| Abstract |
Undisturbed River Rhine sediment cores were taken in May 2016 from the protected area of the small reservoir Altrhein Wyhlen (Germany), near Augst (Switzerland) downstream of Swiss nuclear power plants, at 2 m water depth. Freeze-dried sediment samples (layer thickness of 1 cm) were measured γ-spectrometrically using HPGe-detectors (BEGe-5030 with carbon windows, Canberra), preliminary being sealed into radon-tight aluminium composite foil. Self-absorption in the samples and summation corrections in the spectrum were taken into account. After γ-measurements, the sediment samples were ashed at 520 ᵒC and digested with a borate fusion at 1100 ᵒC. Then, after dissolving the molten samples in nitric acid and precipitating the silicates, Pu was separated using an extraction chromatography TEVA column and measured by ICP-MS (Röllin et al., 2009).
The establishment of an age-depth relation in River Rhine sediments is based on the identification of 137Cs deposition maxima. In the sediment cores the maxima of the 137Cs activity concentration corresponding to the fallout after the Chernobyl accident in April 1986 and to the atmospheric nuclear weapons testing in the 1960s with the maximum in 1963 were well recognized. The assignments to the global fallout from the nuclear weapons testing were confirmed by 241Am and 239Pu. The average 240Pu/239Pu ratio of 0.173±0.007 in sediment core AU16-2 corresponds to the typical ratio observed in the global fallout from the nuclear weapons testing and is, therefore, a strong indication that plutonium has originated mainly from that source. Together with the Chernobyl and nuclear weapons testing maxima, additional 137Cs entries in 19761979 and 1970 were visible in the sediment profiles; they correlate well with the known discharges from the NPPs Mühleberg (ENSI, 2013) and Beznau (ENSI, personal communication), respectively.
An alternative approach in age determination of River Rhine sediments is based on piecewise constant rate supply (CRS) of the unsupported 210Pb (Appleby, 2001). Opposed to the simple CRS model (Appleby and Oldfield, 1978) which assumes a constant 210Pb flux over the whole sediment profile, the piecewise CRS model is applied to different sections of the core separated by well-known 137Cs maxima, e.g. 1986 and 1963. In case of River Rhine sediments (core AU16-4) the 210Pb supply rates for 19862016, 19631986, and before 1963, had to be 280, 290 and 230 Bq/(m2 yr), respectively, in order to achieve an exact agreement between 137Cs data points and the continuous 210Pb time scales. For the sediment core AU16-2 taken a few meters apart, the 210Pb supply rates for 19862016, 19631986, and before 1963, are lower: 150, 130 and 180 Bq/(m2 yr), respectively. Sedimentation rate in core AU16-4 is 0.65 g/(cm2 yr) for 20161986 and it agrees well with the sedimentation rates of 0.62 and 0.69 g/(cm2 yr) for the period 19861995 measured by Albrecht et al. (1998). In core AU16-2, the sedimentation rate for the same period (19862016) is a factor of two lower. Sedimentation rates for the period 19631986 are slightly higher than those between 1986 and 2016 which also corresponds to higher 210Pb fluxes.
More results on depth-age relations in River Rhine sediments, sedimentation rates, 210Pb fluxes, and a comparison of vertical distribution of radionuclides in studied sediment cores (sampled in 2016) with the cores taken from the same spot in 1995 (Albrecht et al., 1998) will be presented at the conference. Also data on sediments from Swiss lakes will be shown.
References:
Albrecht, A., Reiser, R., Lück, A., Stoll, J-M. A., Giger, W., 1998. Radiocesium Dating of Sediments from Lakes and Reservoirs of Different Hydrological Regimes. Environ. Sci. Technol. 32: 18821887.
Appleby, P.G., 2001. Chronostratigraphic technique in recent sediments. In: W.L. Last and J.P. Smol (Eds.), Tracking Environmental Change Using Lake Sediments, Volume 1: Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic Publishers, Dordrecht, The Netherlands: pp. 171203.
Appleby, P.G., Oldfield, F., 1978. The calculation of 210Pb dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5: 18.
ENSI, 2013. Die Fakten zur Radioaktivität im Bielersee. Swiss Federal Nuclear Safety Inspectorate http://www.ensi.ch/de/2013/07/15/die-fakten-zur-radioaktivitat-im-bielersee/ (Date of access: 27.11.2014).
Röllin, S., Sahli, H., Holzer, R., Astner, M., Burger, M., 2009. Pu and Np analysis of soil and sediment samples with ICP-MS. App. Rad. Iso. 67 (5): 821-827. |
