Volatile Fuel Hydrocarbons and MTBE in the Environment
Abstract
Petroleum hydrocarbons (hydrocarbons that result from petroleum products such as oil, gasoline, or diesel fuel) are among the most commonly occurring and widely distributed contaminants in the environment. Volatile hydrocarbons are the lighter fraction of the petroleum hydrocarbons and, together with fuel oxygenates, are most often released from crude oil and liquid petroleum products produced from crude oil. The demand for crude oil stems from the world's ever-growing energy need. From 1970 to 1999, primary energy production of the world grew by 76% (Energy Information Administration, 2001), with fossil fuels (crude oil, natural gas, and coal) accounting for ∼85% of all energy produced worldwide (Figure 1). World crude oil production reached a record 68 million barrels (bbl) per day (1.08×1010 L d-1) in 2000. The world's dependence on oil as an energy source clearly is identified as contributing to global warming and worsening air and water quality. (7K)Figure 1. World primary energy production by source from 1970 to 1999 (Energy Information Administration, 2001). Petroleum products are present in Earth's subsurface as solids, liquids, or gases. This chapter presents a summary of the environmental problems and issues related to the use of liquid petroleum, or oil. The focus is on the sources of volatile hydrocarbons and fuel oxygenates and the geochemical behavior of these compounds when they are released into the environment. Although oxygenates currently in commercial use include compounds other than methyl t-butyl ether (MTBE), such as ethanol (ETOH), most of the information presented here focuses on MTBE because of its widespread occurrence. The environmental impact of higher molecular weight hydrocarbons that also originate from petroleum products is described in (Chapter 9.13, Abrajano et al.).Crude oil occurs within the Earth and is a complex mixture of natural compounds composed largely of hydrocarbons containing only hydrogen and carbon atoms. The minor elements of sulfur, nitrogen, and oxygen constitute less than 3% of most petroleum (Hunt, 1996). Releases to the environment occur during the production, transport, processing, storage, use and disposal of these hydrocarbons. The petroleum industry classifies oil naturally occurring in a liquid phase as either conventional oil or nonconventional oil. Conventional oils can be explored and produced by conventional primary and secondary recovery techniques, whereas nonconventional oils, such as heavy oils, tar sands, and synthetic oils, are more difficult to extract from the host rock ( Tissot and Welte, 1984). The history of liquid petroleum usage dates back centuries. Marco Polo described its use as a fuel for lamps in 1291 ( Testa and Winegardner, 2000). In the mid-nineteenth century, Native Americans used crude oil skimmed from creeks and rivers as a medicinal ointment ( Energy Information Administration, 1999). Commercial production in the United States began in 1859, with kerosene production for use in lamps. At the turn of the century, emphasis shifted to gasoline with the invention of the automobile, and in the 1930s and 1940s a substantial market for heating oil developed.In 2000, over 85% of the world's energy came from fossil fuels (Table 1), with oil production alone supplying 40% of that energy (Edwards, 2001). Oil, with its high British Thermal Unit (BTU) density and ease of transport, is the most valuable fuel in the world today, and the demand for world crude oil is ever increasing. Annual world crude-oil demand increased from 22 billion to 28 billion bbl of oil (BBO) ((3.5-4.5)×1012 L) from 1990 to 2000. Increasing population growth and industrialization in developing countries is expected to drive this annual oil demand up 1.5% annually to ∼38 BBO (6.0×1012 L) by 2030. As demand increases and known oil reserves are depleted, more emphasis will be placed on new discoveries and improved recovery technologies. Sources such as tar sands and oil shales are likely to become more important, bringing environmental problems inherent to the production, transport, and processing of these resources to new sectors of the environment. Table 1. United States and world energy consumption by source (January 2000) Energy SourceUnited StatesWorld BBOEPercentBBOEPercent Petroleum6.84127.140 Natural gas4.224.214.421.2 Coal3.923.216.724.7 Nuclear1.37.94.56.7 Hydroelectric0.733.71.62.4 Biomass3.45 Total7.610069.3100 Source: Edwards (2001). BBOE=billion barrels of oil equivalent. Reserves of crude oil, the raw material used to make petroleum products, are not evenly distributed around the world. The production levels of the major oil-producing nations in the world, shown in Figure 2, are based on data collected by the US Department of Energy's (DOEs) Energy Information Administration (EIA). Nations in the Organization of Petroleum Exporting Countries (OPEC) produce ∼43% of the world's total of nearly 68 million bbl per day (bpd) (1.08×1010 L d-1), with Saudi Arabia as the world's largest producer at 8.3 million bpd (12% of total) (1.33×109 L d-1), followed by Russia and the former Soviet Union at 6.4 million bpd (9%) (1.02×109 L d-1). The United States produced ∼5.7 million bpd (8%) (9.06×108 L d-1). Oil and gas are being produced in virtually all geographic areas intersecting a wide range of ecological habitats, including tropical rain forests, Middle Eastern deserts, Arctic regions, and deep offshore marine environments. (6K)Figure 2. World crude-oil production from leading producers, 1960-2000, in million barrels per day (Energy Information Administration, 2001). The petroleum industry has two main components, exploration/recovery and refinement/delivery. Crude oil is delivered to refineries and refined products are delivered to consumers through a large transportation network that includes tankers, barges, pipelines, and railroads. This transportation network links suppliers and producers across the world. Above-ground tanks, below-ground tanks and caverns, and offshore facilities store petroleum at various stages in this distribution system. The largest underground storage facilities in the US are part of the US. Strategic Petroleum Reserve maintained by the US government for times of supply shortage (Energy Information Administration, 1999). The global and complex nature of this industry means that every environmental compartment (air, land, water, biota) and all nations are affected by petroleum spills and waste products.Petroleum products are released into the environment inadvertently and intentionally at all stages of petroleum use, from exploration, production, transportation, storage, use and disposal (Figure 3). The more volatile components of oil partition preferentially into the atmosphere and hydrosphere, and to a lesser extent, the geosphere (e.g., sorption onto sediments), and biosphere (i.e., bioaccumulation), whereas the higher molecular weight fraction of oil is less soluble and less volatile and impacts the biosphere and geosphere more directly as described by Abrajano et al. (see Chapter 9.13). Usage causes compounds from petroleum products to separate and enter the environment. For example, incomplete fuel combustion in an automobile engine or a home heating furnace results in the release of compounds to the atmosphere, where they can be widely dispersed. Compounds in the exhaust gas of a motor boat partition in river or lake water, allowing for mixing and transport in the water body. These examples are illustrative of chronic low-level and widespread release of compounds referred to as nonpoint sources. (10K)Figure 3. Modes of contamination by petroleum products during production, transport, use, and disposal (after Fetter, 1993). Concentrated releases of compounds to the environment generally are associated with product mishandling and accidents during storage and transportation. For example, gasoline leaking from an underground storage tank (UST) releases compounds to the air, water, and solids of the subsurface. An oil tanker spill at sea can result in the rapid dispersal of large amounts of product. These two examples of more concentrated release are referred to as point sources. However, in the case of a tanker spill, the point release can result in a serious regional problem such as the spill of the Exxon Valdez that spilled oil that spread 750 km from the original spill site and impacted 1,750 km of shoreline (Wolfe et al., 1994).Nonpoint sources can cause compounds to be dispersed throughout the hydrosphere, which can complicate efforts to evaluate the effect of spills. Current sampling and analytical techniques allow for the detection of volatile organic compounds (VOCs) at low concentrations (0.1 μg L-1 and less). As a result, VOCs have been detected frequently in ambient shallow groundwater in urban areas across the country in studies conducted as part of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) program (Squillace et al., 1996). The importance of VOCs at low concentrations in ambient water is unclear. If these low concentrations are the result of plumes emanating from point sources, then concentrations could possibly increase with time. If the source were diffuse (nonpoint), as in the case of atmospheric sources, then changes in VOC concentrations in groundwater over time would be constrained by atmospheric concentrations.Widely distributed and frequent point source spills can be considered nonpoint sources when the scale of concern is large in space or time. For example, the United States generates 1.3 billion gallons (4.9×109 L) of used motor oil each year, of which ∼13% is disposed of improperly (Motor Oil Facts, http://www.epa.gov/seahome/housewaste/src/oilfact.htm). This large quantity of improperly disposed motor oil could be a regional source of MTBE and benzene, toluene, ethylbenzene, and xylenes (BTEX) as used motor oil contains BTEX and MTBE, ∼500-2,000 mg L-1 for BTEX and 100 mg L-1 for MTBE (Baker et al., 2002; Chen et al., 1994). In a survey of 946 domestic wells designed to assess ambient groundwater quality in Maine, MTBE was detected in 15.8% of the wells, mostly at concentrations less than 0.1 μg L-1 (State of Maine Bureau of Health, 1998). Small and widely distributed spills of fuel-related products not necessarily associated with leaking storage tanks were identified as likely sources.
- Publication:
-
Treatise on Geochemistry
- Pub Date:
- December 2003
- DOI:
- 10.1016/B0-08-043751-6/09054-X
- Bibcode:
- 2003TrGeo...9..433C