Organochlorine Compounds in Fish Tissue in the Lower Mississippi River

INTRODUCTION

    The Mississippi River and its tributaries drain nearly 40 percent of the continental United States. Inputs to the river system come from a wide variety of sources including extensive agricultural areas and large metropolitan areas. All of these inputs influence the water quality of the Mississippi River and the organisms which inhabit the river system.
    As part of the National Water-Quality Assessment (NAWQA) program, fish tissue samples were collected at several sites within the Yazoo River Basin. More than half of this 13,000-square-mile basin is an intensively cultivated area, with the primary crops being cotton, soybeans, rice, and corn. This basin represents the largest drainage into the Mississippi River from the east, south of the confluence of the Ohio River. In addition, fish tissue samples were collected from eight sites on the mainstem of the Mississippi River for a NAWQA special study.
    Fish tissue is often used to document the occurrence of hydrophobic compounds, including many pesticides, because fish lipids bioaccumulate these compounds and because the fish integrate the presence of contaminants over time. Previously, fish tissue has been collected at sites within the Yazoo River Basin by state and federal agencies (Mississippi Bureau of Pollution Control, 1984 and 1992; Schmitt and others, 1990), as well as by this NAWQA project (Plunkett and others, 1997). These data have shown that total DDT and toxaphene concentrations in the fish tissue collected from sites within the Yazoo River Basin are among the highest levels found in the United States.

Figure 1. Location of sampling sites on the Mississippi and Yazoo Rivers.

Figure 2. Electrofishing for carp on the Mississippi River.

The purpose of this study was two-fold: (1) to measure concentrations of organochlorine contaminants in fish tissue in the Mississippi River; and (2) to determine if these concentrations were affected by contributions from the Yazoo River Basin. The second objective was addressed by collecting fish tissue samples in the Mississippi River upstream and downstream from the confluence of the Yazoo River.
The authors would like to acknowledge Al Gibson and David Locke, Mississippi Department of Environmental Quality, for their assistance in the collection of the fish samples.

METHODS

    Site selection. Eight sites were chosen for fish collection in the Mississippi River. The sites were located from Mississippi River Mile 495 near Mayersville, Mississippi, to Mississippi River Mile 378 near Waterproof, Louisiana (figure 1). The confluence of the Yazoo River, north of Vicksburg, Mississippi, was the approximate center of the sampling range. Sites 1 through 4 are located upstream from the Yazoo River confluence, whereas sites 5 through 8 are downstream from the confluence. Fish samples were also collected from the Yazoo River near Long Lake, Mississippi.
    Sample collection. The fish were collected by electrofishing between November 19 and 22, 1997 (figure 2). Each sample consisted of a 5 to 8 whole-fish composite of common carp (Cyprinus carpio). Each fish was weighed, standard and total length were measured, sex was determined, and scales were removed for age determination. The fish were individually wrapped in aluminum foil and shipped to the laboratory on dry ice. Duplicate samples were collected at sites 2 and 5. Physical parameters were measured at each site.
    Sample Analysis. Analysis of the samples was performed by the U.S. Geological Survey National Water Quality Laboratory. The composite samples were homogenized and subsampled. Chemical procedures employed included Soxhlet extraction and fractionation followed by analysis by dual capillary column GC/ECD (Leiker and others, 1995). The data presented in this paper have not been age or lipid normalized.

RESULTS

    Physical Parameters. During the sampling period the water temperature of the Mississippi River averaged 11.3°C, and the air temperature averaged 18°C. The average values for pH, specific conductance, dissolved oxygen and dissolved oxygen, saturation were 7.7, 358 mSiemens, 9.9 mg/L, and 90.6 percent, respectively.
    Fish Characteristics. The 75 carp collected had an average weight of 3,035 g. Site averages varied from 2,398 g at site 4 to a high of 3,794 g at site 8. The total length of the fish averaged 576 mm. The mean age of the fish was 4.5 years, ranging from an average of 4.2 years at site 3 to 5.1 years at site 1. The sample contained 41 percent females, with site 8 having only 12.5 percent females while site 6 contained 75 percent females. The correlation between weight or age and pesticide concentration was not significant.
    Organochlorine Results. The primary contaminants found in this study were chlordane, dieldrin, PCBs and DDT metabolites. Concentrations for these compounds for each site are presented in figures 3 through 6. Aldrin, penta-chloroanisole, oxy-chlordane, cis-nonachlor, p,p' methoxychlor, lindane, alpha-, beta-, and delta-BHC, heptochlor epoxide, heptachlor, endrin and DCPA were below laboratory reporting levels in all samples. Toxaphene, trans-nonachlor, mirex, and hexa-chloro-benzene were found in concentrations greater than reporting levels at only one or two sites. Duplicate samples from sites 2 and 5 yielded very similar results.

DISCUSSION

Toxaphene. Toxaphene was measured at 1,600 mg/kg at the Yazoo River site, the only site where the concentration was greater than the laboratory reporting level. Other fish tissue samples taken from stations farther upstream in the Yazoo River Basin have yielded even higher toxaphene concentrations (Plunkett and others, 1997.) These results indicate that although toxaphene persists in the environment near the areas of application, it does not appear to be leaving the Yazoo River Basin in quantities sufficient to influence fish tissue concentrations in the Mississippi River.
Total Chlordane. In the early 1980's there were concerns about chlordane in the Mississippi River south of Memphis, Tennessee (Mississippi Bureau of Pollution Control, 1984). Data from this NAWQA study show that total chlordane persists in the Mississippi and Yazoo Rivers below the 24 mg/kg level (figure 3).

Figure 3. Concentrations of trans- and cis- chlordane.

Dieldrin. Dieldrin was found at low concentrations (8 to 48 mg/kg) at all sites by our study (figure 4) and by the other previously cited researchers. Its common occurrence prevents any detailed discussion about its source.

Figure 4. Concentrations of dieldrin.

Total PCB. All of the samples in this study had detectable levels of PCB (figure 3). Results were higher than the Mississippi River samples of Leiker and others (1991), but consistent with those of Schmitt and others (1990). The PCB's appear ubiquitous, but some of the lowest concentrations found were in the Yazoo River samples, indicating that the Yazoo River Basin is not a likely source of these materials.

Figure 5. Total PCB concentrations.

DDT and its metabolites. Our study, and studies by Schmitt and others, (1990), and by the Mississippi Bureau of Pollution Control (1984) have found total DDT levels much greater in fish tissue in the Yazoo River and other Yazoo River Basin sites than at sites in the Mississippi River (figure 5). There is little difference among the total DDT concentrations Schmitt and others (1990) found in fish collected from fish in the Yazoo and Mississippi Rivers in 1985, concentrations Leiker and others (1995) found in fish in the Mississippi River collected in 1987, or our samples collected from both rivers in 1996, demonstrating a persistence of DDT in the environment more than 25 years after the cessation of the use of DDT for insect control in the United States. However, there is no indication that total DDT levels in fish in the Mississippi River upstream from the Yazoo River are different than levels in fish living downstream from the Yazoo River influence.

Figure 6. Concentrations of DDT and its metabolites.

CONCLUSIONS

Total PCB's, dieldrin, and chlordane persist in fish tissue at low levels at most of the Mississippi River and Yazoo River sites. The occurrence of these compounds is ubiquitous in our study area.
Concentrations of total DDT in fish tissue in the Mississippi River have not appreciably changed from those reported during the 1980's, and are much lower than concentrations in the Yazoo River Basin.
The concentrations of total DDT and toxaphene, are not measurably higher in Mississippi River fish downstream from the Yazoo River confluence, compared with concentrations in fish from upstream sites. These results indicate that these organochlorine compounds do not appear to be leaving the Yazoo River Basin in quantities sufficient to influence fish tissue concentrations in the Mississippi River. This also suggests that these compounds are not sufficiently mobile to be rapidly reducing contamination levels in the Yazoo River Basin.

REFERENCES

Leiker, T.J. , Rostad, C.E., Barnes, C.R., and Pereira, W.E. 1991. A reconnaissance study of halogenated organic compounds in catfish from the lower Mississippi River and its major tributaries. Chemosphere. 23:817-829.

Leiker, T.J., Madsen, J.E., Deacon, J.R. and Foreman, W.T. 1995. Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of chlorinated pesticides in aquatic tissues by capillary column gas chromatography with electron capture detection. U.S. Geological Survey Open-File Report 94-710, 42 pp.

Mississippi Bureau of Pollution Control. 1984. Ambient Biological Monitoring Report 1983-1984 104 p.

Plunkett, M.L., Morris III, F., Oakley, W.T. and Turnipseed, D.P. 1997. Mississippi Water Resources Data-- Mississippi, Water Year 1996: U.S. Geological Survey Water-Data Report MS-96-01, 324 p.

Schmitt, C.J., Zajicek, G.L., and Peterman, P.H. 1990. National Contaminant Biomonitoring Program: Residues of organochlorine chemicals in U.S. freshwater fish, 1976-1984. Archives of Environmental Contamination and Toxicology. 19:748-781.