儘管在幾次降雨過後，PAHs的濃度變低，不過在舖有黏膜層上的涇流，起初所觀察到的濃度竟超過5,000 ppb，遠高於在未舖之地測得10 ppb的濃度值。
Driveways and parking lots may look blacker and shinier with a layer of sealcoating applied to the pavement, but the rainwater running off the surface into nearby streams will be carrying more than oxygen and hydrogen atoms.
New research conducted at the University of New Hampshire Stormwater Center shows that sealcoating contributes to the amount of polyaromatic hydrocarbons entering waterways from stormwater runoff.
More commonly known as PAHs, polyaromatic hydrocarbons are found in diesel and crude oil. The Department of Health and Human Services has determined that some PAHs may reasonably be expected to be carcinogens.
Although small amounts of PAHs are typically found in the waters around the New Hampshire Seacoast, the sudden spike in the hydrocarbon concentrations in water draining from a university parking lot used for research caused Tom Ballestero, UNH associate professor of civil engineering, to be concerned about unknown impacts.
Although it is intended to remain on the pavement surface, much of the sealcoating eventually washes or scrapes off and ends up in nearby streams and rivers, says Alison Watts, affiliate faculty member at the University of New Hampshire Stormwater Center.
Most PAHs do not dissolve easily in water but stick to solid particles and settle to the bottoms of lakes or rivers. Microorganisms can break down PAHs in soil or water after a period of weeks to months, according to the federal Agency for Toxic Substances.
On-site stormwater drains off the parking lot and into a nearby swale. The PAH concentration was measured in the water and sediments coming from the sealcoated and unsealed parking lot sections.
Sealcoating led to a rapid increase in PAH concentrations in the initial runoff - up to 5,000 parts per billion, much higher than the 10 ppb levels released from the unsealed lot, although concentrations decreased after several rainstorms.
Unlike other compounds, PAHs do not break down easily and persist in the environment for decades. Even a small amount of PAHs coming off sealcoated parking lots may overwhelm an aquatic ecological system already stressed by other contaminants.
Increased PAH concentrations in waterways could be a human health issue if people are exposed to the substance regularly. In addition, dust particles coming from a sealcoated driveway could potentially be troublesome for children who play on the sealed surface.
"You don't see people falling over from PAHs in sealcoat, it's not that big of a health issue," Ballestero says. "But it could be a cumulative exposure problem that gets uglier over time."