Radioecology is the study of the environmental effects of radioactivity, but also includes the use of radioactive isotope (radionuclide) tracers for basic ecological research. My research in this area includes the relationships between energy metabolism and radionuclide uptake by animals, comparative studies of radionuclide elimination, how organismal physiology may be adapted to reclaming radioactively contaminated areas, and determining effects of ionizing radiation.
I first became involved in radioecological research in 1985 (one year before the Chornobyl nuclear accident), as a Master's student at the University of Georgia's Savannah River Ecology Laboratory. For my thesis research, I conducted a study of the effect of constant versus variable temperatures on the elimination of the nuclear fission product cesium-137 (137Cs) by yellow-bellied turtles (formerly Pseudemys, now Trachemys scripta) at the U.S. Department of Energy's Savannah River Site (SRS). My work focused on turtles inhabiting Pond B, an 82-ha. lake that formerly served as a cooling reservoir for the plutonium and tritium production reactors on the SRS. A faulty fuel rod from the R reactor, which was stored in unmonitored basin on the site, had in 1963 released large amounts of 137Cs, 90Sr, and lesser amounts of transuranic elements into the basin. These were subsequently discharged into Pond B via the R canal (the thin line entering the bay to the left of the dam). This accident, coupled with unrelated damage to the R Reactor vessel, required that the reactor be permanently shut down in 1964 [click here for a hisory of the Site's operations]. Thirty-five years later, nearly all of the biota of the pond are contaminated with 137Cs at levels approximately 1000 times that of global fallout from nuclear weapons testing. At that time, T. scripta was being evaluated as a possible 'sentinel species' for determining radiological effects in the environment. This was because this turtle is long-lived, possesses a high degree of site fidelity, and has an omnivorous diet. However, this species is poikilothermic (i.e., has a variable body temperature), which complicates predictions of its 137Cs uptake and elimination. However, I found that despite differences in thermal environment and physical activity, 137Cs elimination rate constants of T. scripta held in the laboratory at constant temperatures were statistically indistinguishable from individuals held outdoors in an experimental pond, provided that the laboratory temperature matched the average temperature that the individuals held outdoors could maintain by thermoregulation. My results therefore indicated that ecologically realistic information on the elimination of radionuclides by ectotherms could be obtained from captive animals, if basic information on their temperature preferences were available (Peters and Brisbin 1988).
At this time, I also began measurements of 137Cs uptake in this species, by releasing previously uncontaminated turtles into a semi-aquatic enclosure on Pond B. The long elimination half-times for T. scripta indicated that it was unlikely that an effective equilibrium could be attained before the end of the turtles' active season. Moreover, studies of this and other species indicated that 137Cs uptake by free-ranging animals did not follow the same pattern as in the laboratory, indicating that food availably was substantially affecting radionuclide uptake (Brisbin et al. 1990).
I continued this research over the next four years, by combining the elimination results with data on the temperature-dependent feeding and 137Cs elimination rates of yellow-bellied turtles, measurements of 137Cs concentrations of their diet in Pond B, and information on Pond B temperatures to develop a predictive simulation model of 137Cs body burdens for this species (Peters and Brisbin 1996). Sensitivity analyses of this model indicated that the influence of environmental temperature on feeding rate was the most important factor governing 137Cs concentrations. I validated this model's predictions of effective equilibrium concentrations and patterns of annual cycling of 137Cs body burdens both by measuring the uptake by uncontaminated turtles in the Pond B enclosure, and by measuringthe 137Cs body burdens of more than 400 free-ranging Pond B turtles over a four year period.
Compared with the biochemical similar elements of potassium (K) and rubidium (Rb), cesium (Cs) radioisotopes have long biological half-life in animals, All of these elements are absorbed by animals at comparable efficiencies, however, Cs generally takes several times longer to leave the body than either K or Rb. This difference is important, because it affects the amount of radioactive Cs that can be accumulated by animals (and humans) after accidental releases, and influences the lifetime radiation exposure of organisms in comtaminated habitats. My colleagues and I have completed comparative studies of rubidium (Rb) and cesium (Cs) elimination kinetics in channel catfish, Ictalurus punctatus after intravascular administration (Peters et al.1999). Using toxicokinetic modeling of whole-body and blood concentrations, we have found that Cs elimination is 5 to 7 times slower than for Rb within the same individual. Our results also indicated that there was no difference in the sequestration of these metals within tissues, but it appears that once Cs enters the interior of cells, its rate of exchange with the extracelluar fluid, as well its removal from from this fluid and elimination from the body are also 5 to 7 times slower than for Rb.
Studies of 137Cs kinetics of lower vertebrates are also complicated by the fact that organisms that are chronically exposed to 137Cs have slower biological elimination half-times than do acutely-dosed animals. I have therefore also studied temperature-dependent 137Cs elimination in largemouth bass (Micropterus salmoides) from Pond B (Peters and Newman 1999). David Nestle, a graduate student of mine at the University of Michigan, has recently completed a study of bluegill (Lepomis macrochirus) from Pond B, in which he compared the elimination kinetics and tissue distributions of of acute oral doses of another radioisotope of cesium (cesium-134, 134Cs ) with the elimination of 137Cs accumulated during the lifetime of the fish.
Studies of radionuclide elimination kinetics do not tell the entire story, however. Many radionuclide contaminants persist in the environment for decades, and may decrease in bioavailablity through time. This decrease can be expressed in terms of 'ecological half-lifes' (Te's) within resident species: 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 and ecosystem or render it biologically unavailable. I have recently collaborated with scientists at the Westinghouse Savannah River Laboratory on a study of Te's for 137Cs of fishes in a variety of lentic and lotic ecosystems on the Savannah River Site (Paller et al. 1999). Ecological half-lives were estimated in largemouth bass, sunfishes (Lepomis spp.), and bullhead catfish (Ameiurus spp.) from two reservoirs (Pond B and Par Pond) and three streams on the Savannah River Site. For this study, we examined historical data collected during 1972-1996, following radionuclide releases that occurred primarily during the 1960's and 1970's. Te's in the fishes ranged from 3.2 to 16.7 y, and all were shorter than expected from the half-life for radioactive decay of 137Cs (30.2 y) alone. Fish taxa from the same locations had different 137Cs concentrations (highest in largemouth bass and lowest in sunfishes) but most had similar 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 streams. Te's in lower portions of streams were longer, as were Te's in Pond B. Te's in the Par Pond, which has a much shorter water residence time, were nearly comparable to those in the upper portions of the streams:
I have also developed simulation models to examine whether altering traditional agricultural practices can permit the safe production of poultry and eggs in radioactively contaminated areas, as a possible alternative to subjecting such areas to expensive (and possibly destructive) reclamation. I presented this work in May 1995 at the International Symposium on Environmental Impact of Radioactive Releases (Peters et al. 1995) at the International Atomic Energy Agency's Vienna headquarters, and in more detail at a symposium at Taras Shevchenko University in Kyiv. This work was reported on by several news sources, including New Scientist magazine [click here to view article], the Associated Press [click here to view article], and (inevitably) in Chuck Shepherd's News of the Weird (September 10, 1995, please note that the quote is not mine!)
Finally, in addition to these whole organism studies, I have also worked on understanding radiation effects at the cellular level. With Dr. John Lett at Colorado State University, I modeled repair of cellular damage from cosmic radiation using simulations of multi-enzyme kinetics (Lett and Peters 1992). This work is important in predicting the effects of long-duration exposures by astronauts during residence on space stations or during possible future manned missions to Mars.
Literature Cited
Peters, E.L., and I.L. Brisbin, Jr. 1988. Radiocaesium elimination in the yellow-bellied turtle (Pseudemys scripta). Journal of Applied Ecology 25:461-471. [abstract]
Brisbin, I.L. Jr., M.C. Newman, S.G. McDowell, and E.L. Peters 1990. Prediction of contaminant accumulation by free-living organisms: applications of a sigmoidal model. Environmental Toxicology and Chemistry 9:141-149.
Lett, J.T. and E.L. Peters. 1992. Deoxyribonucleoprotein structure and radiation injury: cellular radiosensitivity is determined by LET[oo]-dependent DNA damage in hydrated deoxyribo-nucleoproteins and the extent of its repair. Advances in Space Research 12:51-58.
Peters, E.L., I.L. Brisbin, Jr. and R.A. Kennamer. 1995. Alternative agriculture as an option to environmental remediation: the production of poultry in radioactively contaminated regions. Pages 523-537 In Proceedings of the International Symposium on Environmental Impact of Radioactive Releases. International Atomic Energy Agency, Vienna, Austria. 8-12 May 1995. [abstract]
Peters, E.L., and I.L. Brisbin, Jr. 1996. Environmental influences on the 137Cs kinetics of the yellow-bellied turtle (Trachemys scripta). Ecological Monographs 66:115-136. [abstract]
Peters, E.L., and M.C. Newman. 1999. 137Cs kinetics of chronically-contaminated largemouth bass (Micropterus salmoides). Health Physics 76:260-268. [abstract]
Paller, M.H., J.W. Littrell, and E.L. Peters. 1999. Ecological half-lives of 137Cs in fishes from the Savannah River Site. Health Physics 77:392-402. [abstract]
Peters, E.L., I.R. Schultz and M.C. Newman. 1999. Rb and Cs kinetics and tissue distributions in channel catfish (Ictalurus punctatus). Ecotoxicology (in press). [abstract] [download paper (PDF format)]
Last Updated: Wednesday, 14 December 2005, 6:00 PM