Hydrologic and Water Quality Evaluation of Four Permeable Pavements in North Carolina, USA

A permeable pavement parking lot in eastern North Carolina consisting of four types of permeable pavement and standard asphalt was monitored from June 2006 to July 2007 for hydrologic differences in pavement surface runoff volumes, total outflow volumes, peak flow rates, and time to peak, and from January 2007 to July 2007 for water quality concentrations. The four permeable sections were pervious concrete (PC), two types of permeable interlocking concrete pavement (PICP) with small-sized aggregate in the joints and having 12.9% (PICP1) and 8.5% (PICP2) open surface area, and concrete grid pavers (CGP) filled with sand. The site was located in poorly drained soils, and all permeable sections were underlain by a crushed stone base layer with a perforated underdrain. All permeable pavements significantly and substantially reduced surface runoff volumes and peak flow rates from those of asphalt (p<0.01). Of the permeable pavements, CGP generated the greatest surface runoff volumes (p<0.01). The PICP1 and CGP cells generated significantly lower outflow volumes than all other sections evaluated (p<0.01), and had the lowest peak flows and the longest time to peak. The response of the PICP1 cell was likely due to an increased base storage volume resulting from an elevated pipe underdrain; whereas, the CGP cell response was attributed to water retention in the sand fill layer. Overall, different permeable pavement sections performed similarly, but were substantially different from asphalt. Subtle differences in the performance of CGP were primarily due to the characteristics of the sand filled media compared to small aggregate typically used in PC and PICP joints and bedding. The pH of permeable pavement subsurface drainage was higher than that of asphalt runoff (p<0.01) with the PC cell having the highest pH values (p<0.01). Permeable pavement subsurface drainage had lower NH₄-N (p<0.01) and TKN concentrations than asphalt runoff and atmospheric deposition. With the exception of the CGP cell, permeable pavements had higher NO_2,3-N concentrations than asphalt (p<0.01), a probable result of nitrification occurring within the permeable pavement profile. The CGP cell had the lowest mean TN concentrations; however, results were not significantly different from those of asphalt runoff. Overall, different permeable pavement sections performed similarly to one another with respect to water quality, but the CGP cell appeared to improve stormwater runoff nitrogen concentrations.