Potential of largemouth bass as vectors of 137Cs dispersal.

We conducted a radio telemetry study on the movements of potentially contaminated largemouth bass between Steel Creek, a restricted access (137)Cs contaminated stream on the Savannah River Site (located in South Carolina, USA), and the publicly accessible Savannah River. Largemouth bass were relatively mobile in lower Steel Creek and the portion of the Savannah River near Steel Creek, and there was considerable movement between these two habitats. Largemouth bass had home ranges of about 500 linear meters of shoreline in the Savannah River but sometimes moved long distances. Such movements occurred primarily during the spawning season, largely upstream, and increased when water levels were changing or elevated. However, approximately 90% of the largemouth bass observations were within 10 km of Steel Creek. The total quantity of (137)Cs transported into the Savannah River by largemouth bass was much less than transported by water and suspended sediments discharged from Steel Creek. We conclude that largemouth bass from the Savannah River Site are unlikely to be responsible for long distance dispersal of substantial radiological contamination in the Savannah River.

Influences on mercury bioaccumulation factors for the Savannah River.

Mercury TMDLs (Total Maximum Daily Loads) are a regulatory instrument designed to reduce the amount of mercury entering a water body and ultimately to control the bioaccumulation of mercury in fish. TMDLs are based on a BAF (bioaccumulation factor), which is the ratio of methyl mercury in fish to dissolved methyl mercury in water. Analysis of fish tissue and aqueous methyl mercury samples collected at a number of locations and over several seasons in a 118-km reach of the Savannah River demonstrated that species-specific BAFs varied by factors of three to eight. Factors contributing to BAF variability were location, habitat, and season-related differences in fish muscle tissue mercury levels and seasonal differences in dissolved methyl mercury levels. Overall (all locations, habitats, and seasons) average BAFs were 3.7 x 10(6) for largemouth bass, 1.4 x 10(6) for sunfishes, and 2.5 X 10(6) for white catfish. Determination of representative BAFs for mercury in fish from large rivers necessitates collecting large and approximately equal numbers of fish and aqueous methyl mercury samples over a seasonal cycle from the entire area and all habitats to be represented by the TMDL.

Ecological half-lives of 137Cs in fishes from the Savannah River Site.

Ecological half-lives (Te's) were estimated for 137Cs in largemouth bass, sunfishes, and bullheads from two reservoirs and three streams on the Savannah River Site, a nuclear weapons material production facility in South Carolina. Ecological half-life is the time required for a given contaminant concentration to decrease by 50% as a result of physical, chemical, and/or biological processes that remove it from an ecosystem or render it biologically unavailable. Te's were estimated from whole-fish 137Cs concentrations in samples collected during 1972-1996, following radionuclide releases that occurred primarily during the 1960's and early 1970's. Te's ranged from 3.2 to 16.7 y, and all were shorter than expected from the half-life for radioactive decay (Tp = 30.2 y) alone. Fish taxa from the same locations differed in mean 137Cs concentrations (highest in largemouth bass and lowest in sunfishes) but, in most cases, exhibited similar 137Cs Te's. Rates of 137Cs removal in fishes were strongly correlated with rates of 137Cs removal in water. The shortest Te's occurred in the upper portions of the streams. Te's in lower portions of the streams were longer, as were Te's in one of the reservoirs. Te's in the second reservoir, which had a much shorter water residence time, were nearly comparable to those in the upper portions of the streams until 1991. At that time, 137Cs concentrations in fishes began to increase following drainage and refilling of the reservoir, which apparently resuspended 137Cs buried in the sediments.