Forty-First Lauriston S. Taylor Lecture: Environmental Radiation and Life-A Broad View.

Since Earth's creation some 4.5 billion years ago, primordial radioactivity has been part of the planet, and radiations from space have continuously impinged on its surface. Primordial radioactivity has helped shape Earth's surface through the heat from radioactive decay energy, and omnipresent natural radiation has likely influenced the origin, and certainly the evolution, of all life forms in our biosphere today. This paper offers a brief discussion of our natural radiation environment and its impacts, from the beginning of life to the present, and provides a broad overview of present day radioecology, which includes the use of radioactive tracers to study ecosystem functions, the fate and transport of radionuclides in the biosphere, and radiation effects on plants and animals. Large releases of radioactivity, although tragic and regrettable, have been studied to increase our knowledge of Earth's basic processes and of radionuclide transport and accumulation in the environment. On a much smaller scale, purposeful use of natural and anthropogenic radioactive tracers has contributed further knowledge. This information has solidified basic concepts and provided data for constructing dynamic models to calculate concentrations of radionuclides in, and radiation doses to, plants and animals. Sealed radiation sources have been used to study effects of chronic exposure on natural biotic communities. Existing transport models and knowledge of radiation effects provide tools to evaluate human health risks and environmental impacts of radioactive releases. Applications have included guidance for environmental protection, radiation litigation, environmental cleanup decisions and informed responses to releases of radioactivity. This paper concludes with a brief discussion of the more urgent knowledge gaps and potential new research approaches.

Pulsed redistribution of a contaminant following forest fire: cesium-137 in runoff.

Of the natural processes that concentrate dispersed environmental contaminants, landscape fire stands out as having potential to rapidly concentrate contaminants and accelerate their redistribution. This study used rainfall simulation methods to quantify changes in concentration of a widely dispersed environmental contaminant (global fallout 137Cs) in soils and surface water runoff following a major forest fire at Los Alamos, New Mexico, USA. The 137Cs concentrations at the ground surface increased up to 40 times higher in ash deposits and three times higher for the topmost 50 mm of soil compared with pre-fire soils. Average redistribution rates were about one order of magnitude greater for burned plots, 5.96 KBq ha(-1) mm(-1) rainfall, compared with unburned plots, 0.55 KBq ha(-1) mm(-1) rainfall. The greatest surface water transport of 137Cs, 11.6 KBq ha(-1) mm(-1), occurred at the plot with the greatest amount of ground cover removal (80% bare soil) following fire. Concentration increases of 137Cs occurred during surface water erosion, resulting in enrichment of 137Cs levels in sediments by factors of 1.4 to 2.9 compared with parent soils. The elevated concentrations in runoff declined rapidly with time and cumulative precipitation occurrence and approached pre-fire levels after approximately 240 mm of rainfall. Our results provide evidence of order-of-magnitude concentration increases of a fallout radionuclide as a result of forest fire and rapid transport of radionuclides following fire that may have important implications for a wide range of geophysical, ecosystem, fire management, and risk-based issues.

Food chains and biogeochemical pathways: contributions of fallout and other radiotracers.

This paper reviews examples of how measurements of global fallout in the environment and related tracer radionuclides have been used to enhance our basic knowledge of biogeochemical processes and food-chain pathways. Because it is these fundamental, natural processes that control the transport and accumulation of such trace substances in the environment, direct measurements of trace substances over time and space reveal strong insights into these processes. The necessity to monitor global fallout transport, although largely motivated by human health concerns, gave rise to a plethora of new information about plants, animals, and natural and agricultural ecosystems and how they function. This review provides a small selection of examples in the areas of plant and animal physiology, productivity and energy transfer in food chains, biogeochemical cycles of certain elements and their analogues, feeding relationships and movements of organisms, and the agriculture-based human food chain. It is concluded that if society is to cope successfully with continued growth of the human population and resource consumption, more knowledge is still required about these fundamental processes. The use of radiotracers can contribute greatly to this need, but current funding priorities, societal attitudes, and onerous regulations on the use of radioactivity may continue to limit such applications.

Contaminant Transport through Agroecosystems: Assessing Relative Importance of Environmental, Physiological, and Management Factors.

Agroecosystems can become contaminated by atmospherically released radionuclides. The subsequent concentrations of radionuclides in foods are dependent on numerous environmental, physiological, and management factors. We compared four approaches for estimating the relative importance of several of these factors in determining concentrations of 131 I and 137 Cs in milk. A series of sensitivity analyses with Monte Carlo and full-factorial sampling designs was conducted on the PATHWAY model, which simulates radionuclide transport through an agroecosystem. Sensitivity of time-integrated concentrations in milk was estimated as a function of the time of year that fallout was deposited and as a function of time following a spring deposition. The dominant parameters affecting time-integrated concentrations of 131 I in milk were the initial fraction of radionuclides deposited on vegetation, timing and amount of pasture consumption, and the production rate of milk. For time-integrated concentrations of the longer-lived 137 Cs in milk, resuspension was a dominant parameter and pasture use was less important. The sampling designs were compared by ranking the parameters to which the model output is sensitive. The three sampling designs based on parameter variances produced sets of ranks that were similar to each other but differed from the ranking produced by the sampling design based on parameter magnitude. The results indicate which data are most crucial for real-time calculations following an accident and how subsequent dose from ingestion can be most effectively reduced, provide insight into model behavior, and help prioritize future research. This paper demonstrates the importance of variance-based sensitivity analysis.