Benefits of porous asphalt overlay on storm water quality

This project documents the impact of a porous asphalt overlay on the quality of highway storm water runoff. A porous asphalt overlay, also known as a permeable friction course (PFC) or open-graded friction course, is a layer of porous asphalt, approximately 50 nun thick, that is often applied on top of conventional asphalt on highways to enhance safety and reduce noise. The quality of storm water runoff from a four-lane divided highway in the Austin, Texas, area was monitored before and after the installation of a PFC. The observed concentrations of total suspended solids and pollutants associated with particulate material were much lower in the runoff from the PFC than that derived from the conventional asphalt surface. Concentration reductions were observed for total suspended solids (92%), total lead (91%), total copper (47%), and total zinc (75%). The concentrations of chemical oxygen demand and total Kjeldahl nitrogen were initially lower in the runoff from the PFC but increased abruptly after about 6 months to concentrations that were similar to the concentrations in the runoff from conventional pavement. The concentrations of dissolved constituents were not significantly different between the two pavement types. The observed concentrations of polycyclic aromatic hydrocarbons were below the detection limit for both pavement types.