Porous pavements are among the most effective treatment methods for reducing pollution in stormwater runoff from pavements.
Although sampling on porous pavement systems has been limited, the available data indicate a high removal rate for total suspended solids (TSS), metals, and oil and grease (Cahill, Adams, & Marm, 2005). Table 1 shows pollution removal efficiencies reported by Cahill.
Table 1: Water Quality Benefits of Porous Pavement with Infiltration (% Removal Efficiency) (Cahill, Adams, & Marm, 2005)
University of New Hampshire
The University of New Hampshire has been monitoring stormwater from a porous asphalt parking lot which was constructed on the campus in 2004. Table 2 shows the pollution removal efficiency for this parking lot (UNH Stormwater Center, 2007). The University reports that the water quality treatment performance of the porous asphalt lot generally has been excellent. It consistently exceeds EPA’s recommended level of removal of total suspended solids, and meets regional ambient water quality criteria for petroleum hydrocarbons and zinc. Researchers observed limited phosphorus treatment and none for nitrogen, which is consistent with other non-vegetated infiltration systems.
They also observed that the system did not remove chloride, but since it drastically reduced the salt needed for winter maintenance, it may prove effective at reducing chloride pollution. They reported that winter maintenance requires “between zero and 25 percent of the salt routinely applied to impervious asphalt to achieve equivalent, or better, deicing and traction.”
Table 2: Pollution removal efficiencies (University of New Hampshire)
The design of the porous asphalt pavement at UNH is different from the standard design due to poor quality soils and high groundwater. The pavement structure is shown below.
University of New Hampshire porous pavement cross section
Centre County/Penn State Visitor Center
Dempsey and Swisher studied the hydrologic and chemical performance of a porous pavement/infiltration system at the Centre County/Penn State Visitor Center. (Dempsey & Swischer, 2003). They reported, “The system consists of porous pavement, a 462-m3 (604 cubic yards) storage/infiltration bed with coarse aggregate (40% porosity), geo-textile filter fabric, and an average 2 m (6.5 feet) of un-compacted soil.” “Eleven storm events generated at least 10 cm standing water in the 1.6 m (5 foot) reservoir, allowing sampling. There has been no surface runoff from the site, and infiltration rates have remained relatively constant at 17 cm/hr (6.7 in/hr).”
“The aggressiveness of the water towards calcium carbonate was considerably reduced upon contact with limestone materials in the pavement and in the reservoir, decreasing the potential for sinkhole development. The concentrations of Zn, Cu, and Pb (zinc, copper, and lead) were low, and the total annual loading of metals onto the soil beneath the reservoir was much less than the annual loading of metals that is allowed during the application of soil-amendments to agricultural soils. Organic loadings were relatively low and there was evidence of an active community of organisms within the reservoir.”
Sampling for water quality was from a sampling well that extended to the top of the geotextile filter fabric so this does not account for any treatment of water infiltrating through the subgrade soils. They also report,
Infiltrating water at the Visitor Center had low chemical oxygen demand (COD) values. This indicates low concentrations of organic materials such as petroleum hydrocarbons. The literature indicates that organic materials will be sorbed and bio-degraded within the top few cm of the sub-grade soil. Macro and microorganisms were consistently observed in the storage/infiltration bed at the Visitor Center. Therefore, there is little potential for contamination of groundwater by organic materials due to normal use of the porous pavement parking lots at the Visitor’s Center.
Water quality on highways using porous surfaces
In addition to improvements in water quality at porous asphalt parking lot sites, studies by the Texas Department of Transportation indicate that runoff from highways can also be improved by use of open-graded pavement surfaces. Their study examined the quality of runoff from a conventional asphalt pavement and one surfaced with porous friction course (PFC). Porous friction course is the name commonly used in Texas for the pavements commonly called open-graded friction course in the U.S. In this project, the porous asphalt mix was placed directly on a dense-graded asphalt pavement so that there was no infiltration. Even so, concentrations of total suspended solids (TSS), total metals, and phosphorus were found to be significantly lower in the runoff from the PFC surface than in the runoff from the conventional impermeable asphalt surface.
Concentrations of TSS as well as the total forms of lead and zinc were one order of magnitude lower from the porous asphalt than from the traditional asphalt in most samples. Average concentrations of TKN, COD, nitrate-nitrite, and the dissolved forms of lead, zinc, and phosphorus showed little change between the two surface types. The researchers concluded, “From these results it is evident that the runoff generated from the PFC surface is of better quality than that from the traditional asphalt surface” (Barrett & Shaw, 2007).
This study indicates that even the porous asphalt surface, which typically is one inch deep or less, removes some of the pollutants normally associated with runoff from pavement.