Water quality characterization of bridge deck runoff in NC
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.
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, DWQ 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.
Background
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.
Objective
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:
- Characterize stormwater runoff quality and quantity from selected representative bridges in North Carolina.
- In order to better understand the effects of stormwater runoff from bridges on receiving waters, (a) determine if the chemistry of bed sediments upstream and downstream from selected bridges differs substantially; (b) measure stream water quality upstream from selected bridges in order to compare bridge deck stormwater concentrations and loads to stream constituent concentrations and loads; and (c) estimate the length of the mixing zone at the bridge deck study sites under a range of flow conditions, where the mixing zone is defined here as the stream reach required for a point source of stormwater entering the stream from the bank to became fully mixed across the stream.
Scope
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.
Background
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.
Objective
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:
- Characterize stormwater runoff quality and quantity from selected representative bridges in North Carolina.
- In order to better understand the effects of stormwater runoff from bridges on receiving waters, (a) determine if the chemistry of bed sediments upstream and downstream from selected bridges differs substantially; (b) measure stream water quality upstream from selected bridges in order to compare bridge deck stormwater concentrations and loads to stream constituent concentrations and loads; and (c) estimate the length of the mixing zone at the bridge deck study sites under a range of flow conditions, where the mixing zone is defined here as the stream reach required for a point source of stormwater entering the stream from the bank to became fully mixed across the stream.
Scope
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.
Approach
The specific tasks to meet the study objectives are as follows:
- Quantify bridge deck runoff volume and quality.
- Quantify stream water quality upstream of selected bridge deck monitoring sites.
- Compare streambed sediment quality at locations upstream and downstream from selected NCDOT bridges.
Task 1. Quantify bridge deck runoff volume and quality
Bridge deck runoff volume and water quality and will be monitored at 15 bridge sites in North Carolina (five in the Blue Ridge, seven in the Piedmont, and three in the Coastal Plain; table 2). Runoff samples will be collected by using an automated sampler, flow will be measured continuously in the collection system discharge pipe by using acoustic velocimetry, and a raingage will be installed at each site.
All pertinent bridge characteristics will be provided to USGS by NCDOT from the NCDOT bridge maintenance database. NCDOT also will provide continuous daily traffic counts at each of the 15 sites during the study. Basins upstream from the bridge sampling site will be characterized by compiling information on physical features (area, slope, etc.), population, land use, point sources, and upstream bridge crossings.
Bridge Deck Runoff Constituents
A broad range of constituent groups will be measured in the bridge deck runoff (table 1). These constituents were measured in at least 20 percent of the 218 highway runoff studies summarized in the National Highway Runoff Data and Methodology Synthesis (Granato, 2003) and include physical properties, solids, nutrients, major elements, trace metals, and PAHs. The number of analytes may be reduced if initial analyses consistently show concentrations that are below the laboratory reporting level. Total bridge deck stormwater loads will be based on total recoverable constituents because 1) there are fewer sample processing steps and therefore fewer opportunities for contamination, 2) North Carolina water quality criteria currently are based on total recoverable concentrations, and 3) most historical data are for total recoverable concentrations. All chemical analyses of water samples will be conducted at the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, with the exception of suspended sediment concentrations, which will be determined at the USGS Kentucky Sediment Laboratory.
Table 1. Water-quality constituents to be analyzed for bridge deck runoff samples.
Constituent group | Analytes |
---|---|
Water samples (12 events at all bridge deck stormwater sites) | |
Physical | Specific conductance, pH |
Solids | Total solids, total dissolved solids, total suspended solids, and suspended sediment concentration |
Nutrients | Total Kjeldahl nitrogen, nitrate+nitrite, ammonia, total phosphorus, ortho-phosphorus |
Major elements | Calcium, magnesium, sodium, potassium, chloride, sulfate, bromide, and alkalinity |
Trace metals | Total recoverable and dissolved: aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Organic compounds | Total and dissolved organic carbon, oil and grease, total petroleum hydrocarbons, 56 semivolatile organic compounds including PAHs and pthalates |
Fluvial sediment (4 samples collected at one bridge deck site) | |
Trace metals | Total-digestion: aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Task 2. Quantify constituent concentrations and loads upstream from bridges
Constituent concentrations represent conditions to which aquatic organisms are exposed. Prevailing water-quality standards also are expressed as concentrations. Therefore, comparison of constituent concentrations in bridge runoff and receiving waters is warranted. In addition, an evaluation of how much the mass of material washed off the bridge deck adds to the instream load is a measure of the relative effect of the bridge on downstream stream water quality. To make this comparison, measured bridge deck constituent loads of suspended sediment, nitrogen, phosphorus, total organic carbon, total dissolved solids, sodium, chloride, and other commonly detected constituents will be compared to instream loads at selected sites.
Four bridge deck monitoring sites that are co-located with USGS stream gages (table 5) will be sampled monthly for one year to characterize instream concentrations and flux. The monthly samples will be supplemented with 6 high-flow events sampled at each site using automated samplers. The four sites represent a gradient of daily traffic counts: Swannanoa River (traffic count is in the 90th percentile of the 15 bridge sites), Little River (75 percentile), Mountain Creek (35th percentile), and Black River (5th percentile). A new, temporary streamflow gage was installed at the Swannanoa River site; USGS gages already exist at the remaining three sites.
Task 3. Compare streambed sediment quality upstream and downstream from bridges
Concentrations of material attached to sediment particles represent water-quality conditions on a time scale from weeks to years, depending on recent hydraulics at the site, whereas a single water sample represents water quality on a time scale from minutes to hours. Bed sediments provide habitat for aquatic organisms and an interface for ground-water and surface-water interactions; thus, they are an important component of stream ecosystems. Streambed-quality data will aid in the interpretation of benthic macroinvertebrate communities at sites near bridges, which are being assessed separately from this study.
Many constituents associated with road runoff preferentially adsorb to particulates, and thus are found in higher concentrations in sediment than in overlying water. These hydrophobic constituents may include several trace metals, nutrients, and persistent organic compounds such as polycyclic aromatic hydrocarbons (PAHs).
Streambed Sediment Constituents
Fine-grained sediments will be analyzed for major and trace elements, total and organic carbon, total nitrogen, phosphorus, and sulfur, and semivolatile organic compounds including PAHs and pthalates (table 2). Both total-digestion and total recoverable concentrations of inorganic elements will be measured. As mentioned previously, total-digestion concentrations reflect the underlying mineralogy of the sediment, while total recoverable concentrations reflect metals that are adsorbed or lightly bound to the mineral matrix. Sediment quality guidelines are variously based on total-digestion or total-recoverable concentrations.
Table 2. Constituents to be analyzed in bed sediment
Constituent group | Analytes |
---|---|
Fine-grained fraction (diameter <63 microns) | |
Nutrients and carbon | Total nitrogen, total phosphorus, total carbon and inorganic carbon |
Major elements | Calcium, magnesium, sodium, potassium, sulfur |
Trace metals | Total-digestion and total recoverable aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Organics | Total organic carbon and semivolatile compounds including polycyclic aromatic hydrocarbons and pthalates |
View Bridge Stormwater Study Sites map
Table 3. Real-time streamgage and precipitation stations co-located with the bridge deck runoff monitoring sites.
Site Name Site Type USGS Station # Period of record
Raingage at Town Creek near Wilmington, NC, Bridge No 90061 | Precipitation | 340813077591645 | April 2009 to April 2010 |
Raingage at Smith Creek near Wilmington, NC, Bridge No 640131 | Precipitation | 341528077550745 | April 2009 to April 2010 |
Raingage at Black River near Tomahawk, NC, Bridge No. 810014 | Precipitation | 344516078172145 | April 2009 to April 2010 |
Raingage at Mountain Creek near Bahama, NC, Bridge No. 310005 | Precipitation | 360908078540745 | May 2009 to April 2010 |
Rainage at Little River near Orange Factory, NC, Bridge No. 310064 | Precipitation | 360829078550945 | May 2009 to April 2010 |
Raingage at Perry Creek near Raleigh, NC, Bridge No. 910124 | Precipitation | 355247078325045 | April 2009 to April 2010 |
Raingage at Mango Creek near Raleigh, NC, Bridge No. 911102 | Precipitation | 354703078304845 | March 2009 to April 2010 |
Raingage at Mallard Creek near Charlotte, NC, Bridge No. 590296 | Precipitation | 351911080450545 | April 2009 to April 2010 |
Raingage at Swift Creek at Garner, NC, Bridge No. 910255 | Precipitation | 354217078392245 | May 2009 to March 2010 |
Raingage at Middle Creek near Fuquay-Varina, NC, Bridge No. 910273 | Precipitation | 353633078411045 | May 2009 to March 2010 |
Raingage at Swannanoa River near Black Mountain, NC, Bridge No. 100494 | Precipitation | 353708082182145 | March 2009 to April 2010 |
Raingage at Big Ivy Creek near Mars Hill, NC, Bridge No. 100734 | Precipitation | 354728082321945 | July 2009 to April 2010 |
Raingage at Dillingham Creek at Barnardsville, NC, Bridge No. 100145 | Precipitation | 354607082260945 | May 2009 to April 2010 |
Raingage at Bolyston Creek at Mills River, NC, Bridge No. 440008 | Precipitation | 352231082325645 | May 2009 to April 2010 |
Raingage at Flat Creek near Weaverville, NC, Bridge No. 100250 | Precipitation | 354306082372645 | July 2009 to April 2010 |
Black River near Tomahawk, NC | Streamflow and Water Quality | 02106500 | October 1951 - present |
Little River at SR 1461 near Orange Factory, NC | Streamflow and Water Quality | 0208521324 | September 1987 - present |
Mountain Creek at SR 1617 near Bahama, NC | Streamflow | 0208524090 | October 1994 - present |
Mountain Creek at SR 1616 near Bahama, NC | Water Quality | 0208524088 | May 2009 - April 2010 |
Swannanoa River near Black Mountain, NC | Streamflow and Water Quality | 03448800 | March 2009 - present |
Below are publications associated with this project.
Characterization of stormwater runoff from bridges in North Carolina and the effects of bridge deck runoff on receiving streams
Below are partners associated with this project.
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.
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, DWQ 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.
Background
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.
Objective
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:
- Characterize stormwater runoff quality and quantity from selected representative bridges in North Carolina.
- In order to better understand the effects of stormwater runoff from bridges on receiving waters, (a) determine if the chemistry of bed sediments upstream and downstream from selected bridges differs substantially; (b) measure stream water quality upstream from selected bridges in order to compare bridge deck stormwater concentrations and loads to stream constituent concentrations and loads; and (c) estimate the length of the mixing zone at the bridge deck study sites under a range of flow conditions, where the mixing zone is defined here as the stream reach required for a point source of stormwater entering the stream from the bank to became fully mixed across the stream.
Scope
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.
Background
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.
Objective
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:
- Characterize stormwater runoff quality and quantity from selected representative bridges in North Carolina.
- In order to better understand the effects of stormwater runoff from bridges on receiving waters, (a) determine if the chemistry of bed sediments upstream and downstream from selected bridges differs substantially; (b) measure stream water quality upstream from selected bridges in order to compare bridge deck stormwater concentrations and loads to stream constituent concentrations and loads; and (c) estimate the length of the mixing zone at the bridge deck study sites under a range of flow conditions, where the mixing zone is defined here as the stream reach required for a point source of stormwater entering the stream from the bank to became fully mixed across the stream.
Scope
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.
Approach
The specific tasks to meet the study objectives are as follows:
- Quantify bridge deck runoff volume and quality.
- Quantify stream water quality upstream of selected bridge deck monitoring sites.
- Compare streambed sediment quality at locations upstream and downstream from selected NCDOT bridges.
Task 1. Quantify bridge deck runoff volume and quality
Bridge deck runoff volume and water quality and will be monitored at 15 bridge sites in North Carolina (five in the Blue Ridge, seven in the Piedmont, and three in the Coastal Plain; table 2). Runoff samples will be collected by using an automated sampler, flow will be measured continuously in the collection system discharge pipe by using acoustic velocimetry, and a raingage will be installed at each site.
All pertinent bridge characteristics will be provided to USGS by NCDOT from the NCDOT bridge maintenance database. NCDOT also will provide continuous daily traffic counts at each of the 15 sites during the study. Basins upstream from the bridge sampling site will be characterized by compiling information on physical features (area, slope, etc.), population, land use, point sources, and upstream bridge crossings.
Bridge Deck Runoff Constituents
A broad range of constituent groups will be measured in the bridge deck runoff (table 1). These constituents were measured in at least 20 percent of the 218 highway runoff studies summarized in the National Highway Runoff Data and Methodology Synthesis (Granato, 2003) and include physical properties, solids, nutrients, major elements, trace metals, and PAHs. The number of analytes may be reduced if initial analyses consistently show concentrations that are below the laboratory reporting level. Total bridge deck stormwater loads will be based on total recoverable constituents because 1) there are fewer sample processing steps and therefore fewer opportunities for contamination, 2) North Carolina water quality criteria currently are based on total recoverable concentrations, and 3) most historical data are for total recoverable concentrations. All chemical analyses of water samples will be conducted at the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, with the exception of suspended sediment concentrations, which will be determined at the USGS Kentucky Sediment Laboratory.
Table 1. Water-quality constituents to be analyzed for bridge deck runoff samples.
Constituent group | Analytes |
---|---|
Water samples (12 events at all bridge deck stormwater sites) | |
Physical | Specific conductance, pH |
Solids | Total solids, total dissolved solids, total suspended solids, and suspended sediment concentration |
Nutrients | Total Kjeldahl nitrogen, nitrate+nitrite, ammonia, total phosphorus, ortho-phosphorus |
Major elements | Calcium, magnesium, sodium, potassium, chloride, sulfate, bromide, and alkalinity |
Trace metals | Total recoverable and dissolved: aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Organic compounds | Total and dissolved organic carbon, oil and grease, total petroleum hydrocarbons, 56 semivolatile organic compounds including PAHs and pthalates |
Fluvial sediment (4 samples collected at one bridge deck site) | |
Trace metals | Total-digestion: aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Task 2. Quantify constituent concentrations and loads upstream from bridges
Constituent concentrations represent conditions to which aquatic organisms are exposed. Prevailing water-quality standards also are expressed as concentrations. Therefore, comparison of constituent concentrations in bridge runoff and receiving waters is warranted. In addition, an evaluation of how much the mass of material washed off the bridge deck adds to the instream load is a measure of the relative effect of the bridge on downstream stream water quality. To make this comparison, measured bridge deck constituent loads of suspended sediment, nitrogen, phosphorus, total organic carbon, total dissolved solids, sodium, chloride, and other commonly detected constituents will be compared to instream loads at selected sites.
Four bridge deck monitoring sites that are co-located with USGS stream gages (table 5) will be sampled monthly for one year to characterize instream concentrations and flux. The monthly samples will be supplemented with 6 high-flow events sampled at each site using automated samplers. The four sites represent a gradient of daily traffic counts: Swannanoa River (traffic count is in the 90th percentile of the 15 bridge sites), Little River (75 percentile), Mountain Creek (35th percentile), and Black River (5th percentile). A new, temporary streamflow gage was installed at the Swannanoa River site; USGS gages already exist at the remaining three sites.
Task 3. Compare streambed sediment quality upstream and downstream from bridges
Concentrations of material attached to sediment particles represent water-quality conditions on a time scale from weeks to years, depending on recent hydraulics at the site, whereas a single water sample represents water quality on a time scale from minutes to hours. Bed sediments provide habitat for aquatic organisms and an interface for ground-water and surface-water interactions; thus, they are an important component of stream ecosystems. Streambed-quality data will aid in the interpretation of benthic macroinvertebrate communities at sites near bridges, which are being assessed separately from this study.
Many constituents associated with road runoff preferentially adsorb to particulates, and thus are found in higher concentrations in sediment than in overlying water. These hydrophobic constituents may include several trace metals, nutrients, and persistent organic compounds such as polycyclic aromatic hydrocarbons (PAHs).
Streambed Sediment Constituents
Fine-grained sediments will be analyzed for major and trace elements, total and organic carbon, total nitrogen, phosphorus, and sulfur, and semivolatile organic compounds including PAHs and pthalates (table 2). Both total-digestion and total recoverable concentrations of inorganic elements will be measured. As mentioned previously, total-digestion concentrations reflect the underlying mineralogy of the sediment, while total recoverable concentrations reflect metals that are adsorbed or lightly bound to the mineral matrix. Sediment quality guidelines are variously based on total-digestion or total-recoverable concentrations.
Table 2. Constituents to be analyzed in bed sediment
Constituent group | Analytes |
---|---|
Fine-grained fraction (diameter <63 microns) | |
Nutrients and carbon | Total nitrogen, total phosphorus, total carbon and inorganic carbon |
Major elements | Calcium, magnesium, sodium, potassium, sulfur |
Trace metals | Total-digestion and total recoverable aluminum, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, and zinc |
Organics | Total organic carbon and semivolatile compounds including polycyclic aromatic hydrocarbons and pthalates |
View Bridge Stormwater Study Sites map
Table 3. Real-time streamgage and precipitation stations co-located with the bridge deck runoff monitoring sites.
Site Name Site Type USGS Station # Period of record
Raingage at Town Creek near Wilmington, NC, Bridge No 90061 | Precipitation | 340813077591645 | April 2009 to April 2010 |
Raingage at Smith Creek near Wilmington, NC, Bridge No 640131 | Precipitation | 341528077550745 | April 2009 to April 2010 |
Raingage at Black River near Tomahawk, NC, Bridge No. 810014 | Precipitation | 344516078172145 | April 2009 to April 2010 |
Raingage at Mountain Creek near Bahama, NC, Bridge No. 310005 | Precipitation | 360908078540745 | May 2009 to April 2010 |
Rainage at Little River near Orange Factory, NC, Bridge No. 310064 | Precipitation | 360829078550945 | May 2009 to April 2010 |
Raingage at Perry Creek near Raleigh, NC, Bridge No. 910124 | Precipitation | 355247078325045 | April 2009 to April 2010 |
Raingage at Mango Creek near Raleigh, NC, Bridge No. 911102 | Precipitation | 354703078304845 | March 2009 to April 2010 |
Raingage at Mallard Creek near Charlotte, NC, Bridge No. 590296 | Precipitation | 351911080450545 | April 2009 to April 2010 |
Raingage at Swift Creek at Garner, NC, Bridge No. 910255 | Precipitation | 354217078392245 | May 2009 to March 2010 |
Raingage at Middle Creek near Fuquay-Varina, NC, Bridge No. 910273 | Precipitation | 353633078411045 | May 2009 to March 2010 |
Raingage at Swannanoa River near Black Mountain, NC, Bridge No. 100494 | Precipitation | 353708082182145 | March 2009 to April 2010 |
Raingage at Big Ivy Creek near Mars Hill, NC, Bridge No. 100734 | Precipitation | 354728082321945 | July 2009 to April 2010 |
Raingage at Dillingham Creek at Barnardsville, NC, Bridge No. 100145 | Precipitation | 354607082260945 | May 2009 to April 2010 |
Raingage at Bolyston Creek at Mills River, NC, Bridge No. 440008 | Precipitation | 352231082325645 | May 2009 to April 2010 |
Raingage at Flat Creek near Weaverville, NC, Bridge No. 100250 | Precipitation | 354306082372645 | July 2009 to April 2010 |
Black River near Tomahawk, NC | Streamflow and Water Quality | 02106500 | October 1951 - present |
Little River at SR 1461 near Orange Factory, NC | Streamflow and Water Quality | 0208521324 | September 1987 - present |
Mountain Creek at SR 1617 near Bahama, NC | Streamflow | 0208524090 | October 1994 - present |
Mountain Creek at SR 1616 near Bahama, NC | Water Quality | 0208524088 | May 2009 - April 2010 |
Swannanoa River near Black Mountain, NC | Streamflow and Water Quality | 03448800 | March 2009 - present |
Below are publications associated with this project.
Characterization of stormwater runoff from bridges in North Carolina and the effects of bridge deck runoff on receiving streams
Below are partners associated with this project.