Characterization of water-quality and bed-sediment conditions in Currituck Sound, North Carolina, prior to the Mid-Currituck Bridge construction, 2011–15

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 two-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. The results of the Final 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. The primary objective of this initial study phase was 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. Between 2011 and 2015, discrete water-quality samples were collected monthly and after selected storm events from five locations in Currituck Sound. The sampling locations were distributed along the proposed alignment of the Mid-Currituck Bridge. Water samples were analyzed for physical parameters and water-quality constituents associated with bridge deck stormwater runoff and important in identifying impaired waters designated as “SC” (saltwater-aquatic life propagation/ protection and secondary recreation) under North Carolina’s water-quality classifications. Bed-sediment chemistry was also measured three times during the study at the five sampling locations. Continuous water-level and wind speed and direction data in Currituck Sound were also collected by the U.S. Geological Survey during the study period. For the water samples, measured concentrations were greater than water-quality thresholds on 52 occasions. In addition, there were 190 occurrences of censored results having a reporting level higher than specific thresholds. All 52 occurrences of concentrations greater than water-quality thresholds were confined to seven different physical properties or constituents, namely pH (25), turbidity (8), total recoverable chromium (6), total recoverable copper (6), dissolved copper (3), total recoverable mercury (2), and total recoverable nickel (2). Concentrations of 17 other constituents were never measured to be greater than their established water-quality thresholds during the study. The focus of the water-quality characterization was on concentrations of constituents identified as parameters of concern in a 2011 collaborative U.S. Geological Survey/North Carolina Department of Transportation study that characterized bridge deck stormwater runoff across North Carolina. The occurrence and distribution of parameters of concern identified in the 2011 study, including pH, nutrients, total recoverable and dissolved metals, and polycyclic aromatic hydrocarbons, and some additional pertinent physical properties (dissolved oxygen, specific conductance, and turbidity), were analyzed in water and bed-sediment samples. Statistical differences were identified between monthly and storm samples for the following physical properties and constituents: pH, dissolved oxygen, specific conductance, turbidity, Escherichia coli bacteria, total recoverable aluminum, and total recoverable iron. Seasonality was observed in pH, specific conductance, dissolved oxygen, turbidity, total phosphorus, and total nitrogen, and total recoverable aluminum, arsenic, iron, lead, and manganese during the study period. The volume and residence time of the water in Currituck Sound are such that the water chemistry is relatively uniform spatially, but variable temporally. One of the most variable constituents in bed sediments was the fraction of fines, those sediments less than 63 micrometers in diameter. Because most, if not all, of the measured constituents were presumably associated with this fraction, bulk-sediment concentrations are determined largely by the amount of fines present. Only four constituents were greater than bed-sediment thresholds: tin (5 samples), barium (4 samples), aluminum (2 samples), and diethyl phthalate (1 sample). The occurrences of concentrations being greater than referenced thresholds could be underestimated for diethyl phthalate, because the reporting level exceeded the threshold Characterization of Water-Quality and Bed-Sediment Conditions in Currituck Sound, North Carolina, Prior to the Mid-Currituck Bridge Construction, 2011–15 By Chad Wagner, Sharon Fitzgerald, and Dominick Antolino 2 Water-Quality and Bed-Sediment Conditions in Currituck Sound Prior to the Mid-Currituck Bridge Construction, 2011–15 in nine samples. Thirty-five constituents had sampled concentrations that were never greater than quality thresholds, although 21 of these constituents (154 instances) had at least one sample with a reporting level that was greater than the corresponding threshold. Finally, 33 constituents had no quality thresholds. This study and previous studies of bed-sediment quality in Currituck Sound, although few, indicate that sedimentation in Currituck Sound near the proposed alignment of the MidCurrituck Bridge is characterized overall by low and transient input, frequent wind-driven resuspension, and little long-term deposition of fines. To the extent that it might alter water depths along the alignment, bridge construction might also alter the deposition and resuspension rates of fine sediments in Currituck Sound in the vicinity of the bridge. The characterization of water-quality and bed-sediment chemistry in Currituck Sound along the proposed alignment of the Mid-Currituck Bridge summarized herein provides a baseline for determining the effect of bridge construction and bridge deck runoff on environmental conditions in Currituck Sound. Introduction Ecological conditions in and around Currituck Sound in northeastern North Carolina and southeastern Virginia have changed substantially since at least the 1980s. Fish-population surveys have indicated a decrease in freshwater species and an increase in estuarine species. These changes are attributed to an increase in salinity in the sound (Southwick and Norman, 1991). It has further been hypothesized (U.S. Army Corps of Engineers, 2001) that the change in the salinity regime is associated with a substantial decline in submerged aquatic vegetation (SAV), although other factors may be responsible for the SAV declines. Beds of SAV provide food for migratory waterfowl, in addition to spawning and nursery habitats for fish species. A decline in SAV beds also contributes to a decline in water quality, in that a decrease in SAV root systems and underwater biomass allows increased resuspension of fine sediments and associated nutrients during wind events (U.S. Army Corps of Engineers, 2001). In 2001, the U.S. Army Corps of Engineers conducted a reconnaissance study of Currituck Sound under Section 905(b) of the Water Resources Development Act of 1986. The purpose of the study was “to determine whether planning for the improvement of water quality, environmental restoration and protection, and related purposes for Currituck Sound should proceed further” (U.S. Army Corps of Engineers, 2001). Among other findings, one conclusion from the study was that Currituck Sound is a “threatened resource” (Fine, 2008). In 2005, a team of scientists was assembled to begin an investigation of the hydrologic and water-quality characteristics of Currituck Sound and its major tributaries. The objectives of the investigation were to document water-quality trends in the sound and collect the data needed for the development of a coupled hydrodynamic and water-quality model. The U.S. Geological Survey (USGS), Elizabeth City State University (ECSU), North Carolina National Estuarine Research Reserve (NERR), and the U.S. Fish and Wildlife Service (USFWS) partnered in data-collection efforts in and around Currituck Sound during 2006 and 2007. Data were collected at nine locations and included continuous measurements of water elevation, velocity, discharge, water temperature, specific conductance, dissolved oxygen, pH, turbidity, and chlorophyll a (at a subset of the nine sites). Water-quality samples also were collected at selected sites in the sound. All data can be obtained through the National Water Information System (NWIS) Web interface at http://waterdata.usgs.gov/nc/nwis or in the USGS annual waterresources data reports (U.S. Geological Survey, 2007, 2008). The USGS also collected data to define the hydrologic and salinity characteristics of the upper Currituck Sound and selected tributaries from 1998 to 1999. The results of this analysis are presented in Caldwell (2001). The North Carolina Turnpike Authority (NCTA), a division of the North Carolina Department of Transportation (NCDOT), is planning to make transportation improvements in the Currituck Sound area by constructing a two-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, N.C. (fig. 1). The results of the Final Environmental Impact Study (FEIS) 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 (North Carolina Turnpike Authority, 2010). Several studies have shown that bridge deck runoff contains a relatively high load of constituents, such as nutrients, solids, pesticides, metals, and polycyclic aromatic hydrocarbons (PAHs) (Driscoll and others, 1988; Dupuis, 2002; Malina and others, 2005; Marsalek and others, 1997; McKenzie and Irwin, 1983; URS Corporation, 2010; Wagner and others, 2011). These substances have the po