Evaluation of Permeable Friction Course (PFC), Roadside Filter Strips, Dry Swales, and Wetland Swales for Treatment of Highway Stormwater Runoff

Stormwater runoff from roadways is a source of surface water pollution in North Carolina. The North Carolina Department of Transportation (NCDOT) is required to implement stormwater control measures (SCMs) in the linear environment. NCDOT has specific interest in evaluating pollutant loads from interstate highways and potential stormwater treatment measures. The research presented herein focuses on monitoring of highway runoff at four sites along Interstate 40 (I-40) in Johnston, Sampson, and Duplin counties. This entire stretch of I-40 had a permeable overlay [known as a permeable friction course (PFC)] applied in November, 1998. The overlay is porous, and allows water to pass through the surface of the pavement, reducing splash during rainfall and allowing for improved vehicle traction (Barrett et al. 2006). Drainage from the PFC was monitored at all four sites to determine highway pollutant concentrations and loads. Roadside filter strips are nearly ubiquitous on highways, as they are constructed to make grade and to hydraulically connect the roadway to the roadside swale. Two roadside filter strips (21.5 ft in width) were evaluated in this study. Finally, four linear roadside swales were monitored to determine their hydrologic and water quality benefits. Two of these swales were dry swales, meaning that they drained inter-event. The other two swales had wetland characteristics, including hydrophytic vegetation, hydric soils, and wetland hydrology. Data collection began in September 2008 and continued through May 2010. Runoff from the highway and the downslope edge of the filter strip was collected in separate slot drains. The drainage was conveyed to a weir and stage recorder, which enabled flow measurement. An outlet structure using a compound weir was installed in each swale and a similar weir and stage recorder was used for flow measurement. Flow-proportional, composite water quality samples were obtained at ten different locations, four at the edge-of-pavement, two at the downslope end of the filter strips, and four at the swale outlets. Monitored water quality parameters included total Kjeldahl nitrogen (TKN), nitrate- and nitrite-nitrogen (NO2,3-N), ammonium nitrogen (NH4-N), organic N (Org-N), total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS). Results showed that PFC sequestered and/or reduced the generation of TSS from the highway surface. Median effluent TSS concentrations were 8 mg/L, 8 mg/L, 9 mg/L, and 17 mg/L, lower than previous studies on standard asphalt highways (Barrett et al. 1998; Sansalone et al. 1998; Kayhanian et al. 2003). Other sediment-bound pollutant (such as phosphorus) concentrations, were reduced to what appeared to be at- or near-irreducible levels. Due to these findings, the authors support further use of PFC on highways throughout North Carolina. The roadside filter strips were shown to increase sediment and sediment-bound pollutant concentrations, due to relatively high slopes, fair vegetative cover, and clean influent. The wetland swales produced lower mean effluent concentrations (by approximately 0.4 mg/L) of TN when compared to the dry swales. Similar trends were not observed for TP and TSS. Therefore, there is the potential for greater nitrogen removal credit for wetland swales. Load reductions of pollutants were generally poor to fair for the roadside filter strips due to substantial measured soil compaction. In fact, TP and TSS loads increased through both filter strips studied. Pollutant loads were generally lowest at the swale outlets, except at site D, where a head cut in the swale caused substantial increases in TP and TSS loads vis-a-vis the edge-of-pavement.