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Nuclear facilities in coastal locations often discharge low-level liquid wastes into the sea and the radioisotopes in these discharges are of interest both in assessing possible environmental impacts and as tracers for coastal processes. The distributions of a range of artificial radionuclides, derived from the authorized discharges from British Nuclear Fuels (BNFL) Sellafield, have been determined in the sediments of an intertidal salt marsh in the Esk Estuary, Cumbria, UK. Where published discharge histories exist (for 137Cs, 238Pu, 239,240Pu and 241Am), the sediment core-profile distributions of these radionuclides have been compared with the releases from Sellafield, and consistent values of the accumulation rate (0.226 ± 0.007 g cm-2 yr-1) are obtained. A quantitative model has been developed, describing association of radionuclides with suspended particulate material, which is then accumulated and mixed in an offshore mud patch before resuspension and deposition in the salt marsh. The model has been used to describe radionuclide distributions observed in both the mud patch and the salt marsh, and to identify isotopes for which post-depositional remobilization or solution transport from the discharge point are important. The behaviour of the commonly studied isotopes (137Cs, 238Pu, 239,240Pu and 241Am) is similar to that observed at this and other nearby locations. The activation product isotope 236U is enhanced in these sediments over the natural baseline by four to eight orders of magnitude, and the results suggest that Sellafield-derived uranium is comparably mobile to 137Cs in these sediments although the processes governing the behaviour of these two elements different. In situ production of 241Am by decay of its 241Pu parent has generated only 17% of the current sediment inventory of this isotope, insufficient to account for the increase over the last 20-25 years, and suggesting that the input material for these sediments is preferentially enriched in Am

The porewater concentrations of the transuranium elements Np, Pu, and Am have been measured over time at a salt marsh in west Cumbria, U.K., and all three show seasonal variations. Pu follows the pattern previously observed at this site very closely, with concentrations reaching a maximum of 3.2 mBq L-1 and a minimum of 0.8 mBq L-1 in 1996, compared with 3.5 mBq L-1 and 1.1 mBq L-1, respectively, in 1994. However, no relationship with dissolved Fe and Mn concentrations was observed in this study. Plutonium and Am concentrations follow similar patterns from April to September, but there are fluctuations in the Am concentration in February which are not observed for Pu. Neptunium concentrations, measured by accelerator mass spectrometry, follow a less clear pattern, although they are at a marked maximum of 0.56 mBq L-1 in February. Changes in Pu and Am concentrations between April and September can be related to changes in the microbial community and suggest that these elements are taken up in biosorption processes. When biomass is low, no relationship is observed between Pu and Am concentrations. There is no correlation between the microbiological data and Np concentrations at any time of year.

Partial specific volume is an important molecular parameter in the measurement of molecular masses by analytical ultracentrifugation and X-ray diffraction studies. Humic substances (aquatic fulvic and humic acids and peat humic acid) were isolated from a natural source (Whitray Fell, UK). The partial specific volumes ( ) have been studied by two independent methods, an analytical ultracentrifugation technique based on simultaneous sedimentation equilibrium measurements in water and deuterium oxide and by high precision digital densimetry. Measurements were made in a range of conditions from 0.01 M to 0.5 M salt concentration and in the humic substance concentration range 0.01 to 2 mg ml−1. Partial specific volumes in the range 0.43 to 0.63 cm3 g−1 were found depending on the samples and conditions. No evidence was found for a dependence of ( ) on humic concentration within the ranges studied. Both methods of measurements gave results in reasonable accord. The ultracentrifugation method was more precise for the aquatic fulvic acid than for the more polydisperse humic acids. For the fulvic sample the reproducibility of the ultracentrifugation method was ± 4.31%, comparable to the average reproducibility of the densimetry method which was ± 4.07% over a range of 12 different systems.