Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) and Azaarenes in Runoff from Freshly Applied Coal-Tar-Based Pavement Sealcoat

Coal-tar-based sealcoat (CT-sealcoat) is extensively applied to asphalt parking lots and driveways in the U.S. and Canada. Toxicity to fish and invertebrates of runoff from pavement to which CT-sealcoat has been freshly applied has been reported, but relatively little is known about how concentrations of chemicals in runoff change in the hours to days following sealcoat application. We measured the concentrations of 16 U.S. Environmental Protection Agency Priority Pollutant polycyclic aromatic hydrocarbons (PAHs) and 7 azaarenes in 9 samples of simulated runoff from a coal-tar-sealed test plot collected at increasing intervals from 5 hours to 16 weeks following application. Azaarenes, several of which are common constituents in coal-tar pitch, and their oxidized derivatives, azaarones, are an emerging group of little-studied heterocyclic chemicals. Runoff samples were collected by spraying 25 L of a diluted groundwater to 10 m² on sealed pavement and retrieving the runoff downgradient where the runoff pooled against spill berms. Unfiltered samples were analyzed by GC/MS following liquid-liquid extraction. In the first sample (t=5 hr), phenanthrene had the highest concentration (130 μg/L) among the 16 PAHs. Concentrations of the lower molecular weight (LMW) PAHs (2 and 3 ring) decreased during the 16 weeks following application, and concentrations of the higher molecular weight (HMW) PAHs (4 to 6 ring) increased, coincident with an increase in the concentration of suspended particulates. In the final sample (t=16 weeks), fluoranthene had the highest concentration (36 μg/L) among the 16 PAHs. Of the azaarenes measured, concentrations of acridine and carbazole (107 and 750 μg/L, respectively) in the initial sample exceeded those of any of the PAHs measured except phenanthrene; acridine and carbazole concentrations decreased over the 5 weeks to <5% of their initial values. Samples of dried sealcoat were analyzed the day of application and 5 weeks later. Samples were scraped from aluminum roofing disks that had been placed on the pavement prior to sealant application, and analyzed by GC/MS. Total PAH concentrations (sum of the 16 PAHs, ΣPAH₁₆) in the dried sealcoat decreased from 93,000 mg/kg several hours after application to 46,000 mg/kg 5 weeks after application; the loss of about 50% is consistent with that reported previously and attributed to volatilization. As in the runoff water, concentrations of PAHs in dried sealcoat collected on the day of application were dominated by phenanthrene (25% of ΣPAH₁₆); 5 weeks following application, as a result of volatilization of the lower molecular weight PAH, the PAH assemblage was dominated by fluoranthene (24% of ΣPAH₁₆). Concentrations of carbazole and dibenzothiophene, the two heterocyclic compounds measured in the dried sealcoat, were equivalent to 4% and 1% of ΣPAH₁₆. Results of the runoff sampling demonstrate that the chemistry of runoff from pavement with CT-sealcoat changes in the days to weeks following application, from a profile dominated by the more soluble and volatile LMW PAHs to one dominated by the more persistent HMW PAHs. The shift from the more bioavailable LMW PAHs to the more carcinogenic HMW PAHs indicates that modes of toxicity might change with time.