RESULTS AND DISCUSSION:

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Polynuclear aromatic hydrocarbons, phthalate esters, and PCBs-

The range of concentrations for selected SVOCs, including PAHs, phthalate esters, and PCBs, from the Barron River Canal and Turner River sites is given in Table 3. Concentrations were below aquatic-life criteria. However, bis(2-ethylhexyl) phthalate (south of I-75) and butylbenzyl phthalate (Turner River) approached within 94 and 70 %, respectively, of the aquatic-life criteria summarized by Gilliom and coworkers (1998). Mixtures of SVOCs and other contaminants found at some sites could behave synergistically to cause adverse biological effects that are not indicated by criteria developed for each individual contaminant (USGS, 1999).

The six most frequently-detected SVOCs, normalized to organic carbon, are plotted against distance along the Barron River Canal to assess potential local sources (Fig. 5). The ratio for bis(2- ethylhexyl)phthalate peaked at the site just south of I-75, indicating a potential source for this widely-used plasticizer. Other patterns in the ratios of SVOCs are difficult to interpret as discussed below. The ratio for bis(2-ethylhexyl)phthalate also varied inversely with the ratio for p-cresol, a disinfectant, fungicide, and insecticide. Another plasticizer, butylbenzyl phthalate, varied in the canal between nearby sites with similar concentrations of organic carbon, and the highest ratio was at the Turner River background site (also shown on Fig. 5 at -5 km). Normalizing trace organics to organic carbon helps to minimize some variations in the effects of sediment composition, but it does not compensate for differences in the size of the organic particles, the chemical composition of the organic matter, or the residence time of sediment at a site, and all of these can have an effect on retention of contaminants in addition to any local inputs of contamination. In addition, approximately two thirds of the concentrations used to compute the ratios were near or below the method reporting levels; therefore, precision is low and may contribute to variability in Fig. 5.

When concentrations of SVOCs were plotted against the concentration of organic carbon in bottom sediment, two groups of compounds emerge with different distributional patterns (Fig. 6). In both groups of compounds, the concentration of SVOCs generally increased with increasing organic carbon content. However, concentrations of the PAH and cresol group (pyrene, 2,6-dimethylnaphthalene, and p-cresol) at the Turner River background site fall well below the trend line for the Barron River Canal samples, whereas concentrations of the esters [bis(2-ethylhexyl)phthalate, di-n-butyl phthalate, and butylbenzyl phthalate] at the Turner River are in line with the general trend in the Barron River Canal samples. Although there were fewer reportable values for fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[k]fluoranthene, benzo[g,h,i]perylene, phenanthrene, phenol, acenaphthylene, anthracene, benzo[a]anthracene, chrysene, and benzo[b]fluoranthene; plots (not shown) of these compounds against organic carbon concentrations were similar to those for pyrene, 2,6-dimethylnaphthalene, and p-cresol shown in Fig. 6. Except for the relatively high (2,500 µg/kg) concentration of bis(2-ethylhexyl)phthalate in the Barron River Canal south of I-75, the highest concentration of the phthalate esters occurred at the Turner River background site that had the highest percentage of fine sediment (74 % less than 63 micrometers) and highest concentration (42 %) of organic carbon. Local inputs of contaminants are probably minor at this background site.

We interpret the different distributional patterns and behavior between the phthalate esters compared to the PAHs and p-cresol to mean that the phthalate esters have a general and widespread source such as atmospheric precipitation or atmospheric transport of phthalate ester vapors and that the PAHs and p-cresol have a local source that does not affect the Turner River, such as State Road 29 (SR-29) that is adjacent to the Barron River Canal along its entire length. Possible source of the PAHs and p-cresol associated with SR-29 could be from leakage of vehicle fluids, vehicle exhaust, or leaching of the roadbed materials such as asphalt. Kriech (1990) suggests that leaching of asphalt road surfaces are not significant sources of trace elements and SVOCs, but that crankcase oil and tire wear material from vehicles are the potential sources of some SVOCs such as naphthalene and phenanthrene.

The association of high PAH concentrations with roadways or traffic is indicated from a bottom-sediment sample collected 23 km east of the Barron River Canal at the heavily traveled Tamiami Trail (US-41) Bridge 105 site in August 1996. This site had the highest concentrations of pyrene (640 µg/kg), benzo(a)pyrene (690 µg/kg), benzo(k)fluorene (580 µg/kg), benz(a)anthracene (490 µg/kg), and chrysene (540 µg/kg) of 7 southern Florida National Water-Quality Assessment sites (R.L. Miller, U.S. Geological Survey, unpublished data) and of the Turner River and 10 Barron River Canal sites.

Figure 7. Ratios of concentrations of selected polynuclear aromatic hydrocarbons to concentrations of organic carbon in bottom sediments along the Barron River Canal and the Turner River, October 1998. [larger image]

The high concentrations of PAH contaminants (up to 50,000 µg/kg) reported by Law Engineering and Environmental Services (1993) for bottom sediments in the Jerome area of the Barron River Canal were not evident in our Jerome bottom sediment samples or in samples downstream of Jerome. The high values reported by Law Engineering and Environmental Services were from samples collected in the Barron River Canal just upstream of the Jerome borrow pit and were confined to a length of less than 300 m of the canal. Our Jerome samples were collected in the canal just downstream of the borrow pit mouth. Although our research did not detect the high PAH concentrations reported by Law Engineering and Environmental Services, we did see higher values for normalized PAH concentrations (for the 6 compounds measured at high concentrations by Law Engineering and Environmental Services) at Jerome and downstream of Jerome than upstream (Fig. 7). In addition to the difference in sampling location, the lower concentrations in our Jerome samples compared with those of Law Engineering and Environmental Services may result from the downstream migration of contaminants, dilution by mixing of sediments, and degradation of these compounds over time. Fluoranthene and pyrene had the highest ratios to organic carbon about 8 km downstream of Jerome perhaps due to downstream migration. Smaller peaks for these and other PAHs occurred at Jerome and at CR-858 (Fig. 7).