The migration and accumulation of radionuclides in the Ravenglass saltmarsh, Cumbria, UK

Abstract

Profiles of ²⁴¹Am, ¹³⁷Cs, ²³⁸Pu, ^239,240Pu & ^24lPu have been studied in four corescollected from the Ravenglass saltmarsh, Cumbria and one core collected from the Irish Sea, near the effluent pipeline of the Sellafield nuclear reprocessing plant. Major and trace elements have also been measured to look at the effects of sediment composition and any redox controls on element redistribution. In addition, almost one hundred surface scrape samples were collected from the Ravenglass saltmarsh and the same radionuclides were measured to study the lateral distribution of radionuclides. Three of the Ravenglass cores have well preserved the history of Sellafield discharge since the plant became operational in 1952. There are no significant post depositional processes and redox changes for all the cores studied. However, one of the saltmarsh cores collected from the front of the marsh shows that surface erosion has occurred. Surface scrape sample results also show that erosion is a significant process which results in the redistribution of sediments within the marsh. The core collected from the Irish Sea adjacent to the effluent pipeline shows sediment mixing has operated and these mixed sediments are the main source of material to the saltmarsh. Using the Sellafield discharge records from 1952 onwards, sediment mixing rates have been estimated using a simple mathematical model and they are inferred to be between 80 % and 90 %. Isotopic ratios of Pu isotopes have been compared to those estimated ratios corresponding to particular sediment mixing rates and show good agreement up to 30 cm below the surface. There is less correlation between Sellafield discharge ratios and measured ratios in cores below 30 cm and this may be due to unreliable discharge data or more likely the advective redistribution of material in the saltmarsh. A contribution of ²⁴¹Am originating from the decay of ^24IPu in cores was determined. The ingrown ²⁴¹Am contributes significantly to the total ²⁴¹Am measured. Based on the Sellafield discharge record, it is estimated that approximately 46 % of total ^24IAm in the system has originated from the decay of ²⁴¹Pu discharged from the plant since 1952. Spatial distribution of radionuclides in the Ravenglass saltmarsh is controlled by sediment accumulation associated with tidal inundation frequency and the clay content in the sediments. Surface erosion also plays a major role in redistributing saltmarsh sediment labelled with older Sellafield discharge within the marsh together with the sediments originated from the River Esk.

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