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Peter Van Metre: Well, we’re out here today because two years ago we measured volatilization of PAHs off of freshly applied coal-tar sealcoat and we found some extraordinarily high rates and numbers. So we want to verify those and we’re also at the same time sampling much more intensively and with several other instruments that we didn’t have last time to try to do the best job we possibly can to quantify this volatilization flux and see how important an air contaminant it is.
Barbara Mahler: At the same time we’re going to be doing some run off experiments so we can look at the potentially toxic effects of storm water runoff from recently seal coated parking lots on aquatic biota.
Somewhere between when you put the product on the surface and when it dries and you scrape it back off you’re losing some of those PAHs. The question is-- are they going into the air?
Peter Van Metre: So this same patch of ground behind us, a 50 ft. by 50 ft. square area, was sealcoated two years ago and we sampled air concentrations right on the surface with a thing called a hat sampler that is a round steel disk that has a high volume air sampler at the center and it samples the surface air layer. We sampled concentrations about four feet off the surface with a second high volume sampler. With those two and with wind speed you can make an estimate of the total flux of gauge as to air, the total amount of volatilization.
What we found from that initial study in our first sample two-hours after the sealcoat was applied was an extremely high concentration of PAHs in air and a really high volatilization rate. We’re back today because in that study we sampled relatively infrequently right after sampling and spread over a whole year and we realized when we look at the results from it that a whole lot of the action, a whole lot of the volatilization was happening in the first few minutes or hours after application.
Barbara Mahler: We’re going to collect samples more frequently than we did the first time around. So we’ll be able to have a much better description of what those PAH concentrations are in the first few days after application.
Peter Van Metre: There is a lot of evidence out there that the runoff if it does rain soon after the sealcoat is applied it is very toxic. There have been two fish kills that we know of when it has rained within a day or so of application of sealcoat, in the streams that those parking lots drained into. So we’re doing a much more controlled experiment than that. We’re going to simulate runoff to try and see how toxic the runoff is and for how long.
Barbara Mahler: Then we’re going to do all of these same types of sampling but we’re going to do them in a few weeks on a surface that has been treated with a different type of sealcoat, one that has an asphalt base as opposed to the coal-tar base.
Peter Van Metre: The coal-tar product has now been banned in Austin where we are, we had to get special permission to do this, and in more than a dozen other US cities and in the State of Washington. The obvious alternative is this asphalt based product. So we also want to test it to see how environmentally friendly or safe it is.
Barbara Mahler: Yeah, it’s important to see how substantial the difference really is from an environmental point of view between the two products.
Douglas Harned: So tell us about the night sample.
Peter Van Metre: About the night sample?
Douglas Harned: Why you’re out here in the middle of the night.
Peter Van Metre: We’re out here in the middle of the night because we want to sample at certain time periods after the sealcoat has been applied. This is 12 hours after so it happens to be 11:00 PM. That’s because we are wondering how toxicity varies in runoff with time afterwards. We want a sample at specific and pretty close time periods after the application of the sealcoat; so this is 12 hours after.
The three major findings I would say of the studies of sealcoat, the first one is simply that it has extremely high concentrations of PAHs and that’s in the product, it’s in dust that’s swept off of the parking lot, it’s in runoff, it’s in air concentrations next to the parking lots. So it’s just a really strong source of this group of contaminants.
Barbara Mahler: A second major finding is that it doesn’t stay where you put it. Coal-tar sealcoat wears off. You can see it on the parking lot around us. You can see the asphalt through the sealcoat and that’s normal and that’s why applicators recommend that you have it reapplied every three, four, or five years. The question is where does it go? We found that it comes off in storm water runoff, it gets tracked into people’s homes, it ends up in storm water detention ponds, it ends up in our lakes and rivers, and it ends up in the air. So there are a number of different environmental compartments that are affected.
Peter Van Metre: To elaborate on one of the things that Barbara just mentioned is a major finding is our studies of lake sediment chemistry around the United States have shown that concentrations of PAHs have been increasing in urban lakes across the U.S. for the last 30 or 40 years and that the dominant source of PAHs to those lakes is the coal-tar based pavement sealcoat.
Title: Paint it Black
Pete Van Metre and Barbara Mahler discuss an experiment to assess release of Polycyclic Aromatic Hydrocarbons (PAHs) from coal tar pavement sealant after application on a parking lot. (Short Version)
Location: Austin, TX, USA
Date Taken: 8/17/2011
Video Producer: Douglas Harned , North Carolina Water Science Center, U.S. Geological Survey
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Additional Video Credits:
Pete Van Metre
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