Savannah River Ecology Laboratory
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Contaminants in Turtles and Alligators on the SRS


Tracey D. Tuberville, David E. Scott, Brian S. Metts, and Stacey L. Lance

 
gaping alligator canoe full of sliders alligator in rad counter common snapper

Project background

Wildlife species are exposed to an increasingly complex and pervasive suite of contaminants, particularly in aquatic environments. Future management and remediation efforts for contaminated aquatic ecosystems will depend in part on predicted risks to wildlife species from contaminant exposure and bioaccumulation. Most efforts to understand their risk of exposure to contaminants has focused on relatively short-lived species (typically < 10-15 yrs). However, these models may not adequately estimate the exposure risks for species with much longer life spans. In the southeastern U.S., American alligators (Alligator mississippiensis) and several turtle species occur in aquatic systems and have estimated longevities of 40-70 years and have diets composed largely of vertebrate prey and/or invertebrates. As a result, alligators and turtles may be good ecological receptors for assessing risks associated with long-term contaminant exposure.

SRS map and example wetlands
Figure 1. Range of aquatic habitat types on SRS.

The Savannah River Site (SRS), near Aiken, South Carolina, USA, is a 800-km2 Department of Energy installation at which some localized areas contain elevated concentrations of radioactive and trace element contaminants. These areas vary in their contaminant history in terms of the timing and types of contaminants introduced into the system. For example, nickel and uranium are the primary contaminants of interest in the Tim’s Branch/Steed Pond area, radioisotopes (especially cesium-137) are of concern in impoundments in the Lower Three Runs Creek area; elements such as arsenic and selenium are elevated in the Beaver Dam Creek system; and elevated mercury is a problem across several sites where environmental conditions (e.g., low pH, high dissolved organic carbon, fluctuating water levels) favor mercury methylation. The SRS also has an incredible diversity and abundance of natural, uncontaminated wetlands, resulting in a continuum of aquatic habitats that vary in size, physical structure, hydrology, vegetation community type, degree of disturbance, and level and type of contamination (Figure 1). Thus, the SRS is an ideal setting to conduct field-based surveys and experiments of the effects of contaminants on wildlife.

American alligators and 11 species of aquatic turtles occur on the SRS. Due to their longevity and potential for chronic exposure, turtles and alligators may serve as reservoirs of contaminants, and because of their mobility and (at least in some species) use of rivers and streams as movement and dispersal corridors, they can potentially transport contaminants offsite. For example, radioactively contaminated turtles have been captured on private lands adjacent to the SRS. Both taxonomic groups are consumed by humans in surrounding communities. Indeed, several turtle species have been a source of food for years, and alligators are now legally hunted and consumed in both South Carolina and Georgia. Therefore, body burdens of certain trace elements and radiological contaminants in wildlife are of concern not only for the wildlife but for the humans that may consume them.

Previous research

Throughout the 1970s and 1980s the turtle community in PAR Pond, Pond B, and other reservoirs in the watershed was studied intensively, including estimates of numbers, demographic parameters, and body burdens of 137Cs. Cesium-137 kinetics in turtles has been well-studied by SREL researchers and the gamma emission of 137Cs allows the estimation of body burdens without destructive sampling. Total body burdens (TBB) of 137Cs were monitored in both yellow-bellied sliders and alligators captured in Par Pond and Pond B reservoirs during the 1980s, providing important historical data for these long-lived reptiles. Comparison of historical TBB with current levels in recaptured animals also provides the opportunity for monitoring changes in TBB over time.

Current research

We conducted broad-based field surveys to document current body burdens of metals and radiological contaminants in alligators and freshwater turtles across the SRS. Field sampling began in April 2010 and we sampled turtles—primarily yellow-bellied sliders—and alligators from a series of aquatic sampling sites that differed in their contaminant types, levels and spatial scales, including references sites not known to be contaminated. Alligators were captured using Murphy traps and by hand; turtles were captured using baited aquatic hoop net traps. We permanently and uniquely marked all animals, took standard morphometric measurements, and measured gamma radiation to determine 137Cs total body burden in subsamples of turtles and alligators from each location. The count data were corrected for background radiation, and these data will be used to determine tissue concentrations of radioisotopes (after adjusting for the physical decay of the 137Cs phantoms and animal geometry effects on counting efficiency). In addition, we collected whole blood, nail, and scute (alligators only) samples for analysis for a suite of metals.


Figure 2.Figure 3.

We captured 220 turtles and 162 alligators from metals-contaminated, radiologically- contaminated, and uncontaminated reference sites across the SRS. Our preliminary data suggest that 137Cs is elevated in yellow-bellied sliders (Figure 2) and alligators (Figure 3) from Pond B and Par Pond, with the highest levels documented at Pond B. In addition, we also documented elevated gamma counts in turtles from a site not previously known to us to be contaminated – the Upper A01 Wetland— our preliminary identification of the spectrum is a thorium isotope present at above background levels due to SRS operations. The primary finding in regards to metals analysis were the extremely high body burdens of selenium in yellow-bellied sliders (presented as means and 95% confidence intervals; Figure 4) and alligators (Figure 5) from the D-Area ash basin. Even though some trace element and radionuclide analyses documented elevated levels, most captured animals appear to be physically healthy. Our data, when combined with long-term mark-recapture and previously collected body burden data, offer the opportunity to study legacy effects of contaminants in aquatic systems


Figure 4.Figure 5.

Future research

Much of the research on the SRS and with reptiles in general has focused on characterizing body burdens and tissue residues of contaminants. Few studies have investigated the biological effects of these contaminants. We are currently exploring a variety of biological endpoints—including hematological profiles, hormone levels, and molecular and cellular damage—that will hopefully provide insight into individual health and fitness using non-destructive sampling techniques. We also plan to incorporate experimental manipulations to control for contaminant exposure, thereby allowing us to more closely link contaminant exposure to observed biological effects.

Acknowledgements

We would like to thank Andrew Grosse, Bess Harris, and Robert Horan for trapping and tissue collection, Judy Greene for assistance with turtle processing, and David Kling for assistance with alligator surveys. Numerous SREL technicians and interns assisted in the field and lab. In addition, Whit Gibbons and Judy Greene generously shared the long-term turtle datasets, and we thank I. Lehr Brisbin and Bobby Kennamer for sharing long-term alligator datasets. Funding was provided by the Area Completions Project of Savannah River Nuclear Solutions, and work was partially supported by the Department of Energy under Award Number DE-FC09-07SR22506 to the University of Georgia Research Foundation.