Twelve Mile Creek

The Twelve Mile Creek-Lake Hartwell system in northwest South Carolina provides an excellent opportunity to examine the presence and relative... 

The plant was discharging PCB-laden effluent into the creek via two serial settling basins. Sediment samples in these basins were found to have average PCB concentrations of 27,300 JLg/g in the upper basin and 7970 xg/g in the lower basin (II). The average total PCB concentration of the effluent in 1976 was 30 xg/L. Subsequent studies of sediments in Twelve Mile Creek and Lake Hartwell showed maximum concentrations in excess of 150 xg/g (dry weight basis) (12). Total PCB concentrations were highest immediately downstream of the source and decreased with distance (11-14). 

Differences in the hydrologic characteristics for the upper and lower portions of Twelve Mile Creek and for Lake Hartwell are believed to have a significant effect on physiochemical and biochemical weathering processes. The upper portion of Twelve Mile Creek (0-32 km) is characterized by relatively shallow waters and high water velocities. In the lower portion (32-39 km), the creek deepens and widens because of the impoundment of Lake Hartwell. Water velocities are much slower in this portion of the creek, and a significant portion of the solids load is deposited in the sediments. Flow from Twelve Mile Creek eventually enters the top of Lake Hartwell, where it mixes with uncontaminated waters from the Keowee River. The waters then move slowly down this 50-km-long reservoir toward the Hartwell Dam. 

Figure 1. Map of the Twelve Mile Creek-Lake Hartwell system showing sample sites and boundaries of the...

Figure 2. Weight percent (peak mass I total mass X 100%) of each peak for an 80% 1016/20% 1254 standard Aroclor mixture and for the top section office Twelve Mile Creek—Lake Hartwell sediment cores. Sample location distances from the PCB source and weight-percent summaries for peaks 1-22 and 25-64 are given. Some peaks are not quantified because of chromatographic interferences (GC peaks 24, 27, and 64) or lack of analytical sensitivity (GC peaks 45, 48, 55, 56, 59, and 62). Peak 15 coelutes with peak 14. Peak 23 is the internal standard aldrin.

Figure 5 shows a more detailed analysis of variations of homologous chlorine groups with depth in sediment. Again two seemingly different systems are revealed for the lower portion of Twelve Mile Creek and for Lake Hartwell. The Twelve Mile Creek samples (G26 and G33) showed DCB... 

Figure 3. Weight-percent data for DCBsy TrCBs, TCBs, and PHHCBs versus distance from the PCB source for the top sections of six Twelve Mile Creek-Lake Hartwell sediment cores. Peaks containing two or more congeners were assigned to a homologous chlorination group on the basis of which congener accounted for most of the total peak mass (as reported in references 25 and 26). Peaks that could not be accurately classified (26, 28, and 31) were...

Figure 4. Weight-percent data for each peak for three sections of two representative cores, G26 (for the lower portion of Twelve Mile Creek) and G56 (for Lake Hartwell). Depth in core and weight-percent summaries for peaks 1-22 and 25-64 are given. Figure 2 gives sample locations and identification of peaks not quantified and sample locations.

Steady-state modeling calculations were performed to examine how congener-specific properties (such as sediment-water partition coefficients, Henry s law constants, and molecular diffusion rates) affect the transport and fate of PCBs. A basic description of the model, along with modeling results, is presented here to further explain the importance of physiochemical weathering processes in controlling the fate and distribution of PCB congeners in Twelve Mile Creek and the upper portion of Lake Hartwell. 

Background Information. For model calculations Twelve Mile Creek is divided into an upper and lower reach (as shown in Figure 1). In... 

The lower reach of Twelve Mile Creek empties into the upper portion of Lake Hartwell, which is described as a series of three mixing basins (see Figure 1). The uppermost basin, which receives inflows from both Twelve Mile Creek and the Keowee River, is physically separated from the second basin by a submerged sill under the U.S. Highway 123 bridge in Clemson, South Carolina. The average annual flow in the Keowee River is approxi-... 

Model Description. The transport and fate of PCBs in Twelve Mile Creek and the upper portion of Lake Hartwell are described by a series of mass conservation equations for solids and PCBs in the water column and in the active sediment layer (given as the top 10 cm of sediment) following the approach described by O Connor (20-22). For solids, the equations for the water column and active sediment layer are given as ... 

For transport terms in eqs 1 and 3, the upper and lower reaches of Twelve Mile Creek were treated as one-dimensional advective (plug flow) systems. The upper basin of Lake Hartwell was modeled as a series of completely mixed reactors, with inflows into the first reactor from Twelve Mile Creek and the Keowee River. Analytical solutions for steady-state distributions of solids and PCB congeners in the water column and the active sediment layer were determined by using approaches outlined by O Connor (20-22). Because Kd values for a specific congener class were taken to be equivalent for the water column and active sediment layer, the solutions are independent of k[ values. 

Figure 7. Model results for PCB particulate concentrations in Twelve Mile Creek-Lake Hartwell sediments versus distance from the source.

Further studies examining time-variable behavior of PCBs in the Twelve Mile Creek-Lake Hartwell system and sensitivity of model calculations to various system parameters are presently being performed. The steady-state modeling results presented in this chapter, however, provide a reasonable base for an initial assessment of the fate of PCBs in the Twelve Mile Creek-Lake Hartwell system. The cumulative removals of PCBs from the system by volatilization and burial are shown as percents of the total PCB... 

Figure 9. Model results for PCB removal by volatilization and burial fluxes in Twelve Mile Creek and the upper portion of Lake Hartwell.

This finding, which is consistent with our analyses of sediment cores, indicates that the lower reach of Twelve Mile Creek serves as an effective... 

Similar results for biodegradative and nonbiodegradative zones were previously reported by Bopp et al. (4) for the Hudson River. Considering the occurrence of altered residues in the more highly contaminated upper Hudson River sediments, Bopp et al. (4) concluded that reductive dechlorination may not be significant at PCB concentrations less than approximately 150 ig/g. This value is much higher than the maximum concentrations seen in both the Lake Hartwell (16 and 7 xg/g) and Twelve Mile Creek (26 and 40 ig/g) cores. 

Steady-state model calculations were performed to further examine physiochemical weathering behavior. Results were consistent with congener distributions in surficial sediments. In general, they showed that a preferential depletion of the lower chlorinated congeners occurred in the upper portion of Twelve Mile Creek, where volatilization was the only removal mechanism. In the lower portion of Twelve Mile Creek and the upper portion of Lake Hartwell, burial of PCBs in deeper sediments played a more important role. A preferential depletion of higher chlorinated congeners occurs when burial is the dominant process by physiochemical weathering. 

From this initial assessment, approximately 65% of the PCB input is calculated to have been removed from the system by volatilization and 15% of the input by burial in deeper sediments, primarily in the lower portion of Twelve Mile Creek. As indicated most clearly by higher levels of specific ortho-substituted DCBs, TrCBs, and TCBs, PCBs buried in the deeper sediments of the creek have undergone further weathering by reductive dechlorination. 

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