Organochlorine compounds and trace elements in fish tissue and streambed sediment in the Mobile River Basin, Alabama, Mississippi, and Georgia, 1998

During the summer of 1998, as part of the National Water-Quality Assessment Program, a survey was conducted to determine which organochlorine compounds and trace elements occur in fish tissues and streambed sediments in the Mobile River Basin, which includes parts of Alabama, Mississippi, Georgia, and Tennessee. The data collected were compared to guidelines related to wildlife, land use, and to 1991 and 1994 National Water-Quality Assessment Program Study-Unit data.

Twenty-one sites were sampled in subbasins of the Mobile River Basin. The subbasins ranged in size from about 9 to 22,000 square miles and were dominated by either a single land use or a combination of land uses. The major land-use categories were urban, agriculture, and forest.

Organochlorine compounds were widespread spatially in the Mobile River Basin. At least one organochlorine compound was reported at the majority of sampling sites (84 percent) and in a majority of whole-fish (80 percent) and streambed-sediment (52 percent) samples. Multiple organochlorine compounds were reported at 75 percent of the sites where fish tissues were collected and were reported at many of the streambed-sediment sampling sites (45 percent). The majority of concentrations reported, however, were less than 5 micrograms per kilogram in fish-tissue samples and less than 1 microgram per kilogram in streambed-sediment samples.

The majority of trace elements analyzed in fish-liver tissue (86 percent) and streambed-sediment (98 percent) samples were reported during this study. Multiple trace elements were reported in all samples and at all sites.

Based on comparisons of concentrations of organochlorine compounds and trace elements in fish-tissue and streambed-sediment samples in relation to National Academy of Science and National Academy of Engineering and Canadian tissue guidelines, probable-effects concentrations, and mean probable-effects concentration quotients for streambed sediment, the potential exists for adverse effects to wildlife at 15 (72 percent) of the sites sampled. The potential for adverse effects at these sites is because of the presence of residues or breakdown products related to polychlorinated biphenyls (PCB’s), chlordane, dichlorodiphenyltrichloroethane (DDT), chromium, lead, and zinc.

The majority of compounds reported (65 percent) were chlordane, DDT, and PCB’s, or their breakdown products. Concentrations of chlordane and heptachlor epoxide in whole-fish tissue were positively correlated to the amount of urban land use in a basin. Total DDT concentrations in whole-fish tissues were positively correlated to agriculture.

The relation of trace elements to land use is not as clear as the relation of organochlorine compounds to land use. This lack of clarity may be due to the possibility of geologic sources of trace elements in the Mobile River Basin and to the ubiquitous nature of many of these trace elements. However, there may be a correlation between the amount of urban land use and concentrations of antimony, cadmium, lead, and zinc in streambed-sediment samples from the Mobile River Basin.

Fewer organochlorine compounds and trace elements were reported in samples from the Mobile River Basin than in samples collected during the 1991 and 1994 National Water-Quality Assessment Program studies. Of the organochlorine compounds analyzed nationally, 57 percent were reported in whole-fish tissue samples collected locally and 41 percent were reported in streambed-sediment samples collected locally, whereas 96 percent and 86 percent, respectively, were reported nationally. Of trace elements analyzed nationally, 86 percent were reported in fish-liver tissue locally and 95 percent were reported in streambed-sediment samples locally, whereas 95 percent and 98 percent, respectively, were reported nationally.

In general, concentrations of organochlorine compounds and trace elements and the frequency with which they were reported in the Mobile River Basin are similar to or less than those reported by 1991 and 1994 Study Units; however, exceptions do exist to this generality.

Exceptions are the higher reporting frequency of heptachlor epoxide, p,p'-DDT, and PCB’s in whole-fish samples, higher quartile concentrations of heptachlor epoxide and p,p'-DDT in whole-fish samples, the higher maximum concentration of heptachlor epoxide in whole-fish samples, and the higher maximum concentration and reporting frequency of trans-chlordane in streambed-sediment samples in the Mobile River Basin. In addition, aluminum, barium, chromium, cobalt, nickel, and strontium were reported more frequently in fish-liver samples locally than nationally, boron concentrations were consistently higher in fish-liver samples locally than nationally, and mercury, silver, and ytterbium were reported more frequently in streambed-sediment samples from the Mobile River Basin than nationally.

The higher reporting frequencies and concentrations of heptachlor epoxide, p,p'-DDT, and trans-chlordane in samples from the Mobile River Basin are probably due to the greater use of these compounds locally than nationally. The reason for the higher reporting frequency of PCB’s is less clear, but may be due to the production of these compounds within the basin.

The higher frequency with which some trace elements were reported in fish-liver tissue and streambed-sediment samples in the Mobile River Basin may be due to geology or anthropomorphic activities. Higher boron concentrations locally are probably due to anthropomorphic activities, because there are no major geological sources for boron in the Mobile River Basin. Potential sources locally for most of these trace elements include irrigation drain water (mercury), coal combustion (mercury and nickel), metallurgy (barium, boron, chromium, and nickel), soaps (boron), and the wood and pulp industry (barium and chromium). Other sources that could enrich samples from the Mobile River Basin are the Black Warrior coal fields (aluminum and mercury) in the northwestern quarter of the basin, which are known to have elevated levels of mercury, and barite deposits (barium) in east-central Alabama.