Anthropogenic radionuclides in tide-washed pastures bordering the Irish Sea coast of England and Wales

The activity concentrations of 137Cs,238 Pu, 239,240Pu and 241Am were measured in root mat and vegetation samples collected from tide washed pastures in 17 estuaries spanning the eastern seaboard of the Irish Sea, extending from the Solway in north-west England to St. David's Head in south Wales. Some of these estuaries had been investigated in previous surveys, but this study is unique in that it covered a wide geographic range using the same sampling and analytical methodology and within a comparatively short time scale. This allows for a valid comparison within the data set of the contamination levels at the different areas. Spatial distributions of the radionuclides were consistent with transport of radionuclides discharged to the Irish Sea from the Sellafield Nuclear Reprocessing Plant, with the highest activities occurring in the Esk estuary (closest to Sellafield) and lowest at the Welsh sites. Measurable activity concentrations of238 Pu and 239,240Pu were found in root mat samples from the Solway estuary to as far south as the Gwyrfai in Wales and showed an average238 Pu/239,240Pu ratio of 0.2, consistent with Sellafield-derived Pu. The ratios of137 Cs/241Am increased with distance from the source, with values of 1:1 in estuaries near Sellafield to ratios between 2 and 5 in estuaries further south and in excess of 10:1 in Wales. This is probably due to the more rapid movement of dissolved137 Cs in Irish Sea waters compared with the actinides. In contrast, 241Am and 239,240Pu behave similarly with consistent ratios of between 2:1 and 3:1, as both are associated with particulates. Dose assessment calculations suggest that external exposure would be a maximum of 530 µSv at the most contaminated spot at the Esk estuary. Relatively lower doses arise from the ingestion of animal products (along the soil-vegetation-grazing animal pathway) due to the low availability of sediment-associated radionuclides for gut transfer. The dose assessment calculations highlight the importance of using the appropriate transfer parameters that take into account this low bioavailability.