Side-by-side evaluation of four level spreader–vegetated filter strips and a swale in eastern North Carolina

Abstract

Level spreader–vegetated filter strips (LS–VFSs) and swales are versatile, low cost stormwater control measures (SCMs). The purpose of this study was to determine the runoff volume and pollutant reductions of four LS–VFSs and a swale in eastern North Carolina (NC). Two vegetated filter strips (VFSs) of 8 m long by 6 m wide (26 ft long by 20 ft wide), two VFSs of 20 m by 6 m (66 ft by 20 ft), and a trapezoidal swale with 3:1 side slopes (0.15 m [0.5 ft] bottom width and 10.4 m [34 ft] long) were constructed. One VFS of each size was amended with a mixture of sand and ViroPhos, a proprietary phosphorus (P) sorptive aggregate. Hydrologic data were collected for rainfall events during the 11-month sampling period. Influent and effluent samples were collected from select rainfall events and analyzed for concentrations of nitrogen (N) and P species, total suspended solids (TSS), cadmium (Cd), copper (Cu), and zinc (Zn).

Significant runoff volume reductions averaging 36%, 59%, 42%, 57%, and 23% were found for the small and large unamended VFSs, the small and large amended VFSs, and the swale, respectively (n = 30). In general, concentrations of total nitrogen (TN) and TSS were reduced, while total phosphorus (TP) increased in nearly all treatments. The reductions in TN concentrations were significant in the amended VFSs and the swale, and TN was found to be irreducible at influent concentrations <1 mg L⁻¹ (<1 ppm). TN load reductions in all treatments exceeded the 30% reduction credit assigned by NC, while only the amended VFSs and the swale exceeded the 35% load reduction credit assigned for TP.TN and TP load reductions were not significantly different between the VFSs and the swale. All treatments exceeded 75% TSS load reduction, far exceeding the 40% TSS removal credit that these systems receive in NC. In general, swale and VFS performance was similar in terms of TN and TP load reduction, while the VFSs significantly outperformed the swale in average volume and TSS load reduction.