Radionuclides in sediments of the Aare and Rhine river system: Fallouts, discharges, depth-age relations, mass accumulation rates and transport along the river

The Aare-Rhine river system with its four nuclear power plants on the banks of these rivers and with its intermediate lakes and reservoirs provide the unique chance to analyze the input of radioactivity into the system thereby furnishing information on the sources, to analyze the transport within the sediment and along the rivers, and to refine unsupported 210Pb dating validated by known discharge maxima. At three locations (Lake Biel, Klingnau Reservoir, old branch of the Rhine) in the Aare and Rhine rivers system downstream of the older nuclear power plants (NPPs) Mühleberg and Beznau, the vertical distributions of 137Cs, 210Pb, 214Pb, 214Bi, 40K, 7Be, 239Pu, 240Pu, 241Am, and 237Np in sediment cores were determined. Depth-age relations using the excess 210 Pb were established with the raw and with the piecewise Constant Rate Supply (CRS) models. A comparison of the piecewise CRS method with the imprints of known discharges showed differences of up to two years. Besides typical 137Cs signals (about 100 Bq∙kg−1) from the atmospheric nuclear weapons testing (NWT) and the Chernobyl fallouts, imprints of known 137Cs discharges (10–70 Bq∙kg−1) from the NPPs were found in the sediments. The 237Np distributions (6–10 Bq∙kg−1) essentially follow the 137Cs NWT distributions. In the sediment downstream the NPP Mühleberg (Lake Biel) a239Pu distribution (