U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3147

January, 2006

Prepared in cooperation with the City of Austin, Texas

Parking Lot Sealcoat: A Major Source of Polycyclic Aromatic 

Hydrocarbons (PAHs) in Urban and Suburban Environments

By Peter C. Van Metre1, Barbara J. Mahler1, Mateo Scoggins2, and Pixie A. Hamilton1

Printed on recycled paper

Collaborative studies by the City of Austin and the U. S. Geological Survey (USGS) have identified 

coal-tar based sealcoat—the black, shiny emulsion painted or sprayed on asphalt pavement such 

as parking lots—as a major and previously unrecognized source of polycyclic aromatic hydrocarbon (PAH) contamination. Several PAHs are suspected human carcinogens and are toxic to aquatic 

life. Studies in Austin, Texas, showed that particles in runoff from coal-tar based sealcoated 

parking lots had concentrations of PAHs that were about 65 times higher than concentrations in 

particles washed off parking lots that had not been sealcoated. Biological studies, conducted by 

the City of Austin in the field and in the laboratory, indicated that PAH levels in sediment contaminated with abraded sealcoat were toxic to aquatic life and were degrading aquatic communities, 

as indicated by loss of species and decreased numbers of organisms. Identification of this source 

of PAHs may help to improve future strategies for controlling these compounds in urban water 

bodies across the Nation where parking lot sealcoat is used.

Particles in runoff from coal-tar based sealcoated parking lots had concentrations of PAHs 

that are about 65 times higher than concentrations in particles washed off parking lots that 

had not been sealcoated. 

What are PAHs, coal tar, and 

sealcoat?

Polycyclic aromatic hydrocarbons 

(or PAHs) are a group of organic contaminants that form from the incomplete 

combustion of hydrocarbons, such as coal 

and gasoline. PAHs are an environmental 

concern because they are toxic to aquatic 

life and because several are suspected 

human carcinogens. 

Coal tar is a byproduct of the coking 

of coal, and can contain 50 percent or 

more PAHs by weight. 

Sealcoat is a black liquid that is 

sprayed or painted on asphalt pavement in an effort to protect and beautify 

the asphalt. Most sealcoat products are 

coal-tar or asphalt based. Many coal-tar 

sealcoat products contain as much as 30 

percent coal tar by weight. Product analyses by the City of Austin indicated that 

coal-tar sealcoat products have median 

concentrations of total PAHs about 70 

times higher than concentrations in 

asphalt-based sealcoat products.

Sealcoat is used commercially and 

by homeowners across the Nation. It is 

applied to residential driveways and to 

parking lots associated with commercial 

businesses (including strip malls and 

shopping centers), apartment and condominium complexes, churches, schools, 

and business parks. The City of Austin, 

Texas, estimates that about 600,000 gallons of sealcoat are applied every year 

in the greater Austin area. National use 

numbers are not available, but commercial availability suggests that asphaltbased sealcoat is commonly used on the 

West Coast and coal-tar based sealcoat 

is commonly used in the Midwest, the 

South, and on the East Coast.
How does sealcoat get from 

parking lots into the environment?

Vehicle tires abrade parking lot 

sealcoat into small particles. These small 

particles are washed off parking lots by 

precip­itation and into storm sewers and 

streams. Sealcoat “wear and tear” is 

visible in high traffic areas within a few 

months after application. Sealcoat manufacturers recommend reapplication every 

2 to 3 years.

What are potential environmental 

and human-health concerns?

PAHs are toxic to mammals 

(including humans), birds, fish, amphibians, invertebrates, and plants. Aquatic 

invertebrates, the insects and other small 

animals that live in streams and lakes, are 

particularly susceptible to PAH contamination, especially the bottom dwellers 

(benthic invertebrates) that live in the 

mud where PAHs tend to accumulate. 

They are an important part of the food 

chain and are often monitored as indicators of stream quality (analogous to the 

“canary in the coal mine” concept). Possible effects of PAHs on aquatic invertebrates include inhibited reproduction, 

delayed emergence, sediment avoidance, 

and mortality, and possible adverse 

effects on fish include fin erosion, liver 

abnormalities, cataracts, and immune 

system impairments. PAHs tend to attach 

to sediment; the Probable Effect Concentration (PEC)—a widely used sedimentquality guideline that is the concentration of a contaminant in bed sediment 

expected to adversely affect benthic (or 

bottom-dwelling) biota—is 22.8 mg/kg 

(milligrams per kilogram) for total PAHs.

Studies by USGS and City of Austin 

did not evaluate human-health risk from 

exposure to sealcoat. Human-health risk 

from environmental contaminants is often 

evaluated in terms of exposure pathways. 

For example, people could potentially 

be exposed to PAHs in sealcoat through 

skin contact with abraded particles from 

parking lots, inhalation of wind-blown 

particles, and inhalation of fumes that 

volatilize from sealed parking lots. PAHs 

in streams and lakes rarely pose a humanhealth risk via drinking water because 

of their tendency to attach to sediment 

rather than dissolve in water. In addition, 

because PAHs do not readily bioaccumulate within the food chain, possible 

human-health risks associated with consumption of fish are low.

What are the concentrations of 

PAHs in runoff from sealed and 

unsealed parking lots?

Concentrations of PAHs in particles 

(including abraded sealcoat along with 

urban dust and other sediment) washed 

off from each of the different surface 

types—including the unsealed parking lots—were greater than the PEC of 

22.8 mg/kg. The average concentration 

in particles washed off parking lots that 

were not sealcoated was 54 mg/kg. This 

is not surprising because runoff from 

parking lots is likely to contain PAHs 

from many sources, including leaking 

motor oil, tire particles, vehicle exhaust, 

and atmospheric deposition. 

Concentrations of PAHs were much 

higher in particles in runoff from parking 

lots sealed with coal-tar based sealcoat 

than from all other types of parking lot 

surfaces. Specifically, the average concentration of PAHs from coal-tar sealed 

lots was 3,500 mg/kg, about 65 times 

higher than the average concentration in 

particles washed off unsealed parking 

lots. The average concentration in parHow did USGS study parking lot runoff?

USGS researchers sampled runoff at 13 parking 

lots in Austin. They also took scraping samples of 

parking lot surfaces to compare source materials 

to particles in the runoff. Scraping samples and 

the water and particles in the runoff samples were 

analyzed for a suite of PAHs, major elements, and 

trace elements. The researchers sprayed water on 

four different types of parking lot surfaces: lots 

sealed with coal-tar based sealcoat (top photo), 

lots sealed with asphalt-based sealcoat, unsealed 

asphalt lots, and unsealed concrete lots. The runoff 

was collected behind spill berms, pumped into 

containers (middle photo) and filtered through 

Teflon filters to collect the particulates for analysis 

(bottom photo). The particulates, the filtered water, 

and samples of sealcoat scraped from the parking 

lot surfaces were analyzed for PAHs at the USGS 

National Water Quality Laboratory in Denver, 

Colorado. Concentrations and yields (the amount 

of PAHs coming off a parking lot per unit area) 

were used to determine levels of contamination in 

runoff from each type of parking lot and the importance of sealed lots as a source of PAHs to urban 

streams. 

Concentrations of total PAHs in particles 

in runoff from sealed parking lots greatly 

exceeded concentrations from unsealed 

parking lots. The bar on each graph is 

the mean concentration. The y-axis is 

logarithmic.

10

100

1,000

10,000

100,000

Total PAHs, in milligrams per kilogram

Asphalt sealed

Coal-tar sealed

Unsealed

Coal-tar parking lot

scraping sample

Probable Effect Concentration

Type of parking lot
ticles washed off parking lots sealed with 

asphalt-based sealcoat was 620 mg/kg, 

about 10 times higher than the average 

concentration from unsealed parking lots 

and 6 times less than the average concentration from coal-tar sealed lots.

The large differences between 

concentrations associated with sealed 

and unsealed parking lots indicate that 

abraded sealcoat is a major and previously unrecognized contributor to PAH 

contamination.

How do PAHs from sealcoat affect 

the quality and biology of streams?

Studies by USGS scientists demonstrated possible connections between 

PAHs in particles washed off sealed 

parking lots and PAHs in suspended sediment in four streams in Austin and Fort 

Worth, Texas. The total mass of PAHs 

(or “load”) expected to wash off sealed 

parking lots was compared to the load of 

PAHs measured in suspended sediment 

in the four streams after rainstorms. The 

load of PAHs estimated to come from the 

sealed parking lots was comparable to the 

measured load in the streams, indicating 

that runoff from sealed parking lots could 

account for the majority of PAHs in these 

streams. Findings also showed that PAHs 

in suspended sediment in the streams 

were chemically similar to those in particles washed off parking lots sealed with 

coal-tar based sealcoat. What would be 

the effect on PAH loading to the streams 

if parking lots were not sealed? Estimates 

How did City of Austin scientists conduct biological studies?

City of Austin biologists conducted laboratory and field studies to evaluate the 

effects of sealcoated parking lots on aquatic communities in area streams. These 

studies included toxicity testing in controlled laboratory experiments that exposed 

organisms to sediment spiked with coal-tar and asphalt-based sealcoat (left 

photo); controlled experiments that used aquariums with diverse natural biological 

communities to which sealcoat was added (middle photo); and field assessments 

of aquatic communities in streams upstream and downstream from inflows of 

runoff from sealcoated parking lots (right photo). 

-5

0

5

10

15

20

Change in total PAHs, in micrograms per gram organic carbon

(upstream to downstream)

Taxa Loss (upstream to downstream)

0 500 1,000 1,500 2,000 2,500

-50

0

50

100

150

Number

200

of organisms lost (upstream to downstream)

0 500 1,000 1,500 2,000 2,500

Change in total PAHs, in micrograms per gram organic carbon

(upstream to downstream)

Field assessments in five selected Austin streams showed loss of species (taxa) and decreases in the number of aquatic organisms 

downstream of coal-tar sealcoated parking lots that can be, in large part, explained by increases in total PAHs.

from the USGS study indicate that total 

loads of PAHs coming from parking 

lots in the studied watersheds would be 

reduced to about one-tenth of their current levels if all of the parking lots were 

unsealed.

Studies by City of Austin biologists 

showed that PAHs in sediment contaminated with abraded sealcoat could be 

adversely affecting aquatic communities. 

Specifically, toxicity testing of organisms 

in the laboratory showed large increases 

in mortality as sealcoat amounts and 

PAH concentrations were increased, and 

that sediment contaminated with coal-tar 

sealcoat was toxic to aquatic life at PAH 

concentrations observed in Austin waterways. Controlled experiments that used 

aquariums with diverse natural biological 

communities showed significant biological degradation in response to the addition of sealcoat particles. Finally, field 

assessments in selected Austin streams 

showed loss of species and decreases in 

the number of aquatic organisms downstream from inflows of runoff from coaltar sealcoated parking lots. These effects 

coincided with increases in concentrations of PAHs in stream sediment below 

sealcoated parking lots. Overall, City 

of Austin scientists have reported PAH 

contamination at levels predicted to be 

toxic to benthic invertebrates in about 13 

percent of sampled Austin creeks. 
How do these findings apply to 

urban lakes and reservoirs?

The concentrations of PAHs in 

lakes and reservoirs across the Nation are 

increasing, as indicated by USGS studies 

of 38 reservoirs and lakes conducted in 

18 metropolitan areas across the country 

(Van Metre and Mahler, 2005). Sediment cores (vertical tubes of mud) were 

collected from reservoir and lake bottoms 

(see photo below); analysis of these cores 

provides a reconstruction of historical water quality over time, much like 

using tree rings to reconstruct historical climate. Runoff carries soil, debris, 

and attached contaminants to lakes and 

reservoirs, which settle to the bottom; as 

the sediment builds up, changes in water 

quality are recorded in the successive 

sediment layers.

USGS findings show that concentrations of total PAHs in the majority of 

lakes and reservoirs in urban and suburban areas across the Nation increased significantly from 

1970 to 2001. 

The increases 

were greatest 

in lakes with 

rapidly urbanizing watersheds (urban 

sprawl); for 

example, 

over the last 

10 years, the 

concentrations 

of PAHs in 

Lake in the Hills (suburban Chicago, Illinois) increased ten-fold as the watershed 

was rapidly developed. Further study is 

needed to assess direct links between the 

use of sealcoat and PAH trends in these 

urban lakes and reservoirs across the 

Nation.

What are the implications of these 

studies?

The study of parking lot surfaces by 

the USGS and the City of Austin show 

that abraded sealcoat could be a major 

source of PAHs to urban and suburban 

water bodies in watersheds across the 

Nation where sealcoat is used. Such findings have implications that extend beyond 

Texas as sealcoat is used nationwide; 

further studies would help to evaluate 

the potential impacts of sealcoat on the 

aquatic environment in other parts of the 

country. Identification of this source may 

influence future strategies for controlling 

PAHs in urban environments. In the past, 

sources of PAHs in urban watersheds 

were thought to be dominated by numerous nonpoint sources, such as leaking 

motor oil, tire wear, vehicular exhaust, 

and atmospheric deposition. Such 

sources are difficult to quantify or control 

because of their diffuse nature. In contrast, sealcoated parking lots are specific 

areas that contribute directly to urban 

stormwater runoff (see photo below), 

and the use of sealcoat is voluntary and 

controllable. To address PAH contamination in streams, the City of Austin 

Council banned the use of coal-tar based 

sealcoat, effective January 2006 (Nancy 

McClintock, written communication, City 

of Austin, November 2005).

Possible alternatives to coal-tar 

based sealcoating of parking lots and 

driveways include the use of concrete and 

unsealed asphalt pavement, and the use of 

asphalt-based sealcoat that contains lower 

levels of PAHs.

Currently, the use of coal-tar based 

sealcoat is not federally regulated. In

1992, the U.S. Environmental Protection 

Agency excluded coke product residues, 

including coal tar, from classification as 

hazardous wastes if they are recycled. 

Under the Resource Conservation and 

Recovery Act, coal-tar based pavement sealants are products that contain 

recycled coal tar and, therefore, are not 

regulated.

Contacts for additional information

Peter Van Metre and Barbara Mahler

U.S. Geological Survey 

8027 Exchange Drive 

Austin, Texas 78754-4733

(512) 927-3506 or pcvanmet@usgs.gov

(512) 927-3566 or bjmahler@usgs.gov

Mateo Scoggins

City of Austin 

Watershed Protection and Development 

Review Department 

505 Barton Springs Road, 11th Floor 

Austin, Texas 78704

(512) 974-1917 or 

mateo.scoggins@ci.austin.tx.us

Links to related publications, data 

and maps

City of Austin Coal Tar Sealant 

Information– 

http://www.ci.austin.tx.us/watershed/

bs_coaltar.htm

USGS frequently asked questions–

http://water.usgs.gov/nawqa/asphalt_

sealers.html

Basic information on the toxicity of 

PAHs to biological organisms, 

U.S. Environmental Protection Agency 

(USEPA)–

http://www.epa.gov/R5Super/ecology/

html/toxprofiles.htm#pahs

General information on PAH 

exposure, Agency for Toxic Substances 

and Disease Registry (ATSDR)– 

http://www.atsdr.cdc.gov/toxprofiles/

phs69.html

References

Mahler, B.J., Van Metre, P.C., Bashara, 

T.J., Wilson, J.T., and Johns, D.A., 

2005. Parking lot sealcoat: An unrecognized source of urban PAHs: Environmental Science and Technology, 

vol. 39, no. 15, p. 5560-5566.

Van Metre, P.C. and Mahler, B.J., 2005. 

Trends in hydrophobic organic contaminants in urban and reference lake 

sediments across the United States, 

1970-2001: Environmental Science 

and Technology, vol. 39, no. 15, p. 

5567-5574.

City of Austin. 2005. PAHs in Austin, 

Texas. (http://www.ci.austin.tx.us/

watershed/downloads/coaltar_draft_

pah_study.pdf) 

1

 U.S. Geological Survey

2

 City of Austin