by projected stream flow volume. The model assumes uniform mixing conditions. 

b Shading indicates concentrations greater than the acute toxicity threshold for aquatic species. c

 Projected occurrence interval based on historical data from OPS hazardous liquids database. 

Table L-4 Chronic Toxicity Threshold for Aquatic Life Compared with Estimated Benzene 

Concentrations following Pipeline Rupture or Leak to Streams

Throughput – 

150,000 bpd 

Stream 

Flow 

Rate 

(cfs) 

Benzene 

Chronic 

Toxicity 

Thresholda 

(mg/L) 

Scenariob 

Projected Occurrence 

Intervalc 

(# yrs between spills) 

Complete 

rupture 

(19,563 barrels) 

1% Leak 

(221 barrels) 

(mg/L) (mg/L) 

Complete 

Rupture 1% Leak 

Moderate Flow 

Stream 

100 1.4 1.0 0.56 189,600 51,496 

High Flow 

Stream 

1000 1.4 0.1 0.06 34,473 9,363 

a The chronic toxicity value for benzene is based on a 7-day chronic toxicity value of 1.4 mg/L for trout. Exposure concentrations were 

estimated over a 7-day period since the chronic toxicity value was based on a 7-day exposure. b Estimated proportion of benzene in the petroleum product is 0.12 percent, and is assumed to be entirely water solubilized in the 

event of a spill. The resulting concentration was calculated by multiplying 0.12 percent of the maximum amount of petroleum 

product divided by 7 days of stream flow volume. The model assumes uniform mixing conditions. c

 Projected occurrence interval based on historical data from OPS hazardous liquids database. 

Table L-5 Human Drinking Water Standard Compared with Estimated Benzene 

Concentrations following Pipeline Rupture or Leak to Streams 

Throughput – 

150,000 bpd 

Stream 

Flow Rate 

(cfs) 

Benzene 

MCL 

(mg/L) 

Scenarioa Projected 

Occurrence 

Intervalc 

(# yrs between spills) 

Complete 

Rupture 

(6250 barrels) 

1% Leak 

(62.5 barrels) 

(mg/L) (mg/L) Complete 

Rupture 

1% 

Leak 

Moderate Flow 

Stream 

100 0.005 2.2b 0.02 51,496 23,303 

High Flow 

Stream 

1000 0.005 0.02 0.002 9,363 4,237 

a Estimated proportion of benzene in the petroleum product is 0.12 percent, and is assumed to be entirely water solubilized in the 

event of a spill. The resulting concentration was calculated by multiplying 0.12 percent of the amount of petroleum product 

released in 1 hour during a pipeline rupture by 24 hours of stream flow. b Shading indicates concentrations that could exceed the maximum containment level (MCL) for drinking water (0.005 mg/L). c

 Projected occurrence interval based on historical data from OPS hazardous liquids database. 

L-5 
References: 

American Petroleum Institute 2000. Robust Summary of Information on Petroleum Gases. 15 August 2000. 

Clark D.G. and Tinson, D.J. 1982. Acute Inhalation Toxicity of some Halogenated and Non-Halogenated 

Hydrocarbons. Human Toxicol. Vol. 1, pp 239-247 

Mackay, D, Paterson, S., & Shiu, W. Y.1992. Generic Models for Evaluating the Regional Fate of Chemicals. 

Chemosphere, 24, 695-717 

ZoBell C.E. 1963. The occurence, effects and fate of oil polluting the sea. Int. J. Air Wat. poll. Pergamon Press 

Vol. 7, pp 173-198. 

L-6