Sediment records in the river Loire estuary

¹ Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire Mixte CEA-CNRS, 91198 Gif-sur-Yvette cedex, France
² Laboratoire Souterrain de Modane, 90 rue Polset, 73500 Modane, France
³ EDF R&D Département Laboratoire National d'Hydraulique et Environnement, BP. 49, 78401 Chatou cedex, France

Abstract

Within the framework of the "Loire river and estuary radioecology" program, radionuclide measurements were undertaken in the various components of the estuarine aquatic ecosystem: dissolved, particulate and sedimentary fractions. In order to investigate the extent of long-term radionuclide accumulation in this area, 8 sediment cores were sampled in the estuary, plus 2 cores on the sea shore near the river mouth; gamma spectrometric measurements were performed at the Laboratoire Souterrain de Modane (LSM, CNRS-CEA) in the French Alps using very low background, high efficiency "well-type" Ge detectors with crystal volumes of 430 and 910 cm³. Such large detectors allow the measurement of samples of about 20g with very high detection efficiencies. Among the artificial radionuclides present in industrial releases, ⁶⁰Co was detected in most samples throughout the cores. Using ¹³⁷Cs and ²¹⁰Pb profiles in the cores, mean apparent accumulation rates were estimated after a normalization to the fine-grain sedimentary fraction which is the main carrier of reactive radionuclides. A sedimentary record of the last 50 years was obtained. ⁶⁰Co activities were always very low, from below 0.1 up to 2 Bq.kg^-1. Plots of ⁶⁰Co versus sedimentary layer ages are presented and compared with annual discharges of nuclear power plants located along the Loire river.