Environ. Sci. Technol. 2002, 36, 2330-2337
Potential Remobilization of 137Cs, 60Co, 99Tc, and 90Sr from Contaminated Mayak Sediments in River and Estuary Environments WILLIAM J. F. STANDRING,* DEBORAH H. OUGHTON, AND BRIT SALBU Isotope Laboratory, Department of Soil and Water Sciences, P.O. Box 5028, Agricultural University of Norway, N-1432 Aas, Norway
Following 50 years of nuclear production at Mayak PA, sediments in storage reservoirs are significantly contaminated. Dam failure or flooding could potentially transport large amounts of sediments, via rivers, to the Ob estuary and Kara Sea. The objectives of this work were to investigate fresh and seawater remobilization of 137Cs, 60Co, 99Tc, and 90Sr from contaminated Reservoir 10 sediments. Sediments were extracted sequentially using synthetic Techa freshwater, seawater, and chemical reagents with increasing dissolution powers. 137Cs and 90Sr freshwater distribution coefficients (apparent Kd) agreed quite well with published values; values for 99Tc were higher and values for 60Co were lower than expected. In seawater, mean apparent Kd values decreased by 94, 77, 48, and 73% (137Cs, 60Co, 99Tc, and 90Sr, respectively), indicating increased radionuclide mobility. Remobilization in seawater was 5, 15, 1, and 23% of total activities (i.e., releases of 165, 11, 0.3, and 170 kBq kg-1 d.w.) for 137Cs, 60Co, 99Tc, and 90Sr, respectively. 137Cs and 99Tc were strongly bound to sediments (60% and 80%, respectively). 60Co and 90Sr were more mobile (70% reversibly bound). In conclusion, Mayak Reservoir sediments could potentially contaminate the Ob estuary due to remobilization of sediment-held radionuclides upon contact with seawater.
Introduction The “Mayak” Production Association (Mayak PA), situated in the Urals mountains, was established in the 1940s to produce weapons-grade plutonium. Mayak PA and its surrounding area is recognized as being significantly contaminated with radionuclides, from both production and accidental discharges: notably the “Kyshtym” accident in 1957 when a storage tank exploded, contaminating approximately 20 000 km2 with more than 4000 Bq m-2 90Sr, and in 1967, when wind erosion of exposed Lake Karachay sediments lead to the dispersal of 20 TBq of radioactive dust around the installation (1). During the period 1949-1956, approximately 100 PBq of liquid radioactive waste was discharged from the Mayak PA facility directly into the river Techa (1). After 1951, direct releases were also channelled into Lake Karachay. Annual discharges have been greatly * Corresponding author phone: (+47) 64948362; fax: (+47) 64948359; e-mail:
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reduced in recent years and were estimated to be approximately 10 PBq in 1995 (2). The river Techa forms the first link in the Techa-IsetTobol-Irtysh-Ob system, which discharges into the Kara Sea (Figure 1). Total amounts (1949-1956) of 137Cs and 90Sr discharged into Reservoir 3 (upstream of Reservoir 10, originally a local pond in the Techa) are estimated to be 13 and 12 PBq, respectively (1). The total amount of 99Tc discharged from Mayak PA into the river Techa is estimated to be 10-103 TBq (3). Two “industrial” reservoirs were created in the river Techa (Figure 1), Reservoir 10 in 1956 (18.6 km2, 77 million m3) and Reservoir 11 in 1963 (44.2 km2, 230 million m3), to avoid contaminated water and sediment entering the lower Techa. The construction of reservoirs flooded the upper river Techa meanders and flood plain. It is estimated that approximately 4 PBq of 90Sr, 2.5 PBq 137Cs, and 82 TBq 60Co are retained in Reservoir 10, compared to approximately 0.6 PBq of 90Sr, 0.6 PBq 137Cs, and 2.5 TBq 60Co retained in Reservoir 11 (1). For Reservoir 10, Mayak PA estimated (1993 data) that about 99% and 96% of the 137Cs and 60Co activities, respectively, were held in sediments, indicating that sediments have acted as a sink for these radionuclides in the reservoir, whereas 90Sr was more mobile with ∼13% in the aqueous phase (1, 4). No information is available regarding 99Tc retention in Reservoir 10. Levels of 137Cs, 60Co, and 90Sr in Techa sediments, downstream from the reservoirs, are several orders of magnitude higher than those expected from global fallout (5), primarily due to discharges from 1949 to 1956. In particular, large areas of the Asanov Swamps (downstream from Reservoir 11) are contaminated with approximately 210 TBq of 137Cs and 40 TBq of 90Sr (1992 data) in surface soils due to flooding of the Techa, prior to the construction of the reservoirs (1). Techa surface sediments (0-4 cm depth), 49 km downstream from the discharge point, have been found to contain 540 Bq kg-1 99Tc and 290 kBq kg-1 137Cs (6). Here, 99Tc activity followed 137Cs in vertical sediment profiles (maxima of 775 Bq kg-1 99Tc and 820 kBq kg-1 137Cs at 14-18 cm depth) and was interpreted as not being present as pertechnetate, indicating the importance of redox processes to Tc retention in sediments (3). This sediment layer was 210Po dated to approximately 1944-1950, which agrees with previous maximum discharges (3). Measurements of 137Cs in sediments collected 169 km and 290 km downstream, indicated a similar sedimentation rate (219Po dating), but maximum 137Cs concentrations occurred at 5-10 cm depth (at 169 km), while in sediments collected at 290 km downstream in the Techa, close to the confluence with the Iset, the maxima for both 137Cs and 239,240Pu occurred in the top 0-5 cm sediment layer, indicating that the lower Techa river received maximum 137Cs contamination loadings 1015 years after the main discharges occurred (6). As 137Cs is predominantly sediment-bound in freshwater systems (e.g., refs 1 and 4) these studies indicate the migration of contaminated particles down through the river system, probably due to flooding. 137Cs has also been documented to be transported into the Kara Sea in floodwaters or incorporated in “dirty” ice, when associated to contaminated suspended sediments (7, 8). No information about sediment transport for 99Tc or 60Co is available. For 90Sr, however, chemical leaching from contaminated soils in the upper Techa catchment has been suggested as a major source of mobile 90Sr into the Techa-Iset-Tobol-Irtysh-Ob river system (9). In the Ob delta system, 137Cs:Pu and 238Pu:239,240Pu ratios in sediment cores from small, seasonally flooded lakes, were 10.1021/es0103187 CCC: $22.00
2002 American Chemical Society Published on Web 05/01/2002
FIGURE 1. Map showing Mayak Production Association (inset) and the Techa-Iset-Tobol-Irtysh-Ob river system, draining into the Kara Sea. mostly attributed to global fallout (10), but the isotope ratios did give evidence of nonfallout sources (11), although they could not be directly related to Mayak activities. Sediments held in Reservoirs 10 and 11 are highly contaminated, and it is assumed that the contaminants can potentially be released to the water phase, dependent on the physicochemical properties of the specific radionuclide, binding mechanisms in sediments and the water composition (7, 12, 13). Among the “worst-case” scenarios for the release of radionuclides from sources at Mayak PA, evaluated by the Joint Norwegian-Russian Expert Group (JNREG), flooding and subsequent dam failure is considered plausible (14, 15). Following a dam failure, resuspended reservoir sediments could be transported over great distances, and radionuclides can be remobilized from sediments via displacement (e.g., ion -exchange: 137Cs, 90Sr) and dissolution (e.g., redox: 99Tc, 60Cosdue to Co association with Fe and Mn oxides/ hydroxides) processes. Such remobilization processes can be more pronounced in areas where different water qualities mix, such as in estuaries (12) and sediments in runoff through the Ob-Yenisey estuaries have been suggested as a potential source of radioactive contamination in the Kara Sea (16). The importance of colloids, capable of being transported long distances, for the transport and behavior of trace metals in the Ob estuary has also been documented (17). The objectives of the present work were to investigate the potential for remobilization of 137Cs, 60Co, 99Tc, and 90Sr from contaminated Reservoir 10 sediments when exposed to synthetic freshwater, similar to Techa spring floodwater, and to seawater, thus simulating a high pH, high salinity estuarine environment. The hypothesis was that transferring contaminated Reservoir 10 sediments from freshwater to seawater environments would increase the mobility of radionuclides. Results are based on laboratory batch experiments where contaminated sediments were exposed to freshwater and then saltwater extracts, followed by sequential extractions to obtain information about the internal distribution of the radionuclides (exchangeable and reversible binding versus slowly reversible and irreversibly bound fractions). This is the first time that highly contaminated sediments from a Mayak Reservoir have been assessed in this way.
Experimental Section Sampling and Sample Preparation. The experiment was carried out using surface sediment samples collected from four sites (A, B, C, and D) on a transect across Reservoir 10 (Figure 1), during the Joint Norwegian-Russian “Mayak” research expedition in 1996. A full account of radionuclide contamination in all sediment and water samples taken during this fieldwork, including estimated radionuclide inventories, can be found in the JNREG publication (15). The bed of Reservoir 10 comprises of both old Techa river sediments and flooded soils. Sediment samples collected across Reservoir 10 are therefore assumed to represent the old, prereservoir Techa riverbed (B and C) and adjacent flooded areas (A and D) formed after the reservoir’s construction. Sediment samples were taken at 7-10 m depth using gravity corers; the samples were quite black in color and weakly anoxic under the surface. Surface sediment samples were taken by scraping off the top layer (approximately 0-3 cm) from the waterlogged cores. Samples were stored cold (∼4 °C) prior to experiments in clean plastic containers with a small volume of reservoir water present. Visible vegetation (root material) and stones were removed from the samples before they were homogenized by stirring, prior to the experiment. Dry weight (105 °C), organic content by loss on ignition (LOI %, 12 h at 550 °C), and surface area (gravimetric B.E.T. method, nitrogen adsorption) were determined on sediment subsamples. Freshwater and Seawater Exposures. Extraction experiments were used to investigate remobilization of 137Cs, 60Co, 99Tc, and 90Sr from the contaminated sediments, when exposed to synthetic Techa freshwater (FW) and seawater (SW) solutions, to simulate the transport of sediments from freshwater into high ionic strength, high pH, estuarine environments. All experiments were conducted at ∼4 °C unless otherwise stated and under aerobic conditions. Three parallels of each sediment sample were used in the experiment (∼0.4 g dry weight per parallel), and sediment-solution ratios of 1:25 (simulating turbid storm flows), similar to those used in previous experiments (7), were adopted. The Techa and the Iset are both classified as small, hydrocarbonate class, rivers with average mineralization and moderate hardness (1). Synthetic Techa freshwater solutions (FW, pH 6.5) were made to mimic Techa floodwaters with VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Major Element Concentrations (mg L-1) Reported for Techa River Water and the Synthetic River Water Used in Batch Experiments Ca2+ Mg2+ Na+ + K+ HCO3- SO42- ClTecha watera (low flow) 62.3 30.7 Techaa (spring floods) 38.8 14.6 synthetic Techa water (FW) 32.8 8.3 a
30.7 23.0 51.9
225 57.7 31.8 166 50.2 17.8 116.6 63.4 35.4
Values reported in JNREG (1997).
respect to major ionic composition and pH (Table 1) using reagent grade chemicals, mixed with Milli-Q water, to concentrations based on recorded values (1). Synthetic seawater solutions (SW) were made (pH 8, ∼28 ‰) using nonassayed sea salts (Sigma) diluted into Milli-Q water, simulating the major ionic composition and pH conditions encountered in estuarine environments. The two solutions were made in one batch and equilibrated at 4 °C prior to experiments. Sediments were first subjected to three consecutive, 7 day exposures to the FW solution (FW 1-3) followed by four consecutive SW exposures (SW 1-4). The first three SW exposures were for 7 days, the last was a 2 months exposure to the seawater solution. Solid-solution separation was achieved by high-speed centrifugation (25 min at 23 700 g). The contact-time dependent, apparent distribution coefficient (Kd) was determined for the pseudoequilibrium systems and compared between the last FW exposure (t ) 7 d) and the first SW exposure (t ) 7 d). Apparent Kd (7 d) values (mL g-1) were calculated by
apparent Kd )
Cs(t) Cw(t)
(1)
where Cs and Cw are the activity concentrations of radionuclide in the dry weight mass of sediment (Bq g-1) and in the exposed solution (Bq mL-1) at the specified contact time (t), respectively. All FW and SW extractions occurred in darkness at 4 °C, with loose centrifuge tube caps to aid aeration. Sequential Extraction Procedure. After FW and SW exposures, sediment parallels were subjected to a modified sequential extraction method (7, 18) using chemical reagents with increasing displacement and dissolution powers, to obtain information about the internal distribution of radionuclides in the sediments. Extraction agents used were as follows: 1 M NH4Ac (pH 5 - HNO3 adjusted, 2 h at room temperature); 0.04 M NH2OH‚HCl in 20% v/v HAc (6 h at 80 °C); 30% H2O2 (pH 2 - HNO3 adjusted, 6 h at 80 °C); 7 M HNO3 (6 h at 80 °C). All extractions were followed by a 10 mL Milli-Q water wash, to stop the extraction reactions. Activity removed in Milli-Q wash solutions was assumed to be similarly associated to the sediment as the preceding extraction fraction. Supernatants were separated from solid phases by high-speed centrifugation (25 min at 23 700 g) before being filtered through 0.45 µm filters, in case of dislodged particles during handling, into preweighed 20 mL counting vials. All plastic and glassware used during experiments had been acid-washed (HNO3) and rinsed in Milli-Q water. Final residues were subjected to extraction with 30 mL of Aqua Regia (12 h at 80 °C), filtered through 0.45 µm membrane filters and diluted up to 100 mL before γ-counting and 90Sr analysis. Analysis of Radionuclides. Total 137Cs and 60Co γ-activity was determined for each parallel prior to experiments, to assess the recovery rates for extraction procedures, using an automatic 3 × 3-in. NaI detector system (Packard, Minaxi Auto-Gamma 5000 series, using Canberra software). Possible 2332
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interferences were controlled using a HP Ge detector (Canberra: resolution 1.9 keV, efficiency 20%). γ-Spectrometry was applied to all extracts and filters from sediments A, B, C, and D, while 99Tc and 90Sr determination was carried out on extracts from the most contaminated sediments (B and C), assumed to represent the old Techa riverbed. 137Cs (t 1/2 30 y, β- 510 keV, 94%: counted via measurement of the daughter nuclide in secular equilibrium with 137Cs, 137mBa, t 60Co (t 1/2 2.6 min, I.T. γ 661 keV, 100%) and 1/2 5.27 y, γ 1173 keV, 100%) were counted directly using gamma spectrometry. Blanks and standards were counted together with extracts to correct for background variations and spectrum drift in the instrument. After gamma counting, 99Tc (t1/2 2.1 × 105 y, β- 292 keV, 100%) in the freshwater (FW) and seawater (SW) extracts (sediments B and C only) was separated from 90Sr and other β-emitters using TEVA resin columns (Eichrom Technologies Inc.). Extraction solutions (FW or SW) were combined with the respective wash solutions and adjusted to pH 7 before being poured through a freshly prepared TEVA column. TEVA resin is an anionic exchanger such that it withholds 99Tc when present as 99TcO4- (19) while allowing cations such as Sr2+ to pass through it. After washing the TEVA resin with 1 M HNO3, 99Tc was eluted using 10 mL of 7 M HNO3 into a new 20 mL counting vial. 99mTc (t1/2 6.0 h, I.T. γ 140 keV, 89%) and 85Sr (t1/2 64.84 d, γ 514 keV, 98%) were used as yield monitors throughout this Tc separation procedure. Average yields for this separation procedure were 92% for 99mTc and 100% for 85Sr. 99Tc in the TEVA eluents (from FW and SW extracts) and in diluted sequential extractions was determined via mass spectrometry using an ICP-MS-ETV (Perkin-Elmer, Elan 6000). 100Ru or 101Ru was determined simultaneously to correct for possible interferences from the 99Ru isotope (13% abundance). The detection limit achieved on the ICP-MSETV was 4 ppt (ng L-1) 99Tc, when measuring TEVA eluents. Matrix effects when measuring 99Tc in the diluted sequential extractions were accounted for using standard additions (20). Details of the separation and analysis method used are found in ref 21. 90 Sr was determined in FW and SW extracts (mixed with their respective Milli-Q washes, previously passed through TEVA columns) and sequential extractions from sediments B and C. Solutions were evaporated to dryness, and the resulting residues were ashed (300 °C for 2 h, 610 °C for 12 h) before being dissolved into 60 mL of 1 M HCl ready for 90Y extraction. YCl was added at the start of the 90Sr procedure 3 (20 mg) as a carrier/yield monitor. Yields (68% average) were determined after complexation (titration) of the stable Y with EDTA (Titriplex). Determination of 90Sr (t1/2 29.1 y, β- 550 keV, 100%) was carried out using a liquid-liquid extraction method described in ref 22. Separation was achieved using 5% HDEHP in toluene, followed by liquid scintillation counting (Packard, Tri-Carb 4530: detection limit ∼0.2 Bq) of the Cerenkov radiation produced by the short-lived, highenergy 90Y daughter nuclide (t1/2 64 h, β- 2250 keV, 100%, assumed to be in secular equilibrium with 90Sr at the time of extraction). Total recoveries of 137Cs and 60Co and activity concentrations of 90Sr were calculated as the sum of all extracts, including the aqua regia extraction of residues. Total 99Tc concentrations presented are the sum of all extracts, excluding the aqua regia (residue) fraction. Standards and blanks were measured during each method to ensure correct calibration of equipment and background deductions.
Results and Discussion Sediment Samples. Table 2 presents dry weights, loss on ignition, surface area, and total activity concentrations of 137Cs, 60Co, 90Sr, and99Tc in the waterlogged, surface sediment samples collected from Reservoir 10. Recovery rates for 137Cs
TABLE 2. Dry Weights, Loss on Ignition, Surface Area, and 137Cs, 60Co, 90Sr, and 99Tc Concentrations (kBq kg-1 Dry Weight) Determined in Sediment Samples (n ) 1)
sediment
dry wt (%)
LOI (%)
Sg (m2 g-1)
Cs-137 (kBq kg-1)
Co-60 (kBq kg-1)
Sr-90 (kBq kg-1)
Tc-99 (kBq kg-1)
A B C D
12.0 9.8 13.3 8.3
46 30 28 66
5.9 20.8 10.3 5.6
3620 5680 4260 1470
56 83 88 42
683 780
15 57
and 60Co after extraction experiments were ∼90%, and the precision between parallels for extractions was generally < 10%. Table 2 shows that sediments B and C had higher radioactive contamination (137Cs and 60Co), indicating that they more closely represent the old Techa riverbed sediments; larger surface areas (approximately 2-4 times greater) and lower organic contents (LOI %) than the two outermost sediments (A and D), which probably represent more boggy, drowned areas adjacent to the Techa river before Reservoir 10 was created. Total dry weight activity concentrations of 137Cs (B and C) were slightly higher than maximum values of 3350 kBq kg-1 reported for Reservoir 10 surface sediments (1, 4) but agree with the other sediment samples collected during 1996 fieldwork (15). 90Sr concentrations were similar to previously reported values (∼700 kBq kg-1). The 90Sr:137Cs ratios, average 0.15 for sediments B and C, were lower than previous observations, ranging from 0.33 to 1.5 (1). Remobilization by Freshwater and Seawater Extractions. Table 1 presents the concentrations of the major ions, Ca2+, Mg2+, Na+, K+, HCO3-, SO42-, and Cl-, present in the synthetic Techa freshwater used in the batch experiments and values for the equivalent concentrations, determined in low water and spring flood Techa river water (1). The synthetic Techa freshwater (FW) provided an adequate match to reported values (1, 3) with respect to the major ionic composition and pH in Techa river water during high flow conditions. Figure 2 shows the percentage removal by successive FW extractions for 137Cs and 60Co (sediments A-D) and for 99Tc and 90Sr (sediments B and C) and that the precision between parallels was acceptable (∼10%). The figure shows that only a very small fraction of the total 137Cs concentration was remobilized by the first FW extract (
