After a century of steadily paving the U.S. landscape with highways, parking lots, sidewalks, and driveways, Americans are gradually beginning to recognize the negative environmental effects of installing so much impervious pavement. Parking lots, for example, present a problem because they collect oil, antifreeze, and other automobile contaminants that are swept into nearby streams with rainfall. This rapid runoff not only carries pollutants to local streams and lakes, it interferes with the natural hydrology that supports aquifer recharge and, ultimately, base flow in streams. One potential remedy is the development of porous pavements that filter rainwater runoff. National Risk Management Research Laboratory (NRMRL) urban watershed researchers are evaluating the performance of one porous pavement system—interlocking concrete pavers with filter/gravel layers. The recent advancement of porous pavements has created a specialized vocabulary, although the terms are often used interchangeably. Porous (or permeable) pavement is a special type of payment that allows rain (and snowmelt) to pass through it, thereby reducing runoff and trapping some pollutants. There are two types of porous pavement—pervious concrete and porous asphalt. Pervious concrete mixes water and cement to a thick paste that coats aggregate particles without the usual sand. This mix creates a system of connected voids that drains rapidly—typically about five gallons per square foot per minute, or more. The ability of pervious concrete to capture and allow storm water to seep slowly into the ground is especially attractive to developers because it may reduce the need for storm sewers, curbing, retention ponds, and other controls required to meet runoff regulations for new construction (www.perviouspavement.org for more information). Porous asphalt pavement consists of coarse aggregate bonded by asphalt cement with sufficient connected voids to make it highly permeable. Both types of porous pavement are typically laid over layers of gravel and crushed stone that act as a storage reservoir. A filter fabric installed beneath gravel layers screens fine soil particles and surface debris. Interlocking concrete paving stone is a low-impact storm water control system made of impermeable pavers, but the spaces between them are backfilled with stone, allowing water to infiltrate and removing pollutants. NRMRL watershed management scientists selected the interlocking pavement system for evaluation because the layers are easily removed for examination and replacement, a necessary condition for long-term monitoring and maintenance. An initial bench-scale (small) study measured discharge volume, flow rate, total suspended solids of influent and effluent, and the performance of a permeable geotextile filter fabric between gravel layers. A set of four paving stone systems were constructed in 3-foot by 2-foot plastic bins: two with filter fabric between gravel layers and two without. Homogenized urban storm water was "rained" onto the systems to simulate years of rain: 4.5 gallons per rain event simulating a 1.2-inch rainfall per event, twice a day, five days a week for 12 weeks. Preliminary results showed that although both types of systems removed total suspended solids, the filter fabric measurably increased the percentage of solids removed. In measurements of flow rate, the average peak infiltration rates were 7.5 percent smaller in systems constructed with filter fabric compared to those without. Further research will examine the role of microbial communities in pollutant removal and carbon presence. The bench-scale evaluation is a preliminary step toward a full-scale parking lot evaluation currently under development. In the larger study, the parking lot exfiltrate will be collected and monitored for selected water quality parameters and pollutants. Following the study of materials and hydraulics, the exfiltrate will be examined to ensure that it has improved sufficiently to release to surface or ground waters. For more information about porous pavement, see the Storm Water Technology Fact Sheet at www.epa.gov/npdes/pubs/porouspa.pdf. For information about EPA's urban watershed management research, go to www.epa.gov/ednnrmrl/.
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