Characterization of Stormwater Runoff from North Carolina Bridges and Determination of Effects on Selected Receiving Waters

Abstract

The North Carolina Department of Transportation and the U.S. Geological Survey collaborated on a study to better understand the effects of stormwater runoff from bridges on receiving waters. The following tasks were performed: (a) characterizing the quality and quantity of stormwater runoff from a representative selection of bridges in North Carolina, (b) measuring the quality of stream water upstream of selected bridges to compare constituent concentrations and loads in bridge deck stormwater with those in the stream, (c) determining whether the chemistry of bed sediment upstream and downstream of selected bridges differed substantially according to the presence or absence of a best management practice for bridge runoff, and (d) estimating the rate at which bridge deck runoff mixed with the receiving stream. The investigation measured bridge deck runoff from 15 bridges for 12 to 15 storms, the quality of stream water for base flow and storm conditions at four of the bridge deck sites, and the chemistry of stream bed sediment upstream and downstream of 30 bridges across North Carolina. Runoff and stream samples were analyzed for a wide range of constituents, including dissolved and total recoverable metals and nutrients, major ions, total suspended solids, suspended sediment, oil and grease, petroleum hydrocarbons, and semi-volatile organic compounds. For 64% of comparisons, concentrations in bridge deck runoff were no different or were less than those measured in receiving waters, and the maximum concentrations of constituents in the bridge deck runoff were rapidly reduced to the ambient stream concentrations, generally within 50 ft downstream of the bridge.