Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia

摘要

The potential sources of anthropogenic radionuclides to the Ob River system of western Siberia include global stratospheric fallout, tropospheric fallout from atomic weapons tests and releases from production and reprocessing facilities. Samples of water, suspended and bottom sediments collected in 1994 and 1995 have been used to characterize the sources and transport of ~(137)Cs, Pu isotopes, ~(237)Np and ~(129)I through the system. For the radionuclides that associate with particles, isotope ratios provide clues to their sources, providing any geochemical fractionation can be taken into account. Activity ratios of ~(239,240)Pu/~(137)Cs in suspended sediments are lower than the global fallout ratio in the Irtysh River before its confluence with the Ob, comparable to fallout in the central reach of the Ob, and greater than the fallout values in the lower Ob and in the Taz River. This pattern mirrors the downriver decrease in dissolved organic carbon (DOC) concentrations. Laboratory adsorption experiments with Ob River sediment and water show that K_d values for Am (and presumably other actinides) are depressed by two orders of magnitude in the presence of Ob DOC concentrations, relative to values measured in DOC-free Ob water. Iodine and cesium K_d values show little or no (less than a factor of 2) dependence on DOC. Mixing plots using plutonium isotope ratios (atom ratios) show that Pu in suspended sediments of the Ob is a mixture of stratospheric global fallout at northern latitudes, tropospheric fallout from the former Soviet Union test site at Semipalatinsk and reprocessing of spent fuel at Tomsk-7. Plutonium from Semipalatinsk is evident in the Irtysh River above its confluence with the Tobal. Suspended sediment samples taken in the Ob above its confluence with the Irtysh indicate the presence of Pu derived from the Tomsk-7 reprocessing facilities. A mixing plot constructed using ~(237)Np/~(239)Pu vs. ~(240)Pu/~(239)Pu shows similar mixtures of stratospheric and tropospheric fallout, with the likely addition of inputs from reprocessing facilities and reactor operations. As with Pu/Cs ratios, Np/Pu ratios could be modified by differential geochemical behaviors of Np and Pu. Dissolved ~(129)I only weakly interacts with particles in the Ob; size-fractionated sampling shows that the colloidal ~(129)I fraction (defined as 1 kDa-0.2 μm) contains generally < 5% relative to that passing a 0.2 μm filter. Iodine-129 concentrations decrease from 8.3*10~9 to 0.65*10~9 atoms 1~(-1) through the Ob system toward the Kara Sea, with highest values in the Tobal River and lowest in the Taz River. The likely source of the elevated ~(129)I in the Tobal is release from the production-reprocessing facilities at Mayak, and decreases downriver are predominantly due to dilution as the various tributaries with low ~(129)I join the