Accumulation of COGEMA-La Hague-derived reprocessing wastes in French salt marsh sediments

Over the past five decades, authorized low-level discharges from coastal nuclear facilities have released significant quantities of artificial radionuclides into the marine environment. In northwest Europe, the majority of the total discharge has derived from nuclear reprocessing activities at Sellafield in the United Kingdom and COGEMA-La Hague in France. At the Sellafield site, a significant amount of the discharges has been trapped in offshore fine sediment deposits, and notably in local coastal and estuarine sediments, and much research has been focused on understanding the distribution, accumulation, and reworking of long-lived radionuclides in these deposits. In contrast, there are few high-resolution published data on the vertical distribution of radionuclides in fine-grained estuarine sediments near, and downstream of, COGEMA-La Hague. This paper therefore examines the vertical distribution of a range of anthropogenic radionuclides in dated salt marsh cores from two estuaries, one adjacent to, and the other downstream of, the COGEMA-La Hague discharge point (the Havre de Carteret at Barneville-Carteret and the Baie de Somme, respectively). The radionuclides examined show a vertical distribution which predominantly reflects variations in input from COGEMA-La Hague (albeit much more clearly at Barneville-Carteret than at the Baie de Somme site), and Pu isotopic ratios are consistent with a La Hague, rather than weapons' fallout, source. Because of sediment mixing, the marshes apparently retain an integrated record of the La Hague discharges, rather than an exact reproduction of the discharge history. Sorption of radionuclides increases in the order ⁹⁰Sr < ¹³⁷Cs < ⁶⁰Co < ^239,240Pu, which is consistent with Kd values reported in the literature. In general, the radionuclide activities observed at the sites studied are low (particularly in comparison with salt marsh sediments near the Sellafield facility), but are similar to those found in areas of fine sedimentation in the central Channel. These marshes are not major sinks for discharged reprocessing wastes.