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 (<3 Bq∙kg^-1) was found, which was solely due to the NWT fallout. Downstream the NPP Beznau (Klingnau Reservoir and an old branch of the Rhine), besides the NWT distribution, also imprints of ²³⁹Pu discharges (up to 7 Bq∙kg^-1) were found within the time interval 1963 to 1986.²⁴⁰Pu/²³⁹Pu ratios revealed that the burn-up times of the nuclear fuel in the NPP (²³⁵U enrichment of 3.5%), from which the discharges stem, should be about 1 year or less. A comparison between the calculated and the measured ¹³⁷Cs/²³⁹Pu ratio revealed no large discrepancies for the Lake Biel and Rhine positions, but in the Klingnau distribution, the calculated ¹³⁷Cs/²³⁹Pu ratio is one order of magnitude larger than the measured one. The reason could be either a natural uranium research reactor as the source, or strong, short-range ²³⁹Pu precipitation after the discharge from the Beznau NPP. The largest ²³⁹Pu peak in the Rhine sediment (1968/70) corresponds to no major peak in the Klingnau sediment. For the NPP Mühleberg discharge of 1982 the ratio of the ¹³⁷Cs deposition in sediments from Lake Biel, Klingnau Reservoir and the Rhine river is about 1 : 0.5 : 2.9. For the 1977/78 ²³⁹Pu deposition the ratio is 1 : 0.02, for the Klingnau Reservoir and the Rhine sediments, respectively. These numbers indicate a long-range transport of Cs and a rather short-range transport of Pu.

Dating of recent sediments from Lago Maggiore and Lago di Lugano (Switzerland/Italy) using 137Cs and 210Pb

This paper presents the results of ¹³⁷Cs and ²¹⁰Pb dating of sediments from the northern and western basins of Lago Maggiore and the southern basin of Lago di Lugano. Vertical distributions of ¹³⁷Cs in sediment cores taken in 2017/18 were compared to those of the sediment cores taken almost two decades ago at the same positions. This comparison showed that ¹³⁷Cs peaks do not alter their shape significantly over time and ¹³⁷Cs remains a reliable time-marker for many decades. ²¹⁰Pb dating using for the first time the piecewise "constant rate supply" (CRS_PW) model constrained with ¹³⁷Cs time-markers is performed for sediment cores of Lago Maggiore and Lago di Lugano. "Event-linked" mass accumulation rates (MARs) were derived from the CRS_PW model. In combination with the dry bulk density, organic matter content, ⁴⁰K and ²¹⁰Pb activity concentrations, these "event-linked" MARs were used for the identification of one or more event layers (turbidites, detrital layers) in 1-cm thick sediment samples. "Continuous" mean MARs which are independent of the event layers were estimated: in Lago Maggiore sediment cores they are 0.066-0.090 g cm^-2 yr^-1; in Lago di Lugano they vary in the range of 0.032-0.057 g cm^-2 yr^-1 with higher values in the time interval 1963-1986 as compared to 1986-present.

Long-term distribution and migration of 137Cs in a small lake ecosystem with organic-rich catchment: A case study of Lake Vorsee (Southern Germany)

In the shallow eutrophic Lake Vorsee with an organic-rich catchment the ¹³⁷Cs activity concentration in water samples (n = 250) and different fish species (n = 631) was determined for a period of 30 years starting soon after the Chernobyl fallout. To understand the distribution and migration of ¹³⁷Cs in the ecosystem of Lake Vorsee, also the ¹³⁷Cs inventory in soils of the catchment area, in sediments, in suspended matter, in water plants, as well as its vertical distribution in soils and sediments were studied. The time dependency of the ¹³⁷Cs activity concentration in water and fish was modelled with a compartment model (AQUASCOPE) as well as with the sum of two exponential functions. Results for effective half-lives of ¹³⁷Cs in water are T_eff1  = 1.2 a and T_eff2  = 7.2 a; for small cyprinidae T_eff1  = 0.6 a and T_eff2  = 8.9 a; and for pike T_eff1  = 1.4 a and T_eff2  = 12.1 a. The vertical distribution of ¹³⁷Cs activity concentration in soils was described by a partial differential equation (diffusion constant D = (1.05 ± 1.16) cm²⋅a^-1 and convection speed v = (0.14 ± 0.07) cm⋅a^-1) and its distribution in sediments by a system of two coupled partial differential equations (fixation rate f = 2.0 d^-1; distribution coefficient K_d^ex = (543 ± 140) L⋅kg^-1; sedimentation rate R_S = (0.030 ± 0.010) g⋅(cm²⋅a)^-1; bioturbation constant D_bio, decreasing linearly with depth with D_bioabs = (3.3 ± 1.0) cm²⋅a^-1 at the surface). According to the extended AQUASCOPE model, the following fractions of the total ¹³⁷Cs inventory were found in the different compartments of the lake ecosystem in 2016: 9.0⋅10^-1 in soils of the catchment area, 1.0⋅10^-1 in sediments, 5.5⋅10^-5 in water, 1.5⋅10^-5 in suspended matter, 1.3⋅10^-5 in watermilfoil, and 1.6⋅10^-6 in fish.

Dating of sediments from four Swiss prealpine lakes with (210)Pb determined by gamma-spectrometry: progress and problems

In this paper the most important problems in dating lake sediments with unsupported (210)Pb are summarized and the progress in gamma-spectrometry of the unsupported (210)Pb is discussed. The main topics of these studies concern sediment samples preparation for gamma-spectrometry, measurement techniques and data analysis, as well as understanding of accumulation and sedimentation processes in lakes. The vertical distributions of artificial ((137)Cs, (241)Am, (239)Pu) and natural radionuclides ((40)K, (210,214)Pb, (214)Bi) as well as stable trace elements (Fe, Mn, Pb) in sediment cores from four Swiss lakes were used as examples for the interpretation, inter-comparison and validation of depth-age relations established by three (210)Pb-based models (CF-CSR, CRS and SIT). The identification of turbidite layers and the influence of the turbidity flows on the accuracy of sediment dating is demonstrated. Time-dependent mass sedimentation rates in lakes Brienz, Thun, Biel and Lucerne are discussed and compared with published data.