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

Abstract

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