Application of natural and artificial radionuclides for evaluation of sedimentation rate in the lake Khuko (West Caucasus)

The aim of study was to evaluate lake sedimentation rate using artificial ¹³⁷Cs and unsupported ²¹⁰Pb. In addition specific activity of ²⁴¹Am and ^239,240Pu in sediments were determined and were used for identification for the possible sources of fallout. Lake Huko located in the approximately 40 km to the north from Sochi in the Caucasus Mountains. The area of the lake is 27 500 m2, the length is 260 m, width 150 m, maximum depth is up to 10 m. Samples were collected in summer 2016. The core was composed of greenish-brown loams with visible banding - alternation of light and dark interlayers. Core was cut on 13 samples with the thickness 0.5 cm and were examined. Gamma-emitted radionuclides (²¹⁰Pb, ²²⁶Ra, ²³²Th, ¹³⁷Cs, ²⁴¹Am) were examined with gamma-spectrometer ORTEC GEM-C5060P4-B with HPGe detector. Alpha spectrometry and ICP MS was used for plutonium determination. Plutonium separation was done using cation-exchange resin after total leaching. To determine sedimentation rate using unsupported ²¹⁰Pb (Goldberg, 1963), a CIC model was applied. ¹³⁷Cs activity depth curve (the main tests of nuclear weapons in 1963 and Chernobyl accident in 1986) were used for correction of ²¹⁰Pb_ex dating. Using both ¹³⁷Cs and ²¹⁰Pbex dating, it was determined that maximum of contemporary sedimentation rate is 0,017 cm/y. The maximum ¹³⁷Cs concentration was found in upper horizon (0-0.5 cm). Maximum age of this layer can be estimated at 30 years, and according to that maximum sedimentation rate over this period is 0.017 cm/y (excluding the loss of upper part of core). Extremely low deposition rates for the last 60 years in the Huko lake is very clear indicator of the very low denudation rates within the catchment area of the lake. High level of ¹³⁷Cs activity (>3 kBq/kg) in the upper layer was quite unexpected, and indicated the presence of intensive radioactive source in the lake Huko bottom sediments. Together with ¹³⁷Cs, a detectable amount of ²⁴¹Am (34-52 Bq/kg) was found, which should be result of nuclear tests or part of Chernobyl accident atmospheric fallout. Significant ²⁴¹Am concentrations indicated that it could be produced from the ²⁴¹Pu radioisotope. According to alpha-spectrometric measurements, the total activity of ^239,240Pu isotopes was detected. For the surface horizon, the specific activity varied from 162.3 to 191.2 Bq/kg for two sample series. The mass-spectrometric analyses results confirmed the specific activities of plutonium isotopes levels. It was amounted to ²³⁹Pu - 85 and ²⁴⁰Pu - 14 ppt. Plutonium isotopes 241 and 238 were not found. So it is likely that the source of plutonium in the Huko lake is not Chernobyl fallout (Lujaniene, 2009). According to isotopic ratios data the global stratospheric fallout in addition with probably some of local explosions (Semipalatinsk, Kapustin Yar,) are sources of artificial radionuclides in the Huko lake. Various conditions for the terrigenous material accumulation by bottom sediments create conditions for the radioactive fallout concentration. (author)