Location of the SC 905 bridge over Buck Creek, the selected DHEC sampling site PD-362

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Collection and Interpretation of Near Real-Time Surface-Water Quality and Flow Data at Selected Sites in the Waccamaw and Pee Dee River Basins Horry County, South Carolina

Project Number: 2519-C6X01
Project Chief: Jim Landmeyer
Cooperator: Horry County Stormwater Division
Period of Project: October 1, 2000 to October 1, 2009

SUMMARY

Location of the road S-26-99 bridge over Chinners Swamp, the selected DHEC sampling site PD-352.

The South Carolina Department of Health and Environmental Control (DHEC) collects and analyzes surface-water samples at sites throughout South Carolina (SC) as part of the Ambient Surface Water Quality Monitoring Network (DHEC ambient sites). Water samples are collected manually once per month at most DHEC ambient sites. These data are used by DHEC to determine long-term water-quality trends, and to assess if surface-water bodies are compliant to State or Federal water-quality concentration-based regulations regarding the support of aquatic life or recreational use.

Although this approach has assisted DHEC officials with the identification of overall gross trends in surface-water quality in SC, a more frequent interval of data collection is needed to support management decisions by County water officials. Specifically, water resource managers in Horry County, SC have developed a Strategic Plan for Water Resources that includes the collection of water-quality data in surface-waters in the County. A more frequent data-collection interval would permit Horry County managers to determine the effect of various environmental factors on certain constituents that often exceed State or Federal levels in some surface waters in the County. Moreover, the collection of water-quality data coupled with stream-flow data would provide the ability to calculate loads of particular constituents as well as provide input to future modeling efforts. Currently, stream-flow information has not been collected at DHEC ambient sites since 1999 (Sally Knowles, DHEC, personal commun., May 18, 2005)). Finally, site-specific rainfall data also are not currently collected at the DHEC ambient sites in Horry County, which constrains the ability to relate rainfall-runoff characteristics to water quality, land use, and changes in either.

This proposed investigation aims to increase the frequency of surface-water quality data collection and initiate stream-flow data collection near existing DHEC ambient sites in Horry County. State-of-the-art water-quality and flow sensors that can collect and report water quality and stream-flow data in near real-time will be installed at 3 sites that are currently on the 303(d) list for SC in the Waccamaw and Pee Dee River Basins in Horry County. To maximize the usefulness of this new data collection, each new site will be located as close as practicable to the existing DHEC ambient sites.

PROBLEM

Buck Creek, Horry County

In coastal South Carolina, Horry County is characterized by two major rivers; the Pee Dee and Waccamaw. As part of DHEC's ambient network previously discussed, 31 sites are manually sampled once per month in Horry County. In 2004, 15 sites (48 percent) were listed on the 2004 303(d) list of non-compliant surface waters for SC. Because the majority of these non-compliant sites are located where surface waters are the border between Horry County and adjacent Counties, this proposed investigation will focus only on those sites within Horry County. As such, table 1 lists 3 DHEC ambient sites in Horry County that were out of compliance in 2004 and representative of the various land uses in the county. As can be seen, in some cases the same exceedence was observed in surface waters characterized by different land uses. For example, Crab Tree Swamp and Chinners Swamp are both characterized by high fecal coliforms (FC), but Crab Tree Swamp drains a predominately urban area, whereas Chinners Swamp is in a forested wetland area (table 1).

These data indicate that other environmental factors in addition to land use may affect surface-water quality trends. For example, a recent investigation by USGS and Horry County revealed that there was a positive relation between turbidity and indicator bacteria (Enterococcus spp.) in 2 coastal watersheds (Landmeyer, in prep). Although one watershed drained an urban area, and one drained a mostly forested wetland, both had high levels of enterococcus during the study. As such, these data and other data (up to 41% of rivers and streams in SC had impairment caused by fecal coliform bacteria; David Chesnut, DHEC, personal commun., May 18, 2005) suggest that high levels of FC may be a natural feature of some Low Country surface waters, such as has been recognized for naturally low levels of dissolved oxygen and pH. The 3 sites listed in table 1 and shown on figure 1 will be the focus of this proposed investigation. The following section provides some background information about each basin to be investigated.

Table 1.-List of Surface Water Basins and DHEC Sites in Horry County to be instrumented as defined by listing on the 303(d) list, with predominant land use (from DHEC ambient network; 2004 Integrated Report Part 1: Listing of Impaired Waters; DHEC, 2004b).

OBJECTIVES AND SCOPE

The objectives of this proposed investigation are to (1) provide water-quality and stream-flow data at 3 sites in Horry County on a near real-time basis, and (2) to interpret the data collected. These objectives will be accomplished through the installation, monitoring, and maintenance of 3 new sites comprised of real-time water-quality and surface-water flow sensors. The water-quality and stream-flow data collected will be used to assess the relation, to the extent possible, between changes in water-quality constituents with changes in stream-flow, precipitation, and land use. During special events, such as peak rainfalls or drought conditions, additional water-quality data, such as indicator bacteria, ions, and nutrients, will be collected and analyzed by scientists with the Coastal Carolina University under a separate agreement with Horry County.

The proposed investigation will focus on three specific surface-water bodies in the Waccamaw and Pee Dee River Basins in Horry County; Buck Creek, Crab Tree Swamp, and Chinners Swamp. The study is envisioned to start in August of Federal Fiscal Year (FY) 2005, and continue through the end of Federal FY 2009. The first year of the study (FY 2005) is comprised of the installation of 3 new sites (table 1). Each site will consist of a multiparameter water-quality sensor (YSI 6600; note that trade names are for identification purposes only and do not constituent an endorsement by the USGS) that can measure surface-water temperature (T), specific conductance (SC), dissolved oxygen (DO), pH, oxidation-reduction potential (ORP), and turbidity. Each site will have a precipitation gage and a data collection platform (DCP), which will be capable of near real-time data transmission by satellite telemetry. The data collected by the water-quality sensors will be calibrated periodically using standard methods as routinely practiced by the USGS, South Carolina Water Science Center. Each site also will be equipped with an acoustic velocity meter (AVM; Sontek Argonaut-SW) that will measure surface-water velocity (figure 4). The AVM can combine water velocity measurements with the cross-sectional area of the channel at each site to calculate channel flows.

APPROACH

Idealization of surface-water flow and level measurement envisioned with installation of the Sontek Argonaut at the 3 selected sampling sites. Two beams measure flow and one beam measures water level.

This proposed investigation will provide continuous surface water-quality, stream-flow, and precipitation data to assist in the assessment of trends in the water-quality conditions of 3 surface waters in Horry County on the 303(d) list. Overall activities will include:

• The water-quality and stream-flow data will be colleted by installing, operating, and maintaining 3 new sites. Water temperature, specific conductivity, dissolved oxygen, pH, ORP, and turbidity will be measured. Stream-flow will be measured using an AVM at each site;
• The water-quality data will be compared to the monthly results collected and analyzed by DHEC as part of their existing ambient network;
• Water-quality and stream-flow data generated will be used to interpret the following possible relations;
  1. Will turbidity increase over time in basins expected to undergo future development (ie, the Long Creek site), relative to turbidity in basins not expected to be developed (ie, Chinners Swamp)?
  2. Will specific conductance increase over time in basins expected to undergo future development (the Long Creek site), relative to turbidity in basins not expected to be developed (Chinners Swamp)?
  3. Can the high fecal coliform levels measured in 2 of the 3 study basins positively related to turbidity?
  4. Are the low DO concentrations measured inversely related to increasing turbidity?
  5. Will surface-water temperatures increase over time in basins expected to undergo future development (the Long Creek site), relative to turbidity in basins that are not expected to be developed (Chinners Swamp)?

This list is provided as a general guide and may change to reflect the data collected.

REFERENCES

SC DHEC, 2004a, State of South Carolina Monitoring Strategy for Calendar Year 2004.

SC DHEC, 2004b, 2004 Integrated Report Part 1: Listing of Impaired Waters.

USGS, Cooperative Water Program, http://water.usgs.gov/coop/