3.0 Suspended Particulate Matter Characteristics in the
Lower Cape Fear River System, 1998-1999

Lynn A. Leonard

3.1  Background
   Suspended particulate matter (SPM) is an important component of water quality. Usually measured as turbidity (inorganic plus organic suspended matter), the presence of SPM diminishes water quality by reducing the light available to aquatic vegetation, and by providing a substrate for the transport of phosphate, ammonium, heavy metals, and some pathogenic bacteria. The Cape Fear River regularly experiences periods of high sediment loading during which turbidity levels exceed the North Carolina state standards of 25 NTU for estuarine waters and 50 NTU for freshwater (Mallin et al. 1997). In the river, areas characterized by high turbidity have also displayed elevated levels of ammonium and phosphorus (Mallin et al. 1997); both of which are associated with sediments and may be recycled from the sediments (Day et al. 1989).
    Previous research suggests that simple measures of turbidity are insufficient to characterize the relationship between SPM and other water quality parameters (e.g. heavy metals, nutrients, pathogens) especially following rain run-off events (e.g. Rao et al. 1993). Certain sediment attributes, specifically particle composition, may exert considerable control over the availability and transport of factors deleterious to water quality. The presence of excess organic matter, for example, can contribute to anoxic conditions in the water column. SPM containing high concentrations of organic matter and certain clay minerals (e.g. montmorillonite with a high cation exchange capacity) has the ability to sorb significant quantities of chemical components. Clays and organic floccules (common constituents of fluvial SPM) tend to concentrate in the smaller size fractions (Horowitz et al. 1990) and adsorb relatively more contaminants per unit volume or mass due to their relatively larger surface-to-mass ratios. Because of the large surface areas of silts, clays, and organic matter and the scavenging nature of oxides, SPM is also the major transport medium for heavy metals in rivers (Louma 1989, Sinclair et al. 1989).
    The enhancement of bacterial sorption in clay-rich soils relative to those with a low clay content is also well documented (Burton et al. 1987; Lebo 1991). Some viruses, such as bacteriophage, also tend to be attracted to certain soil constituents. The latter results from the interaction between the surface charges on the bacteriophage and the clay and organic matter (Gerba et al. 1981). The interaction between particle composition and chemical/biological sorption is especially important in fluvial systems where the presence of particles with diameters less than 70 micrometers can transport significant loads of bacteria even during moderate stream-flow conditions (Rao et al. 1993). As a result, sorption to clay or organic SPM may facilitate the widespread distribution of pollutants in a watershed.
    In August of 1998, the Coastal Sedimentology Laboratory of the UNCW Center for Marine Science Research began a long-term sampling effort in the Cape Fear River as part of the basic monitoring effort associated with the Cape Fear River Program. The long-term objectives of this study are: 1) to determine SPM concentrations (organic plus inorganic fraction) and SPM compositions (organic content and mineralogy) at sites where other physical and chemical parameters are monitored on a monthly basis, 2) to relate SPM levels to tidal and meteorological phenomenon, 3) to relate SPM concentration and composition to the distribution of excess nutrients and pathogens in the watershed, and 4) to identify, when possible, potential sources of SPM (via clay mineralogy) in the watershed that may explain seasonal and aperiodic changes in turbidity. This report will present the baseline SPM data and will summarize the preliminary results of this study after one year of data collection. Analyses of seasonal patterns, long-term trends require additional data and will be the focus of the ongoing work effort. In addition, future work will include a detailed mineralogic study of the clay component in order to identify source areas of the SPM comprising non-point source runoff. Identification of key index minerals will be used to target potential watershed sources of SPM and increase the potential for identification of particulate loading point and non-point sources.

3.2 Methodology
   Two 1-liter water samples were collected monthly on an ebbing tide at the following lower river and estuary sites in the Cape Fear River: NC11, NAV, M42, M54 and M18 (Figure 3.1). The mid-estuarine sites have traditionally experienced high levels of turbidity which have been positively correlated with total nitrogen and total phosphorus (Mallin et al. 1998). Additional samples were collected at B210 and NCF117 in the Black and Northeast Cape Fear Rivers, respectively, for comparative purposes. Each month, samples were collected manually from the upper 50 cm of the water column, stored on ice, and returned to the CMSR for analyses.
    In the laboratory, samples were filtered through pre-weighed, pre-combusted, 1 mm, glass fiber filters and rinsed with deionized water. Samples were dried overnight at 60 degrees Celsius, reweighed, and concentrations determined in mg/l. Filtered samples were then combusted at 450 degrees Celsius for 4 hours to determine organic content. Mineralogies of selected oxidized, uncombusted samples were determined through a combination of microscopic inspection/identification and X-ray diffraction (Bish and Reynolds 1989) using a Philips XRG 2600 X-Ray diffractometer housed in the Department of Earth Sciences.
    Meteorological data were obtained from the State Climate Office of North Carolina and the United States Geological Survey. The rain data presented in this report were collected at the Greensboro Airport, Greensboro, North Carolina and represent the cumulative total rainfall in inches for the thirty days preceding sample collection. These data were selected in order to evaluate the cumulative effect of rainfall and runoff in the upper reaches of the Cape Fear on sediment transport in the lower river basin. Provisional stream flow data for the Cape Fear River at Lock Number 1 were provided by the United States Geological Survey and reported as daily mean values in cubic feet per second.

3.3 Results and Discussion

Suspended Particulate Matter Concentrations
    Monthly total suspended particulate matter (SPM) concentration, organic SPM concentration, inorganic SPM concentration, and percent organic content for each sampling station are shown in Table 3.1. Total SPM concentrations ranged from less than 1 mg/l to as much as 111 mg/l with the greatest concentrations occurring at estuarine (M18, M42, M54) and main stem (NAV, NC11) stations. At these stations, total SPM (inorganic + organic) concentrations were typically less than 40 mg/l throughout the year (Figure 2a). Significantly higher concentrations were observed at NC11 in January (see Table 3.1) following 6.31 inches of recorded rainfall at Greensboro in the thirty days prior to sample collection. SPM concentrations at the Black River (B210) and Northeast Cape Fear River (NC117) sites were extremely low compared to those recorded at other sites (Figure 3.2a). Total SPM concentrations at B210 and NC117 did not exceed 10 mg/l over the sampling period. The lower SPM concentrations observed at these sites resulted from the absence of high-density inorganic clays and the presence of abundant, low density, organic particulate matter. At sites NC117 and B210, the mean annual percent organic contents of SPM were 85 and 97 percent, respectively (Figure 3.2b). These values contrast with annual percent organic means of 26 to 30 percent at estuary stations and 35 to 37 percent at non-estuarine main-stem stations (NAV and NC11).
    Concentrations of inorganic and organic components of SPM for each sampling site are shown in Figure 3.3. As mentioned earlier, SPM at the two blackwater sampling sites (NC117 and B210) consisted almost exclusively of organic material, but in very low concentrations. In contrast, inorganic particulates comprised the bulk of SPM at both the mainstem and estuarine sites. The total concentrations of organic SPM at the estuarine and main stem sites were slightly greater than those observed at NC117 and B210 during the sampling period and may be explained by phytoplankton biomass levels in the river. In past years, the lowest mean phytoplankton biomass concentrations have occurred at the two blackwater stations (Mallin et al. 1998). Primary productivity at these sites is thought to be limited by lower inorganic nutrient concentrations and the presence of darkly colored water. Although conditions are somewhat more favorable to phytoplankton production in the estuary, high turbidity levels associated with periods of high flow and efficient flushing appear to inhibit primary productivity (Mallin et al. 1998) thereby reducing the organic content of SPM.
    The concentration of inorganic SPM in the river (Figure 3.4a and b) did not display any particular seasonal pattern between August 1998 and June 1999. These results are consistent with turbidity data collected in previous monitoring years (Mallin et al. 1998). Inorganic SPM levels at NAV and NC11 were highly variable ranging from a minimum of near-0 mg/l at NAV in October to a maximum of 99 mg/l at NC11 in January. Excessive SPM concentrations at NC11 appear to be strongly influenced by rain induced runoff and will be discussed in a separate section. Turbidity levels at NAV may also be influenced by runoff originating in the upper watershed. The impact of upper watershed runoff at NAV, however, is likely mediated by mixing processes and inputs from black water tributaries unique to this site (Mallin et al. 1998).
    Inorganic concentrations near the river mouth (M18) were frequently equal to or greater than concentrations observed near the region of the estuarine turbidity maximum (ETM). Total suspended solid data collected during the 1997-98 monitoring effort (Mallin et al. 1998), indicated that the highest turbidity levels and greatest inorganic SPM concentrations would coincide with the ETM at site M54. It is possible that this year's SPM data were affected by tidal hydrodynamic processes unaccounted for during the present sampling effort. Although samples were routinely collected during ebbing tides, samples were not collected during tides of similar amplitude. Consequently, changes in current dynamics associated with the fortnightly cycle of the tides and/or position in the estuary may account for the apparent disagreement and should be addressed in future studies.
    Unlike the inorganic data, the concentration of organic SPM in the estuary varied somewhat systematically (Figure 3.4c). Although the concentrations were regularly less than 8 mg/l, organic SPM concentrations appeared to reach maxima in late summer and early spring and minima in early winter and late spring. Although additional yearly data must be collected to establish the statistical validity of these trends, these patterns appear to be consistent with phytoplankton biomass levels measured in the river since 1995. While the Cape Fear does not experience extended algal blooms (Mallin 1994), data from previous monitoring years have shown that phytoplankton biomass levels have been considerably lower during the fall and early winter than during the summer months (Mallin et al. 1998). In addition, spring blooms in February or March are known to occur and may account for the early spring increase in organic SPM observed in the estuary during this study. No seasonal trend in organic SPM could be discerned for the non-estuary stations.

Effect of Rainfall
    Turbidity levels in the Cape Fear River at site M42 have been linked to rainfall in the upper watershed when excessive rain promotes high flow rates in the river proper (Mallin et al. 1998). As a result, watershed rainfall is believed to be an important physical forcing mechanism leading to increased particle delivery to the estuary and subsequent increases in light attenuation. During this study, however, a clear relationship between rainfall and SPM in the estuary could not be discerned. As discussed earlier, the highest SPM concentration recorded during the period of measurement occurred at site NC11 (111 mg/l) in January following a period of high rainfall in the upper watershed. SPM levels at M42, meanwhile, were substantially lower (8 mg/l). One possibility is that rainfall totals in excess of one inch in other portions of the watershed in the three days prior to sampling may have resulted in an increased contribution of low turbidity water from blackwater tributaries below site NAV. Consequently, concentrations at NAV and NC11 were high while concentrations at estuarine sites M54 and M42 were not elevated. Given this and the potential importance of unmeasured tidal parameters, NC11 was chosen to examine the potential effect of rainfall on SPM constituent levels in the river.
    Correlation analyses between rainfall, total SPM, inorganic SPM and organic SPM were conducted using the entire data set from August 1998 to June 1999. As has been the established procedure in previous LCFR monitoring reports, rainfall totals recorded at the Greensboro Airport for the month preceding sampling were used in the analyses. Both the inorganic and organic components of SPM were positively and significantly correlated with rainfall at NC11 (r = 0.773; p = 0.005 and r = 0.627; p = 0.038, respectively). Total SPM was also significantly correlated with rainfall at this site ( r = 0.77; p = 0.005). Although organic SPM levels increased with rainfall, approximately 90% of SPM consisted of inorganic material following periods of excessive rain. Therefore, it is evident that efforts to control non-point source runoff contributions to the lower Cape Fear River should focus primarily on retaining the inorganic fraction of SPM.

Mineralogy
    The high concentrations of SPM at NC11 in January also provided the opportunity to examine the mineralogic composition of the inorganic component. Figure 3.5 shows the results of X-ray analysis of mineral peaks for SPM collected at NC11. This sample consisted of various clay minerals (chlorite, smectite, illite, kaolinite), quartz and pyrite. This mineral assemblage is consistent with one expected for runoff derived from Piedmont and upper Coastal Plain soils; thereby reinforcing the sedimentological linkage between the lower Cape Fear River and its mid and upper components. SPM collected at M18 (lower estuary), in contrast, consisted primarily of silt-sized quartz and calcite. Presumably, the relatively higher density and more chemically reactive clay minerals present at NC11 are removed from suspension via physical and chemical mechanisms at the ETM prior to reaching the lower estuary.

3.4 Summary
   SPM data collected in the lower Cape Fear River indicate that precipitation and season interact to control the composition of material being exported to the coastal ocean. Positive correlations between rainfall in the upper watershed and the various components of SPM in the Cape Fear mainstem suggest that runoff from the Piedmont contributes to sediment loading in the estuary. The mineralogy of suspended sediments at NC11 is consistent with an expected composition for Piedmont soils thereby also suggesting a Piedmont source for the inorganic component. The low inorganic SPM concentrations and high organic percentages observed at N117 and B210 further indicate that little particulate matter enters the lower Cape Fear River from these blackwater systems. Tidal processes and seasonal variations in primary productivity may mediate SPM concentration and composition in the estuary. Efforts to control particulate loading in the lower Cape Fear River should begin in the upper watershed and should focus on the retention of inorganic components.

3.5 References

Bish, D.L. and R.C. Reynolds, Jr. 1989. Sample preparation for X-ray diffraction. In: E.L. Bish and J.E. Post (eds.), Modern Powder Diffraction. Reviews in Mineralogy, 20: 73-97.

Burton, G.A., Jr. D. Gunnison, and G.R. Lanza. 1987. Survival of pathogenic bacteria in various freshwater sediments. Applied Microbiology 53:633-641.

Day, J.W. Jr., C.A.S Hall, W.M. Kemp and A. Yanez-Arancibia. 1989. Estuarine Ecology. John Wiley and Sons, New York, NY.

Gerba, C.P., S.M. Goyal, I. Chech, and G..F Bogdan. 1981. Quantitative assessment of the adsorptive behavior of viruses to soil. Environmental Science and Technology, 15: 940-952.

Horowitz, A.J., F. Rinella, P. Lamothe, T. Miller, T. Edwards, R. Roche, and D. Rickert. 1990. Variations in suspended sediment and associated trace element concentrations in selected riverine cross sections. Environmental Science and Technology 24:1313.

Lebo, M.E. 1991. Particle-bound phosphorus along an urbanized coastal estuary. Marine Chemistry 34:225-246.

Luoma, S.N. 1989. Can we determine the biological availability of sediment-bound trace elements? Hydrobiology, 177: 379-394.

Mallin, M.A. 1994. Phytoplankton ecology of North Carolina estuaries. Estuaries 17:561-574.

Mallin, M.A., M.H. Posey, M.L. Moser, G.C. Shank, M.R. McIver, T.D. Alphin, S.H. Ensign and J.R. Merritt. 1997. Environmental Assessment of the Lower Cape Fear River System, 1996-1997. CMSR Report No. 97-01. Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, NC.

Mallin, M.A., M.H. Posey, M.L. Moser, G.C. Shank, M.R. McIver, T.D. Alphin, S.H. Ensign and J.R. Merritt. 1998. Environmental Assessment of the Lower Cape Fear River System, 1997-1998. CMSR Report No. 98-02. Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, NC.

Rao, S.S., B.J. Dutka, and C.M. Taylor. 1993. Ecotoxicological implications of fluvial suspended particulates. In: SS Rao (ed.) Particulate Matter and Aquatic Contaminants. Lewis Publishers, Boca Raton, FL pp. 157-161.

Sinclair P., R. Beckett, and B.T. Hart. 1989. Trace elements in suspended particulate matter from the Yarra River, Australia. Hydrobiology. 177: 239-248.

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