Mid-Currituck Bridge Water Quality, Currituck Sound, North Carolina

Science Center Objects

The North Carolina Department of Transportation is planning to make transportation improvements in the Currituck Sound area by constructing a three-lane bridge from U.S. Highway 158 just south of Coinjock, North Carolina, to State Highway 12 on the Outer Banks just south of Corolla, North Carolina. From Aug 2011 to early 2018, the USGS will be collecting regular water-quality samples as well as bed-sediment samples to establish baseline water-quality conditions and bed-sediment chemistry of Currituck Sound in the vicinity of the planned alignment of the Mid-Currituck Bridge. These data will be used to evaluate the impacts associated with the bridge construction and bridge deck stormwater runoff.

Data collection

Picture of Wind Speed/Direction and Stage Gage on pier, Currituck Lighthouse
Picture of Wind Speed/Direction and Stage Gage on pier, Currituck Lighthouse
  • Real-time stage, wind speed, and direction (1 location)
  • Monthly water-quality sampling (5 locations)
  • Bed sediment for chemical analysis (5 locations)

All data collected during this project will be available online in the USGS National Water Information System (NWIS).

Background

The North Carolina Turnpike Authority, a division of the North Carolina Department of Transportation, is planning to make transportation improvements in the Currituck Sound area by constructing a 2-lane bridge from US Highway 158 just south of Coinjock, NC to NC Highway 12 on the Outer Banks just south of Corolla, NC.

The results of the Environmental Impact Study associated with the bridge and existing roadway improvements indicated potential water quality and habitat impacts to Currituck Sound related to stormwater runoff, altered light levels, introduction of piles as hard substrate and localized turbidity and siltation during construction.

Picture of scientists taking water sample from Currituck Sound
Picture of scientists taking water sample from Currituck Sound

Previous studies and data have shown that there have already been changes in the ecological conditions in and around Currituck Sound in northeastern North Carolina and southeastern Virginia since at least the 1980s. A 2001 reconnaissance study of Currituck Sound by the U.S. Army Corps of Engineers1 concluded, among other things, that the Currituck Sound is a "threatened resource".

Documented changes include:

  • A decrease in freshwater fish species and an increase in estuarine fish species2
  • An increase in salinity in the sound2
  • A substantial decline in submerged aquatic vegetation1
    • A decline in submerged aquatic vegetation beds decreases food available for migratory waterfowl, in addition to reducing potential spawning and nursery habitats for fish species.
    • A decline in submerged aquatic vegetation beds also contributes to a decline in water quality in that a decrease in root systems and underwater biomass allows increased resuspension of fine sediments and associated nutrients during wind events.

 

Objectives

Picture of gage installation on pier
Picture of gage installation on pier 

The primary objective of this study is to establish baseline bed sediment chemistry and water-quality conditions of Currituck Sound in the vicinity of the planned alignment of the Mid-Currituck Bridge, which will be used to evaluate the impacts associated with the bridge construction and bridge deck stormwater runoff.

Scope

A transect across Currituck Sound in the vicinity of the planned Mid-Currituck Bridge will be intensively sampled at 5 locations over a 26-month period for water-quality parameters pertinent to bridge deck stormwater runoff. Bed sediment chemistry will be measured twice during the study at 5 discrete sampling locations in conjunction with the quarterly benthic macroinvertebrate surveys being conducted by DENR and NCDOT.

References

1. U.S. Army Corps of Engineers, 2001, Reconnaissance report—Currituck Sound, North Carolina: Wilmington, NC, U.S. Army Corps of Engineers, Wilmington District, 15 p.

2. Southwick, Ronald, and Norman, M.D., 1991, Impact of salinity changes on fish populations in Back Bay, Virginia, 1950–1989, in Marshall, H.G., and Norman, M.D., eds., 1991, Proceedings of the Back Bay Ecological Symposium: Norfolk, VA, Old Dominion University, p. 138–147.

Approach

Task 1. Monitor wind speed/direction, stage and velocity data

A wind speed and direction sensor and pressure transducer with satellite telemetry will be deployed in the vicinity of the water-quality sampling transect along the Outer Banks, which will provide real-time wind speed and direction and stage data at the study site.

Acoustic Doppler current profiler (ADCP) velocity data will be used to characterize the circulation dynamics in the vicinity of the planned bridge at the time of each water-quality and bed sediment sampling event. ADCP allows three-dimensional velocities to be measured from approximately 0.3 meters beneath the water surface to within 6 percent of the depth to the bottom.

ADCP measurements will also be used to qualitatively describe the distribution of suspended sediment across the sound during each water-quality sampling event.

Task 2. Conduct water-quality sampling

Discrete water-quality samples will be collected at 5 select locations along a 4-mile transect across Currituck Sound in the vicinity of the planned bridge. Water quality samples will be collected monthly for approximately 26 months prior to the construction of the bridge, which is planned to begin in Spring 2014. The monthly samples will be supplemented with 8 storm events sampled within 48 hours of a significant rainfall (greater than 0.5 inches in 24 hours) in the Upper Currituck basin.

Monthly samples will be planned strategically such that samples are collected during both "ebb" and "flood" wind-driven conditions in order to fully quantify the baseline conditions in the sound. The USGS stage data and wind data will be used to quantify the hydrologic conditions associated with each sampling event.

Constituents

Samples will be analyzed for a wide range of constituents, including metals, nutrients, pH, suspended solids, polycyclic aromatic hydrocarbons (PAHs) and other organic compounds.

Constituent Group              Analytes                                                             
Field measurements           Specific conductance, pH, temperature, turbidity,
                                          salinity, Secchi depth
Solids                                 Total suspended solids
Nutrients                            Total Kjeldahl nitrogen, nitrate+nitrite, ammonia,
                                          and total phosphorus
Metals                                Total recoverable and dissolved: aluminum,
                                          arsenic, cadmium, chromium, copper, iron, lead,
                                          manganese, mercury, nickel, selenium, silver
                                          and zinc
Organic compounds           14 semivolatile organic compounds identified as
                                          parameters of concern in the North Carolina
                                          Bridge Stormwater Project1
Bacteria                              Enterococcus, E. coli (near-surface only) 

 

Task 3. Collect bed sediment for chemical analysis

Picture of scientist taking water sample from Currituck Sound
Picture of scientist taking water sample from Currituck Sound

Bed sediments provide habitat for aquatic organisms and an interface for ground-water and surface-water interactions; thus, they are an important component of aquatic ecosystems. Bed sediments are also important for determining long-term water-quality conditions. Concentrations of material attached to bed sediment particles can 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.

Sampling Locations and Protocols

Bed sediment samples will be collected and analyzed to determine the baseline bed sediment chemistry prior to the construction of the Mid-Currituck Bridge. Samples will be collected twice, at five locations that correspond to the water-quality sampling locations and/or the locations of a benthic macroinvertebrate survey being conducted by DENR and NCDOT.

Constituents

Fine-grained sediments will be analyzed for total concentrations of metals, nitrogen, phosphorus, sulfur, organic carbon, total carbon and SVOCs including PAHs and phthalates.

Constituent Group              Analytes                                                                 
Nutrients and carbon          Nitrogen, phosphorus, sulfur, organic carbon, and total carbon
Metals                                Aluminum, arsenic, cadmium, chromium, copper, iron, lead,
                                           manganese, mercury, nickel, selenium, silver and zinc
Organic                              Semivolatile compounds including polycyclic aromatic hydrocarbons
                                           and phthalates
 

 References

1. URS Corporation, 2012, Stormwater runoff from bridges: Final report to Joint Legislation Transportation Oversight Committee, In fulfillment of Session Law 2008-107 for NC Department of Transportation, available online at: https://connect.ncdot.gov/resources/hydro/Stormwater%20Resources/Stormwater%20Runoff%20from%20Bridges%20-%20May%202012.pdf (PDF, 5 MB)

Related Reports

Mid-Currituck Bridge Study Final Environmental Impact Statement
By North Carolina Turnpike Authority

Final EIS (PDF, 10 MB) · Final EIS Fact Sheet (PDF, 2 MB)