137cs, 239,240 pu and241am in bottom sediments and surface water of lake paijanne, finland
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    137cs, 239,240 pu and241am in bottom sediments and surface water of lake paijanne, finland 137cs, 239,240 pu and241am in bottom sediments and surface water of lake paijanne, finland Document Transcript

    • Journal of Environmental Radioactivity 100 (2009) 468–476 Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad137 Cs, 239,240Pu and 241 Am in bottom sediments and surface water of Lake ¨ ¨Paijanne, FinlandM. Lusa*, J. Lehto, A. Leskinen, T. JaakkolaLaboratory of Radiochemistry, A.I. Virtasen aukio 1, P.O. Box 55, 00014 University of Helsinki, Finlanda r t i c l e i n f o a b s t r a c tArticle history: The concentrations and vertical distribution of 239,240Pu, 241Am and 137Cs in the bottom sediments andReceived 1 May 2008 ¨ ¨ ¨ ¨ water samples of Lake Paijanne were investigated. This lake is important, since the Paijanne area receivedReceived in revised form ¨ ¨ a significant deposition from the Chernobyl fallout. Furthermore Lake Paijanne is the raw water source2 March 2009 for the Helsinki metropolitan area. In addition no previous data on the distribution of plutonium andAccepted 4 March 2009 ¨ ¨ americium in the sediment profiles of Lake Paijanne exist. Only data covering the surface layer (0–1 cm)Available online 11 April 2009 of the sediments are previously available. In the sediments the average total activities were 45 Æ 15 Bq/ m2 and 20 Æ 7 Bq/m2 for 239,240Pu and 241Am, respectively. The average 241Am/239,240Pu ratio wasKeywords:Cesium 0.45 Æ 0.14. The 241Am/239,240Pu ratio is lowest in the surface layer of the sediments and increases asAmericium a function of depth. The 238Pu/239,240Pu ratio of the sediment samples varied between 0.012 Æ 0.025 andPlutonium 0.162 Æ 0.079, decreasing as a function of depth. The average activity in water was 4.9 Æ 0.9 mBq/m3 andBottom sediments 4.1 Æ 0.2 mBq/m3 for 239,240Pu and 241Am, respectively. The 241Am/239,240Pu ratio of water samples was ¨ ¨Lake Paijanne 0.82 Æ 0.17. 239,240Pu originating from the Chernobyl fallout calculated from the average total activities covers approximately 1.95 Æ 0.01% of the total 239,240Pu activity in the bottom sediments. The average total 137Cs activity of sediment profiles was 100 Æ 15 kBq/m2 and 19.3 Æ 1.4 Bq/m3 in water samples. Ó 2009 Elsevier Ltd. All rights reserved.1. Introduction deposited in Finland was approximately half of a percent of the plutonium fallout of the nuclear tests. Similarly the 241Am activity Deposition of 137Cs, 239,240Pu and 241Am from the Chernobyl originating from the Chernobyl accident was approximately 1.7% ofaccident was very unevenly distributed in Finnish lakes and the total activity of 241Am in Finland (Salminen et al., 2005).catchment areas (Kansanen et al., 1991; Paatero et al., 2002; Ilus and ¨ The radioactivity status of Asikkalanselka, the southernmost ´Saxen, 2005; Salminen et al., 2005). This was caused by the differ- ¨ ¨ basin of Lake Paijanne, is an important topic since the Helsinkiences in the areal rainfall conditions. The highest deposition values metropolitan area takes its raw water from this basin. This paperof 239,240Pu and 241Am in Finland were located in the southwestern ¨ describes the present radioactivity situation on the Asikkalanselka,and central parts of the country (Paatero et al., 2002; Salminen et al., twenty years after the Chernobyl accident. Also the radionuclide2005). In August 1986 239,240Pu and 241Am concentrations in the distribution and the radionuclide sources are discussed.surface layer of the bottom sediments (0–1 cm) of Lake Paijanne¨ ¨ In the hydrosphere the prevailing aqueous species of cesium iswere 1.5 Æ 0.1–2.5 Æ 0.2 Bq/m2 and 0.58 Æ 0.05–1.6 Æ 0.2 Bq/m2, the uncomplexed Csþ ion and changes in the pH and Eh do notrespectively (Suutarinen et al., 1993). The maximum 137Cs deposi- affect the speciation of cesium (Lieser and Steinkopff, 1989).tion values, 45–78 kBq/m2, were found in the same region (Arvela Nonetheless, cesium may be adsorbed on surfaces of colloids andet al., 1989). Chernobyl-derived 137Cs deposition is rather high suspended particles, which deposit onto lake bottoms. The abilitycompared to that from the atmospheric nuclear weapons tests in of bottom sediments to bind cesium varies with particle size, and anthe 1950s and 1960s: 1700 Bq/m2 (decay corrected to 1986) (AMAP, increase in 137Cs activity with a decrease in particle size has been1998). Instead the major source of transuranium nuclides in Finland _ _ observed (Lujaniene et al., 2004). According to Lujaniene et al. theis from nuclear weapons test fallout (Paatero et al., 2002; Salminen highest calculated Kd values, i.e. the equilibrium ratio of 137Cs inet al., 2005). The total amount of the Chernobyl-derived 239,240Pu particles compared to 137Cs in water phase, were obtained for particles smaller than 4 mm. This is because the high adsorption of Csþ ions is mainly determined by the clay minerals present in * Corresponding author. Tel.: þ358 9 191 50518; fax: þ358 9 191 50121. sediments (Lieser and Steinkopff, 1989). The main mechanism of E-mail address: merja.lusa@helsinki.fi (M. Lusa). the adsorption is ion exchange and in natural sediments the0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jenvrad.2009.03.006
    • M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 469sorption behavior is dominated by the highly selective exchange average of five months a year from mid December to the beginningsites in clay minerals (Lieser and Steinkopff, 1989). The adsorption of May. The lake is dimictic and oligotrophic.has been reported to be virtually irreversible (Gutierrez andFuentes, 1996). Although the cations with similar charge and ionic 3. Materials and methodsradii are expected to compete with cesium for adsorption sites, the 3.1. Samplingbonding strength found for clay minerals decreases in the sequenceCsþ > Rbþ > Kþ > Naþ (Lieser and Steinkopff, 1989), which is in Sediment samples were taken in 2007 from three sampling sites, one at theagreement with the decrease in ionic radius and the increase in the deepest place of the basin at 51 m (samples A1 and A2) and at two other 32 mhydration enthalpies. deep sites 400 m away from this site, the other to the southeast (A3) and the other to the northwest (A4) (Fig. 2). The locations in Finnish National Coordinate The single most important property affecting the characteristic System and in EUREF-FIN geographical coordinate system are represented inof transuranium elements is their oxidation state. Precipitation, Table 1. Sediment samples were taken with a Limnos gravity corer. The corercomplexation, sorption and colloid formation processes depend on consisted of a series of 1 cm-high rings and an inner tube diameter of 9.3 cm.the prevailing oxidation state (Silva and Nitsche, 1995). All four Sediment samples were divided in situ to 1 cm-thick slices and brought to the laboratory in plastic containers. In the laboratory the sediment samples wereprocesses contribute to the chemical behavior and environmental frozen and freeze-dried. In addition to sediment samples 400 L of surface watertransport properties of actinides, including plutonium and ameri- was collected in 20–50 L plastic canisters from the western shore of Asikka-cium, in the environment. In aqueous solutions with pH and Eh lanselka for 239,240Pu, 241Am and 137Cs analyses. 8 M HNO3 was added to the ¨ranges of natural waters plutonium can be present in four oxidation water samples and their pH was adjusted to 1–2. Samples were kept in a cold-states: III, IV, V, VI (Choppin, 2006). The main part of the dissolved storage room (þ4  C) until analyzed.plutonium in natural waters exists in oxidation state Pu(V), while 3.2. Analysis of americium and plutonium from sediment and water samplesPu(IV) is present in colloidal form (Choppin, 2006). Low pH valuespromote lower oxidation states while higher oxidation states The method for the separation of plutonium and americium from the sedimentbecome more general as the pH increases. Pu(III) is possible in samples included wet ashing, coprecipitation with calcium oxalate and threeprevailing anoxic conditions, e.g. in lower sediments layers. extraction chromatography steps using UTEVAÒ, TRUÒ and TEVAÒ resins (Eichrom Industries) (Fig. 3). 242Pu and 243Am tracers were used for yield determinations. TheAmericium exists in nature in oxidation state III and Am(OH3) sorbs 243 Am tracer used in these analyses contained 2.5% 241Am as an impurity. This wasreadily to nearly all surfaces. subtracted from the 241Am peak in the spectra of the samples. Furthermore, blank analyses were prepared to investigate possible contaminations during the pluto- nium and americium purification steps.2. Study area 242 Pu and 243Am tracers were added to the dried four-gram aliquot samples and the samples were wet-ashed with concentrated HNO3 and HCl to bring the analytes ¨ ¨ ¨nne Asikkalanselka is situated in the southern part of Lake Paija into the solution before further chemical separations. Wet-ashed samples were filtered and plutonium as well as americium were co-precipitated as oxalates as(Fig. 1). The total area of Lake Pa ¨ nne is 1100 km2 and that of ¨ija described earlier by Paatero, Outola and Salminen (Paatero, 2000; Outola, 2002;Asikkalanselka 52.8 km2. The maximum depth in the Asikkalanselka ¨ ¨ Salminen et al., 2005). 100–200 mg of calcium carrier and 2–4 g of oxalic acid werearea is 53 m and its mean depth is 10 m. Pa ¨ nne is ice-covered an ¨ija used for the precipitation. In the case of the upper parts of the sediment profile 137 ¨ ¨nne in Finland and the Fig. 1. Location of Lake Paija Cs fallout from Chernobyl in Finland (fallout map reworked from www.stuk.fi).
    • 470 M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 ¨ Fig. 2. Location of sampling sites A1, A2, A3 and A4 in Asikkalanselka (permission to publish from National Land Survey of Finland).calcium oxalate precipitate was formed when lower amounts of calcium carrier in oxidation state II. The solution was poured into a UTEVAÒ column to separate(100 mg) and oxalic acid (2 g) were used. In the lower parts, however, higher Am(III) and Pu(III) from tetra- and hexavalent impurities. Am(III) and Pu(III)amounts, 200 mg and 2 g, respectively, were needed to precipitate calcium oxalate, remained in the eluent while U(VI), Th(IV) and Np(IV) were retained by the resin.which was probably due to the differences in the matrix concentrations in different Eluent containing Am(III) and Pu(III) was poured to TRUÒ resin to separatedepths. Calcium oxalate precipitation was used to remove disturbing ions such as americium and plutonium from each other. Americium and plutonium was elutedKþ, Fe3þ, Al3þ, Ti4þand PO3À from the samples (Sidhu, 2006; Paatero, 2000). These 4 from TRUÒ resin in stages. First Pu(III) was oxidized with NaNO2 to Pu(IV) whichions remained in solution as the actinides were co-precipitated. In the first place remained in the column while, Am(III) was eluted with 4 M and 9 M HCl. 3þFe was important to remove since it substantially decreases the retention of Thereafter, plutonium was reduced back to Pu(III) with TiCl3 and eluted from theamericium to TRUÒ extraction chromatography resin. column with 4 M HCl. Americium fraction was evaporated to dryness and then The precipitate was calcinated overnight and the ashed sample was dissolved dissolved into 2 M NH4SCN þ 0.1 M HCOOH. This fraction was further purified fromin a small amount of concentrated HNO3 and the solution was evaporated to lanthanides with TEVAÒ resin (Salminen et al., 2005). Lanthanides form tiocyanatedryness. The residue was dissolved into 3 M HNO3 þ 1 M Al(NO3)3 solution and complexes and were not retained in TEVAÒ resin. Am(III) retained into the resin0.6 M Fe(SO3)2 and ascorbic acid were added to separate americium and pluto- and was eluted from the resin with 2 M HCl. Americium fraction was treated withnium from other interfering nuclides with extraction chromatography resins aqua regia and evaporated to dryness. Furthermore americium fraction wasUTEVAÒ, TRUÒ and TEVAÒ as described earlier by Salminen et al. (2004, 2005) treated twice with conc. HNO3 and evaporated to dryness. The residue was dis-(Fig. 3). Al3þ was added, because it complexed with phosphate and thereby pre- solved into 1 M HNO3. Plutonium and americium fractions were co-precipitatedvented the interference of phosphate to neptunium or thorium uptake to UTEVAÒ. with neodymium as trifluorides and the precipitate was collected on a membraneFe(SO3)2 reduced Pu(IV) to Pu(III). Ascorbic acid was added to ensure that iron was filter for alpha counting. The analyses of plutonium and americium from the unfiltered water were started by coprecipitating these nuclides with Fe(OH)3 from the water phase (Paa- ¨ tero, 2000; Pilvio, 1998; Suutarinen et al., 1993). The precipitate was filtered andTable 1 dissolved in 8 M HNO3. Dissolved samples were co-precipitated with calciumThe locations of sampling sites A1, A2, A3 and A4 in the Finnish National Coordinate oxalate and their analysis was continued in the same manner as the sedimentSystem (FNCS) and in EUREF-FIN geographical coordinate system. samples. The americium and plutonium activities were measured from the filters FNCS FNCS EUREF-FIN EUREF-FIN containing the NdF3 precipitates with surface barrier semiconductor detectors. X (p) Y(i) N/lat E/lon The nominal resolution for 241Am and 239,240Pu of the three detectors varied between 20–30 keV. A typical counting time was 5000 min. For a typicalA1 and A2 6,792,301 3,421,983 61 130 53.66500 25 320 39.42200 counting time and counting efficiency of 27% the minimum detectable activityA3 6,792,059 3,422,278 61 130 46.06100 25 320 59.54700 (MDA) was 0.16 Bq/m2 and 0.6 mBq kgÀ1 for the sediment samples and waterA4 6,792,507 3,421,706 61 140 0.11800 25 320 20.55500 samples, respectively.
    • M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 471 Fig. 3. The extraction chromatography procedure used for separating plutonium and americium from sediment samples.3.3. Analysis of cesium from the sediment and water samples For the analysis of 137Cs from the unfiltered water 30 L of water was evaporated to dryness in a 2 L beaker. The residue was dissolved into 8 M HNO3 and 137Cs 137 Cs activity of the sediment samples were determined with a NaI(Tl) activity was measured with a semiconductor detector.detector, which included an automatic sample changer, from the freeze-dried andhomogenized samples in standard 20-ml polyethene counting vials with a samplevolume of 15 ml. For the counting time of 120 min and the counting efficiency of20% with the typical background counts of 39.9 cpm the lowest limit of detection 4. Results and discussion(LLD) for the sediment samples was 9.6 Bq LÀ1. The self-adsorption of gamma rayswas calculated to induce an error of 1–2% but this was not taken into consider- 4.1. Plutonium and americium in sediments and unfiltered wateration in experimental arrangements or calculations. Although the resolution of the samplesNaI(Tl) detector is lower than with semiconductor detectors, the advantage ofthe automatic sampler changer used is that all samples can be loaded into theapparatus at the same time and since the counting efficiency of the detector is The activity concentrations of 239,240Pu and 241Am in sedimenthigh, the counting time is reduced compared with the counting time needed with profiles from the depths of 0–9 cm are presented in Table 2 and insemiconductors. All samples could be measured in less than a week. A disad- Figs. 4 and 5. The total activity of 239,240Pu ranged betweenvantage of the NaI(Tl) detector is the higher inaccuracy of the results. For 29.6 Æ 5.7 and 65.1 Æ 7.1 Bq/m2 and the average total activitycomparison 137Cs from one sediment core (A2) was determined with a semi-conductor detector (Table 6). For profile A2, the average error for the NaI(Tl) was 45 Æ 15 Bq/m2. The average total activity of 241Am wasdetector was 15%. For the same samples the error was on average 1%, when 19.8 Æ 7.1 Bq/m2 and the range was from 11.3 Æ 1.9 to 28.7 Æ 1.7 Bq/semiconductor detector was used. m2. The peak activities were in depths 5–7 cm for both nuclides and
    • 472 M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476Table 2239,240 Pu and 241Am activity concentrations (Bq/m2), 241Am/239,240Pu activity ratios in the bottom sediments of Asikkalanselka at depth 0–9 cm in 2007 and the total activity ¨(S, Bq/m2) of sampling sites A1, A2, A3 and A4. The error indicated for the individual depths is 1s counting error from alpha spectrometry and the error for average values is thestandard deviation of the values. The error for the total activities is the combined error of the individual depths in quadrature. The results under detection limit are indicatedwith a dash (–).Depth A1 A2 239,240 241 241 239,240 239,240 241 241 Pu Am Am/ Pu Pu Am Am/239,240Pu0–1 1.0 Æ 0.2 0.5 Æ 0.2 0.48 Æ 0.27 0.8 Æ 0.1 0.2 Æ 0.1 0.18 Æ 0.091–2 1.5 Æ 0.2 0.8 Æ 0.1 0.56 Æ 0.07 1.4 Æ 0.2 0.7 Æ 0.1 0.50 Æ 0.062–3 2.0 Æ 0.2 1.0 Æ 0.1 0.52 Æ 0.06 1.9 Æ 0.2 1.1 Æ 0.1 0.56 Æ 0.073–4 3.8 Æ 0.5 1.8 Æ 0.1 0.46 Æ 0.04 2.0 Æ 0.2 0.9 Æ 0.1 0.46 Æ 0.064–5 4.9 Æ 0.6 3.0 Æ 0.2 0.60 Æ 0.04 4.0 Æ 0.5 2.8 Æ 0.2 0.70 Æ 0.065–6 11.4 Æ 1.3 4.8 Æ 0.6 0.42 Æ 0.06 12.3 Æ 1.3 5.6 Æ 0.3 0.45 Æ 0.036–7 17.3 Æ 1.8 7.1 Æ 0.9 0.41 Æ 0.05 13.3 Æ 1.4 6.2 Æ 0.3 0.47 Æ 0.037–8 2.0 Æ 0.3 0.7 Æ 0.2 0.34 Æ 0.09 5.8 Æ 0.7 1.8 Æ 0.2 0.31 Æ 0.048–9 – – – 0.5 Æ 0.1 0.4 Æ 0.1 0.80 Æ 0.31S 43.9 Æ 2.4 19.6 Æ 1.1 42.0 Æ 2.1 19.6 Æ 0.6 Average 0.47 Æ 0.09 Average 0.49 Æ 0.19 A3 A40–1 0.5 Æ 0.1 0.1 Æ 0.2 0.18 Æ 0.31 0.4 Æ 0.1 0.1 Æ 0.1 0.19 Æ 0.251–2 1.5 Æ 0.2 0.5 Æ 0.1 0.36 Æ 0.09 2.6 Æ 0.3 1.2 Æ 0.1 0.44 Æ 0.042–3 2.9 Æ 0.4 1.4 Æ 0.2 0.47 Æ 0.08 3.2 Æ 0.3 1.9 Æ 0.1 0.59 Æ 0.043–4 4.0 Æ 1.0 2.3 Æ 0.3 0.58 Æ 0.10 5.1 Æ 0.6 2.8 Æ 0.2 0.55 Æ 0.054–5 7.1 Æ 1.5 1.7 Æ 0.2 0.25 Æ 0.04 9.6 Æ 1.0 4.7 Æ 0.2 0.49 Æ 0.025–6 10.3 Æ 1.5 4.1 Æ 0.5 0.39 Æ 0.05 17.2 Æ 1.9 7.4 Æ 0.3 0.43 Æ 0.026–7 3.4 Æ 0.6 1.0 Æ 0.2 0.31 Æ 0.06 19.2 Æ 2.0 7.0 Æ 0.3 0.36 Æ 0.027–8 0.0 Æ 0.8 0.1 Æ 0.2 – 6.9 Æ 0.8 3.2 Æ 0.3 0.47 Æ 0.048–9 – – – 0.9 Æ 0.2 0.4 Æ 0.2 0.45 Æ 0.19S 29.6 Æ 2.6 11.3 Æ 0.7 65.1 Æ 3.1 28.7 Æ 0.6 Average 0.36 Æ 0.13 Average 0.44 Æ 0.11in these layers 239,240Pu and 241Am originate from the nuclear tests, 4.2. 241 Am/239,240Pu activity ratio in sediments and unfiltered wateras will later be described. The activity concentrations increasedexponentially as a function of depth up to the peak value where No previous data on the distribution of plutonium and ameri-after the activities decreased rapidly. The mineralogy of the sedi- ¨ ¨ cium in the sediment profiles of Lake Paijanne exist. Only datament layers was not further characterized in this study, but it is covering the surface layer (0–1 cm) of the sediments are previouslyknown that the upper layers of the sediment profiles are composed available. Calculated from the Suutarinen et al. (1993) results fromof fine organic matter. In lower stratums the structure gradually the year 1986 the average 239,240Pu activity concentrations ofchanges to more clayey. surface sediments (0–1 cm) from five sampling sites with water The activity concentrations of 239,240Pu and 241Am in the depths of 19–51 m was 2.0 Æ 0.4 Bq/m2 and the average 241Amunfiltered water were low: the average activities being 4.9 Æ 0.9 activity concentration 1.0 Æ 0.1 Bq/m2. In the sampling site corre-mBq/m3 and 4.1 Æ 0.2 mBq/m3, respectively (Table 3). sponding to the depths of sites A1 and A2 in our study theFig. 4. Vertical distribution of 239,240Pu (Bq/m2) in sampling sites A1, A2, A3 and A4 in Fig. 5. Vertical distribution of 241 Am (Bq/m2) in sampling sites A1, A2, A3 and A4 in ¨Asikkalanselka in 2007. ¨ Asikkalanselka in 2007.
    • M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 473Table 3 Concurrently americium can form water soluble lower molecularThe activity concentrations (mBq/m3) of 239,240Pu and 241Am and the 241Am/239,240Pu _ weight complexes with organic matter (Lujaniene et al., 2002). Asactivity ratio in the unfiltered water samples in 2007. mentioned above the upper layers of the sediment samples in our 239,240Sample Pu (mBq/m3) 241 Am (mBq/m3) 241 Am/239,240Pu study are composed of fine organic matter. Assuming theW1 4.3 Æ 1.0 3.0 Æ 1.7 0.71 Æ 0.47 enhancement of the solubility of americium due to complexationW2 5.5 Æ 1.4 5.2 Æ 1.4 0.94 Æ 0.36 and the reduction of plutonium to the less soluble state the aboveAverage 4.9 Æ 0.9 4.1 Æ 0.2 0.82 Æ 0.17 mentioned reactions could explain the differences found in 241 Am/239,240Pu activity ratio of our samples.241 Am/239,240Pu – activity ratio was 0.60 Æ 0.12 in 1986 (Suutarinen Actinides in the different oxidation states may have differentet al., 1993). In our study the average 241Am/239,240Pu – activity ratio migration rates as solubility, sorption and interactions with organicin the surface layer (0–1 cm) of the sediments in these sites was or inorganic ligands depend on the oxidation state. According to0.33 Æ 0.21 in 2007 (Table 2). As 241Pu decays to 241Am with a half- Sokolik et al. (2004) an increase in the amount of soluble organiclife of 14.35 years, the 241Am/239,240Pu activity ratio is presumed to matter favors the mobility of americium and plutonium in soil.increase from 1986 to 2007. Based on our results and those of According to another study of Sokolik et al. (2002) plutonium andSuutarinen et al. (1993) the 241Am/239,240Pu activity ratio in the americium species in soil solutions have different electrical chargesuppermost sediment layer has on the contrary decreased between compared to each other, which results in different migration abilityyears 1986 and 2007. Furthermore the 241Am/239,240Pu activity ratio of these radionuclides. Furthermore in the Sokolik et al. (2002)is lowest in the surface layer and increases as a function of depth study the relative content of anionic species was higher for amer-(Fig. 6). This is probably a consequence of the partial solubility of icium than for plutonium. This may result in higher migration rateamericium from surface sediment layer to the water body. Sanada of americium. When comparing the chemistry of radionuclides inet al. have reported a corresponding increase of 241Am/239,240Pu soil and sediments it should however be taken into account that theactivity ratio in the lower layers of bottom sediment samples from redox and flow conditions in these two media may differ.Pripyat River (Sanada et al., 2002). According to Sanada et al. (2002) According to Salminen et al. the 241Am/239,240Pu ratio in thethis ‘‘implies that its (241Am) behavior is not necessarily similar to Chernobyl deposition was 0.37 (Salminen et al., 2005). In 2007 thisthat of 239,240Pu’’. would correspond to a 241Am/239,240Pu ratio of 2.3. In the weapons In our study the 241Am/239,240Pu activity ratio of the water test fallout in 1963–1965 the 241Am/239,240Pu ratio in lichensamples ranged between 0.71 Æ 0.47 and 0.94 Æ 0.36 and the samples was 0.11 (Jaakkola et al., 1981). Deriving from the physicalaverage ratio was 0.82 Æ 0.17. Based on the 241Am/239,240Pu activity decay of 241Pu this ratio has increased from 0.11 to a ratio of 0.44 inratio both in the sediment and water samples, the relative 2007. Salminen et al. (2005) calculated that the total deposition of 241concentration of 241Am was almost two times higher in the water Am from the Chernobyl accident was only 1.7% of the totalthan in the sediments. This, in addition to the low 241Am/239,240Pu americium deposition in Finland and the rest originated from theratio in the surface sediment layer, strongly indicates to the higher nuclear weapons tests (Salminen et al., 2005). In our study thesolubility of americium compared to plutonium. These results are average 241Am/239,240Pu ratio in the 0–9 cm depth of bottom sedi-in good agreement with the results reported by Lujaniene et al. _ ments was 0.45 Æ 0.14. This corresponds well to the ratio origi-(2002) where higher mobility of Am in comparison with Pu nating from the nuclear tests and indicates that they are the mainisotopes in the Chernobyl soil was estimated via speciation analyses source of plutonium and americium in the sediments, assuming _ _(Lujaniene et al., 2002). Lujaniene et al. (2002) have suggested that that the ratio in lichens is the same as in sediments.the organic matter of soil can reduce the mobility of plutonium notonly due to the sorption of plutonium on it, but also by affecting the 238valence state of plutonium. Humic substances can reduce Pu(V) and 4.3. Pu/239,240Pu activity ratio in sediments and unfiltered waterPu(VI) to less soluble Pu(IV) and in most natural waters plutoniumexists as Pu(IV) (Choppin, 2006). After reduction plutonium According to Holm et al. the 238Pu/239,240Pu activity ratio in theforms hydroxides with very low solubility at oxidation state IV. Chernobyl fallout was 0.47 and in nuclear test fallout 0.026 (Holm et al., 1992). In our study the 238Pu/239,240Pu ratio of the sediment samples varied between 0.012 Æ 0.025 and 0.162 Æ 0.079. The average 238Pu/239,240Pu ratio of the four sampling sites as a function of depth is presented in Fig. 7. In the lowest layers the activity ratio of plutonium isotopes corresponds to the plutonium originating from nuclear weapon tests. In the upper layers the corresponding activity ratio is higher and decreases as a function of depth. On the basis of the 238Pu/239,240Pu activity ratio we deduce that the main portion of plutonium originates from the nuclear weapons fallout and plutonium from the Chernobyl fallout is located in the upper- most parts of the sediment profile. Paatero et al. (2002) have estimated that the Chernobyl-derived 239,240 Pu is approximately 0.5% of the activity of 239,240Pu from nuclear test fallout in Finland. In this study the percentage of Chernobyl-derived plutonium activity was calculated by taking a logarithm from the measured 238Pu/239,240Pu ratios and fitting these values to a linear function (Fig. 8) with an equation (1) R ¼ 10 expðÀ0:08239z À 1:0468Þ (1) 238 239,240 where R is Pu/ Pu and z depth. This was done to describe better the dominating trend in different depths of the sediment 241Fig. 6. Am/239,240Pu activity ratio in the bottom sediments of Lake Paijanne in 2007. ¨ ¨ profile.
    • 474 M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 Table 4 The average fraction of Chernobyl-derived plutonium isotopes 238 and 239,240 in Asikkalanselka bottom sediments (%) and the average 239,240Pu activity originating ¨ from the Chernobyl fallout (Bq/m2). 239,240 Depth The Chernobyl fallout fraction of Pu activity originating from (cm) the total activity of plutonium the Chernobyl fallout (Bq/m2) (%) 0.5 11.8 Æ 2.6 0.08 Æ 0.03 1.5 8.8 Æ 1.9 0.15 Æ 0.05 2.5 6.4 Æ 1.4 0.16 Æ 0.04 3.5 4.3 Æ 0.9 0.16 Æ 0.06 4.5 2.6 Æ 0.6 0.17 Æ 0.07 5.5 1.2 Æ 0.3 0.15 Æ 0.04 6.5 0.00 Æ 0.01 0.000 Æ 0.002 7.5 0.0 Æ 0.2 0.00 Æ 0.03 8.5 0.0 Æ 1.2 0.00 Æ 0.02 Asikkalanselka is 8.05  108 m3 and the total area 5.28  107 m2. ¨ The average activity in unfiltered water was 4.9 Æ 0.9 mBq/m3 and 4.1 Æ 0.2 mBq/m3 for 239,240Pu and 241Am, respectively. TheFig. 7. The average 238Pu/239,240Pu activity ratio of four sampling sites A1, A2, A3 and percentage of the average total activity of 239,240Pu and 241Am in ¨A4 as a function of depth in the bottom sediments of Asikkalanselka in 2007. ¨ water compared to the total activity in Asikkalanselka was calcu- lated using equation (3):   The fraction of Chernobyl-derived plutonium in different depths ab A% ¼  100 (3)was then calculated from the 238Pu/239,240Pu ratios determined ðcd þ abÞwith equation (1) using equation (2) where a is the nuclides’ average activity in water (Bq/m3), b is the total volume of Asikkalanselka (m3), c is the nuclides’ average total ¨Chernobyl derived Puð%Þ ¼ ½0:47R þ 0:026ð1 À Rފ  100 (2) activity in bottom sediments (Bq/m2) and d is the total area ofwhere R is 238Pu/239,240Pu ratio. The percentages of total plutonium ¨ Asikkalanselka. It was calculated that in Asikkalanselka only ¨activity originating from the Chernobyl fallout in various depths of 0.17 Æ 0.07% and 0.32 Æ 0.05% of the total activity of 239,240Pu and ¨ 241bottom sediments of Asikkalanselka are presented in Table 4. The Am, respectively, is in the water column. These values are roughtotal plutonium activities (Bq/m2) from the Chernobyl and nuclear estimates since there are two simplifications in the calculations.test fallout are illustrated in Fig. 9. The 239,240Pu originating from First, the lake mirror area was used in the calculations instead of thethe Chernobyl fallout (Table 4) calculated from the average total actual bottom surface area, which is not known. Secondly, theactivities from the depths of 0–9 cm covers 1.95 Æ 0.01% of the total distribution of radionuclides in the bottom was assumed to be even,239,240 ¨ Pu activity in the bottom sediments of Asikkalanselka. This which is certainly not the case. Fortunately these simplificationscorresponds to a total activity of 0.9 Æ 0.3 Bq/m2 which corre- modify the results into opposite directions, since when the lakesponds well to the levels found in this area by Paatero et al. (2002). mirror area is used the calculated total activity of the bottom The majority of the 239,240Pu and 241Am activities have been sediments is smaller than if the actual bottom surface area wastransported to the bottom sediments. The water volume of used. Furthermore the activities in the central parts of the lake areFig. 8. The logarithmic 238Pu/239,240Pu activity ratios in the bottom sediments of Lake Fig. 9. Plutonium activity (Bq/m2) originating from the Chernobyl and nuclear test ¨ ¨nne in 2007. The 238Pu/239,240Pu activity ratio decreases as a function of depthPaija ¨ fallout in the bottom sediments of Asikkalanselka in 2007. The broken line is nuclearfollowing a linear function y ¼ À0.08239x–1.0468. weapons test plutonium and the solid line Chernobyl-derived plutonium.
    • M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 475Table 5 Table 6137 ¨ Cs activity concentrations in the bottom sediments of Asikkalanselka in the depth A comparison between the results of the sediment profile A2 measured withof 0–9 cm in 2007 and the total activity (S, Bq/m2) of sampling sites A1, A2, A3 and both semiconductor detector and NaI(Tl) detector. The error indicated for the depthsA4. The errors indicated for various depths are 1s counting error from gamma of 0–9 cm is 1s counting error from gamma spectrometry. The error indicated for thespectrometry and for the total activities the error is the combined error of the total activities is calculated from the sum of the individual counting errors of depthsindividual depths in quadratur. of 0–9 cm.Depth (cm) A1 A2 A3 A4 Depth Semiconductor Counting error NaI(Tl) detector Counting error A kBq/m2 A kBq/m2 A kBq/m2 A Bq/m2 Bq/g % Bq/g %0–1 2.6 Æ 0.4 0.8 Æ 0.1 1.5 Æ 0.2 1.5 Æ 0.2 0–1 2.3 1.1 4.8 181–2 5.2 Æ 0.8 5.1 Æ 0.7 5.9 Æ 0.8 10.7 Æ 1.5 1–2 6.5 1.0 6.3 152–3 8.0 Æ 1.2 8.8 Æ 1.3 11.6 Æ 1.6 17.0 Æ 2.4 2–3 6.7 1.0 7.7 143–4 15.5 Æ 2.2 11.8 Æ 1.7 20.4 Æ 2.9 33.5 Æ 4.7 3–4 10.7 0.9 12.0 144–5 28.6 Æ 4.0 43.3 Æ 6.1 22.1 Æ 3.1 41.3 Æ 5.9 4–5 36.3 0.9 32.4 145–6 15.4 Æ 2.2 45.7 Æ 6.5 8.1 Æ 1.1 11.7 Æ 1.7 5–6 21.6 0.8 24.8 146–7 3.8 Æ 0.5 5.2 Æ 0.7 3.0 Æ 0.4 3.4 Æ 0.5 6–7 3.2 0.9 3.7 147–8 2.4 Æ 0.3 5.0 Æ 0.7 1.0 Æ 0.1 1.5 Æ 0.2 7–8 1.6 1.0 1.7 148–9 1.1 Æ 0.2 1.9 Æ 0.3 0.6 Æ 0.1 1.3 Æ 0.2 8–9 0.7 1.1 0.8 159–10 0.7 Æ 0.1 1.4 Æ 0.2 0.4 Æ 0.1 0.6 Æ 0.1 9–10 0.3 1.1 0.4 1610–11 0.5 Æ 0.1 0.6 Æ 0.1 0.3 Æ 0.1 0.4 Æ 0.1 10–11 0.1 3.1 0.2 14S 83.8 Æ 5.3 129.4 Æ 9.3 74.9 Æ 4.8 122.9 Æ 8.3 Total 90.1 0.9 94.9 14presumably larger than the activities on the beach front. As thesevalues are used the calculated total activity of the bottom sedi- The depth profiles of 137Cs in the sampling sites studied showedments is larger than the actual activity. considerable variety. The total activities varied from 75 to 129 kBq/ m2 and the average value was 100 Æ 15 kBq/m2. In the deepest4.4. Cesium in sediments and unfiltered water basin the total activity in sediment from A1 was 65% of the total activity of sediment from A2 even though they were taken only The activity concentrations of 137Cs in sediment samples a few meters from each other. On the other hand, on the basinfrom the depths of 0–9 cm are presented in Table 5 and in fringes the activity of site A3 was only 61% of the activity on theFig. 10. The total activity of sediment profiles ranged between other site of the basin at point A4. In former studies other authors75 Æ 11 and 129 Æ 18 kBq/m2 and the average total activity was have reported the uneven distribution of radionuclides in the100 Æ 15 kBq/m2. ´ sediments. Saxen et al. (1998) and Kansanen et al. (1991) reported The peak activities for 137Cs were in depths of 4–5 cm. The that the 137Cs activity concentrations showed considerable varia-activities decreased rapidly in the sediment layers below the peak ¨ ¨ tions in the bottom sediments of southern Lake Paijanne and othervalue. In 1988 (Saxen et al., 1998) the peak value of 137Cs in a point ´ Finnish lakes. According to Kansanen et al. (1991) the total 137Cscorresponding the sampling sites A1 and A2, was on the surface of activity concentration in Asikkalanselka was 102 400 Bq/m2 in year ¨the sediment profile. In years 1994 and 1997 the peak value was in ´ 1989. Ilus and Saxen (2005) reported that in 2003 the total activitya depth of 1.5–2.5 cm and 2–3 cm, respectively. In 2000 the peak was approximately 90 000 Bq/m2. According to the results of Saxen ´ ´was found in a depth of 3–4 cm (Ilus and Saxen, 2005). 29% of the et al. (1998) the total activity of 137Cs in a point corresponding tototal 137Cs activity was settled to the layers below the maximum our sampling sites A1 and A2 was 135 500 Bq/m2 in 1997. Fromconcentrations and in proportion 37% above the peak values. An physical decay this activity would correspond to an activity ofaverage of 8% more 137Cs activity was in the sediment layers above 108 000 Bq/m2 in year 2007. In our study the average activity of thethe stratums with maximum concentrations than in the lower sampling sites A1 and A2 was 106 600 Æ 5400 Bq/m2. The activitylayers. concentrations of 137Cs in the analyzed unfiltered surface water samples were 18.3 Æ 0.6–20.3 Æ 0.5 Bq/m3. In 1998 137Cs activity in the water of Lake Paijanne was 69 Bq/m3 (Saxen et al., 1998). Taking ¨ ¨ ´ into account the physical decay this would correspond to the activity of 56 Bq/m3 in 2007. This indicates considerable transfer of 137 Cs from the water body to the bottom sediments. Kansanen et al. (1991) reported observations which support the view that redistribution and resuspension of 137Cs had significant ¨ impact in Asikkalanselka. These would cause the focusing of radioactivity in the depressions. In our study clear evidence of the redistribution or focusing of the radioactivity could not be proven. This might be a consequence of low sample number. One sediment profile was also measured with semiconductor detectors to assure the results measured with NaI(Tl) detector (Table 6). The activities measured with the semiconductor detector correspond to the activities determined with the NaI(Tl) detector within the limits of accuracy. The error in NaI(Tl) measurements was, however, more than ten times higher. 5. ConclusionsFig. 10. The vertical distribution of 137Cs (Bq/m2) in sampling sites A1, A2, A3 and A4 in Based on the 238Pu/239,240Pu and 241Am/239,240Pu activity ratios, ¨Asikkalanselka on 9.3.2007. the majority of the 239,240Pu and 241Am activities in the bottom
    • 476 M. Lusa et al. / Journal of Environmental Radioactivity 100 (2009) 468–476 ¨sediments of Asikkalanselka originates from the 1950s and 1960s Holm, E., Rioseco, J., Pettersson, H., 1992. Fallout of transuranium elements following the Chernobyl accident. Journal of Radioanalytical and Nuclearnuclear test fallout. The impact of Chernobyl fallout to the 239,240Pu Chemistry 156 (1), 183–200.and 241Am activities in Asikkalanselka is minor compared to the ¨ Ilus, E., Saxen, R., 2005. Accumulation of Chernobyl-derived 137Cs in bottom sediments ´nuclear test fallout. Only 1.95 Æ 0.01% of the total activity of of some Finnish Lakes. Journal of Environmental Radioactivity 82, 199–221.239,240 Jaakkola, T., Keinonen, M., Hakanen, M., Miettinen, J.K., 1981. Investigation on the ¨ Pu in the bottom sediments of Asikkalanselka was calculated transfer of plutonium and americium from plants to Reindeer and Man in Finnishto have originated from the Chernobyl fallout. This corresponds to Lapland. In: Wrenn, M.E. (Ed.), Actinides in Man and Animals. RD Press, pp. 509–523.the total activity of 0.9 Æ 0.3 Bq/m2. 239,240Pu and 241Am from the Kansanen, P.H., Jaakkola, T., Kulmala, S., Suutarinen, R., 1991. Sedimentation andChernobyl fallout are located in the uppermost parts of the bottom distribution of gamma-emitting radionuclides in bottom sediments of ¨ ¨ southern Lake Paijanne, Finland, after the Chernobyl accident. Hydrobiologysediments. The peak values are found in depths of 5–7 cm for both 222, 121–140.elements and in these layers they originate from the nuclear tests. Lieser, K.H., Steinkopff, T.H., 1989. Chemistry of radioactive Cesium in the hydro- The average 241Am/239,240Pu activity ratio in the water body was sphere and in the geosphere. Radiochimica Acta 46, 39–47. _ ¨ _ Lujaniene, G., Plukis, A., Kimtys, E., Remeikis, V., Jankunaite, D., Ogorodnikov, B.I.,0.82 Æ 0.17 and correspondingly the average value in the bottom 2002. Study of 137Cs, 90Sr, 239,240Pu, 238Pu and 241Am behavior in the Chernobylsediments was 0.45 Æ 0.14. In the sediments the 241Am/239,240Pu soil. Journal of Radioanalytical and Nuclear Chemistry 251 (1), 59–68.activity ratio was lowest in the surface layer and increased as _ ´ Lujaniene, G., Joksas, K., Silobritiene, B., Morkuniene, R., 2004. Physical and chemical characteristics of 137Cs in Baltic Sea. In: Radionuclides in the Envi-a function of depth. Furthermore the 241Am/239,240Pu ratio of the ronment, International Conference on Isotopes in Environmental Studies:surface layer of the sediments has decreased during the last 11 Aquatic Forum 2004. Elsevier, Monaco, pp. 25–29.years. This probably reflects the solubility of 241Am from the sedi- Outola Iisa, 2002. Effect of industrial pollution on the behaviour of 239,240Pu, 241Am and 137Cs in forest ecosystems. Report Series in Radiochemistry 21/2002,ments into the water body. Helsinki. The depth profiles of 137Cs in the sampling sites studied showed Paatero, J., 2000. Deposition of Chernobyl Derived Transuranium Nuclides andconsiderable variety. The total activities ranged between 75 Æ 11 and Short-Lived Radon-222 Progeny in Finland. Finnish Meteorological Institute129 Æ 18 kBq/m2 and the average total activity was 100 Æ 15 kBq/m2. Contributions No. 28. Finnish Meteorological Institute, Helsinki, p. 55 þ Appendixes.The peak activities for 137Cs were in a depth of 4–5 cm. This peak ¨ Paatero, J., Jaakkola, T., Ikaheimonen, T.K., 2002. Regional distribution of Chernobyl-corresponds to the Chernobyl fallout in 1986. The activities derived plutonium deposition in Finland. Journal of Radioanalytical and Nucleardecreased rapidly in the sediment layers below the peak value. No Chemistry 252 (2), 407–412. ¨ Pilvio R., 1998. Methods for the determination of low-level actinide concentrationsclear evidence of the redistribution or focusing of 137Cs activity could and their behaviour in the aquatic environment. Report Series in Radiochem-be proven. This might be a consequence of a low sample number. istry 10/1998, Helsinki, p. 43 þ Appendixes. In bottom sediment samples the peak values of 137Cs were on Salminen, S., Outola, I., Jaakkola, T., Pulli, S., Zilliacus, R., Lehto, J., 2004. Method for determining plutonium in air filters in detection of nuclear activities. Radio-average 2 cm farther up the sediment profile than those of 239,240Pu chimica Acta 92, 467–473.and 241Am. This is because 137Cs originated mainly from the Cher- Salminen, S., Paatero, J., Jaakkola, T., Lehto, J., 2005. Americium and curiumnobyl fallout while plutonium and americium came from nuclear deposition in Finland from the Chernobyl accident. Radiochimica Acta 93, 771–779.weapons tests. Sanada, Y., Matsunaga, T., Yanase, N., Nagao, S., Amano, H., Takada, H., Tkachenko, Y., 2002. Accumulation and potential dissolution of Chernobyl-Acknowledgements derived radionuclides in river bottom sediment. Applied Radiation and Isotopes 56, 751–760. The authors wish to thank Stewart Makkonen-Craig for the Saxen, R., Jaakkola, T., Rantavaara, A., 1998. 137Cs and 90Sr in the Southern Part of ´ ¨ ¨ Lake Paijanne and its catchments. Radiochemistry 40 (6), 522–528.language revision of the manuscript. Sidhu, R., 2006. Radiochemical Procedures for the Determination of Sr, U, Pu, Am and Cm. Nordic nuclear safety research, Radchem, ISBN 87-7893-185-1.References Silva, R.J., Nitsche, H., 1995. Actinide environmental chemistry. Radiochimica Acta 70/71, 377–396.AMAP, 1998. AMAP Assessment Report, Arctic Pollution Issues, Arctic Monitoring Sokolik, G.A., Ovsyannikova, S.V., Kimlenko, I.M., 2002. Effect of humic substances and Assessment Programme. 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