Examination of the solid phase speciation of 239,240Pu, 241Am and 137Cs in seabed sediments from
Thule, NW Greenland using the NIST standard sequential protocol

Ciara McMahon1,Kenneth Inn1, Julie Lucey2, Luis León Vintró2, Peter Mitchell2 and Henning Dahlgaard3

 1 National Institute of Standards and Technology, MS 8462, Gaithersburg, MD 20899
2 Department of Experimental Physics, University College Dublin, Ireland
3 Risø National Laboratory, Roskilde, Denmark

Traditionally, measurements of environmental contamination have focused on the determination of total radionuclide concentrations. It is clear, however that total concentration does not adequately describe the environmental behavior or potential bioavailability of radiological contaminants. Rather, the time dependent spread and uptake of contaminating radionuclides is a function of their geochemical "partitioning" or "speciation" within the sediment matrix. Therefore, for long-term risk assessment, regulatory bodies need information that includes an evaluation of the mobility and bioavailability of radionuclides.

In 1968 a US B-52 bomber carrying four plutonium-bearing weapons crashed on sea ice at Thule Air Base (NW Greenland), causing the release of kilogram quantities of plutonium oxide to the snow-pack and underlying seabed sediments. The geochemical partitioning of 239,240Pu, 137Cs and 241Am in seabed sediments, collected in the course of a collaborative research expedition to Thule in August 1997, was examined using the NIST standard sequential extraction protocol. This protocol separates the activity into six operationally-defined geochemical fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, acid soluble and residual. The data clearly show that, even thirty years after the accident, a substantial proportion of the plutonium in the sediments is in a non-exchangeable form. This is consistent with the observation that virtually no weapons-grade plutonium is present in the overlying water column, while the measured 238Pu/239,240Pu activity ratios suggest weapons-grade plutonium as the main source of contamination to the sediments. The 137Cs (of mainly global fallout origin) was found to be tightly bound, being associated with primary and secondary minerals within the matrix. In contrast, three main partitions of 241Am were identified, namely association with the carbonate, organic and acid-soluble fractions. This observed association with carbonate and organic matter is similar to that observed by other workers for more temperate regions.