Environmental Radiotracers

John Robbins

Although this is no longer considered a current GLERL research project, John Robbins continues to work on completing his many research endeavors in his capacity as a GLERL Scientist emeritus. Please see the Research Programs page for a list of current research projects.

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Background and Objectives

The Environmental Radiotracers (ERT) Project employs natural and artificial radionuclides to identify and model important particle transport processes in diverse systems including the Laurentian and other Great Lakes, smaller freshwater bodies, wetlands and coastal marine environments. The project is an outgrowth of the recognition that, in such systems, many contaminants and nutrients in the water move in association with inorganic and organic particles, including plankton species and their remains. Particle-associated constituents settle through the water column to underlying sediments, where they may be mixed and resuspended by currents or biological action, and are ultimately lost by burial in accumulating sediments. Along such pathways as these, rates of particle transport often can be determined using particle-associated radionuclides because of their built-in clocks, relative ease of measurement or accurately known loading histories. Radionuclide studies often inform development of quantitative process models and this aspect has been emphasized in the ERT Project.

Since its inception in the early 1980s, this technique-based project has given particular attention to radiometric dating of sediments. In addition to the role that sediments play in the regulation of nutrients and contaminants in aquatic systems, they frequently possess retrievable records of present and historical changes in ecosystem status and constituent loads due to natural or human causes. The development, application and evaluation of radionuclide dating methods address numerous scientific and public concerns about the changing status of ecosystems, environmental contamination and global climate change.

Changes in contamination, biological status and physical characteristics of lakes and coastal marine systems during the past century often lead to significant changes in the composition of accumulating sediments. Reconstructing the history of such changes from sedimentary records (paleolimnology) is an important part of present efforts to understand human impacts on ecosystems and undertake appropriate remedial strategies. To this end, it is critical to have reliable and accurate methods of dating sediments. The Great Lakes Environmental Research Laboratory has been at the forefront of developing and testing radiotracer methods for sediment dating.

The ERT Project consists of a series of sub-projects with differing duration, environments and collaborators, which generally share the above objectives. Sub-projects are a mix of internally-funded activities and formal externally-funded inter-agency agreements. Over the lifetime of this project there have been about 30 sub-projects in diverse systems and geographic areas.

Products for this Project

(Publications, Milestone Reports)

Geographic Coverage

Lake Athabaska (Canada), Lake Baikal (Russia), Lake Constance (Germany/ Austria/Switzerland), Coeur D,Alene Lake (Idaho), Florida Bay, Lake George (US/Canada), Great Slave Lake (Canada), Lake Ladoga (Finland/Russia), Lake Oahe (S. Dakota), Lake Rockwell (Ohio), Lake Sniardwy (Poland), Lake Tahoe (California/ Nevada), Lake Winnipeg (Canada), Terrace Lake (Colorado), The Everglades (Florida), The Gulf of Mexico, The Keweenaw Waterway (Michigan), The Laurentian Great Lakes (US/Canada).

Formal and Informal Institutional Collaborations (with abbreviations)

Radionuclides Emphasized in the Environmental Radiotracers Project

Lead-210. This naturally occurring radioactive isotope of lead (t1/2 = 22 years) is part of the uranium decay series that includes radium. This latter radionuclide occurs widely in soils and rocks and produces the well-publicized radioactive, chemically inert gas, radon, which leaks into the atmosphere. Decay of radon (t1/2= 4 days) produces 210Pb, which is efficiently removed by precipitation, transferred to water bodies and rapidly incorporated into sediments where it decays on burial. As a result, a layer of sediment, for example containing half as much 210Pb than at the surface, would be considered 22 years older than sediments deposited at the surface and so forth. This simple idea has often proven to work well in many of the hundreds of studies that have employed the 210Pb method during the past several decades. However the method is never routine because sediments may accumulate at variable rates, be subject to near-surface mixing due to biological or physical processes, and even the rate of delivery of 210Pb to sediments may vary as a result of physical or geochemical processes in aquatic systems. Some of the sub-projects described involve verification of the method, offer approaches to improving its accuracy and attempt to link 210Pb dating to system specific physical and biogeochemical processes. In other sub-projects the method is applied to obtain modern rates of sediment accumulation, rates and ranges of sediment mixing and residence times of particles in sediment surface mixed layers. In some cases the 210Pb has been used to date cores where there is an interest in linking sedimentary records of water body contamination to historical loading from urban or industrial sources.

Cesium-137. This man-made radionuclide (t1/2=30 years) was delivered to aquatic systems primarily through atmospheric fallout of debris from above ground testing of nuclear weapons mostly in the mid-1960s. In the Great Lakes, 137Cs has attached to fine particles and cleared out of the water within a few years after fallout, but small amounts continue to be reintroduced through resuspension of surface sediments. In cores from selected areas of many lake and coastal marine sediments as well as wetland soils, there is a clear peak present at depths corresponding to the fallout maximum in 1963-1964. Thus the 137Cs is useful in verifying sediment cores dates based on 210Pb, but perhaps more important, 137Cs mimics the behavior of many non-degradable contaminants circulating in aquatic systems. Since its loading to the Great Lakes as well as other continental sites, is well-known and 137Cs is unambiguously and easily determined in sediments, this fallout radionuclide can be used to track the movements of contaminants through systems during the past 40 years or so. Some of the sub-projects described below exploit 137Cs to establish or verify sediment accumulation rates while others exploit its particle-tracking capability to characterize the long-term fate of contaminants. In the Great Lakes as in may other aquatic systems, particles that were labeled by 137Cs in the mid sixties are in the process of being buried and replaced by new, largely uncontaminated particles from erosion of shoreline of watershed sources. As a result 137Cs is useful in characterizing mechanisms and times of recovery of water bodies from past contaminant insults. Another fallout radionuclide mentioned in several sub-projects is plutonium, primarily the long-lived isotope, 239Pu, (t 1/2 =24,400 years). Plutonium was co-dispersed with 137Cs in nuclear testing events and deposited essentially in fixed proportion to radiocesium onto land and water surfaces. Although Pu is chemically dissimilar to cesium, both elements have labeled suites of particles that have had comparable transport and fate in the Great Lakes and many other freshwater systems.

Beryllium-7. In contrast with the above radionuclides, naturally occurring 7Be has a comparatively short half-life (t1/2 = 53 days) and results from the smashing (spallation) of stratospheric nitrogen and oxygen nuclei by cosmic rays. Tropospheric concentrations of 7Be and rates of deposition onto surface water have an annual cycle that depends on a combination of its seasonally variable efficiency of transfer from the stratosphere and on regional weather patterns. In the Great Lakes area, the amount of 7Be transferred to water varies almost sinusoidally each year, with maximum deposition in May and minimum in December-January. In several recent sub-projects, the seasonal variability and short half-life of this particle-tracing radionuclide has been used to characterize transport processes and their rates on weekly to monthly time scales.

Status of Currently Active Sub-projects

Descriptions of currently active sub-projects are provided below, including sections on background, recent accomplishments, their significance and plans for the forthcoming year. In cases where interpretations are provided, they may not have been subject to a formal peer review process and should therefore be considered as tentative.

SP-00 Saginaw Bay

SP-1 Lake Erie Reference Sites

SP- 2 Composition and Flux of Settling Matter

SP-3 Recent Sedimentation Rates in Lake Ontario

SP-9 Sediment Focusing in Lake Erie

SP-11 Sediment Records of Contamination and Biologic Responses in Lake George

SP-16 Gamma Scan System

SP-33 Copper Mining Impacts in the Keweenaw Peninsula and Lake Superior

SP-37 Information Content of Sediments Accumulated in a Wrecked Ship

SP-39 Radionuclides, Metals and Organic Contaminants in Lake Ladoga Sediments

SP-43 Sediments in Florida Bay

SP-44 Accumulation and Mixing of Sediments in Lake Michigan

SP-45 Soil Accretion Rates in South Florida Wetlands

SP-47 Mississippi Basin Carbon Project

SP-48 Episodic Resuspension Events in Lake Michigan (EEGLE)

SP-49 Vertical Sediment Transport Models for Lake Tahoe

SP-50 Metal Contaminant Chronology of Recent Sediments near the Great Barrier Reef, Northern Australia

SP-51 Sedimentary Evidence for Recent Die-Off of Diporeia in Lake Michigan

Last updated: 2006-05-11 mbl

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