South Atlantic Water Science Center - North Carolina Office
Period of Project
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Water quality characterization of bridge deck runoff in NC
This project was completed in 2012. These pages are for historical purposes only.
On July 1, 2008, the North Carolina General Assembly passed House Bill 2436, Session Law 2008-107, Stormwater Runoff from Bridges Section 25.18.(a,b,c). This bill requires the North Carolina Department of Transportation (NCDOT) to study 50 bridges to (1) quantify the constituents in stormwater runoff from bridges across the state, (2) evaluate the treatment practices that can be used to reduce constituent loadings to surface waters from bridges, and (3) determine the effectiveness of the evaluated treatment practices.
There is evidence that bridge deck runoff has a relatively high loading of a variety of constituents such as nutrients, solids, pesticides, metals, and polycyclic aromatic hydrocarbons (PAHs). Information on the quality of bridge deck runoff in North Carolina is, however, lacking. Stormwater permits are designed to reduce nonpoint source loadings of anthropogenically derived constituents to surface waters. Permits for bridges in NC, however, must be based on data collected from studies which were conducted 10 – 20 years ago and in other parts of the U.S. As a result permit requirements for bridges in NC may be unnecessarily conservative or inadequate for protecting receiving water quality. Added to this need for more detailed, current, and local bridge deck runoff data are the requirements of House Bill 2436.
The primary objective of this investigation is to identify the loading of selected constituents in stormwater runoff from representative bridges across North Carolina. Working collaboratively, NCDOT and USGS identified other study objectives which could provide information valuable in helping understand the effects of bridge deck runoff on receiving water quality and in managing stormwater runoff from bridges.
The objectives for this investigation, are as follows:
This investigation will measure bridge deck runoff from 15 bridges across NC. Bridges will represent a range of physiographic and climatic conditions, a range of average daily traffic (ADT), and a range in size. Runoff from both concrete deck and asphalt deck bridges will be sampled. The goal is to sample 12 runoff events at each bridge during the study. Samples will be analyzed for a wide range of constituents, including nutrients, major and trace metals, oil and grease, and semivolatile organic compounds.
Bridges at which runoff will be sampled are fitted with a collection system so that all bridge runoff flows through a single pipe, thereby facilitating sampling. Discharge from the collection system flows across a grass swale or through a pond before entering the stream.
Bottom sediment quality will be measured at 30 sites, 15 of which will be the bridge deck runoff monitoring sites, and 15 of which will be at bridges in which runoff discharges from scuppers (essentially a series of pipes along the curb to drain the bridge) directly into the stream. This type of drainage system is much more common across NC than the collection system. Samples at each bridge will be collected once at a location upstream from the bridge and at a second location downstream from the bridge. Bed sediment will be analyzed for nutrients, major and trace metals, and semivolatile organic compounds. Both total and total recoverable concentrations of inorganic elements will be measured.
Four streams at bridge deck runoff sites will be sampled intensively in order to estimate annual loadings of suspended sediment, nutrients and other commonly detected constituents. Stream concentrations and loads will be compared to bridge deck runoff concentrations and loadings at these sites in order to provide insight into the relative contribution of bridge deck runoff to total stream quality.