Transportation and Water Pollution

I. Overview

While air pollution is the most visible and studied environmental consequence of transportation system, water pollution and wetlands issues are also of crucial importance in the transportation and environment nexus. Fuel, particle, and salt-laden runoff from streets, highways, and storage facilities results in damage to public water supplies, ponds, lakes and surface streams, roadside soil, vegetation and trees, and infrastructure and vehicles. The role of wetlands in water purification, the management of surface water runoff, and wetlands as habitat preserves for numerous species are all being closely studied.
Roadways tend to bisect watersheds. Water quality impacts attributed to erosion, sedimentation, and polluted runoff associated with highway construction, operation, and maintenance may be limited to the adjacent streams. But in the watershed downstream, the impact from the road may also contribute to other forms of water pollution. Watersheds are therefore both directly and indirectly impacted by transportation. It is for this reason that a watershed approach has become the most widely accepted direction of study of most water and transportation research.

The main federal legislation concerning water quality is the Federal Water Pollution Control Act of 1972, as amended by the Clean Water Act (CWA) of 1977. This act along with its amendments regulates discharges of pollutants from both point and non-point sources. The Environmental Protection Agency (EPA) and many States have issued regulations implementing the CWA goal of achieving and maintaining a high standard of water quality in surface and ground waters. The CWA also recognized the importance of wetlands.

The Clean Water Act gives states the responsibility to monitor and assess their waters and report the results to the EPA. Monitoring is also done to fulfill specific regulatory requirements, such as those of the National Pollutant Discharge Elimination System (NPDES) permitting process. The Federal Highway Adminstration (FHWA) and the Environmental Protection Agency (EPA) are intended to be proactive leaders working in partnership with the states and local governments and resource agencies to develop joint training sessions, research, and information clearinghouses.

Considerable progress has been made in the last two decades in reducing some of the most obvious water-related impacts of transportation, including leaking fuel, petroleum from underground storage tanks, and solid waste. Yet serious and new challenges remain.

Science of the Problem

The transportation sector's role in water pollution is neither well defined nor well understood by scientists. Traditional water quality concerns have focused on bacterial contaminants, oxygen depleting wastes and sediment loads from human activities. As concerns transportation issues, the source of water pollution originate from both non-point as well as point sources.
The evaluation of ground water quality is complex and assessing the quality of ground water resources has many elements. An accurate and representative assessment of ambient ground water conditions ideally requires a well planned and carefully executed monitoring plan. Such plans are expensive and may not be compatible with state administrative, technical, and programmatic initiatives.

Water contamination often is discovered long after it has occurred. This means that the practices of today may have effects on water quality well into the future, well before we understand the full ramifications of transportation and water issues.

II. The challenges of scientific evaluation

One reason for the difficulty of studying water pollution due to transportation is the slow movement of ground water through aquifers. Contaminants in the ground water do not mix or spread quickly, but remain concentrated in slow-moving, localized plumes that may persist for many years. In some cases, contaminants introduced into the subsurface may be detected several years later.

Ground water quality assessments are performed using comparable data groupings. Data most closely approximating actual ground water quality conditions (e.g., untreated ground water) are given special consideration in these assessments. Scientists focus on organic compounds and metals as most indicative of ground water degradation from transportation.

Organic compounds are usually cited as among the top contaminants. VOCs can be released to the environment through a number of different ways. Generally, organic compounds are released to ground water via disposal practices, and fuel and oil spills. As reported in their 1996 State Water Quality Reports, it was disposal practices that generated the most concern among states concerning water and transportation.

The organic compounds that pose the greatest threat to ground water quality are those that are relatively soluble, not easily converted to the vapor state, and not subject to chemical or biological degradation. Their presence in ground water is becoming increasingly pervasive and a cause for national concern due to the carcinogenic effects of many of the organic compounds.

Toxic pollutants also ply an important role. The top five toxic pollutants (by volume) generated by the transportation industry include xylene, glycol ethers, toluene, methyl isobutyl ketone, and N-butyl alcohol. The final phase of motor vehicle manufacture is painting. Various solid and liquid wastes may be generated throughout painting operations. The wastes are usually the result of paint application and drying, cleanup operations, and disposal of leftover and unused paint as well as containers used to hold paints. There are solvents used to clean equipment and metal parts, and are used in many coatings and finishes.

Other techniques have been developed at the state level for evaluating the expected impact from road construction. A prioritization system, which compares the impacts of one outfall to another and makes a determination of their overall impacts, was developed in the Prioritization Method for Retrofitting Highway, prepared by the Water Quality Unit of WSDOT in 1993. The prioritization system seeks to examine the type and size of the water body to which stormwater will be discharged. The system determines and assigns a value of expected impact on water resources based upon a wide range of characteristics of the planned road.

Direct Impacts

According to the EPA, transportation affects water quality directly in four ways: 1) road construction and maintenance, including the creation of impervious surfaces can adversely affect water quality due to faster rates of runoff, lower groundwater recharge rates, and increased erosion; 2) pollutants such as vehicle exhaust, oil, and dirt, and deicing chemicals, are deposited to roadways and other impervious surfaces; 3) leaking underground storage tanks release petroleum to groundwater; and 4) oil spills, especially in the marine sector affect the water quality of inland waterways and coastal areas.

Road Construction

Roadway construction and maintenance activities can have both direct impacts to both supply and water quality characteristics of the project area. Dredging is undertaken to improve navigation for water-borne transportation. Damage to habitat can occur because of disturbance and removal of bottom material and disposal of dredged material. The U.S. Army Corps of Engineers dredges and disposes of about 200 to 300 million cubic yards of material annually from congressionally-authorized navigation improvement and maintenance projects.

In addition, permit applicants (e.g., port authorities, terminal owners, industries, and private individuals) dredge an additional 100 million cubic yards annually from navigation projects (i.e., ports, berths, and marinas). Dredged material can be disposed of in open water or on land. Open water disposal can alter bottom habitats, decrease water quality, and adversely affect marine organisms. Each year approximately 60 million cubic yards of dredged material is disposed of in the ocean at designated sites.

A variety of impacts are possible, ranging from the erosion of disturbed soils to the chemical pollutants associated with highway maintenance practices. The operation of highways causes other potential pollution sources created by the chemical and biological contaminants present in roadway stormwater runoff.

The construction of surface transportation facilities creates impervious surfaces which can harm water quality in a number of ways including:

Runoff from roadways, runways, and parking lots can produce increased pollutant loadings to wetlands and streams unless the runoff is treated.
An increase in the amount of impervious surfaces results in lower recharge rates for groundwater.
Construction of surface transportation facilities contributes to increased erosion, which also occurs as a result of increased runoff from impervious surfaces.
Impervious surfaces are associated with all modes of transportation, but are most extensive for the highway system and airports. The growth in road mileage in the United States has been relatively slow, increasing by 19 percent from 1945 to 1997. Lane mileage in the United States has grown faster, from an estimated 2,693,604 miles in 1984 to 3,968,813 miles in 1997: a 47 percent increase.
Operation and maintenance of highways involves various activities. Highway operations and maintenance also involves activities such as painting of bridges, re-striping of highways, de-icing of roadway facilities during winter weather conditions, and application of pesticides to roadside vegetation.

Vehicle maintenance and support facilities include motor freight terminals, bus yards, fuel storage tanks, and auto fueling and service stations, all of which have environmental impacts. Each of these activities is associated with potentially harmful environmental impacts.

Since most highway sections lie within or cross a watershed, all phases of project development have the potential for impacting both surface and underground water resources. Highway project planning, location, and design activities can greatly influence future uses of water resources in localities by determining patterns of growth, secondary development, and water supply distribution.


Pollutants to water from transportation take two different forms. The pollutants are either directly leaked into the natural water system, or pollutants are air-borne and then deposited. Runoff pollutants from vehicles include particulates and heavy metals from exhaust fumes, copper from brake pads, tire and asphalt wear deposits, and drips of oil, grease, antifreeze, hydraulic fluids, and cleaning agents. Indirectly, vehicles also contribute to polluted runoff by carrying solids from parking lots, urban roadways, construction sites, farms, and dirt roads.

Use of deicing chemicals facilitates travel during winter weather conditions, and is particularly important for highways and airports. Rock salt is the principal deicing agent used in winter road maintenance throughout the nation. Environmental impacts of road salt include adverse effects to roadside vegetation, harm to soil structure, and potential impacts on drinking water and aquatic life. The actual amount of salt applied to roads nationally is not known, but statistics on road salt sales are available. Approximately 16 million tons of highway salt were sold in 1997.

Aircraft deicers and anti-icers used in North America are based on glycols such as ethylene glycol, diethylene glycol, and propylene glycol, while runway and taxiway deicers are typically formulated with a combination of urea, glycols, sodium formate, and/or potassium acetate. Glycols are biodegradable but can reduce the oxygen available to aquatic life in waters to which deicing and anti-icing fluids are released. Urea degrades to ammonia and nitrate, which are toxic and can contaminate drinking water. It is estimated that 11.5 million gallons of deicing products are used annually in aviation.

Wastewater is another water quality issue associated with transportation. Facilities such as gas stations, maintenance shops, service stations, and freight terminals impact water quality through runoff of gas, oil, and dirt; spills during refueling; waste releases to sewer systems; and cleaning of freight tank interiors. Truck, railcar, and ship cargo interiors that carry fluids must be washed, resulting in the output of spent cleaning fluids, water treatment system sludge, and tank residues.

Deicing is a significant contributor to highway runoff problems, particularly in northern states where cold weather necessitates greater use of de-icing chemicals. Rock salt is a common deicing agent used in winter road maintenance throughout the nation. The use of road salt allows highway travel during snow conditions and is important for delivery of vital goods and services (including emergency support vehicles which save lives) to large segments of the country. Although salt is cheap and effective, it can cause rather serious adverse environmental effects. The environmental impacts of road salt include effects on roadside vegetation, harm to soil structure, and impacts on drinking water and aquatic life.

The effect of deicing runoff is not limited to roadside vegetation. In some cases, 90% of the salt applied to the streets enters the city the city sewerage system and then local aquifers and watersheds in general. Aquatic life in those water systems subsequently suffers.

Leaking Tanks and Spills

Leaking storage facilities result in as much as 20 million gallons of oil leaking into water sources each year. Underground storage tanks (USTs) are used to store fuel at gas stations and other facilities, as well as other chemicals, and can be a major source of groundwater contamination. Releases from tanks and piping occur from corrosion of older, unprotected steel tanks and piping, or from cracks in tanks made from other materials.

Overfilling and spillage during refueling are also responsible for accidental releases. The UST regulations that EPA issued in 1988 established a number

of corrective action requirements for UST owners and operators, including the requirement to clean up soil and groundwater as needed to protect human health and the environment. EPA regulations required that by 1998 all existing USTs have spill protection through catchment basins, automatic shutoff devices, overfill alarms, and mandatory corrosion protection for steel tanks and piping.

A number of factors affect the probability of water pollution due to leaking. Number of leaking underground storage tanks (USTs), type and quantity of materials released from leaking USTs, spill protection mechanisms, cleanup efforts initiated and completed, location of groundwater table, sensitivity of local ecosystems, and treatment of drinking water. There has been a decrease in the number of active petroleum USTs in the U.S. as petroleum UST systems have been closed and environmental standards have become stricter.

(Source: EPA Office of Water)

Indirect Impacts

Air pollution can contribute directly to water pollution. Average daily traffic also has a strong influence on the quality of runoff
in urban areas. Contaminants are deposited on roadway surfaces, median areas, and rights-of-way from atmospheric fallout and vehicle exhaust. During storm events, rainwater washes out atmospheric pollutants and upon surface impact (or snowmelt) and picks up roadway deposits and runs off into receiving water bodies. Because average daily traffic levels are higher in urban areas than rural locations, pollutant levels of water due to traffic are significantly higher in urban areas.

Sprawl development patterns associated with continued road building spread pollution over a larger area and reduce the ability of natural systems to absorb polluted runoff. This generates a great deal of water contamination through the filling and draining of wetlands, the removal of vegetative buffers and trees and shrubs which contributed to water cleansing, and increased use of lawn fertilizers. Suburban sprawl creates the need for more driving which results in increased water pollution from emissions.

The U.S. transportation system is leading to the paving over of up to one-half of the land area of our cities and suburbs in the form of roads, parking and driveways. Paved areas and vehicle storage already account for more developed space in the U.S. than does housing. Although comprehensive studies of parking are rare, a commonly used planning rule is that each motor vehicle registered in a region will occupy six parking spaces: one at home, one or more for guests, one at the worksite, one at one's place of worship or favorite weekend haunt, and one at the mall.

Other Information

The site below contains one of the most comprehensive list of links to a wide range of transportation-related environmental issues.

The Office of Water of the US Environmental Protection Agency is responsible for transportation and water issues. The graphic above links directly to "The Quality of Our Nation's Water: Water Quality Report."

The Water Transportation and the Maritime Industries is directly involved in water.

Office of Oceanic and Atmospheric Research of NOAA
The Office of Oceanic and Atmospheric Research, the primary research arm of NOAA, conducts and directs research in atmospheric, coastal, marine, and space sciences through its own laboratories and programs, and through networks of university-based programs.

III. Government Agencies and Sources

A. Federal

1. Federal Highway Administration

The Federal Highway Administration (FHWA) provides policies, technical and research assistance that incorporate water quality and transportation issues together into a coordinated management approach. The FHWA’s environmental work is handled by several different offices.

The FHWA’s Office of Intermodal and Statewide Programs is responsible for intermodal and statewide planning, geographic information systems, the national highway system, scenic byways, and bicycle and pedestrian programs.
The FHWA’s Office of Highway Information Management maintains data on motor fuel, vehicles, highway finance and road performance, and other transportatation related data.
An important program run by the FHWA is the Transportation Environmental Research Program (TERP). The purpose of TERP is to fund research in transportation and environmental issues at universities and colleges. TERP’s research is designed to provide insight on the policy decisions taken by the FHWA.

2. The United States Environmental Protection Agency
The EPA has a wide range of sources on water pollution related to transportation issues.

B. State

1. Hydraulics Branch of WSDOT

The Hydraulics Branch is organized as part of the Design Office of the Washington State Department of Transportation (WSDOT). The Branch is part of the Environmental and Engineering Service Center. The purpose of the Hydraulics Branch is to provide technical support in the disciplines of hydrology and hydraulics. Technical support is provided to regional WSDOT offices.

2. Water Quality Unit of WSDOT

The Water Quality Unit of WSDOT's Environmental Affairs Office has the dual responsibility of providing adequate mitigation for the water and sediment impacts resulting from transportation projects while allowing WSDOT to conduct its core business of constructing and maintaining transportation facilities statewide.

The action plan of the Water Quality Unit includes the following goals:

protect areas that provide water quality benefits (e.g., wetlands, aquatic ecosystems, riparian areas, wellhead sites, etc.), and protect areas that are susceptible to erosion (e.g., unstable soils, karst materials, landslide areas, fragile stream banks).
develop erosion and sediment control strategies at the planning and design state to be implemented during construction, and operation, and maintenance.
ensure the proper use, storage, and disposal of toxic substances at construction sites and maintenance facilities; develop oil spill contingency plans and clean up procedures.
identify watershed pollution reduction opportunities to reduce pollutant concentrations and volumes entering surface waters.
promote the use of vegetative methods to control erosion and other feasible methods to reduce pollutant loadings and total suspended solids from reaching surface waters.
perform water quality monitoring to assess pollution load reduction and changes in water quality.

3. Washington State Department of Ecology

An innovate agreement has been reached between the Washington State Department of Ecology and WSDOT. It can be viewed directly.

Implementation Agreement between The Washington State Department of Ecology and the Washington State Department of Transportation Regarding the Use of Water Quality Modification General Order (PDF file)

C. City

The Transportation and Natural Resources Library specializes on the topic of transportation issues in King County. Library hours are 9 a.m. - 5 p.m., Monday through Friday. The library’s reference desk’s telephone number is (206) 684-1129 and e-mail is The library can be visited at 821 Second Avenue, Seattle, WA 98104-1598.

IV. Other Sources

A. Center for Transportation and the Environment

The Center for Transportation and the Environment (CTE) is a US Department of Transportation University Transportation Center (UTC) and is located at North Carolina State University. Through programs of research, education, and technology transfer, CTE seeks to mitigate the impacts of surface transportation on the environment.

Center for Transportation and the Environment
at Institute for Transportation Research and Education (ITRE)
North Carolina State University
Centennial Campus Box 8601
Raleigh, NC 27695-8601
Main phone: 919-515-8899

B. Transportation Action Network

The Transportation Action Network website is one of the largest non-government collection of website links to environment and transportation issues.

Prepared by Nathaniel Trumbull and Professor Christine Bae. Last updated 01/28/2000.

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