The history of plutonium and cesium-137 contamination of the Ob River delta sediments

Abstract

Much of the nuclear activity of the former Soviet Union took place within or adjacent to the confines of the Ob River drainage basin. These activities include weapons production and reprocessing at Chelyabinsk-65 (also called Chelyabinsk-40 and Mayak) and the Siberian Radiochemical Plant of Tomsk-7, nuclear weapons testing at Semipalatinsk and uranium mining and milling on the Ishym River. These sites have been the locations of accidental and planned releases of major amounts of radioactive materials since the dawn of the nuclear era. More important, these sites contain vast amounts radioactive waste, in storage, released to the environment, and injected into geologic formations at relatively shallow depth. In total there is thought to be on the order of 2.5 billion curies of radioactive materials presently located within the Ob River basin. Since the Ob is one of the largest rivers flowing into the Arctic, there has been considerable concern over past delivery of radioactive contaminants to the Arctic and the potential for much larger future releases. This project was initiated as part of the Arctic Nuclear Waste Program (ANWAP), administered through the Office of Naval Research, to attempt to characterize the time history of radioactive contaminant transport in the Ob River and to identify the sources of those contaminants. The project has focused on the use of sediment cores to define the distributions of the particle reactive isotopes Cs-137, Pu-239,240, and Pu-238 with depth in sediments. To preserve a temporal record, the sediments need to be deposited more or less continuously and to not be subject to mixing. Meeting these criteria led to a focus on sampling in the shallow "sor" and oxbow lakes that are abundant in the flood plain of the Ob delta as sources of undisturbed sediment. To gain access to the lakes on the flood plain it was necessary to design and build a shallow draft vessel that could navigate the shallow channels and marshes that connect the lakes to the main channels and still accommodate coring operations. An inflatable catamaran was built for this purpose and used during the month of July, 1994 for field operations. A Russian Fisheries Protection vessel served as a base of operations and means of transport between stations for the catamaran. Samples were collected at 17 sites from the delta to the Ob Estuary. Eight of the cores collected were analyzed in detail for the isotopes noted above and for Pb-210; seven of these were from the Ob delta and estuary, and one was from the Taz estuary, a river with no nuclear facilities in its watershed. The patterns of distribution of Cs-137 with depth in most of the cores are very similar. Distributions of Pu-239,240 closely follow those of Cs-137. The depth distribution pattern in the Taz is also essentially the same as those observed in the Ob cores. All of the cores are characterized by relatively low surface activities, a strong subsurface maximum, and activities that decrease sharply to below detection beneath the maximum. The depth of the subsurface maximum varies in response to differing sedimentation rates as do the absolute activities observed in a given core. Consistent with the similarity of the Cs-137 and Pu-239,240 distributions, Pu-239,240/Cs-137 ratios are constant in most cores; they are also very similar in all of the cores, save possibly for the Taz which is slightly higher. The distributions are qualitatively similar to those reported in other areas and attributed to a global fallout source. Time scales of deposition established on the basis of excess Pb-210 are consistent with a fallout source, placing the introduction of Cs-137 and Pu-239,240 at approximately 1950 and the subsurface maxima at around 1963, the time of maximum fallout deposition reported for Fairbanks and Anchorage, Alaska. The Pu/Cs ratios, corrected to 1994, are also consistent with a fallout origin and much greater than that reported for contaminated sediments in the vicinity of the weapons plants in this region. These studies support several conclusions regarding the deposition of the radioactive contaminants studied and the sources of these nuclides. 1) The sediments of many "sor" and oxbow lakes on the flood plain preserve a record of particle reactive contaminant transport down the Ob River. There is little or no evidence of mixing of the record in many of the cores collected. These sediments should preserve the temporal histories of other particle reactive contaminants as well. 2) Based on the patterns and time scale of deposition, the Pu/Cs ratios, and the similarity of the Ob and Taz distributions, the primary source of the Cs-137 and Pu isotopes in these sediments is global fallout. 3) Uncertainties in the analyses could pennit perhaps 25% of the isotopes studied to be derived from a Mayak-like source. However, the data do not require any source other than global fallout. Consequently, we conclude that local sources make little if any contribution to these sediments.