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Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia
Earth and Planetary Science Letters 179 (2000) 125^137 www.elsevier.com/locate/epsl
Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia J. Kirk Cochran a; *, S. Bradley Moran b , Nicholas S. Fisher a , Thomas M. Beasley c , James M. Kelley d a
Marine Sciences Research Center, State University of New York Stony Brook, NY 11794, USA Graduate School of Oceanography, The University of Rhode Island, Narragansett, RI 02882, USA Environmental Measurements Laboratory, US Department of Energy, 201 Varick St., New York, NY 10014, USA d Paci¢c Northwest National Laboratory, Battelle Blvd. MS P7-07, Richland, WA 99352, USA b
c
Received 5 August 1999; accepted 5 April 2000
Abstract 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 Kd 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 Kd 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 Wm) contains generally 6 5% relative to that passing a 0.2 Wm filter. Iodine-129 concentrations decrease from 8.3U109 to 0.65U109 atoms l31 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
* Corresponding author. Tel.: +1-516-632-8733; Fax: +1-516-632-8820; E-mail: [email protected] 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 1 0 - 2
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system. Fluxes of 129 I to the lower Ob at Salekhard are 6 1% of the releases of this radionuclide from the nuclear fuel reprocessing facilities at Sellafield, UK and La Hague, France. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: Ob River; human activity; radioactive isotopes; fallout
1. Introduction The Ob River in western Siberia has a mean discharge of 1.4U104 m3 s31 and is the third largest Russian river. The Ob watershed is extensive (V3U106 km2 ) and has received radioactive contaminants from global fallout from nuclear weapons testing, local fallout from weapons testing at Novaya Zemlya and Semipalatinsk and, perhaps most signi¢cantly, discharges from nuclear facilities located on or near rivers that drain into the Ob. In particular, releases of radioactivity from Russian production reactors and reprocessing facilities such as Tomsk-7 and Mayak are potential sources of anthropogenic radionuclides (137 Cs, 90 Sr, 239;240 Pu, 241 Am, 60 Co, 237 Np, 129 I and others) to the Ob system. The Ob drains into the Kara Sea and thus may represent an important pathway for delivery of anthropogenic radionuclides to the Arctic Ocean (Fig. 1). The Ob River is characterized by fairly high particle loads (tens of milligrams per liter), and under such conditions, many of the radionuclides released to the Ob watershed (e.g. 137 Cs, Pu, 241 Am) would be expected to associate predominantly with particles, particularly in the freshwater portion of the system. Less reactive radionuclides such as 90 Sr or 129 I will be transported more in the dissolved phase. Dai and Martin [1] documented high dissolved (DOC) and colloidal organic concentrations on the Ob and showed that colloidal material (de¢ned as the size fraction from 104 Da to 0.4 Wm) was important in transporting trace metals such as Fe, Ni and Cu in the Ob. Moran and Woods [2] showed that Cu, Cr and Ni in suspended and bottom sediments of the Ob were present in crustal abundances, while Cd and Pb had additional (anthropogenic) sources. In this paper we present results of analyses of anthropogenic radionuclides in the freshwater portion of the Ob River. For the particle-reactive radionuclides (e.g. 137 Cs, Pu isotopes, 237 Np) our
emphasis has been on the suspended particle reservoir and for the soluble radionuclides (129 I), on the dissolved fraction. Our goal is to attempt to characterize the sources and transport of these radionuclides in the Ob system. 2. Sample collection and analysis 2.1. Station locations and sampling Water samples were collected in the Ob, Irtysh, Tobal and Taz Rivers, northwestern Siberia, in July^August 1994 and in June 1995 (Fig. 1). The Russian Fisheries Protection vessel RS300#168, based in Salekhard, was used for the sampling. Suspended sediment samples of 10^15 g were collected by ¢ltering 50^200 l from a water depth of 1^2 m through 0.5 Wm cartridge ¢lters (Microwynd DPPPZ1). Additional samples (V100 ml) were taken for suspended particulate matter concentration and particulate organic carbon. The former were ¢ltered through pre-weighed 0.4 Wm Nuclepore ¢lters and the latter through pre-combusted Whatman 25 mm glass ¢ber ¢lters (0.7 Wm). Size-fractionated sampling was conducted for 129 I. Approximately 60 l of surface water were pumped using a Flotec pump through acidcleaned tubing connected to a 0.2 Wm Gelman Maxi capsule ¢lter. The `dissolved' ¢lter-passing fraction ( 6 0.2 Wm) was connected to a cross-£ow ¢ltration system with an Osmonics 1000 nominal molecular weight (NMW) spiral wound membrane (polysulfone) in a PVC housing. Cross£ow ¢ltration (CFF) was conducted in a recirculating mode, resulting in collection of colloidal (1000 NMW^0.2 Wm) and truly dissolved ( 6 1000 NMW) size fractions. The wetted parts of the system were PVC, silicon, Te£on and polyethylene, and both system and ¢lters were acid-
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ing a small box corer. Approximately 10 g of sediment were removed from the upper 0.5^1 cm of the core and stored in plastic vials until analysis. 2.2. Sample analyses
Fig. 1. Map showing stations sampled in the Ob River system. (A) Overview of system showing locations of weapons test sites at Novaya Zemlya and Semipalatinsk and nuclear fuel and weapons production/reprocessing facilities at Mayak and Tomsk-7. (B) Detail of area studied showing sampling locations. Stations marked with a ¢lled circle were sampled in 1994. Samples indicated with a ¢lled square were sampled in 1995. All stations were in the freshwater portion of the system when sampled. Map provided courtesy of T. Kenna (Woods Hole Oceanographic Institution).
cleaned prior to use. A new CFF ¢lter and housing were used at each station to limit cross-contamination between stations. In addition to the water column sampling, sur¢cial bottom sediments were collected in 1994 us-
2.2.1. Radionuclides Suspended sediment samples from the large volume ¢ltration were ashed (550³C for 24 h) to eliminate organic matter. The samples were assayed for 137 Cs by non-destructive gamma spectrometry using a low background germanium detector. The detector was calibrated with NIST Standard Reference Material #1645 (River Sediment) measured in the same geometry as the samples. The standard was measured before and after each set of samples. Uncertainties were calculated as 1c based on counting statistics. Plutonium isotopes (239, 240, 241, 242) and 237 Np were measured on the ashed sediment samples by thermal ionization mass spectrometry (TIMS). Approximately 1 g aliquots were totally dissolved with HCl and HF, and Pu and Np were radiochemically puri¢ed and mounted for TIMS [3]. Measurements of 129 I were made by accelerator mass spectrometry at the IsoTrace Laboratory, Canada [4]. Aliquots (450 ml) of the 0.2 Wm ¢ltered fraction (`dissolved') as well as the 1000 NMW^0.2 Wm and 6 1000 NMW (truly dissolved) fractions were analyzed. The measurements were normalized with respect to ISOT-2 reference material (129 I/127 I ratio = 1.174 þ 0.022U10311 ). Samples of bottom sediment collected in 1994 also were analyzed. Partition coe¤cients (Kd ) for Am, Cs and I were experimentally determined using 241 Am, 137 Cs and 131 I tracers, following protocols described elsewhere [5]. Bottom sediments collected in 1994 in the Ob were used in the experiments. Wet sediment was added to ¢ltered (0.2 Wm) Ob River water and the tracers were added. Kd values (l water/kg sediment) were calculated after the tracers had reached an apparent equilibrium with respect to partitioning, usually after 1^2 days. Values of Kd also were determined using sediment collected in the Kara Sea and ¢ltered Atlantic Ocean water. In an e¡ort to evaluate
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Fig. 2. Speci¢c activities (Bq g31 ) of 137 Cs (A), 239;240 Pu (B) and the 239;240 Pu/137 Cs activity ratio (C) in suspended sediment ( s 0.5 Wm) of the Ob River system. The data from 1994 and 1995 are plotted together and are grouped as follows: the Irtysh River above its con£uence with the Tobal (sample 95-12), the Tobal River above its con£uence with the Irtysh (95-10), the Irtysh after its con£uence with the Tobal (95-9, 7, 6), the Ob River above (95-4, 5) and below (95-1, 2; 94-1^11) its con£uence with the Irtysh, and the Taz River (94-13).
the possible e¡ect of dissolved organic carbon on Kd , a second set of Ob River experiments was run using Ob River water that had been UV-irradiated to photo-oxidize the DOC [6]. 2.2.2. Suspended solids, dissolved and particulate organic carbon Total suspended solids were determined gravimetrically after drying the 0.4 Wm Nuclepore ¢lter samples (1994). Particulate organic carbon (POC) was determined after drying the Whatman GF/F ¢lters at 60³C for 24 h and exposing them to fuming HCl to eliminate carbonate carbon. Filter samples were combusted in a Carlo Erba EA1108 Elemental Analyzer, and blank determinations were made using clean ¢lters and those subjected to HCl fuming [7]. Precision is estimated to be þ 5%. DOC was measured on aliquots of the 6 0.2 Wm fraction of the size-fractionated samples taken for 129 I. Samples were acidi¢ed in the ¢eld with 50% H3 PO4 and stored in the dark until analysis. DOC was measured using a Shimadzu 5000 TOC Analyzer. The uncertainty, based on replicate measurements, is estimated as þ 2%.
3. Results 3.1. Pu isotopes,
137
Cs and
237
Np
Cesium-137 activities and atom concentrations of Pu isotopes in suspended sediments of the Ob system are given in table 1 in the EPSL Online Background Dataset1 . The atom concentrations of 239 Pu and 240 Pu have been converted to activities and summed (239;240 Pu) to facilitate comparison with prior work (Fig. 2). All samples were collected in the freshwater portion of the system. Both Pu and Cs activities show considerable variability, exceeding factors of 10 for both radionuclides (Fig. 2). The lowest activities for both Pu and Cs are seen in the Irtysh River. The greatest activities for both are evident in the delta region of the lower Ob (94-11) and in the Taz River (9413). These latter samples overlap the 1993-94 sampling stations of Baskaran et al. [8]. Our 239;240 Pu
1
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Fig. 3. Atom ratios of
240
Pu/239 Pu (A) and
237
129
Np/239 Pu (B) in suspended sediment ( s 0.5 Wm) of the Ob River system.
values are somewhat greater than theirs (0.43^ 0.59 vs. 0.25^0.29 mBq g31 ) are, but the 137 Cs values (9^15 vs. 9^14 mBq g31 ) are comparable in the two data sets. The 239;240 Pu/137 Cs activity ratio (Fig. 2C and table 1 in the EPSL Online Background Dataset, see footnote 1) shows considerable variation, with high values in the lower Ob and Taz Rivers. In the Tobal and in the Irtysh after its con£uence with the Tobal (samples 95-10, 95-6, 9) the ratios appear to be lower than elsewhere in the system. The sample taken in the Irtysh at station 95-7 has a high Pu/Cs ratio but the large uncertainty in the 137 Cs makes it di¤cult to consider this sample in Table 1 Laboratory determination of Kd values for sediment from di¡erent stations in the Ob River and Kara Sea Location
Am
Cs
I
Kara Sea Ob Rivera 94-1 94-3 (+DOC)b 94-3 (3DOC) 94-5 94-8 94-11 (+DOC) 94-11 (3DOC) 94-13
1.1U105
2.5U102
^ 2.7U103 2.0U105 nm nm 7.0U103 2.1U105 nm
3.0U102 9.0U102 7.8U102 1.2U103 8.0U102 1.7U104 1.4U104 3.0U103
3.0U101 1 1.0U102 2.7U102 3.0U102 2.1U102 2.5U102 2.0U103 4.0U103 1.7U103
nm: no measurement. a Measurements made with bottom sediment collected at the stations indicated. b +DOC = Kd measured in natural Ob River water; 3DOC = Kd measured in UV-irradiated Ob River water.
the context of the others and it is not plotted in Fig. 2C. Atom concentrations of the Pu isotopes and 237 Np show variations in the suspended sediment reservoir of the Ob system (table 1 in the EPSL Online Background Dataset, see footnote 1). The highest concentrations are seen in the lower Ob including the delta region north of station 94-8 and in the Taz. Suspended sediments have greater speci¢c concentrations (atoms g31 ) than local bottom sediments (table 1 in the EPSL Online Background Dataset, see footnote 1), consistent with resuspension of a ¢ne-grained fraction with greater surface area. Isotope (atom) ratios of 240 Pu/ 239 Pu show signi¢cant variations (Fig. 3A) with the lowest 240 Pu/239 Pu ratios in the Ob above its con£uence with the Irtysh (95-4, 5). The sample taken in the Irtysh above its con£uence with the Tobal (95-12, Fig. 3A) is also low. The 237 Np/ 239 Pu ratios are more variable, with the highest value seen in the Irtysh above its con£uence with the Tobal (95-12, Fig. 3B) and low values in the Tobal and Taz. 3.2. Iodine-129 Iodine-129 shows strong gradients through the Ob River system (Fig. 4, table 2 in the EPSL Online Background Dataset, see footnote 1). In the Tobal River above its con£uence with the Irtysh, `dissolved' 129 I concentrations exceed 8U109 atoms l31 . Values decrease progressively towards the Kara Sea, with the lowest value (0.65U109
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atoms l31 ) seen in the Taz River (Fig. 4). In particular, the 129 I concentrations decrease at each con£uence of tributaries. The size-fractionated samples show that generally 6 5% of the 129 I is present in the colloidal fraction (table 2 in the EPSL Online Background Dataset, see footnote 1), relative to the 129 I that passes a 0.2 Wm ¢lter (`dissolved'). However, in the Taz River the colloidal 129 I is as great as 20% of the `dissolved' fraction. Iodine-129 concentrations in bottom sediments collected in 1994 range from 0.6 to 7.2U108 atoms g31 . Values of Kd calculated from the sediment and `dissolved' 129 I concentrations range from 0.4U102 to 6U102 (table 2 in the EPSL Online Background Dataset, see footnote 1). 3.3. Concentrations of suspended particles, POC and DOC Suspended particle concentrations in the Ob system are high, as previously reported [1]. Values measured in 1994 ranged from 14 to 174 mg l31 (see table 2 in the EPSL Online Background Dataset, see footnote 1). Excluding the latter number, which was measured shortly after a storm, the mean suspended sediment concentration of the lower Ob is 46 þ 19 mg l31 . POC ranges from
Fig. 5. Dissolved organic carbon ( 6 0.2 Wm) in the Ob River system.
0.08 to 0.27 mmol l31 , with no clear trend downriver (see table 2 in the EPSL Online Background Dataset, see footnote 1). In contrast, DOC shows a strong gradient from values as great as V2.5 mmol l31 in the Irtysh River to a minimum of 0.5 mmol l31 in the Taz (Fig. 5). Values in the Irtysh after its con£uence with the Tobal (95-6, 7, 9) are noticeably greater than elsewhere in the system and appear to increase downstream before decreasing to lower values in the Ob. In the Ob River proper, DOC ranges from 0.7 to 1.2 mmol l31 . 3.4. Distribution coe¤cients
Fig. 4. 129 I in size-fractionated samples of Ob River water. Samples were ¢ltered through 0.2 Wm ¢lters to collect `dissolved' iodine then through cross-£ow ¢ltration to separate colloidal 129 I (1000 NMW^0.2 Wm) from truly dissolved ( 6 1000 NMW).
Values for distribution coe¤cients (Kd ) of americium, cesium and iodine are given in Table 1. In unmodi¢ed Ob River water, Kd values range from 2.7 to 7U103 for 241 Am, 8U102 to 1.7U104 for 137 Cs and 1U102 to 2U103 for 129 I. Distribution coe¤cients measured after irradiating Ob River water with UV to destroy DOC showed the greatest change for Am, with values about
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two orders of magnitude higher. Cs and I showed less than a factor of two change in Kd measured in UV-irradiated river water vs. non-irradiated water. 4. Discussion 4.1. Sources of Pu and Cs Our sampling of suspended sediments in the Ob system presents snapshots of radionuclide concentrations and ratios during high river £ow in 1994 and 1995. We seek to identify the sources of anthropogenic radionuclides to the system as seen in the suspended sediments. Speci¢c activities or concentrations (Bq or atoms g31 ) are of limited use because grain size and composition of the suspended sediments play an important role in governing the activity. Isotope ratios are of greater use in identifying sources, but the ratios can be a¡ected by di¡erential geochemical behaviors with respect to solution^particle interactions. Interannual variability also may a¡ect comparisons of data taken in two years from di¡erent portions of the river system. However, the dissolved 129 I data suggest that the two years of sampling (1994 and 1995) are comparable (see below). Several sources contribute anthropogenic radionuclides to the Ob system. First, global stratospheric fallout from the atmospheric testing of atomic weapons has added anthropogenic radionuclides directly to the Ob and its watershed. Tropospheric fallout from weapons test sites in the former Soviet Union is also a likely source. Such sites were located in Novaya Zemlya in the Kara Sea and at Semipalatinsk in Kazakhstan. The latter is likely to be of greater signi¢cance for much of the Ob system because Bradley and Jenquin [26] have documented 137 Cs-contaminated soils adjacent to the Irtysh River near Semipalatinsk. The production and reprocessing facilities at Mayak and Tomsk-7 constitute direct inputs of anthropogenic radionuclides to the Ob River system. The former, in particular, has been documented as releasing about 100 PBq of liquid waste
131
to the Techa River during the period 1948^1956 and to nearby Lake Karachay since 1951 [9]. The `Kyshtym' accident in 1957, in which a storage tank exploded, as well as the desiccation of Lake Karachay in the 1960s further increased the releases of anthropogenic radionuclides from Mayak and led to contamination in soils taken near the facility [3,9]. Relative to our sampling sites, releases from Mayak might be evident in the Tobal River initially and be seen eventually at all stations except those on the Irtysh before its con£uence with the Tobal (95-12) and the Ob prior to its con£uence with the Irtysh (94-4, 5). In contrast, releases from Tomsk-7 might appear in the Ob before and after its con£uence with the Irtysh (Fig. 1). To determine the possible in£uences of these multiple sources of anthropogenic radionuclides to the Ob we consider Pu/Cs isotope ratios in separate portions of the system. Samples taken prior to the Irtysh^Ob con£uence have 239;240 Pu/ 137 Cs activity ratios 6 0.020, somewhat low relative to global fallout (Fig. 2C). In 1962 the global fallout value was approximately 0.012 [10,11], and correction for 137 Cs decay to the date of sample collection yields a value of V0.025. The 239;240 Pu/ 137 Cs activity ratios in sediments along the Techa River near Mayak are characterized by low values, V0.0030 on average [12]. Such material could help account for the low values seen in our samples, but the sample taken in the Tobal closest to Mayak (95-10) shows a value comparable to fallout within the uncertainty (0.019 þ 0.006). Indeed, much of the Ob River south of Salekhard, including the portion prior to its con£uence with the Irtysh, shows Pu/Cs activity ratios that are indistinguishable from global fallout mean (0.032 þ 0.006 for stations 95-2, 1; 94-1, 2, 4, 5). The lower Ob River, including the Taz (stations 94-8, 10, 11, 13), shows ratios higher, on average, than global fallout. This pattern is consistent with that determined by Sayles et al. [13], who sampled lakes throughout the Ob system. Occasional high Pu/Cs ratios are seen in the sediment cores analyzed for Cs and Pu by Sayles et al. [13], and these are mostly con¢ned to samples taken in the Taz. Sayles et al. [13] concluded however that low ac-
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tivity ratio material similar to that seen by Trapeznikov et al. [12] in the sediments near Mayak contributed at most a small fraction of Pu and Cs inventories to the lower Ob system. A complicating factor in attempts to reconstruct sources from Pu/Cs activity ratios is the possibility of di¡ering geochemistries of these radionuclides. Our data on distribution coe¤cients (Table 1) highlight several factors that may be important. Cesium is adsorbed onto particles in freshwater with Kd values ranging from 3U102 to 1.7U104 . The values were determined using bottom sediment, and much of the station-to-station variation is explained qualitatively by grain size. Stations with coarser-grained bottom sediments characteristically have lower Kd values. The Kd for Pu was not measured in the present study, but values in the literature show it to adsorb to a much greater extent than Cs [14]. Americium is considered to be even more reactive than Pu with respect to uptake onto particle surfaces [14], yet distribution coe¤cients for Am in the Ob are within about a factor of two of those determined for Cs. One reason for this similarity may be the high DOC content of water in the Ob system. Measurement of Kd for Am after destruction of the DOC shows values higher by factors of 30^70, comparable to those obtained with Kara Sea water (Table 1). It is likely that Pu also would show a similar dependence of Kd on DOC. The portion of the Ob system with the greatest DOC values is the Irtysh River before its con£uence with the Ob. Thus it is possible that Pu/Cs activity ratios are depressed in this area because of complexation and mobilization of Pu by dissolved organic ligands. Sampling of dissolved Pu and Cs would be necessary to resolve whether the observed ratios in the suspended sediments re£ect this process or the input of low ratio material from Semipalatinsk. 4.2. Plutonium isotope ratios in the Ob system The 240 Pu/239 Pu, 241 Pu/239 Pu and 242 Pu/239 Pu atom ratios also are potentially useful indicators of the sources of Pu to the Ob (Fig. 3A, and table 1 in the EPSL Online Background Dataset, see footnote 1). Unlike the 239;240 Pu/137 Cs ratio, the
Pu atom ratios are not a¡ected by di¡erential geochemical behavior, since isotopes of the same element are used. Moreover, the measurement sensitivity a¡orded by TIMS permits high precision measurement of the isotope ratios. There is signi¢cant variation in all the Pu isotope ratios (240/239, 241/239 and 242/239) in the Ob system. For example, relative to the value expected from northern latitude (30^71³N) global fallout (0.180 þ 0.014; [15]), 240 Pu/239 Pu ratios are markedly low in the Irtysh above its con£uence with the Tobal and in the Ob prior to its con£uence with the Irtysh (Stations 95-5, 4; Fig. 3A). Values that are somewhat depressed relative to the global fallout value persist throughout the Ob downstream of its con£uence with the Irtysh. The sample collected in the Taz, which has no known sources of Pu other than global fallout, shows the global value (Fig. 3A). An e¡ective approach for determining sources of Pu to the Ob watershed is through the use of isotope ratio plots [3,15,16]. Plots of 241 Pu/239 Pu and 242 Pu/239 Pu vs. 240 Pu/239 Pu reveal mixing line trends that indicate sources (Fig. 6A,B). As indicated above, one end-member for Pu mixing trends is global input from atmospheric testing of atomic weapons. The global component of fallout is derived from material that was injected into the stratosphere and distributed over the Earth through the atmospheric circulation. Regional variation in fallout £uxes has been observed (e.g. [17], and the variation in isotope ratios corresponding to this variation has been tabulated by Kelley et al. [15]. A second component of fallout is low altitude (tropospheric) material whose deposition is con¢ned regionally to areas proximal to test sites. Potential inputs to the Ob system could arise from weapons tests conducted at the Novaya Zemlya archipelago in the Kara Sea and at Semipalatinsk, adjacent to the Irtysh River. Low ratios of 240 Pu/239 Pu and 241 Pu/239 Pu have been measured in sediments of Chernaya Bay in Novaya Zemlya [27]. Pu inputs from testing in this area are likely to a¡ect only the lower Ob River while tests at Semipalatinsk could supply material to the upper Ob via the Irtysh. A third component of Pu in the Ob system arises from processed reactor
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Fig. 6. Isotope ratio mixing plots of (A) 241 Pu/239 Pu vs. 240 Pu/239 Pu, (B) 240 Pu/239 Pu in suspended and bottom sediments of the Ob River system.
fuel. Such sources include the reprocessing/production complexes at Mayak and at Tomsk-7. Releases from the former have contaminated the Techa River [12], while the latter constitutes a source of Pu contamination to the Ob River before its con£uence with the Irtysh (Fig. 1). For purposes of constructing mixing plots, we take as end-members the northern latitude soil results of Kelley et al. [15] as representative of global stratospheric fallout. Analyses of soils at Mayak and Semipalatinsk [3] are used to represent Pu derived from reprocessing and close-in fallout, respectively. Relative to these end-members, Pu isotope ratio mixing plots for the Ob system show signi¢cant fractions of non-global Pu in the samples (Fig. 6A,B). The Pu isotope ratios for many of the samples, particularly those collected on the Ob prior to its con£uence with the Irtysh and the Irtysh prior to its con£uence with the Tobal, show values that are lower than the global fallout value. The values generally follow the trend de¢ned by mixing of Pu from global fallout and reprocessed fuel (as represented by Mayak soil), although it is di¤cult to di¡erentiate the latter from close-in fallout represented by the soils taken at Semipalatinsk. In particular, the samples taken in the Ob River prior to its con£uence with the Irtysh (95-4, 5) show the largest fractions of non-fallout Pu. Plutonium from closein fallout from Semipalatinsk has low ratios, but is likely to be con¢ned to the watershed of the Irtysh River. Assuming that the Pu isotope signa-
242
Pu/239 Pu vs.
240
Pu/239 Pu and (C)
133
237
Np/239 Pu vs.
ture of releases from Tomsk-7 is comparable to that recorded at Mayak, the low ratios at stations 95-4 and 5 in the Ob are more likely due to releases from the reprocessing of spent fuel at the Tomsk-7 facilities. Other stations with low Pu isotope ratios include 95-12 on the Irtysh before its con£uence with the Ob and 95-2 on the Ob after its con£uence with the Irtysh. The former can be explained as transport of Pu from Semipalatinsk while the latter is likely due to continued transport of Pu originating from fuel reprocessing at Tomsk-7 (seen upriver at stations 95-4 and 5). Interestingly, the suspended sediment recovered from the Tobal shows no clear evidence of Pu originating at Mayak. Similarly the Irtysh below its con£uence with the Tobal also shows little in£uence of the Pu from Semipalatinsk seen in station 9512. 4.3. Neptunium-237 An approach similar to that taken with the Pu isotopes may be applied to 237 Np. This isotope is produced in nuclear devices via an (n, 2n) reaction on 238 U and production is greatest during the testing of high yield thermonuclear devices [15]. The testing of such devices at Novaya Zemlya produced high 237 Np/239 Pu atom ratios (0.48 þ 0.07) in northern latitude fallout [15]. A mixing line on the plot of 237 Np/239 Pu vs. 240 Pu/239 Pu is de¢ned with the end-members taken
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as stratospheric fallout as recorded in northern tier soils and reprocessing material as typi¢ed by soil taken near Mayak (Fig. 6C). Tropospheric fallout from Semipalatinsk is also plotted [3,15,16]. The data show several clusters of data: one along the mixing line, one with Np/Pu ratios lying above and one with values below the line. The values that cluster along the line tend to be those from the Ob River itself before and after its con£uence with the Irtysh (95-4, 5 and 95-1, 2, 941, 2, 4, 5, 8, 10). Stations on the Irtysh downstream of its con£uence with the Tobal (95-6, 7 and 9) lie closest to the northern tier soil endmember. Samples from the Ob River itself appear to represent a mixture of northern latitude fallout and Mayak material transported down the Ob. The suspended sediment sample from the Irtysh upstream of its con£uence with the Tobal (95-12) as well as bottom sediments from the lower Ob (94-1, 5 and 10) all have elevated 237 Np/239 Pu ratios and depressed 240 Pu/239 Pu ratios. This pattern is consistent with inputs from a reprocessing facility waste stream [15]. In the case of the Irtysh sample, however, it is unclear what the source of such material might be. The sample taken in the Tobal nearest Mayak has an unusually low 237 Np/ 239 Pu ratio but a 240 Pu/239 Pu ratio like northern latitude fallout (95-10; Fig. 3). Such an isotopic signature is characteristic of releases from reactor operations [15] and the Mayak facilities represent a possible source of such releases. It is interesting to note that suspended sediment from the Ob delta (94-11) and suspended and bottom sediments from the Taz (94-13) also possess a similar isotopic signature. As with Pu/Cs ratios, discussion of possible sources of Np to the Ob system is dependent on the assumption that the 237 Np/239 Pu isotope ratio re£ects the source without geochemical fractionation. Neptunium has been shown to be less reactive in its oxidized (V) state than as the more reduced IV and III forms [18,19]. Thus if Np is released in mixed valence states to the Ob system, progressive oxidation may mobilize it from particles and produce lower values as particles `age' in their transport to the Kara Sea. There is no clear evidence of such a trend in the data, although the lower 237 Np/239 Pu ratios in the Ob
delta (94-11) and in the Taz (94-13) may be due to Np loss. 4.4. Iodine-129 The pattern of 129 I in the Ob River system provides clear evidence of the release of this radionuclide from the Mayak facilities at Chelyabinsk (Fig. 4). `Dissolved' 129 I concentrations are greatest (V8U109 atoms l31 ) in the Tobal River before it joins the Irtysh. The size-fractionated sampling shows that s 70% of the 129 I that passes a 0.2 Wm ¢lter is truly dissolved in most of the Ob system (Fig. 4, table 2 in the EPSL Online Background Dataset, see footnote 1). In contrast to the Tobal results, dissolved 129 I concentrations in the Irtysh above its con£uence with the Tobal and in the Ob above its con£uence with the Irtysh are low, V2U109 (95-12) and V1U109 (95-4, 5) atoms l31 , respectively. As these rivers merge, ¢rst with the Tobal and then with the Irtysh, the Mayak 129 I signal is progressively diluted (Fig. 4). Downriver from the con£uence of the Ob and Irtysh, there is little change in 129 I concentration (values range from 2.2 to 2.8U109 atoms l31 ) until the broad delta portion of the system is reached (94-11, 14, 16). The lowest concentrations are measured in the Taz River (0.7U109 atoms l31 ). The near constancy in 129 I concentrations in the Ob from its con£uence with the Irtysh until it enters the delta provides a measure of the interannual variability in the system. Two of the samples in this reach of the river were collected in 1995 (95-1, 2) and the remainder in 1994. There is little di¡erence between the two sets of data. In contrast, Raisbeck (personal communication, 1995) measured 7U108 atoms l31 in a sample collected in 1993 near Salekhard and 8U108 atoms l31 in a sample collected in the lower Ob near station 94-14. These values are two to four times lower than our measurements on samples collected in 1994, and the di¡erences may highlight interannual variability in the system. None of our stations taken in 1995 is a precise reoccupation of a 1994 station. However, 95-1 and 94-1 are spatially the closest and are from the same part of the river. These stations have essentially
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the same 129 I concentration in the two years (2.72U109 vs. 2.78U109 atoms l31 ), and the pattern suggests that our 1994 and 1995 data sets are comparable. The likelihood that the decreasing129 I concentration downriver is caused by dilution of the signal as the various tributaries join the system can be checked by comparison with a soluble tracer such as 90 Sr. Paluszkiewicz et al. [20] modeled the concentrations and £uxes of 90 Sr through the Ob in 1985, a year in which the £ow at Salekhard was somewhat higher than average. The calculated 90 Sr concentrations of the Tobal were about four times those of the Ob at Salekhard during the time of year when our samples were taken. These results are comparable to the di¡erence of a factor of three in 129 I concentrations in the Tobal (95-10, 11) and Ob at Salekhard (94-5). The extent to which model results using discharge data from 1985 can be applied to our sampling in 1994^1995 is unclear because discharge data from the mid-1900s are unavailable. The possibility also exists that some of the decrease in 129 I, in the lower Ob, is due to removal of iodine from solution. Iodine is weakly adsorbed to particles, with Kd values measured in the laboratory ranging from 1U102 to 2U103 (Table 1). The latter, higher values are derived using sediment collected in the Ob delta and the Taz River, stations that show lower dissolved 129 I. We are able to calculate Kd values from the ¢eld data as well, using the 129 I concentrations measured in river water (`dissolved') and in bottom sediments (table 2 in the EPSL Online Background Dataset, see footnote 1). Values are equal to or less than those measured in the lab, but always agree within a factor of four. Both laboratory and ¢eld calculations of Kd use bottom sediments, but estimates from the ¢eld data assume that sorption equilibrium is attained between bottom sediments and overlying water. In oxic waters, iodine is present as the unreactive iodate (V) anion, but under reducing conditions it can be reduced to I3 . Such a transformation could occur in bottom sediments if the pore waters are anoxic. Complexation of I with organic matter also may play a role. Laboratory Kd determinations show less than a factor of two increase
135
in Kd after oxidation of DOC (Table 1), but the decrease in DOC concentrations in the lower Ob (Fig. 5) may be linked to enhanced removal of I from solution. Additional 129 I data exist from the Yenisei River [21], the Kara and Barents Seas [22] and the open Arctic [23]. The Yenisei River, also in Siberia, is the second largest Russian river in terms of discharge (V2U104 m3 s31 ) and is subject to releases of anthropogenic radionuclides from the Krasnoyarsk-26 nuclear complex. The Yenisei had 129 I concentrations ranging from 1.8U108 to 4.5U108 atoms l31 in 1993 [21], about one order of magnitude lower than we observed in the Ob in 1994 and 1995. 129 I concentrations in the Ob are comparable to those measured in Kara and Barents Sea surface waters (V9^25U108 atoms l31 [22]), but exceed values in surface waters of the open Arctic Ocean (0.2^8U108 atoms kg31 [21]). 4.5. Fluxes to the Kara Sea We are able to make some estimates of inputs of anthropogenic radionuclides from the Ob River to the Kara Sea. The most important component of the £ux with respect to large-scale dispersion in the Arctic is likely to be the dissolved £ux. The £ux of particles transported through the river system is di¤cult to predict because particles can be trapped in depositional areas such as the reach between the con£uence with the Irtysh and Salekhard [24]. 129 I is present in the Ob mostly in solution, and we have su¤cient data to estimate the dissolved £ux. The mean annual discharge of the Ob is V1.4U104 m3 s31 (4.25U1014 l yr31 [20]). As this value applies to the river after all tributaries have joined it, we use 129 I concentrations from the lower river to calculate the £ux. `Dissolved' 129 I values in the lower Ob range from 0.65 to 2.7U109 atoms l31 , yielding a £ux of 0.3^1.1U1024 atoms yr31 [25]. The annual discharge of the Yenisei is V2U104 m3 s31 , and using this value and 129 I concentrations of 1.8^ 4.5U108 atoms l31 [21] yields a £ux of 0.1^ 0.3U1024 atoms yr31 from this river to the Kara Sea. The combined 129 I £ux from the Ob and Yeni-
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sei Rivers, 0.4^1.4U1024 atoms yr31 , may be compared with other releases to the Arctic. The major releases of 129 I have been from the reprocessing facilities at La Hague, France and Sella¢eld, UK. The combined annual releases from La Hague and Sella¢eld have ranged from 1.68 to 4.67U1026 atoms yr31 over the period 1978^ 1991 [4] with an average of V2.6U1026 atoms yr31 during this time. The £ux from the Ob and Yenisei is 0.2^0.5% of this value. We have only particle data for the other radionuclides measured, but estimates of the dissolved £ux can be made assuming sorption equilibrium is attained. For example, 237 Np has an average concentration in the lower Ob (94-8, 10, 11) of V7U107 atoms g31 . For a Kd for Np of 5U103 [14], the dissolved concentration of 237 Np is predicted to be V1U107 atoms l31 . Using the discharge estimates for the Ob given above, the dissolved £ux is V4U1021 atoms yr31 . In comparison, discharges of 237 Np from Sella¢eld average V2U1025 atoms yr31 during the period 1992^1994 [23]. The £ux of 237 Np from the Ob is only 0.02% of the Sella¢eld discharge, a factor of 10 lower than that for 129 I. Fluxes of 137 Cs and Pu are likely to be similarly low. Acknowledgements We are grateful to Drs. Hugh Livingston and Fred Sayles for allowing us to participate in their 1994 and 1995 sampling trips on the Ob River and for subsequent discussions of the results. The sampling expeditions would not have been possible without the expert organizational and logistical skills of Gera Panteleyev, who tragically lost his life during the 1995 sampling expedition. The 129 I measurements were made by Dr. Linas Kilius at the IsoTrace Laboratory (University of Toronto, Ont., Canada). Linas also died during the course of this research, and we dedicate this paper to the memory of these two colleagues. Wendy Woods (University of Rhode Island) participated in the 1995 expedition to collect the suspended sediment samples. We are grateful to Steven Smith and Joanne Goudreau (Woods Hole Oceanographic Institution) and Matthew Monetti
(Environmental Measurements Laboratory) for help in the ¢eld. Dr. Olga Medkova participated in the cruises and facilitated shipping of gear and samples to the ¢eld site. David Hirschberg and Kim Roberts (MSRC, SUNY, Stony Brook) assisted in the laboratory and Tim Kenna (Woods Hole Oceanographic Institution) provided the base map of sampling stations. This research was supported by Grants N000149410 and N000149511197 from the O¤ce of Naval Research, as part of the Arctic Nuclear Waste Assessment (ANWAP) Program. This is Contribution No. 1188 from the Marine Sciences Research Center.[EB] References [1] M.-H. Dai, J.-M. Martin, First data on trace metal level and behavior in two major Arctic river-estuarine systems (Ob and Yenisey) and in the adjacent Kara Sea, Russia, Earth Planet. Sci. Lett. 131 (1995) 127^141. [2] S.B. Moran, W.L. Woods, Cd, Cr, Cu, Ni and Pb in the water column and sediments of the Ob-Irtysh Rivers, Russia, Mar. Pollut. Bull. 35 (1997) 270^279. [3] T.M. Beasley, J.M. Kelley, K.A. Orlandini, L.A. Bond, A. Aarkrog, A.P. Trapeznikov, V.N. Pozolotina, Isotopic Pu, U, and Np signatures in soils from Semipalatinsk21, Kazakh Republic and the Southern Urals, Russia, J. Environ. Radioact. 39 (1998) 215^230. [4] G.M. Raisbeck, F. Yiou, Z.Q. Zhou, L.R. Kilius, 129 I from nuclear fuel reprocessing facilities at Sella¢eld (UK) and La Hague (France): potential as an oceanographic tracer, J. Mar. Systems 6 (1995) 561^570. [5] N.S. Fisher, P. Bjerregaard, S.W. Fowler, Interactions of marine plankton with transuranic elements. I. Biokinetics of neptunium, plutonium, americium, and californium in phytoplankton, Limnol. Oceanogr. 28 (1983) 432^447. [6] F.A. Armstrong, P.M. Williams, J.D. Strickland, Photooxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications, Nature 211 (1966) 481^487. [7] S.M. Pike, S.B. Moran, Use of Poretics 0.7 Wm pore size glass ¢ber ¢lters for determination of particulate organic carbon in seawater and fresh water, Mar. Chem. 57 (1997) 355^360. [8] M. Baskaran, S. Asbill, P. Santschi, J. Brooks, M. Champ, D. Adkinson, M.R. Colmer, V. Maeyev, Pu, 137 Cs and excess 210 Pb in Russian Arctic sediments, Earth Planet. Sci. Lett. 140 (1996) 243^257. [9] G.C. Christensen, G.N. Romanov, P. Strand, B. Salbu, S.V. Malyshev, T.D. Bergan, D. Oughton, E.G. Drozhko, Y.V. Glagolenko, I. Amundsen, A.L. Rudjord, T.O.
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Bjerk, B. Lind, Radioactive contamination in the environment of the enterprise `Mayak' PA: Results from the joint Russian-Norwegian ¢eld work in 1994, in: P. Strand, B. Howard, A. Cook (Eds.), Environmental Radioactivity in the Arctic, Sci. Total Environ. 202 (1997) 237^248. H.D. Livingston, V.T. Bowen, Pu and 137 Cs in coastal sediments, Earth Planet. Sci. Lett. 43 (1979) 29^45. H.L. Beck, P.W. Krey, Radiation exposure in Utah from Nevada nuclear tests, Science 220 (1983) 18^24. A.V. Trapeznikov, A. Aarkrog, N.V. Kulikov, S.P. Nielsen, V.N. Pozolotina, G. Polikarpov, V.N. Trapezhikova, M.Ya. Chebotina, P. Yushkov, Radioactive contamination of the Ob River system from the nuclear enterprise `Mayak' in the Urals, in: P. Strand, E. Holm (Eds.), Environmental Radioactivity in the Arctic and Antarctic, Norwegian Radiation Protection Authority, Osteras, 1993, pp. 135^150. F.L. Sayles, H.D. Livingston, G.P. Panteleyev, The history and source of particulate 137 Cs and 239;240 Pu deposition in sediments of the Ob River delta, Siberia, in: P. Strand, B. Howard, A. Cook (Eds.), Environmental Radioactivity in the Arctic, Sci. Total Environ. 202 (1997) 25^41. Sediment Kd s and Concentration Factors for Radionuclides in the Marine Environment, International Atomic Energy Agency, Tech. Document #257, Vienna, 1985, 73 pp. J.M. Kelley, L.A. Bond, T.M. Beasley, Inventory and composition of Pu isotopes and 237 Np in soil. In: Proceedings from the International Symposium on Marine Pollution, IAEA, Monaco, 1998, in press. T.M. Beasley, J.M. Kelley, T.C. Maiti, L.A. Bond, 237 Np/ 239 Pu ratios in integrated fallout: a reassessment of the production of 237 Np, J. Environ. Radioact. 38 (1998) 133^ 146. E.P. Hardy, P.W. Krey, H.L. Volchok, Global inventory and distribution of fallout plutonium, Nature 241 (1973) 444^445. E.A. Bondietti, C.W. Francis, Geological migration potentials of technetium-99 and neptunium-237, Science 203 (1979) 1337^1340. J.M. Cleveland, T.F. Rees, K.L. Nash, Neptunium and
[20]
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americium speciation in selected basalt, granite, shale and tu¡ ground waters, Science 221 (1983) 271^273. T.L. Paluszkiewicz, L.F. Hibler, P. Becker, A. Mandych, M.C. Richmond, S. Thomas, An assessment of the £ux of 90 Sr contamination through the Ob' River and estuary to the Kara Sea, in: P. Strand, B. Howard, A. Cook (Eds.), Environmental Radioactivity in the Arctic, Sci. Total Environ. 202 (1997) 43^56. T.M. Beasley, L.W. Cooper, J.M. Grebmeier, L.R. Kilius, H.-A. Synal, 36 Cl and 129 I in the Yenisei, Kolyma and Mackenzie Rivers, Environ. Sci. Technol. 31 (1997) 1834^1836. G.M. Raisbeck, F. Yiou, Z.Q. Zhou, L.R. Kilius, H. Dahlgaard, Anthropogenic 129 I in the Kara Sea, in: P. Strand, E. Holm (Eds.), Environmental Radioactivity in the Arctic and Antarctic, Norwegian Radiation Protection Authority, Osteras, pp. 125^128. T. Beasley, L.W. Cooper, J.M. Grebmeier, K. Aagaard, J.M. Kelley, L.R. Kilius, 237 Np/129 I atom ratios in the Arctic Ocean: Has 237 Np from Western European and Russian fuel reprocessing facilities entered the Arctic Ocean?, J. Environ. Radioact. 39 (1998) 255^278. T.L. Paluszkiewicz, L.F. Hibler, M.C. Richmond, D.J. Bradley, An assessment of the £ux of radionuclide contamination through the Ob and Yenisei Rivers and estuaries to the Kara Sea, in: P. Strand (Ed.), Proceedings of the International Conference on Environmental Radioactivity in the Arctic, Norwegian Radiation Protection Authority, Osteras, 1995. S.B. Moran, J.K. Cochran, N.S. Fisher, L.R. Kilius, 129 I in the Ob River, in: P. Strand, A. Cooke (Eds.), Environmental Radioactivity in the Arctic. Scienti¢c Committee of the International Conference on Environmental Radioactivity in the Arctic, Oslo, 1995, pp. 75^78. D.J. Bradley, U.P. Jenquin, Radioactive Inventories and Sources for Contamination of the Kara Sea by Riverine Transport, Report PNWD-2316, Paci¢c Northwest Laboratory, Richland, WA, 1995. J.N. Smith, K.M. Ellis, L. Polyak, G. Ivanov, S.L. Forman, S.B. Moran, 239;240 Pu transport into the Arctic Ocean from underwater nuclear tests in Chernaya Bay, Novaya Zemlya, Cont. Shelf Res. (1999) submitted.
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Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia J. Kirk Cochran a; *, S. Bradley Moran b , Nicholas S. Fisher a , Thomas M. Beasley c , James M. Kelley d a
Marine Sciences Research Center, State University of New York Stony Brook, NY 11794, USA Graduate School of Oceanography, The University of Rhode Island, Narragansett, RI 02882, USA Environmental Measurements Laboratory, US Department of Energy, 201 Varick St., New York, NY 10014, USA d Paci¢c Northwest National Laboratory, Battelle Blvd. MS P7-07, Richland, WA 99352, USA b
c
Received 5 August 1999; accepted 5 April 2000
Abstract 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 Kd 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 Kd 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 Wm) contains generally 6 5% relative to that passing a 0.2 Wm filter. Iodine-129 concentrations decrease from 8.3U109 to 0.65U109 atoms l31 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
* Corresponding author. Tel.: +1-516-632-8733; Fax: +1-516-632-8820; E-mail: [email protected] 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 1 0 - 2
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system. Fluxes of 129 I to the lower Ob at Salekhard are 6 1% of the releases of this radionuclide from the nuclear fuel reprocessing facilities at Sellafield, UK and La Hague, France. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: Ob River; human activity; radioactive isotopes; fallout
1. Introduction The Ob River in western Siberia has a mean discharge of 1.4U104 m3 s31 and is the third largest Russian river. The Ob watershed is extensive (V3U106 km2 ) and has received radioactive contaminants from global fallout from nuclear weapons testing, local fallout from weapons testing at Novaya Zemlya and Semipalatinsk and, perhaps most signi¢cantly, discharges from nuclear facilities located on or near rivers that drain into the Ob. In particular, releases of radioactivity from Russian production reactors and reprocessing facilities such as Tomsk-7 and Mayak are potential sources of anthropogenic radionuclides (137 Cs, 90 Sr, 239;240 Pu, 241 Am, 60 Co, 237 Np, 129 I and others) to the Ob system. The Ob drains into the Kara Sea and thus may represent an important pathway for delivery of anthropogenic radionuclides to the Arctic Ocean (Fig. 1). The Ob River is characterized by fairly high particle loads (tens of milligrams per liter), and under such conditions, many of the radionuclides released to the Ob watershed (e.g. 137 Cs, Pu, 241 Am) would be expected to associate predominantly with particles, particularly in the freshwater portion of the system. Less reactive radionuclides such as 90 Sr or 129 I will be transported more in the dissolved phase. Dai and Martin [1] documented high dissolved (DOC) and colloidal organic concentrations on the Ob and showed that colloidal material (de¢ned as the size fraction from 104 Da to 0.4 Wm) was important in transporting trace metals such as Fe, Ni and Cu in the Ob. Moran and Woods [2] showed that Cu, Cr and Ni in suspended and bottom sediments of the Ob were present in crustal abundances, while Cd and Pb had additional (anthropogenic) sources. In this paper we present results of analyses of anthropogenic radionuclides in the freshwater portion of the Ob River. For the particle-reactive radionuclides (e.g. 137 Cs, Pu isotopes, 237 Np) our
emphasis has been on the suspended particle reservoir and for the soluble radionuclides (129 I), on the dissolved fraction. Our goal is to attempt to characterize the sources and transport of these radionuclides in the Ob system. 2. Sample collection and analysis 2.1. Station locations and sampling Water samples were collected in the Ob, Irtysh, Tobal and Taz Rivers, northwestern Siberia, in July^August 1994 and in June 1995 (Fig. 1). The Russian Fisheries Protection vessel RS300#168, based in Salekhard, was used for the sampling. Suspended sediment samples of 10^15 g were collected by ¢ltering 50^200 l from a water depth of 1^2 m through 0.5 Wm cartridge ¢lters (Microwynd DPPPZ1). Additional samples (V100 ml) were taken for suspended particulate matter concentration and particulate organic carbon. The former were ¢ltered through pre-weighed 0.4 Wm Nuclepore ¢lters and the latter through pre-combusted Whatman 25 mm glass ¢ber ¢lters (0.7 Wm). Size-fractionated sampling was conducted for 129 I. Approximately 60 l of surface water were pumped using a Flotec pump through acidcleaned tubing connected to a 0.2 Wm Gelman Maxi capsule ¢lter. The `dissolved' ¢lter-passing fraction ( 6 0.2 Wm) was connected to a cross-£ow ¢ltration system with an Osmonics 1000 nominal molecular weight (NMW) spiral wound membrane (polysulfone) in a PVC housing. Cross£ow ¢ltration (CFF) was conducted in a recirculating mode, resulting in collection of colloidal (1000 NMW^0.2 Wm) and truly dissolved ( 6 1000 NMW) size fractions. The wetted parts of the system were PVC, silicon, Te£on and polyethylene, and both system and ¢lters were acid-
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127
ing a small box corer. Approximately 10 g of sediment were removed from the upper 0.5^1 cm of the core and stored in plastic vials until analysis. 2.2. Sample analyses
Fig. 1. Map showing stations sampled in the Ob River system. (A) Overview of system showing locations of weapons test sites at Novaya Zemlya and Semipalatinsk and nuclear fuel and weapons production/reprocessing facilities at Mayak and Tomsk-7. (B) Detail of area studied showing sampling locations. Stations marked with a ¢lled circle were sampled in 1994. Samples indicated with a ¢lled square were sampled in 1995. All stations were in the freshwater portion of the system when sampled. Map provided courtesy of T. Kenna (Woods Hole Oceanographic Institution).
cleaned prior to use. A new CFF ¢lter and housing were used at each station to limit cross-contamination between stations. In addition to the water column sampling, sur¢cial bottom sediments were collected in 1994 us-
2.2.1. Radionuclides Suspended sediment samples from the large volume ¢ltration were ashed (550³C for 24 h) to eliminate organic matter. The samples were assayed for 137 Cs by non-destructive gamma spectrometry using a low background germanium detector. The detector was calibrated with NIST Standard Reference Material #1645 (River Sediment) measured in the same geometry as the samples. The standard was measured before and after each set of samples. Uncertainties were calculated as 1c based on counting statistics. Plutonium isotopes (239, 240, 241, 242) and 237 Np were measured on the ashed sediment samples by thermal ionization mass spectrometry (TIMS). Approximately 1 g aliquots were totally dissolved with HCl and HF, and Pu and Np were radiochemically puri¢ed and mounted for TIMS [3]. Measurements of 129 I were made by accelerator mass spectrometry at the IsoTrace Laboratory, Canada [4]. Aliquots (450 ml) of the 0.2 Wm ¢ltered fraction (`dissolved') as well as the 1000 NMW^0.2 Wm and 6 1000 NMW (truly dissolved) fractions were analyzed. The measurements were normalized with respect to ISOT-2 reference material (129 I/127 I ratio = 1.174 þ 0.022U10311 ). Samples of bottom sediment collected in 1994 also were analyzed. Partition coe¤cients (Kd ) for Am, Cs and I were experimentally determined using 241 Am, 137 Cs and 131 I tracers, following protocols described elsewhere [5]. Bottom sediments collected in 1994 in the Ob were used in the experiments. Wet sediment was added to ¢ltered (0.2 Wm) Ob River water and the tracers were added. Kd values (l water/kg sediment) were calculated after the tracers had reached an apparent equilibrium with respect to partitioning, usually after 1^2 days. Values of Kd also were determined using sediment collected in the Kara Sea and ¢ltered Atlantic Ocean water. In an e¡ort to evaluate
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Fig. 2. Speci¢c activities (Bq g31 ) of 137 Cs (A), 239;240 Pu (B) and the 239;240 Pu/137 Cs activity ratio (C) in suspended sediment ( s 0.5 Wm) of the Ob River system. The data from 1994 and 1995 are plotted together and are grouped as follows: the Irtysh River above its con£uence with the Tobal (sample 95-12), the Tobal River above its con£uence with the Irtysh (95-10), the Irtysh after its con£uence with the Tobal (95-9, 7, 6), the Ob River above (95-4, 5) and below (95-1, 2; 94-1^11) its con£uence with the Irtysh, and the Taz River (94-13).
the possible e¡ect of dissolved organic carbon on Kd , a second set of Ob River experiments was run using Ob River water that had been UV-irradiated to photo-oxidize the DOC [6]. 2.2.2. Suspended solids, dissolved and particulate organic carbon Total suspended solids were determined gravimetrically after drying the 0.4 Wm Nuclepore ¢lter samples (1994). Particulate organic carbon (POC) was determined after drying the Whatman GF/F ¢lters at 60³C for 24 h and exposing them to fuming HCl to eliminate carbonate carbon. Filter samples were combusted in a Carlo Erba EA1108 Elemental Analyzer, and blank determinations were made using clean ¢lters and those subjected to HCl fuming [7]. Precision is estimated to be þ 5%. DOC was measured on aliquots of the 6 0.2 Wm fraction of the size-fractionated samples taken for 129 I. Samples were acidi¢ed in the ¢eld with 50% H3 PO4 and stored in the dark until analysis. DOC was measured using a Shimadzu 5000 TOC Analyzer. The uncertainty, based on replicate measurements, is estimated as þ 2%.
3. Results 3.1. Pu isotopes,
137
Cs and
237
Np
Cesium-137 activities and atom concentrations of Pu isotopes in suspended sediments of the Ob system are given in table 1 in the EPSL Online Background Dataset1 . The atom concentrations of 239 Pu and 240 Pu have been converted to activities and summed (239;240 Pu) to facilitate comparison with prior work (Fig. 2). All samples were collected in the freshwater portion of the system. Both Pu and Cs activities show considerable variability, exceeding factors of 10 for both radionuclides (Fig. 2). The lowest activities for both Pu and Cs are seen in the Irtysh River. The greatest activities for both are evident in the delta region of the lower Ob (94-11) and in the Taz River (9413). These latter samples overlap the 1993-94 sampling stations of Baskaran et al. [8]. Our 239;240 Pu
1
http://www.elsevier.nl/locate/epsl; mirror site: http://www.elsevier.com/locate/epsl
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Fig. 3. Atom ratios of
240
Pu/239 Pu (A) and
237
129
Np/239 Pu (B) in suspended sediment ( s 0.5 Wm) of the Ob River system.
values are somewhat greater than theirs (0.43^ 0.59 vs. 0.25^0.29 mBq g31 ) are, but the 137 Cs values (9^15 vs. 9^14 mBq g31 ) are comparable in the two data sets. The 239;240 Pu/137 Cs activity ratio (Fig. 2C and table 1 in the EPSL Online Background Dataset, see footnote 1) shows considerable variation, with high values in the lower Ob and Taz Rivers. In the Tobal and in the Irtysh after its con£uence with the Tobal (samples 95-10, 95-6, 9) the ratios appear to be lower than elsewhere in the system. The sample taken in the Irtysh at station 95-7 has a high Pu/Cs ratio but the large uncertainty in the 137 Cs makes it di¤cult to consider this sample in Table 1 Laboratory determination of Kd values for sediment from di¡erent stations in the Ob River and Kara Sea Location
Am
Cs
I
Kara Sea Ob Rivera 94-1 94-3 (+DOC)b 94-3 (3DOC) 94-5 94-8 94-11 (+DOC) 94-11 (3DOC) 94-13
1.1U105
2.5U102
^ 2.7U103 2.0U105 nm nm 7.0U103 2.1U105 nm
3.0U102 9.0U102 7.8U102 1.2U103 8.0U102 1.7U104 1.4U104 3.0U103
3.0U101 1 1.0U102 2.7U102 3.0U102 2.1U102 2.5U102 2.0U103 4.0U103 1.7U103
nm: no measurement. a Measurements made with bottom sediment collected at the stations indicated. b +DOC = Kd measured in natural Ob River water; 3DOC = Kd measured in UV-irradiated Ob River water.
the context of the others and it is not plotted in Fig. 2C. Atom concentrations of the Pu isotopes and 237 Np show variations in the suspended sediment reservoir of the Ob system (table 1 in the EPSL Online Background Dataset, see footnote 1). The highest concentrations are seen in the lower Ob including the delta region north of station 94-8 and in the Taz. Suspended sediments have greater speci¢c concentrations (atoms g31 ) than local bottom sediments (table 1 in the EPSL Online Background Dataset, see footnote 1), consistent with resuspension of a ¢ne-grained fraction with greater surface area. Isotope (atom) ratios of 240 Pu/ 239 Pu show signi¢cant variations (Fig. 3A) with the lowest 240 Pu/239 Pu ratios in the Ob above its con£uence with the Irtysh (95-4, 5). The sample taken in the Irtysh above its con£uence with the Tobal (95-12, Fig. 3A) is also low. The 237 Np/ 239 Pu ratios are more variable, with the highest value seen in the Irtysh above its con£uence with the Tobal (95-12, Fig. 3B) and low values in the Tobal and Taz. 3.2. Iodine-129 Iodine-129 shows strong gradients through the Ob River system (Fig. 4, table 2 in the EPSL Online Background Dataset, see footnote 1). In the Tobal River above its con£uence with the Irtysh, `dissolved' 129 I concentrations exceed 8U109 atoms l31 . Values decrease progressively towards the Kara Sea, with the lowest value (0.65U109
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atoms l31 ) seen in the Taz River (Fig. 4). In particular, the 129 I concentrations decrease at each con£uence of tributaries. The size-fractionated samples show that generally 6 5% of the 129 I is present in the colloidal fraction (table 2 in the EPSL Online Background Dataset, see footnote 1), relative to the 129 I that passes a 0.2 Wm ¢lter (`dissolved'). However, in the Taz River the colloidal 129 I is as great as 20% of the `dissolved' fraction. Iodine-129 concentrations in bottom sediments collected in 1994 range from 0.6 to 7.2U108 atoms g31 . Values of Kd calculated from the sediment and `dissolved' 129 I concentrations range from 0.4U102 to 6U102 (table 2 in the EPSL Online Background Dataset, see footnote 1). 3.3. Concentrations of suspended particles, POC and DOC Suspended particle concentrations in the Ob system are high, as previously reported [1]. Values measured in 1994 ranged from 14 to 174 mg l31 (see table 2 in the EPSL Online Background Dataset, see footnote 1). Excluding the latter number, which was measured shortly after a storm, the mean suspended sediment concentration of the lower Ob is 46 þ 19 mg l31 . POC ranges from
Fig. 5. Dissolved organic carbon ( 6 0.2 Wm) in the Ob River system.
0.08 to 0.27 mmol l31 , with no clear trend downriver (see table 2 in the EPSL Online Background Dataset, see footnote 1). In contrast, DOC shows a strong gradient from values as great as V2.5 mmol l31 in the Irtysh River to a minimum of 0.5 mmol l31 in the Taz (Fig. 5). Values in the Irtysh after its con£uence with the Tobal (95-6, 7, 9) are noticeably greater than elsewhere in the system and appear to increase downstream before decreasing to lower values in the Ob. In the Ob River proper, DOC ranges from 0.7 to 1.2 mmol l31 . 3.4. Distribution coe¤cients
Fig. 4. 129 I in size-fractionated samples of Ob River water. Samples were ¢ltered through 0.2 Wm ¢lters to collect `dissolved' iodine then through cross-£ow ¢ltration to separate colloidal 129 I (1000 NMW^0.2 Wm) from truly dissolved ( 6 1000 NMW).
Values for distribution coe¤cients (Kd ) of americium, cesium and iodine are given in Table 1. In unmodi¢ed Ob River water, Kd values range from 2.7 to 7U103 for 241 Am, 8U102 to 1.7U104 for 137 Cs and 1U102 to 2U103 for 129 I. Distribution coe¤cients measured after irradiating Ob River water with UV to destroy DOC showed the greatest change for Am, with values about
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two orders of magnitude higher. Cs and I showed less than a factor of two change in Kd measured in UV-irradiated river water vs. non-irradiated water. 4. Discussion 4.1. Sources of Pu and Cs Our sampling of suspended sediments in the Ob system presents snapshots of radionuclide concentrations and ratios during high river £ow in 1994 and 1995. We seek to identify the sources of anthropogenic radionuclides to the system as seen in the suspended sediments. Speci¢c activities or concentrations (Bq or atoms g31 ) are of limited use because grain size and composition of the suspended sediments play an important role in governing the activity. Isotope ratios are of greater use in identifying sources, but the ratios can be a¡ected by di¡erential geochemical behaviors with respect to solution^particle interactions. Interannual variability also may a¡ect comparisons of data taken in two years from di¡erent portions of the river system. However, the dissolved 129 I data suggest that the two years of sampling (1994 and 1995) are comparable (see below). Several sources contribute anthropogenic radionuclides to the Ob system. First, global stratospheric fallout from the atmospheric testing of atomic weapons has added anthropogenic radionuclides directly to the Ob and its watershed. Tropospheric fallout from weapons test sites in the former Soviet Union is also a likely source. Such sites were located in Novaya Zemlya in the Kara Sea and at Semipalatinsk in Kazakhstan. The latter is likely to be of greater signi¢cance for much of the Ob system because Bradley and Jenquin [26] have documented 137 Cs-contaminated soils adjacent to the Irtysh River near Semipalatinsk. The production and reprocessing facilities at Mayak and Tomsk-7 constitute direct inputs of anthropogenic radionuclides to the Ob River system. The former, in particular, has been documented as releasing about 100 PBq of liquid waste
131
to the Techa River during the period 1948^1956 and to nearby Lake Karachay since 1951 [9]. The `Kyshtym' accident in 1957, in which a storage tank exploded, as well as the desiccation of Lake Karachay in the 1960s further increased the releases of anthropogenic radionuclides from Mayak and led to contamination in soils taken near the facility [3,9]. Relative to our sampling sites, releases from Mayak might be evident in the Tobal River initially and be seen eventually at all stations except those on the Irtysh before its con£uence with the Tobal (95-12) and the Ob prior to its con£uence with the Irtysh (94-4, 5). In contrast, releases from Tomsk-7 might appear in the Ob before and after its con£uence with the Irtysh (Fig. 1). To determine the possible in£uences of these multiple sources of anthropogenic radionuclides to the Ob we consider Pu/Cs isotope ratios in separate portions of the system. Samples taken prior to the Irtysh^Ob con£uence have 239;240 Pu/ 137 Cs activity ratios 6 0.020, somewhat low relative to global fallout (Fig. 2C). In 1962 the global fallout value was approximately 0.012 [10,11], and correction for 137 Cs decay to the date of sample collection yields a value of V0.025. The 239;240 Pu/ 137 Cs activity ratios in sediments along the Techa River near Mayak are characterized by low values, V0.0030 on average [12]. Such material could help account for the low values seen in our samples, but the sample taken in the Tobal closest to Mayak (95-10) shows a value comparable to fallout within the uncertainty (0.019 þ 0.006). Indeed, much of the Ob River south of Salekhard, including the portion prior to its con£uence with the Irtysh, shows Pu/Cs activity ratios that are indistinguishable from global fallout mean (0.032 þ 0.006 for stations 95-2, 1; 94-1, 2, 4, 5). The lower Ob River, including the Taz (stations 94-8, 10, 11, 13), shows ratios higher, on average, than global fallout. This pattern is consistent with that determined by Sayles et al. [13], who sampled lakes throughout the Ob system. Occasional high Pu/Cs ratios are seen in the sediment cores analyzed for Cs and Pu by Sayles et al. [13], and these are mostly con¢ned to samples taken in the Taz. Sayles et al. [13] concluded however that low ac-
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tivity ratio material similar to that seen by Trapeznikov et al. [12] in the sediments near Mayak contributed at most a small fraction of Pu and Cs inventories to the lower Ob system. A complicating factor in attempts to reconstruct sources from Pu/Cs activity ratios is the possibility of di¡ering geochemistries of these radionuclides. Our data on distribution coe¤cients (Table 1) highlight several factors that may be important. Cesium is adsorbed onto particles in freshwater with Kd values ranging from 3U102 to 1.7U104 . The values were determined using bottom sediment, and much of the station-to-station variation is explained qualitatively by grain size. Stations with coarser-grained bottom sediments characteristically have lower Kd values. The Kd for Pu was not measured in the present study, but values in the literature show it to adsorb to a much greater extent than Cs [14]. Americium is considered to be even more reactive than Pu with respect to uptake onto particle surfaces [14], yet distribution coe¤cients for Am in the Ob are within about a factor of two of those determined for Cs. One reason for this similarity may be the high DOC content of water in the Ob system. Measurement of Kd for Am after destruction of the DOC shows values higher by factors of 30^70, comparable to those obtained with Kara Sea water (Table 1). It is likely that Pu also would show a similar dependence of Kd on DOC. The portion of the Ob system with the greatest DOC values is the Irtysh River before its con£uence with the Ob. Thus it is possible that Pu/Cs activity ratios are depressed in this area because of complexation and mobilization of Pu by dissolved organic ligands. Sampling of dissolved Pu and Cs would be necessary to resolve whether the observed ratios in the suspended sediments re£ect this process or the input of low ratio material from Semipalatinsk. 4.2. Plutonium isotope ratios in the Ob system The 240 Pu/239 Pu, 241 Pu/239 Pu and 242 Pu/239 Pu atom ratios also are potentially useful indicators of the sources of Pu to the Ob (Fig. 3A, and table 1 in the EPSL Online Background Dataset, see footnote 1). Unlike the 239;240 Pu/137 Cs ratio, the
Pu atom ratios are not a¡ected by di¡erential geochemical behavior, since isotopes of the same element are used. Moreover, the measurement sensitivity a¡orded by TIMS permits high precision measurement of the isotope ratios. There is signi¢cant variation in all the Pu isotope ratios (240/239, 241/239 and 242/239) in the Ob system. For example, relative to the value expected from northern latitude (30^71³N) global fallout (0.180 þ 0.014; [15]), 240 Pu/239 Pu ratios are markedly low in the Irtysh above its con£uence with the Tobal and in the Ob prior to its con£uence with the Irtysh (Stations 95-5, 4; Fig. 3A). Values that are somewhat depressed relative to the global fallout value persist throughout the Ob downstream of its con£uence with the Irtysh. The sample collected in the Taz, which has no known sources of Pu other than global fallout, shows the global value (Fig. 3A). An e¡ective approach for determining sources of Pu to the Ob watershed is through the use of isotope ratio plots [3,15,16]. Plots of 241 Pu/239 Pu and 242 Pu/239 Pu vs. 240 Pu/239 Pu reveal mixing line trends that indicate sources (Fig. 6A,B). As indicated above, one end-member for Pu mixing trends is global input from atmospheric testing of atomic weapons. The global component of fallout is derived from material that was injected into the stratosphere and distributed over the Earth through the atmospheric circulation. Regional variation in fallout £uxes has been observed (e.g. [17], and the variation in isotope ratios corresponding to this variation has been tabulated by Kelley et al. [15]. A second component of fallout is low altitude (tropospheric) material whose deposition is con¢ned regionally to areas proximal to test sites. Potential inputs to the Ob system could arise from weapons tests conducted at the Novaya Zemlya archipelago in the Kara Sea and at Semipalatinsk, adjacent to the Irtysh River. Low ratios of 240 Pu/239 Pu and 241 Pu/239 Pu have been measured in sediments of Chernaya Bay in Novaya Zemlya [27]. Pu inputs from testing in this area are likely to a¡ect only the lower Ob River while tests at Semipalatinsk could supply material to the upper Ob via the Irtysh. A third component of Pu in the Ob system arises from processed reactor
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Fig. 6. Isotope ratio mixing plots of (A) 241 Pu/239 Pu vs. 240 Pu/239 Pu, (B) 240 Pu/239 Pu in suspended and bottom sediments of the Ob River system.
fuel. Such sources include the reprocessing/production complexes at Mayak and at Tomsk-7. Releases from the former have contaminated the Techa River [12], while the latter constitutes a source of Pu contamination to the Ob River before its con£uence with the Irtysh (Fig. 1). For purposes of constructing mixing plots, we take as end-members the northern latitude soil results of Kelley et al. [15] as representative of global stratospheric fallout. Analyses of soils at Mayak and Semipalatinsk [3] are used to represent Pu derived from reprocessing and close-in fallout, respectively. Relative to these end-members, Pu isotope ratio mixing plots for the Ob system show signi¢cant fractions of non-global Pu in the samples (Fig. 6A,B). The Pu isotope ratios for many of the samples, particularly those collected on the Ob prior to its con£uence with the Irtysh and the Irtysh prior to its con£uence with the Tobal, show values that are lower than the global fallout value. The values generally follow the trend de¢ned by mixing of Pu from global fallout and reprocessed fuel (as represented by Mayak soil), although it is di¤cult to di¡erentiate the latter from close-in fallout represented by the soils taken at Semipalatinsk. In particular, the samples taken in the Ob River prior to its con£uence with the Irtysh (95-4, 5) show the largest fractions of non-fallout Pu. Plutonium from closein fallout from Semipalatinsk has low ratios, but is likely to be con¢ned to the watershed of the Irtysh River. Assuming that the Pu isotope signa-
242
Pu/239 Pu vs.
240
Pu/239 Pu and (C)
133
237
Np/239 Pu vs.
ture of releases from Tomsk-7 is comparable to that recorded at Mayak, the low ratios at stations 95-4 and 5 in the Ob are more likely due to releases from the reprocessing of spent fuel at the Tomsk-7 facilities. Other stations with low Pu isotope ratios include 95-12 on the Irtysh before its con£uence with the Ob and 95-2 on the Ob after its con£uence with the Irtysh. The former can be explained as transport of Pu from Semipalatinsk while the latter is likely due to continued transport of Pu originating from fuel reprocessing at Tomsk-7 (seen upriver at stations 95-4 and 5). Interestingly, the suspended sediment recovered from the Tobal shows no clear evidence of Pu originating at Mayak. Similarly the Irtysh below its con£uence with the Tobal also shows little in£uence of the Pu from Semipalatinsk seen in station 9512. 4.3. Neptunium-237 An approach similar to that taken with the Pu isotopes may be applied to 237 Np. This isotope is produced in nuclear devices via an (n, 2n) reaction on 238 U and production is greatest during the testing of high yield thermonuclear devices [15]. The testing of such devices at Novaya Zemlya produced high 237 Np/239 Pu atom ratios (0.48 þ 0.07) in northern latitude fallout [15]. A mixing line on the plot of 237 Np/239 Pu vs. 240 Pu/239 Pu is de¢ned with the end-members taken
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as stratospheric fallout as recorded in northern tier soils and reprocessing material as typi¢ed by soil taken near Mayak (Fig. 6C). Tropospheric fallout from Semipalatinsk is also plotted [3,15,16]. The data show several clusters of data: one along the mixing line, one with Np/Pu ratios lying above and one with values below the line. The values that cluster along the line tend to be those from the Ob River itself before and after its con£uence with the Irtysh (95-4, 5 and 95-1, 2, 941, 2, 4, 5, 8, 10). Stations on the Irtysh downstream of its con£uence with the Tobal (95-6, 7 and 9) lie closest to the northern tier soil endmember. Samples from the Ob River itself appear to represent a mixture of northern latitude fallout and Mayak material transported down the Ob. The suspended sediment sample from the Irtysh upstream of its con£uence with the Tobal (95-12) as well as bottom sediments from the lower Ob (94-1, 5 and 10) all have elevated 237 Np/239 Pu ratios and depressed 240 Pu/239 Pu ratios. This pattern is consistent with inputs from a reprocessing facility waste stream [15]. In the case of the Irtysh sample, however, it is unclear what the source of such material might be. The sample taken in the Tobal nearest Mayak has an unusually low 237 Np/ 239 Pu ratio but a 240 Pu/239 Pu ratio like northern latitude fallout (95-10; Fig. 3). Such an isotopic signature is characteristic of releases from reactor operations [15] and the Mayak facilities represent a possible source of such releases. It is interesting to note that suspended sediment from the Ob delta (94-11) and suspended and bottom sediments from the Taz (94-13) also possess a similar isotopic signature. As with Pu/Cs ratios, discussion of possible sources of Np to the Ob system is dependent on the assumption that the 237 Np/239 Pu isotope ratio re£ects the source without geochemical fractionation. Neptunium has been shown to be less reactive in its oxidized (V) state than as the more reduced IV and III forms [18,19]. Thus if Np is released in mixed valence states to the Ob system, progressive oxidation may mobilize it from particles and produce lower values as particles `age' in their transport to the Kara Sea. There is no clear evidence of such a trend in the data, although the lower 237 Np/239 Pu ratios in the Ob
delta (94-11) and in the Taz (94-13) may be due to Np loss. 4.4. Iodine-129 The pattern of 129 I in the Ob River system provides clear evidence of the release of this radionuclide from the Mayak facilities at Chelyabinsk (Fig. 4). `Dissolved' 129 I concentrations are greatest (V8U109 atoms l31 ) in the Tobal River before it joins the Irtysh. The size-fractionated sampling shows that s 70% of the 129 I that passes a 0.2 Wm ¢lter is truly dissolved in most of the Ob system (Fig. 4, table 2 in the EPSL Online Background Dataset, see footnote 1). In contrast to the Tobal results, dissolved 129 I concentrations in the Irtysh above its con£uence with the Tobal and in the Ob above its con£uence with the Irtysh are low, V2U109 (95-12) and V1U109 (95-4, 5) atoms l31 , respectively. As these rivers merge, ¢rst with the Tobal and then with the Irtysh, the Mayak 129 I signal is progressively diluted (Fig. 4). Downriver from the con£uence of the Ob and Irtysh, there is little change in 129 I concentration (values range from 2.2 to 2.8U109 atoms l31 ) until the broad delta portion of the system is reached (94-11, 14, 16). The lowest concentrations are measured in the Taz River (0.7U109 atoms l31 ). The near constancy in 129 I concentrations in the Ob from its con£uence with the Irtysh until it enters the delta provides a measure of the interannual variability in the system. Two of the samples in this reach of the river were collected in 1995 (95-1, 2) and the remainder in 1994. There is little di¡erence between the two sets of data. In contrast, Raisbeck (personal communication, 1995) measured 7U108 atoms l31 in a sample collected in 1993 near Salekhard and 8U108 atoms l31 in a sample collected in the lower Ob near station 94-14. These values are two to four times lower than our measurements on samples collected in 1994, and the di¡erences may highlight interannual variability in the system. None of our stations taken in 1995 is a precise reoccupation of a 1994 station. However, 95-1 and 94-1 are spatially the closest and are from the same part of the river. These stations have essentially
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the same 129 I concentration in the two years (2.72U109 vs. 2.78U109 atoms l31 ), and the pattern suggests that our 1994 and 1995 data sets are comparable. The likelihood that the decreasing129 I concentration downriver is caused by dilution of the signal as the various tributaries join the system can be checked by comparison with a soluble tracer such as 90 Sr. Paluszkiewicz et al. [20] modeled the concentrations and £uxes of 90 Sr through the Ob in 1985, a year in which the £ow at Salekhard was somewhat higher than average. The calculated 90 Sr concentrations of the Tobal were about four times those of the Ob at Salekhard during the time of year when our samples were taken. These results are comparable to the di¡erence of a factor of three in 129 I concentrations in the Tobal (95-10, 11) and Ob at Salekhard (94-5). The extent to which model results using discharge data from 1985 can be applied to our sampling in 1994^1995 is unclear because discharge data from the mid-1900s are unavailable. The possibility also exists that some of the decrease in 129 I, in the lower Ob, is due to removal of iodine from solution. Iodine is weakly adsorbed to particles, with Kd values measured in the laboratory ranging from 1U102 to 2U103 (Table 1). The latter, higher values are derived using sediment collected in the Ob delta and the Taz River, stations that show lower dissolved 129 I. We are able to calculate Kd values from the ¢eld data as well, using the 129 I concentrations measured in river water (`dissolved') and in bottom sediments (table 2 in the EPSL Online Background Dataset, see footnote 1). Values are equal to or less than those measured in the lab, but always agree within a factor of four. Both laboratory and ¢eld calculations of Kd use bottom sediments, but estimates from the ¢eld data assume that sorption equilibrium is attained between bottom sediments and overlying water. In oxic waters, iodine is present as the unreactive iodate (V) anion, but under reducing conditions it can be reduced to I3 . Such a transformation could occur in bottom sediments if the pore waters are anoxic. Complexation of I with organic matter also may play a role. Laboratory Kd determinations show less than a factor of two increase
135
in Kd after oxidation of DOC (Table 1), but the decrease in DOC concentrations in the lower Ob (Fig. 5) may be linked to enhanced removal of I from solution. Additional 129 I data exist from the Yenisei River [21], the Kara and Barents Seas [22] and the open Arctic [23]. The Yenisei River, also in Siberia, is the second largest Russian river in terms of discharge (V2U104 m3 s31 ) and is subject to releases of anthropogenic radionuclides from the Krasnoyarsk-26 nuclear complex. The Yenisei had 129 I concentrations ranging from 1.8U108 to 4.5U108 atoms l31 in 1993 [21], about one order of magnitude lower than we observed in the Ob in 1994 and 1995. 129 I concentrations in the Ob are comparable to those measured in Kara and Barents Sea surface waters (V9^25U108 atoms l31 [22]), but exceed values in surface waters of the open Arctic Ocean (0.2^8U108 atoms kg31 [21]). 4.5. Fluxes to the Kara Sea We are able to make some estimates of inputs of anthropogenic radionuclides from the Ob River to the Kara Sea. The most important component of the £ux with respect to large-scale dispersion in the Arctic is likely to be the dissolved £ux. The £ux of particles transported through the river system is di¤cult to predict because particles can be trapped in depositional areas such as the reach between the con£uence with the Irtysh and Salekhard [24]. 129 I is present in the Ob mostly in solution, and we have su¤cient data to estimate the dissolved £ux. The mean annual discharge of the Ob is V1.4U104 m3 s31 (4.25U1014 l yr31 [20]). As this value applies to the river after all tributaries have joined it, we use 129 I concentrations from the lower river to calculate the £ux. `Dissolved' 129 I values in the lower Ob range from 0.65 to 2.7U109 atoms l31 , yielding a £ux of 0.3^1.1U1024 atoms yr31 [25]. The annual discharge of the Yenisei is V2U104 m3 s31 , and using this value and 129 I concentrations of 1.8^ 4.5U108 atoms l31 [21] yields a £ux of 0.1^ 0.3U1024 atoms yr31 from this river to the Kara Sea. The combined 129 I £ux from the Ob and Yeni-
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sei Rivers, 0.4^1.4U1024 atoms yr31 , may be compared with other releases to the Arctic. The major releases of 129 I have been from the reprocessing facilities at La Hague, France and Sella¢eld, UK. The combined annual releases from La Hague and Sella¢eld have ranged from 1.68 to 4.67U1026 atoms yr31 over the period 1978^ 1991 [4] with an average of V2.6U1026 atoms yr31 during this time. The £ux from the Ob and Yenisei is 0.2^0.5% of this value. We have only particle data for the other radionuclides measured, but estimates of the dissolved £ux can be made assuming sorption equilibrium is attained. For example, 237 Np has an average concentration in the lower Ob (94-8, 10, 11) of V7U107 atoms g31 . For a Kd for Np of 5U103 [14], the dissolved concentration of 237 Np is predicted to be V1U107 atoms l31 . Using the discharge estimates for the Ob given above, the dissolved £ux is V4U1021 atoms yr31 . In comparison, discharges of 237 Np from Sella¢eld average V2U1025 atoms yr31 during the period 1992^1994 [23]. The £ux of 237 Np from the Ob is only 0.02% of the Sella¢eld discharge, a factor of 10 lower than that for 129 I. Fluxes of 137 Cs and Pu are likely to be similarly low. Acknowledgements We are grateful to Drs. Hugh Livingston and Fred Sayles for allowing us to participate in their 1994 and 1995 sampling trips on the Ob River and for subsequent discussions of the results. The sampling expeditions would not have been possible without the expert organizational and logistical skills of Gera Panteleyev, who tragically lost his life during the 1995 sampling expedition. The 129 I measurements were made by Dr. Linas Kilius at the IsoTrace Laboratory (University of Toronto, Ont., Canada). Linas also died during the course of this research, and we dedicate this paper to the memory of these two colleagues. Wendy Woods (University of Rhode Island) participated in the 1995 expedition to collect the suspended sediment samples. We are grateful to Steven Smith and Joanne Goudreau (Woods Hole Oceanographic Institution) and Matthew Monetti
(Environmental Measurements Laboratory) for help in the ¢eld. Dr. Olga Medkova participated in the cruises and facilitated shipping of gear and samples to the ¢eld site. David Hirschberg and Kim Roberts (MSRC, SUNY, Stony Brook) assisted in the laboratory and Tim Kenna (Woods Hole Oceanographic Institution) provided the base map of sampling stations. This research was supported by Grants N000149410 and N000149511197 from the O¤ce of Naval Research, as part of the Arctic Nuclear Waste Assessment (ANWAP) Program. This is Contribution No. 1188 from the Marine Sciences Research Center.[EB] References [1] M.-H. Dai, J.-M. Martin, First data on trace metal level and behavior in two major Arctic river-estuarine systems (Ob and Yenisey) and in the adjacent Kara Sea, Russia, Earth Planet. Sci. Lett. 131 (1995) 127^141. [2] S.B. Moran, W.L. Woods, Cd, Cr, Cu, Ni and Pb in the water column and sediments of the Ob-Irtysh Rivers, Russia, Mar. Pollut. Bull. 35 (1997) 270^279. [3] T.M. Beasley, J.M. Kelley, K.A. Orlandini, L.A. Bond, A. Aarkrog, A.P. Trapeznikov, V.N. Pozolotina, Isotopic Pu, U, and Np signatures in soils from Semipalatinsk21, Kazakh Republic and the Southern Urals, Russia, J. Environ. Radioact. 39 (1998) 215^230. [4] G.M. Raisbeck, F. Yiou, Z.Q. Zhou, L.R. Kilius, 129 I from nuclear fuel reprocessing facilities at Sella¢eld (UK) and La Hague (France): potential as an oceanographic tracer, J. Mar. Systems 6 (1995) 561^570. [5] N.S. Fisher, P. Bjerregaard, S.W. Fowler, Interactions of marine plankton with transuranic elements. I. Biokinetics of neptunium, plutonium, americium, and californium in phytoplankton, Limnol. Oceanogr. 28 (1983) 432^447. [6] F.A. Armstrong, P.M. Williams, J.D. Strickland, Photooxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications, Nature 211 (1966) 481^487. [7] S.M. Pike, S.B. Moran, Use of Poretics 0.7 Wm pore size glass ¢ber ¢lters for determination of particulate organic carbon in seawater and fresh water, Mar. Chem. 57 (1997) 355^360. [8] M. Baskaran, S. Asbill, P. Santschi, J. Brooks, M. Champ, D. Adkinson, M.R. Colmer, V. Maeyev, Pu, 137 Cs and excess 210 Pb in Russian Arctic sediments, Earth Planet. Sci. Lett. 140 (1996) 243^257. [9] G.C. Christensen, G.N. Romanov, P. Strand, B. Salbu, S.V. Malyshev, T.D. Bergan, D. Oughton, E.G. Drozhko, Y.V. Glagolenko, I. Amundsen, A.L. Rudjord, T.O.
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