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-VFS) are versatile, low cost stormwater control measures with high community acceptance in urban settings. Their effectiveness has not been well studied, however, in eastern North Carolina. Four LS-VFSs and a swale in Wilson, North Carolina were evaluated to determine their pollutant removal efficiencies. Two VFSs of 8 m x 6 m and two VFSs of 20 m x 6 m were constructed. One VFS of each size was amended with ViroPhosTM, a specialized phosphorus sorptive aggregate provided by EnviRemed. Influent and effluent samples were collected over a ten-month period and analyzed for nitrogen, phosphorus, and total suspended solids (TSS) concentrations. The data was analyzed to determine the effects of VFS size and soil amendment in an urban, eastern North Carolina setting. Total nitrogen (TN) concentrations were significantly reduced in each of the amended treatments and the swale. TN was found to be irreducible when influent concentrations were less than 1 mg/l. TP concentrations significantly increased through the unamended VFSs, most likely due to the low influent concentrations and the high P-Index of the native soils. TSS concentrations were significantly reduced by all treatments when influent concentrations were greater than 10 mg/l. Size did not have a significant effect on pollutant concentration reduction, however, the ViroPhos amendment had a significant effect on TN, TP, and TSS reduction (p<0.0001). The reduction in TN and TSS and the smaller increase in TP in the amended VFSs may have been due to physical settling within the VFS, in addition to any effect of the ViroPhos amendment.