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Introduction Great quantities of petroleum fuels, especially aviation turbine fuels, are stored and used at Air Force bases throughout the U.S. and around the world. Unfortunately, fuel storage systems and associated components can be sources of environmental contamination when leaks or spills occur and fuel products are released into the environment. This fact sheet presents an introductory discussion of the basic composition and properties of common categories of fuels and the environmental contamination problems that can result from fuel releases. It is intended to provide Air Force environmental personnel with a basic understanding of the occurrence and characterization of fuel contamination in soils and groundwater. This information will also provide Air Force environmental personnel with an understanding of some of the issues associated with non-compliant air emissions from fuel operations. Important Terms Alcohol - An organic compound that has an oxygen/hydrogen (OH) group bonded to a hydrocarbon group. Biodegradation - The alteration of petroleum by living microorganisms that naturally reside in the environment. Boiling Point - The temperature at which a liquid changes to a vapor or gas. Crude Oil - Petroleum as found in the earth, before it is refined into petroleum products. Gasoline - A refined petroleum product used as fuel for reciprocating engines, primarily automobiles. Hydrocarbons - Organic compounds composed of only carbon (C) and hydrogen. Refined petroleum products are complex mixtures of hydrocarbons. Alkanes are saturated hydrocarbons in which all C-C bonds are single bonds. Alkenes are unsaturated hydrocarbons that have at least one C-C double bond. Aromatics are hydrocarbons that have a C6 ring analogous to that of benzene. Kerosene - A colorless, low-sulfur petroleum product that burns without producing much smoke. It is the primary ingredient in most jet fuels. LNAPL (Light Non-Aqueous Phase Liquid) - a low-solubility liquid that "floats" on water. Gasoline ("free product") behaves as a LNAPL when in contact with water. Naptha - A refined petroleum product used as a solvent, cleaner, and gasoline component. Petroleum - A naturally occurring, oily, flammable liquid composed of hydrocarbons and found associated with gas in natural underground reservoirs. Vapor Pressure - The pressure at which a liquid and its vapor are in equilibrium at a given temperature. The more volatile a liquid and the lower its boiling point, the higher its vapor pressure. The vapor pressure of a liquid also increases with its temperature. Viscosity - The degree to which a fluid resists flow under a standard force. The more viscous a liquid, the more resistant it is to flow. Generally, viscosity decreases as the temperature of a liquid increases. Volatility - A measure of how quickly a substance forms a vapor at ordinary temperatures and pressure. Characteristics Common to Fuels Petroleum-based fuels commonly stored and used at Air Force installations can be grouped into the following four categories based on their use: 1) aviation fuels (turbine fuels and aviation gasoline); 2) diesel fuel; 3) heating oils; and 4) motor gasoline. This section presents basic information about petroleum-based fuels in general and some of the many physical and chemical properties they share. Properties that may be used to characterize environmental contamination caused by one category of fuel in comparison to another are discussed in later sections. Petroleum fuels ignite and burn readily, and produce a great deal of heat and power in relation to their weight. The one composition requirement common to all petroleum fuels is that they consist entirely of hydrocarbon molecules (hydrogen and carbon) except for small amounts of impurities and/or additives. The refining of crude oil, also known as fractional distillation, produces a range of petroleum compounds that are primarily characterized by their boiling points and molecular weights. Short chain, single ring, or "light" hydrocarbons, are more volatile, less viscous, and have lower boiling points than long chain, multiple ring, or "heavy" hydrocarbons. Denoting the number of carbon atoms (CX) in a hydrocarbon molecule is a way to describe its weight relative to other hydrocarbon molecules. Natural gas is composed primarily of the light hydrocarbons methane (C1), ethane (C2), propane (C3), and butane (C4). Gasoline typically contains hydrocarbons in the C4-C10 range. Kerosene and aviation fuels fall primarily into the C4-C19 range. Diesel fuels are composed primarily of hydrocarbons in the C8-C21 range. Heating oils, the most common being Fuel Oil No. 2, are similar to diesel fuels, but are less refined and fall into the heavier C15-C22 range. Lubricating oils, hydraulic fluids, tars, and other heavy residual petroleum compounds are made up of hydrocarbon molecules in the C20-C60 range and larger. Some of the heavier oil fractions are further refined by a process called "cracking" to yield increased amounts of the high-demand lighter petroleum products. Most petroleum fuels are mixtures of hundreds of different hydrocarbon compounds. The exact number and proportions of these compounds in a particular fuel may vary; therefore, most fuels are formulated to meet general property limits, rather than a specified chemical composition. An exception to this is missile fuel, which consists of either a single synthesized hydrocarbon compound or a precise mixture of such compounds. For a particular fuel product, governing standards may limit upper and lower percentage composition of certain hydrocarbons as required to meet performance criteria. Propulsion system demands, such as fluidity, combustion properties, corrosion protection, and impurity limits, are the primary determinants of standardized fuel formulations. Volatility is a property of fuels that affects its ability to vaporize and form a combustible mixture with air. This property results in a quantifiable characteristic called vapor pressure. Vapor pressure and volatility are important properties to be considered by designers of fuel storage and delivery systems because they can lead to unwanted evaporative fuel losses or "fugitive" vapors. Lighter fuels such as gasoline and aviation fuel tend to be more volatile and have higher vapor pressures than heavier fuels such as diesel and heating oil at the same temperature and pressure.
Fuel Impurities
Gums are high molecular weight compounds containing hydrogen, carbon, oxygen, and usually sulfur and nitrogen. They are formed when the hydrocarbon molecules in stored fuels are oxidized or polymerized after exposure to air, sunlight, and/or elevated temperatures. When gums precipitate from the fuel, they can clog and form deposits on vital engine components such as filters and injectors, causing mild to severe engine performance problems. Anti-oxidant fuel additives can prevent the formation of gums.
Metals formed during certain refining processes can oxidize and contribute to the formation of filter clogging gums in any type of fuel. This problem is addressed by using a metal deactivator additive.
Microbial contamination occurs after fuels leave the refinery since the refining process sterilizes fuel. Microbes, including algae, bacteria, and fungi feed on the fuel and use the water in the fuel for their oxygen supply. They can multiply and plug fuel filters with an odorous slime. Some of the microbes can also produce corrosive acid byproducts. Minimizing water content and treating with a biocide additive will control microbial growth in fuel.
Sediment is a common contaminant of fuels and usually consists of rust, mineral scale, sand, dirt, and other insoluble impurities. To address this problem, fuels are filtered upon delivery into bulk and operating storage systems to remove as much sediment as possible before the fuel is delivered to the end user.
Sulfur compounds can be corrosive to metals in fuel systems and are controlled by the total sulfur content limits found in the fuel specification.
Water is a very common fuel impurity. Fuel can become contaminated with water during shipping and storage. Water can condense from the fuel itself, may leak into fuel containers from the outside, or it may be present in containers before they are filled with fuel. Water in fuel may also contain other impurities that can cause corrosion problems and damage filters, pumps, and injectors. Water is denser than fuel and can be removed as it collects at the bottom of a storage container.
Fuel Additives
Alkyl lead was a common gasoline additive until the late 1960s used to obtain higher octane ratings and reduce engine "knock." Lead additives have been reduced or entirely phased out of most automotive gasoline formulations due to the environmental hazards associated with lead-containing exhaust emissions. As lead additives have been phased out of gasoline formulations, other oxygenating additives are now used to boost octane ratings and control knock, as well as reduce harmful exhaust emissions. Leaded automotive gasoline typically contained one or more grams per liter (>1,000 parts per million [ppm]) of alkyl lead. Today, unleaded automotive gasoline contains only a few ppm of lead. Aviation gasoline (Avgas) continues to contain significant concentrations of alkyl lead, typically at levels greater than 1,000 ppm.
Anti-oxidants are primarily used to prevent gum formation in gasolines and aviation fuels.
Biocides may be added to any type of fuel to kill microbes when their growth becomes a recurring problem.
Conductivity additives increase the electrical conductivity of gasolines, aviation, and diesel fuels, thereby reducing the buildup of static charges during mixing, transfer, and shipment.
Corrosion inhibitors protect against corrosion during pipeline transfer and storage of fuels. They have also been found to improve the lubricity, or capacity to reduce friction of fuels. Corrosion inhibitors are used primarily in gasolines, aviation fuels, and diesel fuels.
Detergent additives prevent the buildup of gum deposits in engines and extend fuel injector life. They also help keep fuel filters clean. Detergent additives are primarily found in diesel fuels and automotive gasolines.
Icing inhibitors are used primarily in aviation fuels to prevent the formation of ice crystals from entrapped water in the fuel at freezing temperatures encountered during high altitude flight. Icing inhibitors have also been found to be an effective barrier to microbiological growth. Diethylene glycol monomethyl ether is specified for most military aviation fuels as an icing inhibitor.
Metal deactivators prevent metal contaminants in any type of fuel from oxidizing with hydrocarbons and other compounds to form gums or precipitates.
Oxygenates are oxygen-containing hydrocarbons that are added to automotive gasoline to boost the octane rating, reduce the smog-forming tendencies of exhaust gases, and suppress engine knock. The increased oxygen content promotes more complete combustion, thereby reducing tailpipe emissions. Common oxygenating additives are methyl tertiary butyl ether (MTBE) and ethanol.
Thermal stability additives reduce fuel fouling of critical jet engine components. Thermal stability refers to the ability of the fuel to be used in a system without degradation. Thermal stress results in fuel breakdown that can cause carbon build-up on engine nozzles, afterburner spray assemblies, and manifolds. In some instances, fuel degradation changes the spray pattern in the combuster or afterburner, which leads to damage of engine components, flameouts, and augmentor anomalies.
The standardization of military aviation turbine fuels has been maintained through the Air Standardization Coordinating Committee (ASCC), composed of the United States, United Kingdom, Canada, Australia, New Zealand, and the North Atlantic Treaty Organization (NATO). The worldwide use of American, British, French, Canadian, Dutch, and other western nations' aircraft and engines has further aided in the standardization of military jet fuels. The International Air Transport Association and similar organizations have helped to standardize commercial jet fuels, which are very similar to military jet fuels.
Common types of military aviation turbine fuels (turbo-jet or turbo-prop) are identified by grade designations as discussed below. Specific information on additive packages can be obtained from the Military Specification(s) that govern each type of fuel and from Technical Order (T.O.) 42B-1-1, "Quality Control of Fuels." Table 1 presents a summary of some chemical and physical requirements for common turbine fuels.
JP-4 is typically composed of about 50-60% gasoline and 40-50% kerosene, is highly volatile, and contains hydrocarbons in the C4-C16 range. JP-4 was the primary fuel of the USAF for decades; however, it has been phased out in favor of JP-8 (see below). By specification, it contains a full additive package including a corrosion inhibitor, anti-icing, and anti-static compounds. An optional additive is a metal deactivator.
JP-5 is a low-volatility (C10-C19 range) jet fuel with a relatively high flash point (for shipboard safety reasons) and is designed for use in aircraft aboard Navy aircraft carriers. Anti-icing, anti-oxidant and anti-corrosion additives are required in the formulation of JP-5. An optional additive is a metal deactivator.
JP-8 was developed to be less volatile and explosive than JP-4. (Commercial Jet A-1 fuel is equivalent to JP-8). In 1996, the USAF completed conversion from JP-4 to JP-8 fuel. It also contains a full additive package including a corrosion inhibitor, anti-icing, and anti-static compounds. Optional additives are an anti-oxidant and a metal deactivator.
JP-8+100 is an improved JP-8 fuel with additional "fuel injector cleaner"-type additives. This new fuel has been demonstrated to significantly reduce engine and fuel system operation and maintenance costs for a variety of aircraft. The new additive also increases the thermal stability of the fuel. (When thermal stability is compromised, fuel breaks down into gums, varnishes, carbon deposits, and coke.)
Earlier aviation fuels no longer in use may have contained a greater percentage of gasoline vs. kerosene in their formulations. JP-1, JP-2, and JP-3 have not been used for several decades or never made it past the experimental stage. JP-6, similar to JP-5, was developed in 1956 for the XB-70 experimental aircraft and has not been in use since the XB-70 program was canceled. Another specialty fuel developed in 1956 is JPTS, a highly stable kerosene fuel developed for the U-2. This fuel is still used in the U-2 and the newer TR-1. JP-7, developed in the late 1960s for the SR-71, is not a distillate fuel, but rather is composed of specially processed blending stocks. This results in a very clean hydrocarbon mixture that is very low in sulfur, nitrogen, and oxygen impurities typically found in conventional aviation fuels. JP-7 has poor lubricating properties and requires the addition of a special fuel lubricity agent.
Aviation turbine fuels are typically stored in large above ground or underground tanks near flightlines. It may be transported to the installation by pipeline, railcar, or tanker truck.
Aviation Gasoline
Diesel Fuels Diesel fuel is formulated for engines found in buses, trucks, ships, and locomotives, which are commonly heavier and more powerful than gasoline engines. Diesel fuel, which is less volatile than gasoline and is made up of heavier petroleum fractions, ignites by compression in the cylinder rather than by a spark. In the same way that gasolines are labeled with an octane rating, the ignition performance rating of diesel fuel is called the cetane number. Diesel fuel is generally classified into the following three grades: Diesel No. 1 is relatively volatile (C8-C19) when compared to other diesel fuels. It is used in high-speed diesel engines that vary in speeds and loads, and was originally formulated to meet the specifications for Detroit Diesel Series 71 engines in city buses. Diesel No. 2 is composed of lower volatility, heavier (C9-C21) petroleum compounds than Diesel No. 1. It is used in high-speed diesel engines involving high loads and uniform speeds such as automobiles and trucks. Diesel No. 4 is the most viscous and least volatile diesel blend and is a mixture of heavy diesel distillates and residual (C25+) fuel oils. It is for use in low-and medium-speed services involving sustained loads and constant speeds. It is mainly used by large stationary power generators, locomotives, and ships. Heating Oils Heating oil is a clean burning, non-explosive, highly efficient fuel that produces negligible amounts of smoke and soot emissions and will not burn in a liquid state. Heating oils are graded 1 through 6, with Fuel Oil No. 2 (C15-C22) being one of the most common. Fuel Oil No. 6 (Bunker C) is composed entirely of heavy residual hydrocarbons and is a black viscous fuel that must be heated before it can be pumped and burned. Heavier fuel oils are sometimes blended with used lubricating oil or waste oils, which can contain elevated levels of cadmium and lead. As the name suggests, heating oils are primarily used in burners and furnaces used to heat buildings and residences. It is typically stored in above ground or underground tanks near the point of use. Gasoline (Mogas) Automotive gasoline (Mogas) is a very sophisticated fuel and is often a blend of separately distilled petroleum products. Gasoline is very volatile and produces large amounts of vapor at ordinary temperatures. Gasoline contains hydrocarbon compounds in the C4-C10 range. The major component (60-80%) of gasoline consists of the alkanes, which are stable and burn cleanly. Aromatic compounds comprise about 20-40% of gasoline formulations; however, these compounds are gradually being replaced with other, less polluting, octane boosters such as MTBE and ethanol. There are several toxic compounds in gasoline, the most notable of which are lead and benzene. Benzene is a confirmed carcinogen. Other suspected carcinogens in gasoline include other aromatics, ethylene dibromide, and oxygenating additives. The major toxic risks from gasoline come from breathing tailpipe, evaporative, and refueling emissions. Mogas is typically stored at installations in underground/above ground tanks located near motor fleet operations or vehicle maintenance areas. Fuel Releases and the Environment As stated in the Department of Defense's Military Handbook 1022 (MIL-HDBK-1022), "Petroleum Fuel Facilities," 30 June 1997, "It is the firm policy of the DoD to design and construct fueling facilities in a manner that will prevent damage to the environment by accidental discharge of fuels, their vapors or residues. Designs must comply with foreign government, national, state, and local environmental protection regulations that are in effect at a particular facility." MIL-HDBK-1022 also states "Protection of the natural waters against pollution from discharge of petroleum is achieved by complying with federal, state, and local regulations." In the U.S. and its territories, the applicable federal environmental regulations are:
Fuels are typically released to the subsurface environment when storage and delivery systems leak or when sudden accidental spills occur. Fuel storage tanks, above ground as well as underground, have historically been made of steel, which is known to corrode and leak if not properly outfitted with anti-corrosion devices. Steel piping can also corrode and piping leaks are commonly the result of failures at couplings and joints. Fiberglass is becoming much more common as tank and piping material in new fuel storage and delivery systems. It does not corrode; however, it is not always as structurally strong as steel. On Air Force installations, fuel storage and delivery systems are typically located near flightlines, maintenance areas, vehicle fueling areas, and in the case of heating oil, near homes and buildings. NOTE: Double liners are now required for all underground tanks and containment dikes are required for above ground storage tanks. Subsurface fuel leaks can be very slow, often going completely unnoticed, or they can be fast enough to show up as discrepancies on fuel inventory reports. Many leaks are discovered when fuel storage and delivery systems are serviced, overhauled, or replaced to comply with regulatory upgrade requirements. Sometimes a leak is discovered when an environmental regulatory agency requires that a system be inspected when fuel contaminants are detected in nearby soils, surface water bodies, or drinking water wells. Large accidental fuel spills can occur when fuel overflows during delivery, when above ground storage systems fail catastrophically, and when fuel tanks on aircraft and vehicles are ruptured. Large fuel spills pose immediate health and explosion risks and are generally addressed quickly by installation fire control and/or emergency response personnel. For this reason, sudden large spills may not produce the same level of environmental damage that can result from invisible, long-term leaks from underground fuel system components.
Environmental Fate of Subsurface Fuel Releases
Although hydrocarbons are not very soluble in water, very small dissolved concentrations (1 part per million) can give water a very strong fuel smell. Some of the more toxic components of fuels are considered harmful to human health and the environment when dissolved in drinking water at concentrations as low as one part per billion. A small amount of fuel can contaminate a very large quantity of water. Groundwater migrates horizontally in the subsurface at a rate typically ranging between a few inches to a few feet per day. Coupled with the fact that the groundwater surface can also rise and fall, this can bring undissolved fuel in continuous contact with previously uncontaminated groundwater. Since groundwater usually migrates horizontally in one general direction (with minor seasonal fluctuations), the contaminated portion of the groundwater frequently takes on a three-dimensional elongated shape that may widen and disperse with distance from the source, explaining why the term "plume" is frequently used to describe contaminated groundwater zones.
Fuel additives have turned out to be the worst offenders as far as toxicity is concerned. For example, MTBE is very persistent in the environment and is used as a "marker" for plume definition.
Characterizing Fuel Contamination in the Environment
A variety of common laboratory analytical methods are used to detect petroleum compounds in samples collected from soil, groundwater, and surface water. Most of these methods are able to identify only a select few of the many hundreds of hydrocarbon compounds that may be present in fuel-contaminated media, primarily the compounds that are well documented to pose the greatest environmental and health risks. The data collected is valuable in determining where contamination may or may not be present, as well as how and where it may migrate with time.
In addition to determining the nature and extent of fuel contamination in the environment so it can be remedied, it is sometimes important to establish, to the degree possible, the exact location and time frame of the fuel release. This information is usually required for establishing legal liability for the contamination, which is frequently a necessary prerequisite to obtaining cleanup funding, winning an insurance claim, or initiating a cost recovery action.
As discussed in the above section, petroleum releases are subject to a variety of often unpredictable physical and chemical changes. New analytical techniques, including "hydrocarbon fingerprinting" and "forensic analysis," are being applied to the investigation of fuel contamination in the environment. Some fuel products, even after undergoing years of "weathering" in the environment, retain certain chemical characteristics that allow them to be identified and distinguished from other products. Clearly, knowledge of the original composition and properties of the fuel product can be an important part of this identification process.
A laboratory technique known as capillary column gas chromatography is frequently used to characterize fuels in the environment. The data from the chromatography analysis is presented as a series of peaks drawn by a pen recorder on graph paper. The peaks represent individual hydrocarbon compounds in order of molecular weight, the lightest ones being recorded first. The size of the area under the peak represents the relative quantity of each compound in the sample. This image is a type of "fingerprint" for the contaminant that can be compared to a similar image representing the product in an unweathered state. Major categories of petroleum products, such as gasoline, diesel, kerosene, or lubricating oil, can usually be distinguished from one another in environmental samples with fingerprinting methods. Further definition of the contaminant may be possible when the sample is analyzed for traces of original fuel additives that may serve as markers. Fingerprinting methods may be more difficult to apply when the sample contains a mixture of fuels or other volatile organic contaminants. However, it is important to note that interpreting this type of data is very subjective and qualitative, and depends on the experience of the individuals involved.
Evaporative Fuel Emissions
Fuel vapors evaporate and are released to the air any time fuel is transferred from one container or system to another. Evaporation is most likely to occur during the routine refueling of vehicles and aircraft, and accounts for many thousands of tons of volatile vapors released in the U.S. every year. The high volatility and vapor pressure of the highly refined fuels used in vehicles and aircraft contributes to this problem. Regulatory limits have been placed on gasoline volatility in recent years to control evaporative emissions of hydrocarbons, many of which are EPA air toxics (pollutants, including carcinogens, that cause adverse health effects). In addition, certain states require specialized vapor recovery devices on fueling systems.
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