PARKING LOT SEALANTS: AN UNRECOGNIZED SOURCE OF PARTICULATE PAH IN URBAN RUNOFF

Concentrations of PAHs in urban aquatic sediments have been increasing since the 1970s. Numerous urban sources of PAH have been identified, but uncertainty remains as to the relative importance of different sources and the specific causes of upward trends. One potentially important source of urban PAH has been overlooked: parking lot sealants. Most commonly used sealants contain 25 to 30 percent coal tar, which is 60 to 80 percent PAH by weight. To investigate the mobility of particles from abraded parking lot sealants and to quantify their contribution to urban PAH in sediment, we collected samples of simulated runoff from 13 urban parking lots (daily vehicle use) and four test plots (newly sealed, no vehicle use). The surfaces were either coal-tar-emulsion sealed, asphalt-emulsion sealed, or unsealed (asphalt pavement or cement). Particles filtered from the simulated runoff were analyzed for PAH. The mean total PAH (sum of 12 parent PAH and 2-methylnaphthalene) in particulates washed from parking lots with a coal-tar-emulsion sealant was 3,500,000 μg/kg, more than five times that in particulates washed from lots with an asphalt-emulsion based sealant (620,000 μg/kg), and 65 times that in particulates washed from unsealed lots (54,000 μg/kg). Concentrations in particles washed from the test plots were of a similar magnitude. Mean estimated yields of total particulate PAH were 20 times higher for the coal-tar-sealed parking lots than from the coal-tar-sealed test plots and 160 times higher for the asphalt-sealed lots than the asphalt-sealed test plot, demonstrating the importance of abrasion from vehicle use in mobilizing sealant fragments. The contribution of PAHs in runoff from sealed parking lots to urban waterways was investigated for four small urban watersheds. Sealed parking lots were mapped for each watershed, and the PAH loads contributed by those parking lots for a rain event were estimated on the basis of the yields measured from the runoff experiments. Comparison of the estimated loads to those measured on suspended sediment in the streams suggests that sealed parking lots could be the dominant source of PAHs in some urban watersheds.