Retrofitting with Bioretention and a Bioswale to Treat Bridge Deck Stormwater Runoff

Abstract

Stormwater runoff from roadways is a source of surface water pollution in North Carolina. The North Carolina Department of Transportation (NCDOT) is required to implement stormwater BMPs in the linear environment. NCDOT has specific interest in runoff from bridge decks, which is often discharged through drainage holes in the deck directly to the stream below. This research focuses on retrofit stormwater BMPs for bridge deck runoff management. Two bioretention cells and a bioswale were constructed in the easement of a bridge deck on I-540 at Mango Creek. One bioretention cell was adequately sized based on current North Carolina design guidance, while the other was undersized by one-half. Undersized bioretention cells could often be used in retrofit situations; therefore, it is important to understand how an undersized bioretention cell performs with respect to hydrology and water quality. Both bioretention cells employed 0.9 m (3 ft) of fill media, and had an internal water storage layer (IWS) of 0.6 m (2 ft). The bioswale was designed to convey the 2-year storm event. Runoff was piped from the northbound and southbound lanes to the bioretention cells and bioswale, respectively. Data collection began in October 2009. Weirs and stage recorders were used to monitor inflow to and outflow from each BMP. Flow-proportional, composite water quality samples were obtained at the inlet and outlet of each BMP. Monitored water quality parameters include TKN, NO2-3-N, NH4-N, TN, TP, TSS, Cu, Zn, and Pb. For small storms (those with less than 1 in [2.5 cm] rainfall depth), flow volume reductions for the standard and undersized bioretention cells were 50% and 27%, respectively. This shows the hydrologic importance of sizing bioretention cells appropriately when space is available. Average concentrations of TN (0.74 mg/L), TP (0.12 mg/L), and TSS (31 mg/L) from the bridge decks were relatively low when compared to other highways in North Carolina. Median effluent concentrations for the standard bioretention cell were lower than those for the undersized bioretention cell for all nutrient forms and sediment. Pollutant loads of TN, TP, and TSS were reduced to a much greater extent by the standard bioretention cell due to improved volumetric runoff reductions. The bioswale had similar influent and effluent concentrations for TN, TP, and TSS. Reductions in flow volume for the bioswale were not observed, resulting in poor pollutant load reduction.