Quantifying the Water Quality and Hydrologic Benefits of Two Bioswales Receiving Highway Runoff in North Carolina

Bioswales are an emerging stormwater control measure (SCM) used to treat runoff from highways and parking lots. This SCM consists of an engineered media, gravel layer, and underdrain system beneath a vegetated channel, which helps to promote exfiltration while conveying runoff. This study evaluated how well two bioswales (BS2 and BS4) retrofitted with check dams mitigated stormwater runoff from a highway in Wake County, North Carolina. Inflow, overflow, and underdrain water quality and quantity data were collected using automated sampling equipment and weirs or estimated using runoff equations. Exfiltration was estimated as the difference between inflow and the sum of overflow and underdrain discharge. Outflow was calculated by summing the overflow and underdrain volumes at each time step. BS2 and BS4 were monitored for 61 and 53 storm events, respectively. Of those events, BS2 and BS4 exfiltrated 13 and 23 storms, respectively. For small storms (<19 mm), the mean percentage of runoff that was exfiltrated by BS2 and BS4 was 73% and 85%, respectively. There were also significant (α=0.05) differences between the bioswales’ inflow and overflow, underdrain, or outflow runoff volumes and peak discharges. These results suggest bioswales with check dams can manage stormwater. The mean total nitrogen overflow, underdrain, and outflow concentrations for BS2 were 0.84, 0.66, and 0.70 mg/L, respectively, and 2.10, 1.53, and 1.75 mg/L, respectively, for BS4. The mean total phosphorus overflow, underdrain, and outflow concentrations for BS2 were 0.16, 0.11, and 0.12 mg/L, respectively, and 0.44, 0.26, and 0.35 mg/L, respectively, for BS4. None of the concentrations were significantly different, limiting conclusions that can be drawn from this study.