I have a confession. I love to rollerblade! My heart soars when I am out on a beautiful day and I see a perfectly smooth, jet-black asphalt trail stretching out in front me. I know that I can glide with the breeze on my face and relax a bit because I don’t need to keep an eagle-eye watch out for those miserable cracks that can send me careening off course. So where’s the confession? Surely, inline skating is not illegal, indecent or immoral?
Well, for the past eight years, I have worked as the Environmental Education Coordinator at the Rice Creek Watershed District with a central focus of educating the general public about stormwater runoff and the negative effects it has on our waters. And, as it turns out, those lovely coal-tar sealed asphalt trails (driveways and parking lots too) that I (secretly) adore contain polycyclic aromatic hydrocarbons (PAHs) that are doing a number on our lakes.
What’s the Issue?
Studies have identified coal-tar based sealcoat – the black, shiny emulsion painted or sprayed on asphalt pavement – as a major and previously unrecognized source of PAH contamination of our lakes. Particles in stormwater runoff from coal-tar based sealcoated parking lots had concentrations of PAHs that were about 65 times higher than the concentration in particles washed off parking lots that had not been sealcoated.
What are potential environmental and human-health concerns?
Several PAHs are toxic to aquatic life and are suspected human carcinogens. Biological studies indicate that PAH levels in sediment contaminated with abraded sealcoat are deadly to plants and animals that live in our lakes. As a result, there are less species and fewer numbers of organisms in these degraded aquatic communities.
PAHS are toxic to mammals (including humans), birds, fish, amphibians, invertebrates and plants. Aquatic invertebrates are particularly susceptible to PAH contamination – especially the bottom dwellers (benthic invertebrates) that live in the mud where PAHs accumulate. They are an important part of the food chain and are often monitored as indicators of stream quality. PAHs can inhibit reproduction, delay emergence and cause mortality in aquatic invertebrates. Fish can be affected as well, with fin erosion, liver abnormalities, cataracts and immune system impairments.
The human health risk from environmental contaminants is often evaluated in terms of exposure pathways. For example, people can be exposed to PAHs in sealcoat through skin contact, inhalation of wind-blown particles and fumes that volatilize. Luckily, however, PAHs in lakes and streams rarely pose a human health risk via drinking water since PAHs attach to sediment rather than dissolving in the water. In addition, because PAHs do not readily bioaccummulate within the food chain, the risk of exposure through eating fish from contaminated lakes is relatively low as well.
How does sealcoat get from parking lots into the environment?
Tires grind down parking lot sealcoat into small particles, which are then washed off of 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, which is why sealcoat manufacturers recommend reapplication every two to three years.
How were the studies done?
There have been various studies performed. One study by U.S. Geological Survey (USGS) researchers sampled runoff at 13 parking lots and compared particles in the runoff to scraping samples from the parking lots. The researchers sprayed water on four different types of parking lot surfaces: lots sealed with coal-tar based sealcoat, lots sealed with asphalt-based sealcoat, unsealed asphalt lots and unsealed concrete lots. The runoff was collected, pumped into containers and filtered to collect particulates for analysis. It was found that the concentration of total PAHs in runoff from sealed parking lots was much greater than that from unsealed parking lots.
Now that we know this, what should we do?
Since April of 2010, about a dozen cities have banned coal-tar sealants, including the east metro communities of White Bear Lake, Roseville, Maplewood and Inver Grove Heights. As a result, many retail stores have already removed coal-tar based sealants from their shelves.
If you are resealing a driveway or parking lot, be sure to request that the contractor use an asphalt-based sealant, which is a less toxic alternative. Better yet, explore alternatives to an asphalt driveway such as pervious pavers or porous asphalt.
While it is true that there are additional sources of PAHs, including tire debris, gasoline, oil, and asphalt, coal-tar sealcoats are a relatively large source of pollutants for urban water bodies across the country. We still have a long way to go to eradicate PAHs from our environment, but eliminating the use of coal-tar sealcoats is a low hanging fruit and a step in the right direction.
Guest Writer: Dawn Pape is the Environmental Education Coordinator for the Rice Creek Watershed District. She can be reached at (763) 398-3078 or firstname.lastname@example.org.