Chapter 6 - Energy and the Environment

Chapter 6 - Energy and the Environment

Introduction

Busy arterial street in Washington, DC.

Busy arterial street in Washington, DC.

The U.S. Department of Transportation, under its human and natural environment strategic goal, is committed to protecting and enhancing communities and the natural environment affected by transportation. The economic and societal benefits provided by transportation also generate environmental impacts, and the sector's dependence on fossil fuels is at the root of many of these environmental problems. Construction of transportation infrastructure and facilities, and vehicle manufacturing, maintenance, use, and disposal affect the environment as well.

Transportation energy use has grown at 1.5 percent per year for the past two decades. Still, this growth rate is slower than that of the Gross Domestic Product and passenger-miles of travel, reflecting in part a general decline in the energy intensity of almost all modes. Today, however, the transportation sector consumes a greater share of petroleum (66 percent) than it did in 1973 (50 percent). The use of alternative and replacement fuels to reduce foreign oil dependence and environmental impacts has grown, but, despite incentives in place to promote these fuels, they still accounted for only a small faction of total motor vehicle fuel use in 1999.

Growth in energy consumption is causing a corresponding increase in greenhouse gas (GHG) emissions. The transportation sector emitted 1,819 million metric tons of carbon dioxide in 1999, an increase of 14.9 percent since 1990. Three-quarters of GHG emissions come from the use of highway vehicles. In addition to GHG emissions, transportation remains a primary source of emissions of three of the six air pollutants regulated under the Clean Air Act: carbon monoxide, nitrogen oxides, and volatile organic compounds. However, with the exception of nitrogen oxides, these emissions have been declining since 1990.

Using transportation vehicles generates noise and can result in hazardous materials and oil spills. New estimates suggest that, since 1992 when U.S. requirements for quieter aircraft went into effect, the percentage of the population exposed to airport noise has been cut in half. Another area of concern is oil spills-an average of 1.8 million gallons of oil is spilled into U.S. waters each year. In 1998, 51 percent of this oil was cargo carried by marine vessels, pipelines, railcars, and tank trucks.

Transportation infrastructure and its maintenance can also be sources of environmental damage. Each year the U.S. Army Corps of Engineers dredges an average of 271 million cubic yards of sediments from navigation channels. Contaminated sediments are confined in various ways, while the balance may be used beneficially to nourish beaches and wetlands. Sediments become contaminated from pollutants released into the nation's waters. Similarly, petroleum stored in underground tanks has a history of leaking into soils and ultimately into surface and underground waters. By 2000, after a decade of focused effort by the U.S. Environmental Protection Agency (EPA), there were almost 163,000 known petroleum tank releases around the country waiting to be cleaned up. The leaking of methyl-tertiary-butyl-ether (MTBE), largely from underground storage tanks, was recognized in 2000 as an issue serious enough for EPA to ask the U.S. Congress to ban or reduce its use as an additive in gasoline.

Rubber tires and lead-acid batteries are two of several transportation wastes quantified on an annual basis. States and local governments often promote the establishment of systems to recycle these wastes at the end of their lifetime. However, while over 93 percent of the lead content of batteries was reused in 1998, only 24 percent of the 4.5 million tons of scrap tires generated were recycled. This left almost 3.5 million tons of car, truck, and motorcycle tires to be disposed of in landfills or incinerated.

Preservation of wetlands, urban sprawl, invasive species, and environmental justice are areas of concern that have emerged fairly recently. The United States has an estimated 105 million acres of wetlands. An equal amount may have been lost since the 1600s, drained to develop rural and urban areas. Transportation impacts wetlands when roads and railroads are built and people and cargo are moved through them and when airports and other facilities are placed in them.

How land is used for transportation is also a part of urban sprawl. Here, transportation enables and follows the radial development of commercial, industrial, and residential communities out from urban areas. Transportation also enables the legal, illegal, and unintended importation of non-native species of plants and animals. Once established, many of these species cause environmental and economic damage; in some cases, killing off native species. Disproportionately using land in minority and low-income communities for transportation and other infrastructure has given rise to the environmental justice movement. Today, environmental justice has become part of U.S. Department of Transportation operations. The central aim is to redress lack of meaningful participation by these communities in transportation decisionmaking.

Energy Use

As the economy has grown, so too has transportation energy use. From 1980 to 1999, transportation energy use grew from 19.7 quadrillion (quads) British thermal units (Btu) to 26.0 quads, an annual growth rate of 1.5 percent [1]. The overall growth rate is lower than that of the economy (as measured by Gross Domestic Product (GDP)) and the growth rate in passenger-miles, but not population (figure 1). It is influenced by a combination of factors, including changes in transportation intensity of U.S. GDP, vehicle fuel efficiency, and personal travel propensity.

For decades, the transportation sector has accounted for between 25 percent and 27 percent of total U.S. energy consumption (table 1). In 1998, highway vehicles accounted for about 80 percent of transportation energy use. Passenger cars use about 42 percent of the sector's total, followed by light trucks with 20 percent, and heavier trucks with 17 percent. Among the nonhighway modes, air transportation is the biggest and fastest growing energy user. The pipeline mode, which accounted for about 3 percent of total transportation energy use, is the only mode that does not depend directly on petroleum. Typically, pipelines use natural gas and/or electric pumps to move products (figure 2).

Source
1. U.S. Department of Energy, Energy Information Administration, Monthly Energy Review, May 2000, DOE/EIA-0035(2000/05) (Washington, DC: 2000).

Petroleum Consumption

In the United States, petroleum consumption has risen faster in the transportation sector than in any other since 1973, before the first oil embargo. Continued growth in transportation activities has contributed, in large part, to the increase in oil consumption. While the oil price shocks of 1973-74 and 1979-80 depressed demand for a while, they did little to shake transportation's dependence on oil. Only a small fraction of transportation's energy needs are met by nonpetroleum sources, such as natural gas, methanol, and ethanol. Nonpetroleum sources are used primarily as gasoline blending agents to meet requirements of the Clean Air Act Amendments of 1990.

From 1973 to 1999, the residential and commercial buildings sector cut petroleum use in half, and the utilities sector reduced oil use by more than 60 percent. Over the same period, industrial sector oil use hovered between 4 million barrels per day (mmbd) and 5 mmbd, primarily because petroleum is an important feedstock for the petrochemicals industry. In contrast, oil usein the transportation sector rose from 9.05 mmbd in 1973 to 12.75 mmbd in 1999, an increase of about 41 percent. Due to these changes in consumption patterns among sectors, transportation today accounts for two-thirds of total U.S. petroleum demand compared with about 50 percent before 1973 [1] (figure 1).

The U.S. Department of Energy expects the heavy concentration of oil demand in the transportation sector to continue. Between 1998 and 2020, overall U.S. petroleum consumption is projected to increase by 6.2 mmbd. Transportation demand, particularly for "light products,"1 accounts for much of this projected rise in consumption [2].

1Light products include gasoline, diesel, heating oil, jet fuel, and liquefied petroleum gases.They are more difficult and costly to produce than heavy products, such as fuel oil.

Sources
1. U.S. Department of Energy, Energy Information Administration, Annual Energy Outlook 2000, DOE/EIA-0383(2000) (Washington, DC: December 1999).
2. ____. Annual Energy Review 1999, available at www.eia.doe.gov/emeu/aer/petro.html as of June 23, 2000.

Alternative and Replacement Fuels

Spurred by energy and environmental legislation, the use of alternative and replacement fuels in motor vehicles is growing, but not enough to indicate a trend away from the use of petroleum in the transportation sector. Between 1992 and 1999, estimated alternative fuel use grew by 5.8 percent annually (table 1). Nevertheless, alternative fuels comprise a tiny fraction of total motor vehicle fuel use-0.17 percent in 1992 and 0.21 percent in 1999. This growth is in proportion to the rise in the number of alternative fuel vehicles (table 2) [2].

Replacement fuels-alcohols and ethers (oxygenates)-are blended with gasoline to meet the requirements of the Clean Air Act Amendments of 1990. They comprise a larger proportion of the motor fuel market than alternative fuels, as shown in figure 1. Unlike petroleum, which is composed entirely of hydrogen and carbon atoms, alcohols and ethers contain oxygen and are derived from energy sources other than petroleum.

In areas where carbon monoxide emissions are a problem, fuel providers have been required since 1992 to add oxygenates to gasoline to promote more complete combustion. Gasoline that contains oxygenates is referred to as reformulated gasoline (RFG). Beginning in 1994, areas failing to attain air quality standards for ozone were required to use RFG, which must contain 2 percent oxygen by weight. In 1999, oxygenates made up 3.2 percent of the gasoline pool [2].

The most popular oxygenate is methyl-tertiary-butyl-ether (MTBE), a combination of methanol and isobutylene. Natural gas is used to make MTBE. The discovery of MTBE in drinking water supplies has raised concerns about its use as a gasoline additive, however. At this time, MTBE is not classified as a carcinogen, but studies have shown that it can cause cancer in animals. Moreover, trace amounts of MTBE in water supplies produces an unpleasant odor and taste [1]. Many issues remain regarding the use of MTBE as a replacement fuel. These are just a few that decisionmakers must face when considering the use of alternative and replacement fuels. (MTBE is discussed in more detail in the environment section.)

Sources
1. U.S. Department of Energy (USDOE), Energy Information Administration (EIA), Annual Energy Outlook 2000, DOE/EIA-0383(2000) (Washington, DC: December 1999).
2. ____. Alternatives to Traditional Transportation Fuels 1998, available at www.eia.doe.gov/cneaf/solar.renewables/alt_trans_fuel98/table10.html, as of May 31, 2000).

World Crude Oil Prices

World oil prices tripled between January 1999 and July 2000 as a result of oil production cutbacks by the Organization of Petroleum Exporting Countries (OPEC), with the cooperation of Mexico, Norway, and Russia (figure 1). This oil price hike prompted concern that oil dependence may once again become a serious concern for the transportation sector and the economy as a whole.

The economic costs of previous oil price shocks are estimated to be in the trillions of dollars. Of course, the size of economic losses depends on the importance of oil in the economy and the ability to substitute other energy sources for oil. The transportation sector, with its inelastic demand, has shown little movement toward replacing oil with other energy sources.

A Bureau of Transportation Statistics (BTS) analysis of the economic impact of the 1999-2000 increase in fuel prices concluded that, to drive the same distance and produce the same Gross Domestic Product as in 1999, U.S. households and businesses would spend an additional $67 billion (28 percent more) on transportation fuel in 2000. Households would absorb more than half of the additional cost and for-hire transportation firms about one-third. The rest of the cost would be absorbed by nontransportation firms. On a per household basis, U.S. households would have to spend $344 more in 2000 on motor fuel to travel the same distance as in 1999 [1]. In fact, households spent, on average, $1,550 on motor fuel, or $312 more than the average in 1999 (figure 2). The difference between what BTS estimated households would spend and what they actually spent may have been caused by reduced household travel in response to higher gasoline prices, the use of more efficient vehicles, or switching to other modes of transportation. However, data are not available at this time to substantiate these or other possible explanations.

Source
1. U.S. Department of Transportation, Bureau of Transportation Statistics, "The Economic Impact of the Recent Increase in Oil Prices," Transportation Indicators: A Prototype, May 2000.

Energy Intensity of Passenger Travel and Freight Transportation

The amount of energy required to carry passengers and freight has declined. Between 1980 and 1998, automobile energy use per passenger-mile of travel (pmt) by car fell by 12 percent (figure 1). This has occurred even though the average fuel economy of new car and light truck fleets leveled off in the 1990s [1].

Commercial air carriers reduced energy use per passenger-mile by more than 30 percent over the 1980 to 1998 period, due largely to higher occupancy [1]. Flying a full plane requires considerably less than twice the amount of fuel of a half-full one but yields twice the passenger-miles. Airlines have been increasingly successful in filling their planes; in some cases, reconfiguring seating to fit more passengers. Moreover, although newer airplanes are more efficient, this probably has less effect on energy intensity than the greater number of passengers.

The energy intensity of Amtrak intercity rail and intercity bus declined as well (-22 percent and -33 percent, respectively). At 713 British thermal unit (Btu) per pmt in 1998, intercity buses are considered the most energy-efficient mode of transportation. Energy use per pmt on transit buses, however, increased 51 percent over this period to 4,238 Btu per pmt [1].

Because of data limitations and availability, less is known overall about the energy intensity of freight transportation, particularly the waterborne and heavy truck modes. Some data are available, however. Energy use per vehicle-mile has decreased, albeit slowly. The decrease in energy use per vehicle-mile combined with a general increase in truck size and weight limits suggest that truck energy use per ton-mile has also decreased [2]. For rail freight, energy intensity declined about 38 percent between 1980 and 1997 [3].

It is important to note that intermodal comparisons should be considered approximations. Modal data are collected in different ways and based on different assumptions. Passenger-mile data are more relevant for passenger vehicles, while vehicle-mile or ton-mile data are more relevant for freight vehicles. Modes also perform different functions and serve different travel markets.

Sources
1. Davis, Stacy C., Transportation Energy Data Book, Edition 19 (Oak Ridge, TN: Oak Ridge National Laboratory, 1999), table 2.12.
2. U.S. Department of Transportation, The Changing Face of Transportation (Washington, DC: 2000).
3. U.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Statistics 1999 (Washington, DC: 1999).

Car and Light Truck Fuel Efficiency

Passenger cars and light trucks are more fuel-efficient today than they were in 1978, when fuel economy standards were first implemented. Technologies like fuel injection engines, lockup torque in transmissions, and improved rolling resistance of tires have played a major role in this change. Between 1978 and 1988, new passenger car average fuel economy shot up from 19.9 miles per gallon (mpg) to 28.8 mpg, while light trucks improved somewhat from 18.2 mpg (1979) to 21.3 mpg. Since then, new car fuel economy has remained flat (figure 1).

The Corporate Average Fuel Economy (CAFE) standard for new cars has held constant at 27.5 mpg since 1990 (table 1). On average, new foreign and domestic cars and light trucks meet the CAFE standard, but several high-end imports were below it in 1999 [3].

In recent years, efficiency gains have been offset by increases in vehicle weight and power and by consumer shifts to less efficient vehicles, such as light trucks, especially sport utility vehicles, minivans, and pickup trucks. For example, the average weight of new cars (foreign and domestic) rose from a low of 2,805 pounds in 1987 to 3,116 pounds in 1999. The average weight of new cars today is still lower than the 3,349-pound weight of new cars in 1978. Furthermore, in response to consumer demand for new high performance cars, the ratio of horsepower to 100 pounds of weight increased from 3.98 in 1987 to 5.21 in 1999. For the domestic car fleet, the average is even higher-5.30 horsepower per 100 pounds [3].

The popularity of light trucks continues to grow. Twice as many cars as light trucks were sold in the United States in 1990. By 2000, however, almost an equal number of cars (8.8 million) and light trucks (8.5 million) were sold [2]. Clearly, many consumers are finding what they want in light trucks rather than cars: roominess, more carrying capacity, greater visibility, and a perception of safety (at least for themselves). However, this trend has implications for energy consumption and for emissions, because light trucks, on average, are less fuel-efficient than cars.

A variety of technologies can increase motor vehicle fuel efficiency. In the near term, improving vehicle aerodynamics to reduce drag and lowering the rolling resistance of tires can make a difference. In the longer term, improved electronic transmission controls that allow optimum gear selection for peak efficiency and continuously variable transmissions can provide additional efficiency gains. Several automobile manufacturers are marketing hybrid engines, cars that run alternatively on gasoline and electric-powered engines. These cars are rated at 48 to 64 mpg (combined highway and city) for the 2001 model year.More advanced technologies, such as the diesel hybrid engine, are expected to obtain even higher efficiencies [1].

Sources
1. U.S. Department of Energy and U.S. Environmental Protection Agency, Energy Technology and Fuel Economy, available at http://www.fueleconomy.gov, as of February 2001.
2. Compiled from data provided by Ward's AutoInfoBank as cited in U.S. Department of Transportation, Bureau of Transportation Statistics, "Transportation Indicators," February 2001.
3. U.S. Department of Transportation, National Highway Traffic Safety Administration, Automotive Fuel Economy Program, Twenty-Fourth Annual Report to Congress, Calendar Year 1999 (Washington, DC: June 2000).

Transportation Environmental Indicators

Indicators are quantitative data that can be used to assess the magnitude of problems, help set priorities, develop performance measures and track progress toward goals, or educate stakeholders. To assess problems and measure progress, trend data are needed. The full range of environmental impacts of the transportation system is extensive but few good indicators are available.

Figure 1 is a conceptual diagram of the environmental impacts of transportation from a life cycle perspective. Phases (or stages) of transportation include fuel production, vehicle manufacturing, fixed infrastructure development, travel (or vehicle use), maintenance, and disposal. Activities occurring during the phases result in environmental outcomes (e.g., pollution releases and changes in wetlands acreage). Outcomescan, in turn, affect the environment and human health, creating impacts that are usually negative (e.g., cancers, birth defects, asthma, stunted tree growth, and fish kills). Impacts, which can be chronic or acute, are highly dependent on two variables: concentration and exposure.

Activity, outcome, or impact data can be the source for indicators. However, outcome indicators are most commonly used for transportation. Activities are only indirectly related to environmental consequences. Increases in passenger car vehicle-miles traveled may or may not result in increased pollutant releases. Similarly, the volume of oil transported by marine vessels is not indicative of harm caused by oil spills at sea. Most available impact data do not directly identify sources, such as transportation. For instance, the U.S. Environmental Protection Agency (EPA) reports annually on changes in the nation's air quality (described elsewhere in this chapter). These data come from monitoring stations that measure concentrations of pollutants in the atmosphere. The sources of the pollutants may be factories, powerplants, dry cleaning facilities, printing shops, storage tanks, and so on, as well as vehicles.

Readily available data for outcome indicators are not comprehensive, especially for all modes and life cycle phases. National trend data for outcomes are estimated, modeled, or collected only for some pollutants.

The most often used transportation environmental indicator comprises six criteria air pollutants regulated under the Clean Air Act (described elsewhere in this chapter). The data are estimated annually by EPA and show the relative outcome contribution of these pollutants by mode (except pipelines) during the travel phase. With the emergence of the global climate change issue, an additional indicator is available. EPA and the Energy Information Administration now annually estimate the amount of six greenhouse gases emitted by the transportation sector.

Transportation Sector Carbon Emissions

Most scientists believe that rising concentrations of greenhouse gases in the Earth's atmosphere could cause global climate change. Greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, can occur naturally or can be produced by human activities. Carbon dioxide is the predominant greenhouse gas produced by human activity, accounting for 83 percent of all U.S. greenhouse gas emissions in 1999. Nearly all carbon dioxide emissions are produced by the combustion of fossil fuels. Thus, there is a high correlation between energy use and carbon emissions [1].

Today, almost all of transportation's energy needs are supplied by oil. The combustion of petroleum in the transportation sector alone is responsible for about 27 percent of all greenhouse gases emitted in the United States. Figure 1 shows carbon emissions from energy consumption in transportation and other sectors. From 1990 to 1999, transportation-related carbon dioxide emissions grew 14.9 percent. This is less than the commercial sector growth, about the same as the residential sector, but more than that recorded in the industrial sector. In absolute numbers, however, transportation sector carbon emissions grew the most-about 64.3 million metric tons of carbon (mmtc) during this period. Table 1 breaks down transportation sector emissions by fuel type.

According to the U.S. Department of Energy, carbon emissions from energy consumption could rise from 1,485 mmtc in 1998 to 1,979 mmtc by 2020 under a "business as usual" scenario. The transportation sector could contribute about 710 mmtc, or 36 percent of the total. The transportation emissions growth rate is higher than that of other end-use sectors due to several factors, including expected increases in travel and a leveling off of light-duty vehicle fuel efficiency [2].

Because of growing concerns about the potentially adverse impacts of global climate change (e.g., loss of coastal land to rising oceans and greater frequency of violent weather), several international meetings have addressed global climate change. In 1997, parties to the United Nation's Framework Convention on Climate Change met in Kyoto, Japan, and agreed to the Kyoto Protocol setting emissions targets for individual industrialized countries. Since then, the Convention parties have met in Buenos Aires, Bonn, and the Hague to discuss unresolved issues, such as international emissions trading, rules for joint implementation projects and the clean development mechanism, and compliance and data development issues.

The United States signed the Kyoto Protocol, but it has not been submitted to the U.S. Senate for advice and consent. Many members of Congress have expressed opposition to the Protocol for several reasons. Some say redirecting our resources to meeting this goal could reduce economic growth and hurt U.S. competitiveness relative to countries that have not made binding commitments to reducing emissions (i.e., developing countries). Proponents of control argue that steps need to be taken now to reduce future impacts of climate change and that emissions reductions could create opportunities for businesses selling energy-efficient technologies.

Ultimately, strategies to reduce greenhouse gas emissions will affect all energy-using sectors of the economy, including transportation. In 1999, the U.S. Department of Transportation established a Center for Climate Change and Environmental Forecasting to identify and evaluate options to reduce emissions from and impacts on transportation.

See box to see how EPA and EIA data differ.

Sources
1. U.S. Department of Energy, Energy Information Administration, Annual Energy Outlook 2000, DOE/EIA-0383(2000) (Washington, DC: December 1999).
2. ____. Emissions of Greenhouse Gases in the United States 1999, DOE/EIA-0573(99) (Washington, DC: October 2000).

Key Air Pollutants

Despite significant increases in the U.S. population, gross domestic product, and vehicle-miles traveled since 1980, carbon monoxide (CO), volatile organic compounds (VOC), particulates, and lead emissions have declined, leading to improved air quality. These decreases are due primarily to vehicle tailpipe and evaporative emissions standards established by the U.S. Environmental Protection Agency (EPA), improvements in vehicle fuel efficiency, and the ban on leaded fuel for motor vehicles. Only nitrogen oxide (NOx) emissions, which contribute to the formation of ground-level ozone, remain above their 1990 level (figure 1). In September 1998, EPA issued a rule to reduce NOx emissions in 22 eastern states, but implementation was held up by a court case.

Although progress has been made in reducing transportation-related pollutants, mobile sources still account for a sizable percentage of several key pollutants. In 1998, for example, transportation contributed about 61 percent of all CO emissions, 41 percent of NOx, 36 percent of VOC, and 13 percent of lead [2]. With the exception of lead, highway vehicles were the primary transportation source of these pollutants. The use of lead in aircraft fuel is responsible for nearly all transportation-related lead emissions. The Federal Aviation Administration, EPA, and the aviation industry are examining ways to reduce lead emissions. Figure 2shows 1998 emissions by mode.

In 1997, EPA added ammonia to its National Emission Trends database, which covers both mobile and stationary sources of pollution. Gaseous ammonia reacts in the air with sulfur dioxide and NOx to form ammonium sulfate and nitrate particles that are found in particulate matter of 2.5 microns in diameter or smaller. In 1998, mobile sources, primarily onroad gasoline-powered vehicles, accounted for about 8 percent of total ammonia emissions.

The decline in emissions of some pollutants from transportation vehicles directly impacts the nation's air quality, which is a measure of the concentration of pollutants in the atmosphere. Since 1980, air quality trends show continuous improvement nationwide. Maps compare nonattainment areas1 in September 1996 and 1999. Over this period, the number of nonattainment areas for one or more pollutants declined from 174 to 119. The number of people living in nonattainment areas and, therefore, exposed to poor air quality declined from 127 million to just under 103 million [1, 3].

1 Areas where air pollution levels persistently exceed national air quality standards.

Sources
1. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Quality and Emissions Trends Report, 1995 (Research Triangle Park, NC: 1997).
2. ____. National Air Pollutant Emission Trends: 1900- 1998, EPA 454/R-00-002 (Research Triangle Park, NC: March 2000), also available at http://www.epa.gov/ttn/chief/trends/, as of May 18, 2001.
3. ____. National Air Quality and Emissions Trends Report, 1998, 454/R-00-003 (Research Triangle Park, NC: March 2000), also available at http://www.epa.gov/oar/aqtrnd98/, as of May 18, 2001.

Mobile Source Toxic Air Pollution Emissions

Transportation vehicles emit hazardous air pollutants (HAPs). These chemicals have the potential to cause serious health effects in humans, such as cancer, reproductive disorders, and developmental and neurological problems, and cause damage to the ecosystem. The U.S. Environmental Protection Agency (EPA) estimated that 4.3 million tons of 188 HAPs (as specified in the Clean Air Act Amendments of 1990) were released nationwide in 1996. Half of the total came from mobile sources; the balance, from stationary sources, such as factories, powerplants, and shops.

Mobile source air toxics are constituents of or impurities in petroleum feedstock, are formed during the fuel combustion process or afterward in the atmosphere, or result from engine wear. Benzene, toluene, and xylene, for instance, are volatile organic compounds (VOC) that occur naturally in petroleum. They are concentrated during the gasoline refining process and then released during vehicle fueling and in vehicle exhaust.

The air concentration of 13 mobile source air toxics (MSATs) has been monitored at some sites, nationwide.1 The annual average concentration of benzene declined 37 percent in metropolitan areas between 1993 and 1996 (figure 1), while toluene decreased by 44 percent. EPA attributes these decreases to the use of reformulated fuel in many parts of the country [2]. The nationwide trends for other MSATs, such as 1,3-butadiene and styrene, do not show appreciable air quality improvements or are relatively flat.

EPA issued a ruling in December 2000 designating 21 HAPs as MSATs [1] (table 1). Among this set of HAPs, transportation is the source of 86 percent of the nation's air emissions of methyl-tertiary-butyl-ether (MTBE), 84 percent of ethylbenzene, 79 percent of xylene, 76 percent of benzene, 74 percent of toluene, 70 percent of acetaldehyde, and 60 percent of 1,3-butadiene. EPA is relying on vehicle-based programs already in place or proposed (e.g., reformulated gasoline and sulfur control requirements) to significantly reduce onroad MSAT emissions. In addition, EPA set new gasoline toxic emissions performance standards to reduce emissions such as benzene. For nonroad emissions, EPA plans to conduct research to improve emissions inventory data before proposing any control programs.

1 The total number of monitoring sites varies by pollutant. There are 84 sites measuring benzene and 78 measuring toluene in metropolitan areas. Thirty of the nation's air toxic monitoring locations are in California.

Sources
1. U.S. Environmental Protection Agency (EPA), National Vehicle and Fuels Emission Laboratory, "Control of Emissions of Hazardous Air Pollutants from Mobile Sources," 65 Federal Register 48058, 2001
2. _____. Office of Air Quality Planning and Standards, National Air Quality and Emissions Trends Report, 1998, EPA 454/R-00-003 (Research Triangle Park, NC: 2000).

Aircraft Noise

A series of U.S. laws passed between 1968 and 1990 established aircraft noise as an environmental pollutant. The entire fleet of large civil subsonic turbojet airplanes operating at airports in the contiguous United States were converted to the quieter Stage 3 status by the end of 1999 (see box). Individually, all operators had met or exceeded the interim compliance requirement of the Airport Noise and Capacity Act of 1990. As shown in figure 1, throughout the transition period (1992 to 1999), operators made steady progress toward the final goal [1].

National data and modeling of airport noise indicate there has been a decline in the numbers and percentage of the population exposed to noise levels of 65 dB of day-night noise levels and above since 1975 (figure 2). During this same period, annual aircraft departures rose from 4.5 million to over 8 million [2]. Four different methodologies (for 1995, 1980 to 1985, 1990, and 1990 to 1998) were used to estimate the exposure data. Actual data do not exist for many of the nation's airports, such as New York City's John F. Kennedy International Airport. Airport authorities are not required to produce noise exposure maps and reports except when applying for federal funds to participate in the voluntary Noise Compatibility Program. Accordingly, the current Nationwide Airport Noise Impact Model, which is used to estimate 1995 data and beyond, contains approximations of noise contours for many commercial airports, including 14 of the nation's 50 busiest. These 14 airports accounted for about one-quarter of all air carrier operations in 1998 [3].

Despite declines in exposure and aircraft noise, 58 percent of the airport operators surveyed in 2000 cited noise as their most serious environmental challenge [3]. Aircraft operators have two ways to meet Stage 3 standards: put new aircraft built to the standards in service and retrofit existing aircraft engines with hushkits to muffle noise. Among those surveyed, most airport operators claimed that the continuing noise problem is caused by the retrofitted aircraft because they are still noisier than aircraft built to meet Stage 3 standards [3]. Federal Aviation Administration data on Stage 3 aircraft do not differentiate between these two types of aircraft.

Sources
1. U.S. Department of Transportation, Federal Aviation Administration, Report to Congress: 1998 Progress Report on the Transition to Quieter Airplanes (Washington DC: August 1999).
2. U.S. Department of Transportation, Office of the Secretary, The Changing Face of Transportation, draft for public comment, September 2000, fig. 5-31.
3. U.S. General Accounting Office, Aviation and the Environment: Airport Operations and Future Growth Present Environmental Challenges, GAO/RCED-00-153 (Washington, DC: August 2000), also available at http://www.gao.gov/reports.htm, as of May 2001.

Oil Spills

Failures in transportation systems (vessels, pipelines, highway vehicles, and railroad equipment) or errors made by operators can result in spills of oil and hazardous materials. Better information is available about the extent of spill incidents than about the overall consequences of these spills on the environment and human health. The impact of each spill, for instance, will depend on the concentration and nature of the pollution, the location and volume of the spill, weather conditions, and the environmental resources affected.

When an oil spill occurs in U.S. waters, the responsible party is required to report the spill to the U.S. Coast Guard. The Coast Guard collects data on the number, location, and source of spills, volume and type of oil spilled, and the type of operation that caused spills. Between 1994 and 1999, an annual average of 2.1 million gallons of various types of oil were spilled by all sources (figure 1). While it varies from year to year, not all the oil spilled is cargo. In 1998, for instance, 51 percent of the volume of oil spilled was cargo being moved by transportation equipment such as tankers, barges, pipelines, railroads, and tank trucks [1].

Data for 1989 to 1998 show that marine vessel and pipeline spills varied considerably each year, from a low of 40 percent of the total volume spilled in 1991 to a high of 90 percent in 1989 [2] (table 1). A major incident in any one year can cause major fluctuations in data from year to year. For instance, the 10 million gallon Exxon Valdez spill represented 91 percent of the crude oil spilled in 1989. In 1996, a pipeline ruptured, spilling about 958,000 gallons of intermediate fuel oil into the Reedy River in South Carolina. This one incident represented 98 percent of the total volume of oils spilled into U.S. waters by pipelines that year.

Sources
1. American Petroleum Institute, Oil Spills in U.S. Navigable Waters: 1989-1998 (Washington, DC: Feb. 22, 2000).
2. U.S. Department of Transportation, U.S. Coast Guard, Polluting Incident Compendium, 2000, available at http://www.uscg.mil, as of June 7, 2000.

Dredged Material

The nation's ports and navigation channels must be regularly dredged to maintain proper depths to accommodate shipping. In conducting this work between 1990 and 1999, the U.S. Army Corps of Engineers produced an average of 271 million cubic yards of dredged materials per year at an annual average cost of $572 million [1].U.S. port authorities spend an additional $100 million per year on average to dredge their berths and connecting channels [2], but data on the total amount of material dredged are only occasionally available.

In 1998, the U.S. Environmental Protection Agency (EPA) estimated that about 10 percent (about 1.2 billion cubic yards) of the sediment underlying the nation's surface water was "sufficiently contaminated with toxic pollutants to pose potential risks to fish and to humans and wildlife who eat fish" [3]. Further, EPA noted that about 3 million to 12 million cubic yards of material dredged each year are "sufficiently contaminated to require special handling and disposal." National Army Corps of Engineers data, which are aggregated on an annual basis from individual dredging contracts, do not identify how much material is contaminated, although the data show how dredged material is managed (figure 1). Several of the reporting categories (especially confined, open water/upland, and mixed) may include contaminated sediments.

Sources
1. U.S. Army Corps of Engineers, Water Resources Support Center, Navigation Data Center, Dredging Information System, 2000, available at http://www.wrsc.usace.army.mil/ndc, as of February 2001.
2. U.S. Department of Transportation, Maritime Administration, Office of Ports and Domestic Shipping, United States Port Development Expenditure Report (Washington, DC: November 1999).
3. U.S. Environmental Protection Agency, Office of Water, EPA's Contaminated Sediment Management Strategy, EPA-823-R-98-001 (Washington, DC: April 1998).

Leaking Underground Storage Tanks

Underground tanks for storing petroleum products, such as fuels for transportation, have a history of leaking petroleum into the nation's underground water. The U.S. Environmental Protection Agency (EPA) started collecting annual data on the problem and its resolution in fiscal year (FY) 1990 under the Underground Storage Tank Program. By the end of FY 2000, EPA regions reported that there were 713,666 active underground tanks in the nation and that almost 1.5 million tanks had been closed [3].

Between 1990 and 2000, the numbers of confirmed releases of petroleum from underground storage tanks (USTs) climbed at an annual rate of 17 percent (table 1). By the end of FY 2000, cleanups had been initiated at 89 percent of the confirmed release sites, and 61 percent of those cleanups had been completed. Almost 163,000 known releases still need to be cleaned up.

As with oil spills in U.S. waters, these data do not reveal the overall environmental impact of releasing petroleum products into underground and surface waters. Furthermore, the data gathered show the number of incidents rather than volume; thus, even the full extent of the problem is not clear. For example, the amount of water contaminated by releases from underground tanks is not known.

In 2000, concern arose about the leaking of methyl-tertiary-butyl-ether (MTBE) from storage tanks and other sources. MTBE is a constituent of reformulated gasoline, which is used in nonattainment areas of the country to lower ozone levels. Once released, MTBE moves rapidly through underground water. This substance has been detected in drinking water, with the highest levels in areas of the country using reformulated gasoline. The major source of groundwater contamination from MTBE appears to be from leaking USTs [1]. Other sources include aboveground tanks, pipelines, and recreational boats. An expert panel convened by EPA to review the MTBE issue reported that, while 80 percent of USTs have been upgraded, there continue to be reports of releases from some of them due to "inadequate design, installation, maintenance, and/or operation." EPA has proposed regulations that would ban or limit the use of MTBE as a gasoline additive [2].

Sources
1. U.S. Environmental Protection Agency, Achieving Clean Air and Clean Water: The Report of the Blue Ribbon Panel on Oxygenates in Gasoline, Executive Summary and Recommendations (Washington, DC: July 27, 1999).
2.____. "EPA Administrator Carol M. Browner Remarks as Prepared for Delivery," press release, Mar. 20, 2000.
3. U.S. Environmental Protection Agency, Office of Underground Storage Tanks, Corrective Action Measures Archive, Jan. 13, 2000, available at http://www.epa.gov/swerust1/cat/camarchv.htm, as of May 2001.

Transportation Wastes

As highway vehicles, aircraft, marine vessels, and railroad locomotives and cars and their various parts are maintained over their lifetime, numerous wastes are generated. At the end of their useful life, transportation equipment is generally dismantled with some portions recycled and the rest discarded. The U.S. Environmental Protection Agency (EPA) makes estimates, based on a material flows model, on the amounts of municipal solid waste (MSW) generated each year. In 1998, the United States generated 220 million tons of MSW [3]. Some transportation wastes (e.g., tires and batteries) are included in these data, but others, such as transportation equipment, discards from automobile dismantling operations, and motor oils, are not included.

Most trend data available on transportation wastes pertain to highway vehicles. For instance, EPA makes annual estimates on the disposal of lead-acid batteries and rubber tires from passenger cars, trucks, and motorcycles (figure 1 and figure 2). Recovered batteries and tires are reused in some form and therefore do not end up in municipal waste landfills and incinerators. Batteries are dismantled, and, in 1997, over 93 percent of the lead content and a significant portion of the polypropylene casings were recycled [3].

The transportation sector consumed an estimated 1,176 million gallons of lubricants in 1997 [1]; these motor oils become wastes throughout a vehicle's life cycle. Means of disposal include burning as fuel, dumping illegally, landfilling, rerefining, and incinerating. Trend data on disposal are not collected and the most recent data were estimated for 1991.

The used motor oils that are burned as fuel or incinerated contribute to the transportation sector's carbon dioxide emissions. EPA estimates that 50 percent of the carbon value in lubricants used by the transportation sector is ultimately released. The balance is sequestered in the products [2].

Sources
1. U.S. Department of Energy, Energy Information Administration, State Energy Data Report 1997, 1997, available at http://www.eia.doe.gov/pub/state.data/pdf/SEDR97.pdf, table 15.
2. U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1997, EPA 236-R-99-003 (Washington, DC: 1999), table A-10.
3. U.S. Environmental Protection Agency, Office of Solid Waste, Characterization of Municipal Solid Waste in the United States: 1998 Update (Washington, DC: July 1999).

Wetlands

It is only during the last few decades that the United States has considered wetlands a natural resource worth enhancing and preserving. This change in thinking occurred after the loss by draining of an estimated half of the wetlands acreage believed to exist in the 1600s [5]. In 1993, the Administration issued a "no net loss of wetlands" policy statement, and four years later called for a national goal of an annual net gain of 100,000 acres of wetlands by 2003.

There were an estimated 105.5 million acres of wetlands in 1997, according to a study done by the U.S. Fish and Wildlife Service of the U.S. Department of the Interior (DOI), in cooperation with other agencies [3]. The study responded to problems the General Accounting Office found in previous estimates done separately by DOI and the Department of Agriculture [2]. In the new study, DOI estimated that a net loss of almost 650,000 acres of wetlands occurred between 1986 and 1997. Urban development, which includes transportation activities, accounted for an estimated 30 percent of all wetland losses.

Transportation infrastructure and use has contributed to the loss of wetlands. Today, developers of roads, airports, rail systems, and marine facilities must determine, as part of the National Environmental Policy Act process, whether their projects will impact wetlands. If so, they may need to obtain a Clean Water Act Section 404 permit from the U.S. Army Corps of Engineers.

The only time the Corps has collected nationwide data on permits was between May 1997 and September 1998 (table 1), when it was restructuring permits. According to these data, multiple residential projects received the most permits (20 percent) and impacted 26 percent of the wetlands acreage listed. Transportation ranked second in total number of requested and authorized permits (18 and 19 percent, respectively) and in total wetlands acreage impacted. While these data provide some insight into transportation infrastructure's relative impact on wetlands, there are no data on impacts from runoffs of salt, oils, and rubber from highways and other facilities, and air pollutants emitted by vehicles, locomotives, airplanes, and vessels as they move along or through wetland areas. Furthermore, data showing total numbers of acres provide no information on the quality of the remaining wetlands as measured by their value to society [1].

The Corps does not break down wetlands permit data by mode. The Federal Highway Administration has collected data on wetlands acreage impacted by the Federal Aid Highways system since 1996 (table 2). However, Federal Aid Highways constitute only 4 percent of the total miles of public roads in the country [4]. Although the Federal Aviation Administration (FAA) does not collect data on wetlands impacted by airports, airport runway expansion often involves an evaluation of wetland impacts. As part of its dredging program, the Corps identifies the amount of material dredged from navigation channels used each year to nourish wetlands. From 1994 to 1999, an average of 30 million cubic yards per year (7 percent of all Corps-dredged material) was used for this purpose.

When it is determined that a transportation project will impact a wetland, federal policy requires compensatory mitigation to restore, create, or enhance wetlands. According to Federal Aid Highway Program data, 2.7 times as many acres of wetlands were created than lost between 1996 and 2000 during highway construction or maintenance. Corps data for May 1997 to September 1998 show that 995.8 acres of wetlands were mitigated under transportation projects, resulting in a ratio of 1.7:1. FAA evaluation of mitigation projects focuses on ensuring that habitats are not created that would attract wildlife known to affect aircraft operations.

Sources
1. U.S. Congress, Congressional Research Service, "Wetlands Issues," Mar. 15, 2000.
2. U.S. Congress, General Accounting Office, Wetlands Overview: Problems with Acreage Data Persist, GAO/ RCED-98-150 (Washington, DC: July 1998).
3. U.S. Department of the Interior, Fish and Wildlife Service, Status and Trends of Wetlands in the Conterminous United States: 1986 to 1997 (Washington, DC: December 2000).
4. U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 1998 (Washington, DC: 2000), table HM-14.
5. U.S. Environmental Protection Agency, Office of Water, "Status and Trends," available at http://www.epa.gov/owow/wetlands/vital/status.html, as of May 2001.

Urban Sprawl

There is no standard definition of "sprawl." However, the word generally applies to the expanding growth of suburbs and exurbs around a mature urban area. The characteristics of sprawl can include:

. dispersed commercial and industrial sites;

. low-density residential population;

. single-use zoning (i.e., separate residential areas; shopping centers; strip commercial, industrial, and office parks);

. noncontiguous or leapfrog development; and

. heavy reliance on highway vehicles for transportation.

Local and state governments directly impact local growth through activities such as transportation planning and land-use zoning. However, the federal government plays an indirect role in local growth decisions through spending programs, tax policies, regulatory activity, and administrative actions [4].

Over the past several decades, academics, plan.ners, local government officials, environmentalists, and others have debated the positive and negative impacts of sprawl. However, wide disagreement still exists on many issues (table 1). In addition, a 1998 Transportation Research Board review of studies on sprawl found that various costs and benefits have been identified but are not fully quantified [2]. Data used to prove and disprove whether sprawl exists and is detrimental include: comparisons between population growth and growth of developed land, loss of farmland, changes in vehicle-miles traveled, construction of new homes, the percentage of Americans living in metropolitan areas, and housing costs.

Potential environmental impacts of sprawl are listed in table 2, but, again, are not quantified. While most experts agree that road congestion can be a symptom of sprawl, their environmental impact analyses can differ. Pollutant emissions rates vary with vehicle speed, and the optimal speed varies by pollutant. It is generally agreed that emissions rates are higher during stop-and-go, congested traffic conditions than during free flow conditions [3].

Despite the data gaps and uncertainties of sprawl, state and local governments began in the 1990s to consider and, in some cases, foster "smart growth" and "livable communities" ideas and principles. Both of these concepts apply to urban, as well as suburban and exurban, areas. Indeed, some analyses suggest that the late 20th century decline in urban areas is linked to suburban growth [1].

As a first step toward quantifying livability, the Bureau of Transportation Statistics is supporting a National Research Council study to develop a set of Livability Indicators.

Sources
1. National Governors' Association, Growing Pains: Quality of Life in the New Economy, 2000, available at http://www.nga.org, as of May 2001.
2. National Research Council, Transportation Research Board, The Costs of Sprawl-Revisited (Washington DC: National Academy Press, 1998).
3. U.S. Department of Transportation, Federal Highway Administration, Transportation Air Quality: Selected Facts and Figures (Washington DC: January 1999).
4. U.S. General Accounting Office, Community Development: Local Growth Issues-Federal Opportunities and Challenges, GAO-RCED-00-178, 2000, available at http://www.gao.gov, as of March 2001.

Transportation and Environmental Justice

The Environmental Justice (EJ) movement began in the United States in 1982 during protests over the siting of a landfill in a predominately African-American and low-income county in North Carolina. The issue was and is unfair treatment of minority and low-income communities with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies.

In 1994, the Clinton Administration issued Executive Order 12898 on Environmental Justice, requiring all federal agencies to make EJ part of their missions. The U.S. Department of Transportation (DOT) subsequently issued a DOT Order to Address Environmental Justice in Minority Populations and Low-Income Populations in 1997. This order applies to all policies, programs, and activities undertaken, funded, or approved by DOT components. Although EJ has generated controversy for federal agencies, the actual number of administrative complaints filed has been relatively low. At any given time, DOT usually has fewer than a dozen complaints pending, under investigation, or being processed through alternative dispute resolution.

Viewed narrowly, EJ applied to transportation would be about avoiding, minimizing, and mitigating disproportionately high environmental and health impacts from transportation in disadvantaged neighborhoods where they exist. Transportation EJ has, however, evolved to encompass a broader array of issues, some of which have environmental bases or outcomes. Among the issues are: meaningful participation in transportation planning; access to job-related transportation, especially transit services; the location of infrastructure, such as, highways and interchanges, bus barns, and waste transfer stations, that may adversely impact neighborhoods; the connection between vehicle air pollution and asthma; transportation noise; and the lack of complaint investigation and findings in favor of complainants.

Examples of transportation-related EJ cases include:

. Access to Appropriate Public Transportation. A civil rights suit was brought against the Los Angeles Country Metropolitan Transportation Authority (MTA) by two groups, the NAACP Legal Defense and Education Fund and Environmental Defense, for spending a disproportionate amount of money on a rail system that served mainly white residential neighborhoods. In a 1996 consent decree, MTA agreed to invest over $1 billion in bus system improvements over 10 years [1].

. Impact of Trucks on Residential Neighborhoods. The Federal National Environmental Justice Advisory Council conducted fact-findings on the siting and operations of waste transfer stations (WTSs) in New York City and Washington, DC, in 1998 and 1999. WTSs tend to be in urban areas and clustered in low-income and minority communities. They are facilities where municipal waste is unloaded from collection vehicles and then stored temporarily before being reloaded onto long-distance transport vehicles for shipment to landfills or incinerators. WTSs increase noise, odor, litter, and traffic in a neighborhood and impact air quality because of idling diesel-fueled trucks and from particulate matter such as dust and glass [2].

. Involvement in Transportation Planning. In metropolitan Atlanta, EJ groups raised concerns about disparate impacts related to job access, storm-water runoff cleanup, exposures to small particle air toxics and diesel exhaust, pedestrian safety, noise, and community destruction by highways. DOT now has an EJ equity analysis underway of Atlanta's regional transportation planning process. The first phase began in late 1999 and is examining the public participation and data-collection aspects of Atlanta's process. The second phase will be a quantitative analysis of the distribution of transportation benefits and burdens to low-income and minority communities.

. Location of Waste Dumps. The presence of National Priorities List (NPL) toxic waste sites (more commonly called Superfund sites) in disadvantaged communities and the slow pace at which they are cleaned up has been a major issue for EJ groups. There are over 1,000 Superfund sites, and many of them result from activities related to transportation. Among the sites are facilities that treated wood products with preservative, some of which became ties for railroad tracks; petroleum refineries; barge cleaning and maintenance operations; tire recycling facilities; and railroad yards. Five of the 12 sites added to the NPL in July 2000 are related to transportation: one preserved wood, two refined petroleum, and two maintained marine vessels [3].

Despite the fact that concerned communities, advocacy groups, and government agencies have used various data-collection and analysis methods to try to quantify (and prove or disprove) environmental injustice, these studies have usually produced contradictory and contentious findings. Problems with the studies have involved spatial scale issues (e.g., whether to use ZIPcode or census data to define concentrations of low-income and/or minority populations) and difficulties in modeling the complexities of the human impact of transportation. Another problem has been determining cause and effect, since demographics of neighborhoods change over time. In addition, a risk assessment may be needed to determine whether public health is being impacted from, for example, a landfill in the community and, if so, to what extent.

Sources
1. Environmental Defense, Transportation Equity in Los Angeles: The MTA and Beyond, 2000, available
at http://www.environmentaldefense.org/programs/Transportation/Equity/b_justice.html, as of March 2001.
2. U.S. Environmental Protection Agency, National Environmental Justice Advisory Council, A Regulatory Strategy for Siting and Operating Waste Transfer Stations, EPA 500-R-00-002 (Washington, DC: March 2000).
3. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, New Final NPL Sites, 2000, available at http://www.epa.gov/superfund/sites/newfin.htm, as of March 2001.

Introduction of Harmful Species

Transportation and world trade enables the introduction of both desirable and harmful non-native animal and plant species and path-ogens into the United States. Non-native species arrive via air or marine transportation from locations around the globe and via surface transportation from Canada and Mexico. They are imported legally or illegally or arrive as stowaways among cargo or shipping equipment.

Once inside the country, non-native species may become invasive and adversely affect native species. Not all such introductions have been problematic, however. Non-native corn, wheat, rice, other food crops, and livestock now generate more than 98 percent of the U.S. food system [2]. More than 205 known non-native species were introduced or first detected in the United States in the 1980s and early 1990s. Of these, 59 were expected to cause economic or environmental harm [4]. Estimated annual costs and damages total more than $100 billion dollars each year [3].

Methods to control the entry of non-native species include placing harmful species on "black lists" and regulating pathways. The United States tries to control entry primarily using the species-by-species approach. However, one pathway-ship ballast waters released into the Great Lakes -is regulated. Other pathways include ship dunnage; ship and air cargo containers (and the cargo itself), shipping crates, and packaging; personal baggage and clothing; and military equipment returning from overseas conflicts.

Transportation is not always the carrier of non-native species, and proving it is can be challenging. For instance, several theories surround the entry in 1999 of the West Nile Virus. One is that the virus was carried across the Atlantic Ocean from Europe by migratory birds. Because the virus was first reported in the United States in the Queens borough of New York City, another theory suggests that it arrived (via humans or animals) aboard aircraft landing at John F. Kennedy or LaGuardia Airports [1]. By the summer of 2000, the virus (with mosquitoes as its vector) had spread to 11 states (see map). A few examples of non-native species introductions for which transportation was apparently the enabler are the following:

. The zebra mussel entered the United States and was deposited in the Great Lakes in the 1980s when ship ballast water was discharged from European freighters. It has spread to 20 states, as far south as the mouth of the Mississippi River. While filter-feeding by this mussel has improved water quality in the Great Lakes, it has cost an estimated $3.1 billion over 10 years for control efforts and to fix damages to water intake pipes, filtration equipment, and electric generating plants.

. The brown tree snake is thought to have been introduced to Guam in military cargo shipments just after World War II. More recently, the snake has been found in the wheel wells of aircraft departing and arriving from Guam. The population of the snake has increased tremendously, causing the extinction of native Guam birds. In addition, it damages electrical and telephone grids, resulting in power outages that have cost Guam an estimated $1 million a year.

. The Asian long-horned beetle may have arrived in packing materials or pallet wood in shipments from China. In New York State and the Chicago area, the beetle is attacking a broad range of tree species, eventually killing them.

A number of aquarium fish and plants have been legally imported and then inadvertently or purposefully released. The aquatic plant, hydrilla, was imported for use in aquariums and discarded into the wild in Florida. It is now likely being spread across the United States by plant fragments attached to recreational boats.

The globalization of trade, increased volume of cargo shipments, and rising tourism increase the chances of more accidental introductions [2]. In addition, technological changes in transportation have altered the way non-native species are introduced. For instance, dry ship ballast was a significant pathway for insects and plants in the 1880s. Ships now use water as ballast and, while this pathway is still important today, port areas may no longer be the major point of entry for non-native species. Today, containerized cargo from both ships and aircraft are not always unpacked prior to arrival at an inland destination, creating the potential for release of stowaways in other locations in the United States.

Sources
1. The Lancet, "Genetic Analysis of West Nile New York 1999 Encephalitis Virus," vol. 354, Dec. 4, 1999.
2. Pimentel, D., L. Lach, R. Zuniga, and D. Morrison, "Environmental and Economic Costs Associated with Non-Indigenous Species in the United States," Cornell University, College of Agriculture and Life Sciences, Ithaca, NY, available at http://www.news.cornell.edu/releases/Jan99/species_costs.html, as of May 2001.
3. U.S. Congress, Congressional Research Service, Harmful Non-Native Species: Issues for Congress, RL30123 (Washington, DC: Sept. 15, 1999).
4. U.S. Congress, Office of Technology Assessment, Harmful Non-Indigenous Species in the United States, September 1993, available at http://www.ota.nap.edu, as of May 2001.