Comparing Bridge Deck Runoff and Stormwater Control Measure Quality in North Carolina

Bridge deck runoff sometimes directly discharges through deck drains to water bodies. As such, the runoff is usually not treated; however, recent pressures have led Departments of Transportation to install closed pipe drainage systems beneath bridges to deliver stormwater to a stormwater control measure (SCM). This can be costly both in terms of up-front and long-term maintenance capital. This study compared bridge runoff concentrations of nutrients, sediment, and heavy metals to effluent concentrations from six commonly used SCMs. Runoff quality samples from 15 bridges in North Carolina were collected and compared to those from 41 different SCMs across North Carolina. The SCMs examined in this study were permeable friction course (PFC) overlays, wet retention ponds (WP), bioretention cells (BRC), vegetated filter strips (VFS), constructed stormwater wetlands (CSW), and grassed swales (GS). Bridge deck runoff concentrations were not statistically different from SCM effluent concentrations for total nitrogen (TN). For total phosphorus (TP), all SCMs produced effluent concentrations lower than bridge runoff concentrations, although only PFC, BRC, and WP did so significantly. For total suspended solids (TSS), median effluent concentrations from the SCMs were significantly and substantially lower (a difference of more than 15 mg/L) than those from bridges. Comparison against water quality threshold concentrations developed for North Carolina suggested that BRC and WP were best for TN treatment and that PFC, WP, and BRC were appropriate for TP treatment. For TSS, all six SCMs were capable of improving the bridge runoff. Similar results were observed for copper, lead, and zinc; BRC, GS, and CSW were able to reduce total metals concentrations significantly. Dissolved metal concentrations appeared difficult to reduce with current SCM technology. These results suggest that for certain pollutants, treatment of bridge runoff may yield improvement. However, the appropriateness of installing SCMs to treat bridge deck runoff must account for the increased cost of closed pipe drainage systems beneath bridges and limited space in the right-of-way and weighed against the relative ease of retrofitting stormwater treatment infrastructure into other transportation corridors.