3.0 BOD Characteristics of
Stream Stations in the
Lower Cape Fear River System Under Non-Storm Event Conditions
by
Michael A. Mallin
and Scott H. Ensign
Center for Marine
Science
University of
North Carolina at Wilmington
Wilmington, NC
28409
3.1 Introduction
One of the major water quality concerns in the Cape Fear basin is the summer hypoxia (low dissolved oxygen levels) found in various areas of the system (Mallin et al. 1999a; 1999b; 1999c; NCDENR 2000). The driving force of summer hypoxia is inputs of biochemical oxygen demand (BOD) from both point and non-point sources. Point sources are permitted by the NC Division of Water Quality to release a prescribed amount of BOD into the system through their discharges, and are required to report actual amounts discharged. Earlier research by the LCFRP (Mallin et al. 1999b) suggested that a significant amount of BOD loading to the system is may be derived from non-point sources. However, there is currently no information available on the variability of BOD loading to the system from non-point sources.
Estimating BOD loads requires both assessment of BOD concentrations in the water and computation of stream flow at the time of analysis. During the period May 2000 through April 2001 we collected monthly BOD and streamflow data at six locations in the Black River watershed. These collections were made on a pre-set schedule, so they are not storm event samples, but primarily represent base flow conditions.
3.2 Materials and
Methods
Samples were collected monthly from the following stations: Colly Creek (COL), Great Coharie Creek (GCO), Little Coharie Creek (LCO), Hammond Creek (HAM), Browns Creek (BRN), and Six Runs Creek (6RC). COL, GCO, LCO and 6RC are located in the Black River Basin and HAM and BRN empty into the mainstem of the Cape Fear River (Fig. 1.1).
A bucket and rope were used to collect water mid-stream from a bridge. Acid-cleaned 1L plastic bottles were filled from the bucket and stored on ice for transport to the laboratory. In the lab, a warm-water bath was used to raise the temperature of the 1L bottles to 20o C. Samples were then aerated by rapid mixing to ensure adequate initial dissolved oxygen of the sample water. Duplicate 300 mL BOD bottles were filled with sample water, and air bubbles were removed from the shoulder of the bottles by tapping them with an acrylic BOD bottle stopper. Samples were incubated for 20 days at 20.0o C Dissolved oxygen was read at the time of setup, day 5, and day 20 using a YSI 57 meter and 5905 probe calibrated to air saturation (see APHA 1995). Sample pH was also recorded at setup, day 5, and day 20. BOD5 and BOD20 measurements (as mg/L BOD) were calculated by subtracting dissolved oxygen on day 5 and 20, respectively, from the initial dissolved oxygen.
Flow was measured mid-stream from a bridge using a Marsh-McBirney Flo-Mate Model 2000. A lead weight and fin apparatus were used to keep the flow sensor motionless in the water column and pointed into the current. Flow data were obtained as m/s. A lead line with 0.5 m gradations was used to measure depth at 3 m intervals across the stream. From these measurements average depth was computed. Average depth was multiplied by stream width to obtain the cross-sectional area of the creek in m2. Volume of flow was calculated by multiplying flow by cross-section area of the stream to obtain m3/s, subsequently converted to m3/day.
BOD5 and BOD20 were converted to lbs BOD/m3, and multiplied by daily flow. BOD loading was computed as lbs BOD/day.
3.3 Results and
Discussion
During the sampling period there was virtually no difference among stations for either mean or median BOD concentrations (Table 3.1). BOD5 averaged around 1.0 mg/L and BOD20 averaged around 2.5 mg/L. Colly Creek had one unusually high BOD incident that caused it to maintain the highest variability during this period (Table 3.1).
BOD loading to the main river channels differed considerably, ranging from a low of 33 lbs/day at Hammonds Creek to a high of 978 lbs/day at Six Runs Creek (Table 3.1). This wide range was due to the broadly differing streamflow regimes among these streams. In other words, the largest creeks such as 6RC, COL and GCO had much greater BOD loading to the system than the small creeks such as HAM and BRN.
As mentioned, these data were not collected during storm events and thus do not represent stormwater runoff situations. Instead, they represent base flow conditions. It is likely that the major amount of stream BOD input occurs during and shortly after rain events. Earlier storm related research demonstrated the large difference in BOD concentrations between unimpacted streams and anthropogenically impacted streams in the lower Cape Fear Basin (Mallin et al. 1999c). Clearly, information is needed on the impacts of rural stormwater runoff on the concentration and loading of BOD from the various land-use types in stream watersheds of the lower Cape Fear basin.
3.4 References
Cited
APHA. 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington, D.C.
Mallin, M.A., L.B. Cahoon, M.R. McIver, D.C. Parsons and G.C. Shank. 1999a. Alternation of factors limiting phytoplankton production in the Cape Fear Estuary. Estuaries 22:985-996.
Mallin, M.A., M.H. Posey, M.L. Moser, L.A. Leonard, T.D. Alphin, S.H. Ensign, M.R. McIver, G.C. Shank and J.F. Merritt. 1999b. Environmental Assessment of the Lower Cape Fear River System, 1998-1999. CMSR Report No. 99-01, Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, N.C.
Mallin, M.A., M.H. Posey, G.C. Shank, M.R. McIver, S.H. Ensign and T.D. Alphin. 1999c. Hurricane effects on water quality and benthos in the Cape Fear Watershed: Natural and anthropogenic impacts. Ecological Applications 9:350-362.
NCDENR. 2000. Cape Fear River Basinwide Water Quality Plan. North Carolina Department of Environment and Natural Resources, Division of Water Quality, Water Quality Section, Raleigh, NC, 27699-1617.
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