Monitoring the Water Quality Benefits of a Triangular Swale Treating a Highway Runoff

Abstract

Swales are among the most commonly used stormwater control measures (SCMs) worldwide. In 2009, the North Carolina DOT constructed a grassed swale in Knightdale, North Carolina, in the right-of-way of Interstate 540 near Mango Creek. The swale had a rock-lined forebay, v-shaped cross-section, a sinuosity of 1.1, 2% longitudinal slope, and 37-m centerline length. The swale was vegetated with tall fescue sod and partly below the Interstate 540 southbound bridge deck. Flow-volume weighted, composite water quality samples were collected for more than 30 runoff events at the inlet and outlet of the swale and tested for nitrogen species [total Kjeldahl nitrogen (TKN), ammonia-nitrogen (TAN), and nitrate- and nitrate-nitrogen (NO3)], total phosphorous (TP), and total suspended solids (TSS). The mean effluent total nitrogen, total phosphorous, and total suspended solids concentrations released by the swale were 0.99, 0.16, and 39 mg/L, respectively. Particle size distributions analyzed for three storms showed that the swale and its forebay most easily transported particles in the range of 0.4–40 μm and most effectively trapped particles in the range of 100–2,000 μm in diameter. Although this swale reduced the pollutants associated with coarse particulates (typical of highway runoff), coupling swales with other SCMs is recommended to meet target total nitrogen (TN) and TP thresholds.