Stormwater Quality Benefits of a Porous Asphalt Overlay

This project documents the impact of a porous asphalt overlay on the quality of highway stormwater runoff. A porous asphalt overlay, also known as a permeable friction course (PFC) or open graded friction course (OGFC), is a layer of porous asphalt approximately 50 mm thick which is often applied on top of conventional asphalt highways to enhance safety and reduce noise. The quality of stormwater runoff from a four-lane divided highway in the Austin, Texas area was monitored before and after the installation of a PFC. 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%). 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 runoff from conventional pavement. Concentrations of dissolved constituents were not significantly different between the two pavement types. Observed concentrations of polycyclic aromatic hydrocarbons (PAHs) were below the detection limit for both pavement types. Michael E. Barrett Christopher B. Shaw 3