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Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs
Applied Geochemistry xxx (2016) 1e6
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Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs Shigeo Uchida*, Keiko Tagami Biospheric Assessment for Waste Disposal Team, National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 12 December 2016 Accepted 26 December 2016 Available online xxx
Values of the sediment-seawater distribution coefficient (Kd) in Japanese coastal areas were compared for stable and radioactive Sr and Cs, respectively, in order to clarify whether or not the constant exchangeable fraction in sediment of 20% for the ocean margin (the IAEA recommended method) for all elements was reasonable. Global fallout origin 90Sr and 137Cs concentrations in sediment and seawater were obtained from open data sources as collected in 1964e2010 and 319 and 1506 Kd values were calculated, respectively. When the values were classified into about 10 y intervals from the 1960's to 2000's, Kd values of the two most recent decades did not show any difference for 90Sr and 137Cs. The geometric means of the most recent Kd values in 2000e2010 were 1.2 102 L kg1 for 90Sr and 5.8 102 L kg1 for 137Cs. The stable Sr and Cs concentration data in sediment and seawater samples collected in 2000e2011 were used to calculate stable Sr and Cs Kd values based on the IAEA's recommended method and values were compared. It was found that the stable Sr Kd (geometric mean ¼ 5.3) was much less than that of Kd-90Sr, while stable Cs Kd (geometric mean ¼ 1.7 103) was higher than that of Kd-137Cs. Thus, it was judged to be unsuitable to estimate the Kds of these radionuclides by apply a factor of 0.2 to the total concentration of Sr and Cs in Japanese coastal sediment, assuming 20% was the exchangeable fraction. It is likely that applying the constant exchangeable fraction percentage to all elements will sometimes lead to overestimated or underestimated values. © 2016 Elsevier Ltd. All rights reserved.
Editorial handling by Prof. M. Kersten. Keywords: Marine environment Radiocesium Radiostrontium Exchangeable fraction Stable element
1. Introduction The distribution coefficient (Kd, L kg1) of radionuclides in sediment-seawater systems is of importance in estimating radionuclide concentrations in sediment and seawater after their anthropogenic release to a marine environment. Such values are useful to estimate migration of the radionuclides in sediment, their transfer in sediment-seawater-seafood-human systems, and the radiation doses to marine organisms from the radionuclides in sediment (external dose) and from the radionuclides in their bodies (internal dose). However, unfortunately, Kd values of many radionuclides in the natural marine environment are not available because it is impossible to measure all the radionuclides in seawater and sediment, only some radionuclides originating from global fallout (including hydrogen bomb tests) and nuclear fuel recycling facilities have been measured (Assinder et al., 1985;
* Corresponding author. E-mail address: [email protected] (S. Uchida).
Oughton et al., 1997; Carroll et al., 1999; Skipperud et al., 2000; Tagami and Uchida, 2013). It is necessary to consider more radionuclides which can be potentially discharged to the marine environment. Thus, to estimate the Kd values, IAEA (2004) employed stable element information to derive Kd values for many elements. Other analogue approaches to derive marine Kd values are applying the Kd values obtained from another element (e.g., K and Cs) and freshwater Kd values for the same element. However, further studies are needed to use these analogue approaches to the marine environments. For example, for the former case, distribution of K and Cs in coastal sediment were different (Terada et al., 1986) so that the Kd values would be different; and for the latter case, salinity is generally high in the marine environments than that in the freshwater environments and thus the different salinity conditions should affect the Kd values for many elements. Although Kd is simply explained as a ratio between concentration of a radionuclide in sediment (Bq kg1-dry) and that in water (Bq L1), the concept of Kd translates an equilibrium ratio between radionuclides sorbed onto solid phase and dissolved in liquid
http://dx.doi.org/10.1016/j.apgeochem.2016.12.023 0883-2927/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
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S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
phase. Therefore, setting the percentage of the exchangeable fraction is the key parameter to determine reliable Kd values by measuring concentrations of stable elements. IAEA (2004) adopted the assumption that 20% of the total element concentration in pelitic coastal sediment (clays and silts) represented the exchangeable phase components for all elements (except carbon) in the ocean margins. The current assumption is that the percentage that is exchangeable is similar for all areas of the world. In the long-term, the interaction dynamics may provoke that the exchangeable fraction of stable and radioactive elements become similar, but each element may have its own exchangeable fraction percentage. It was not clear whether the value was truly applicable in real environments for a long time because 20% was an assumed value. Indeed, the following comments were made in IAEA (2004): “The value of 20% is intended to take account both the varying proportions of coarse material (which is not generally involved in exchange processes) in coastal sediments and the proportion of the element associated with politic fractions available for exchange with the aqueous phase”. In this study, therefore, using coastal area data in Japan we tested the suitability of the estimated exchangeable fraction as the assumed value for stable Cs and Sr (expressed as NSr and NCs in this study) and global fallout 137Cs and 90Sr data in seawater and sediment to provide appropriate radiocesium and radiostrontium marine Kd values. We focused on these two elements because they
Kd
1
L kg
2. Materials and method Concentration data of global fallout 137Cs and 90Sr in coastal sediment and seawater were collected in 1964e2010 (NRA, 2016) off 24 prefectures in Japan. Seawater and sediment data sets were selected by checking the sampling site as well as the sampling date; the sites and dates in the respective sets should be the same, however, for the sampling collection dates, a difference within two days was accepted. If the sampling site information indicated large areas (e.g., off Fukushima), then the data were not used; the present survey needed to identify the exact sampling place. Using these data criteria, the data sets we selected for 1984 to 1994 were not the same as reported by Kasamatsu and Inatomi (1998) reported for 12
Kd
1
L kg
To obtain Kd for stable elements, we used two data sources. The first one was our recent co-authored paper on Kd derived from stable element data in sediment and seawater concentrations in Japanese coastal areas (Takata et al., 2016). In that study, sediment and seawater were collected from 19 coastal sites from 2007 to 2011 (shallower than 61 m) and 86 datasets for NSr and 83 datasets for NCs were used for Kd calculation. The second one was a report by Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ-AIST, 2010), including element concentration data in about 5000 Japanese coastal sediment samples. We selected sediment data collected from the similar sampling depth and year of the first data source (Takata et al., 2016), that is, samples collected shallower than 61 m in 2000e2007; finally, the total number of selected data was 300 for each element. Unfortunately, seawater concentration data for NSr and NCs were not available in the GSJ-AIST report (2010), therefore, we applied the world average data in seawater (CRC Press, 2012) for NSr (7.9 mg L1) and NCs (3 104 mg L1) although these values may vary widely with location. We used the factor 0.2 in the following equation recommended by IAEA (2004) to deduce Kd values in ocean margins from measurements of stable elements.
Concentration per unit mass of sediment mg kg1 dry weight ¼ Concentration per unit mass of seawater mg L1
have been considered for dose assessment for planed releases from nuclear power plants and nuclear waste disposal facilities; in Japan, all nuclear power stations locate on the coast.
and seawater (Bq L1).
(2)
However, to test the equation, we changed the factor from 0 to 1.0 by 0.1 increments and compared these results with the Kd values of global fallout 137Cs and 90Sr. To compare the obtained Kd values, t-test (for two datasets) and ANOVA test (for three or more datasets) were carried out using KaleidaGraph software (Synergy Software, version 4.1.4). 3. Results and discussion 3.1. Global fallout
90
Sr
Before comparison of the Kd values, we looked at activity concentration changes of 90Sr in sediment and seawater samples from Japanese coastal environments from 1964 to 2010; a plot of the changes in shown in Fig. 1. The 90Sr concentrations in seawater and
Concentration per unit mass of sediment Bq kg1 dry weight ¼ Concentration per unit mass of seawater Bq L1
regions in Japan. Using the selected data for each radionuclide, best fit exponential trend lines in sediment and seawater were computed using KaleidaGraph software (Synergy Software, version 4.1.4). We used the selected data sets and simply calculated the Kd (L kg1) as the concentration ratio between sediment (Bq kg1-dry)
0:2
(1)
sediment samples decreased with time exponentially. Thus effective half-life (Teff) values were calculated for 90Sr and the values were 15.4 ± 0.2 y in seawater and 20.8 ± 1.7 y in sediment anthropogenic. Povinec et al. (2005) reported that the Teffs of 90Sr in North Pacific seawater were 10e16 y; Kasamatsu and Inatomi (1998) reported that the value in the coastal seawater of Japan
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
-1
Concentration, Bq kg -dry or Bq L
-1
S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-3
90
Sr-seawater, Bq L
90
Sr-sediment, Bq kg -dry
equilibrium conditions. Indeed, for the experiment with added 85Sr, the radionuclide in sediment was readily exchangeable form; however, global fallout 90Sr in sediment was less mobile (Oughton et al., 1997). From the results, we could assume that after a long contact period in sediment, 90Sr partially became less mobile. Because of the presence of immobile 90Sr in sediment, Kd values after the 70's would be expected to increase because of dilution of these radionuclides in seawater; Kd values increased in the 80's, but it became constant afterwards. Probably migration of 90Sr to a deeper layer, sedimentation of less contaminated particles onto the contaminated sediment surface, physical and/or biological mixing might affect the dilution of 90Sr in sediment. Further research is necessary to discuss the Kd change processes in details.
-1 -1
-4
10 1960
1970
1980
1990
2000
2010
Sampling date 90
Fig. 1. Global fallout Sr activity concentration change in sediment and seawater collected in coastal areas of Japan from 1964 to 2010.
ranged from 12.7 to 21.0 years based on data from the 1984-1994 observation period. Thus the present data agreed well with those reported previously. Kd values of 90Sr (Kd-90Sr) were then calculated using equation (1); a total number of 319 Kd-90Sr values were obtained. The geometric mean (GM) of Kd-90Sr, 9.0 101 L kg1, was within the range of previously reported values of 1.8 100e1.35 102 L kg1 by Nyffeler et al. (1984), Carroll et al. (1999) and Rudjord et al. (2001). We also calculated Kd-90Sr using the global fallout 90Sr data in Korean coastal areas reported by Yang et al. (2002), and the values were 4.6 102e4.9 102 L kg1, within the Kd-90Sr range of the Japanese coastal area. Then the Japanese Kd-90Sr values were summarized by year for 1964e1969 (60's), 1970e1979 (70's), 1980e1989 (80's), 1990e1999 (90's) and 2000e2010 (00's) as shown in Table 1. The results showed a log-normal distribution or close to a log-normal distribution (data not shown). Thus, logarithms of the data were chosen for statistical analysis. According to the Teff data, a slight Kd-90Sr increase with time was expected; however, statistically, the data obtained in the 80's, 90's and 00's decades showed no difference, although the data in the 70's showed statistically lower values (ANOVA, p < 0.01). The GM Kd-90Sr values (L kg1) in the 60's, 70's, 80's, 90's and 000 were 9.8 101, 7.2 101, 1.2 102, 1.3 102, and 1.2 102, respectively. For the Kd-90Sr values in the 60's, because the amount of global fallout was the highest among the 10 y intervals, it may affect the distributions of 90Sr in sediment and seawater samples. After 60's, less 90Sr deposited to the marine surface, therefore, the calculated Kd-90Sr values would taken under
Table 1 Sediment-seawater distribution coefficient (Kd) values for
90
Sr and
Element
Year
N
Median
90
All 1964e1969 1970e1979 1980e1989 1990e1999 2000e2010 All 1964e1969 1970e1979 1980e1989 1990e1999 2000e2010
319 64 155 31 36 33 1506 80 445 352 313 316
9.0 9.0 7.5 1.2 1.2 1.2 4.5 7.3 2.6 3.3 4.9 5.8
Sr
137
Cs
101 101 101 102 102 102 102 102 102 102 102 102
3
3.2. Global fallout
137
Cs
Similar to 90Sr, we looked at global fallout 137Cs activity concentration changes in seawater and sediment in the coastal areas of Japan, and the results are shown in Fig. 2. The calculated Teff in seawater was 14.9 ± 0.1 y, and that was almost the same as for 90Sr, while the calculated Teff in sediment was 15.8 ± 0.4 y, and that was slightly shorter than that for 90Sr. Teffs of 137Cs in seawater reported here were the same as previously reported values, that is, 10.6e16.5 y by Povinec et al. (2005) and 13.4e31.2 y by Kasamatsu and Inatomi (1998). Using the 137Cs data, we obtained a total number of 1506 data for Kd of 137Cs (Kd-137Cs), and the summarized results from the 60's to 00's data are shown in Table 1. The Kd values (L kg1) showed a
Fig. 2. Global fallout 137Cs activity concentration change in sediment and seawater collected in coastal areas of Japan from 1964 to 2010.
137
Cs in Japanese coastal areas. GM 9.0 9.8 7.2 1.2 1.3 1.2 4.3 6.8 3.3 3.5 4.9 5.8
101 101 101 102 102 102 102 102 102 102 102 102
GSD
Min.
1.9 1.7 1.9 1.6 1.7 1.9 2.5 2.0 2.7 2.5 2.3 2.1
6.0 2.7 6.0 3.5 5.8 1.4 1.3 8.0 4.6 4.0 1.3 2.7
Max. 100 101 100 101 101 101 101 101 101 101 101 101
6.1 6.1 2.3 2.7 4.1 3.5 5.5 5.5 4.0 4.6 3.9 4.2
5% 102 102 102 102 102 102 103 103 103 103 103 103
2.6 3.4 2.1 5.1 4.7 3.3 6.9 1.7 4.6 5.7 9.5 1.3
95%
101 101 101 101 101 101 101 102 101 101 101 102
3.1 2.8 2.5 2.9 3.6 4.3 2.6 2.6 2.4 2.2 2.5 2.5
102 102 102 102 102 102 103 103 103 103 103 103
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
3.3. Comparison of stable element and global fallout radionuclide Kd In order to compare the Kd values of stable Sr (Kd-NSr) and Kd-90Sr, and also for stable Cs (Kd-NCs) and Kd-137Cs, we adopted Kd-90Sr and Kd-137Cs values collected in the 00's which would be comparable to the sediment and seawater sampling years for stable elements as shown in the materials and method section. We calculated Kd-NSr and Kd-NCs using equation (2) and the results for all data are summarized in Table 2. Similar to Kd-90Sr and Kd-137Cs, Kd-NSr and Kd-NCs values were close to log-normal distributions (data not shown). The Kd-NSr and Kd-NCs ranged 1.0e7.7 101 L kg1 and 2.1 101e1.0 105 L kg1, respectively. For comparison, recommended Kd values for Sr and Cs by IAEA (2004) were 8 and 4 103 L kg1, respectively. We also calculated the Kd-NSr and Kd-NCs using data in sediments of the Upper Gulf of California (Shumilin et al., 2002) and the world average seawater data (CRC press, 2012), and the obtained Kd-NSr and Kd-NCs were 4.4 and 3.5 103 L kg1, respectively, which were within the range of the present results. The Kd-NCs values observed in the Japanese coastal areas varied four orders of magnitude; to compare the Kd values of stable and radioactive isotopes, it would be better to have
reasonably narrow range datasets. Then, to decrease the Kd-NCs range, the data in logarithm for both NSr and NCs were treated by removing outliers; 26 and 7 data were removed from the Kd-NSr and Kd-NCs datasets. The results of datasets without outliers are also summarized in Table 2. The GM values of Kd-NSr and Kd-NCs before and after removal of outliers did not changed much, for example, from 5.9 to 5.3 L kg1 for Kd-NSr, and 1.6 103 to 1.7 103 L kg1 for Kd-NCs, however, geometric standard deviation (GSD) decreased by 0.3e0.4 for both elements. Thus, in this study, the datasets without outliers were used for further discussion. Comparisons between Kd-NSr and Kd-90Sr and between Kd-NCs and Kd-137Cs are shown in Fig. 3. It was clear that considering 20% of total Sr or Cs was exchangeable, the obtained Kd-NSr and Kd-NCs values were statistically different from Kd-90Sr and Kd-137Cs (t-test, p < 0.001). More than one order of magnitude difference was found between the geometric mean (GM) values of Kd-90Sr (GM ¼ 1.2 102 L kg1) and Kd-NSr (GM ¼ 5.3 L kg1); GM of Kd-NSr was smaller than that recommended in IAEA report (2004) (KdSr ¼ 8 L kg1). For the case of Kd-NCs, the GM value was 1.7 103 L kg1, while that for Kd-137Cs was 5.8 102 L kg1. The recommended value in IAEA report (2004) is 4 103 L kg1 for Cs in the ocean margins. The 5e95% confidence intervals for Kd-NSr and Kd-NCs were 1.4e20 L kg1 and 3.6 102e7.7 103 L kg1, respectively, and the recommended values by IAEA (2004) were within these ranges. Thus, the lower GM values of Kd-NSr and Kd-NCs than the recommended values for Sr and Cs in IAEA report (2004) were possibly due to the lower concentrations of these elements in coastal sediments in Japan. Although the GM values of Kd-NSr and Kd-NCs were slightly different from the IAEA report values, larger differences were found between Kd values of stable and radioactive isotopes of Sr and Cs. Previously, we also found a difference between Kd values of stable and radioactive Co; the GM values of Kd of stable Co was 3 105 L kg1 in IAEA report (2004) and the GM of Kd of radioactive Co in Japanese coastal areas was 1.2 103 L kg1 (Tagami and Uchida, 2013). From these results, we
4.0
3.0 p<0.001 d
log-normal distribution or close to a log-normal distribution (data not shown). The GM Kd-137Cs values in the 60's, 70's, 80's, 90's and 00's were 6.8 102, 3.3 102, 3.5 102, 4.9 102, and 5.8 102 L kg1, respectively. Previously reported Kd values for Cs ranged from 1.4 102 to 9.5 102 L kg1 (Nyffeler et al., 1984; Carroll et al., 1999; Rudjord et al., 2001), thus the data presented in this study were within the reported range. Compared to the recommended value in IAEA report (2004) (Kd ¼ 4 103 L kg1), the values reported in the present study were 5e12 times lower. When collection period differences were examined based on the logarithms of the Kd values, Kd-137Cs values were lower in the 70's and 80's compared to the 60's, 90's and 00's (ANOVA, p < 0.01). We saw that in the 60's, Kd-137Cs values were the highest among all periods, but the values decreased once in the 70's and then increased again. Thus, interestingly, similar to Kd-90Sr, the respective GM value of Kd-137Cs was the lowest in the 70's. According to laboratory experiments (Oughton et al., 1997; Børretzen and Salbu, 2002), Kd of 137Cs increased with time in a one-year observation period and the sequential extraction results showed that the 137Cs fraction in irreversible sites also increased. Thus, similar to 90Sr, 137 Cs partially became less mobile after a long contact period in sediment, but for the Kd-137Cs values in the 60's, probably these would be affected by global fallout. Except the 60's results, indeed, GM values indicated a continuous increase, which is in agreement with Cs interaction mechanisms. Lujaniene et al. (2010) reported different sorption behaviors of 137Cs for different particle sizes of sediment collected from the Baltic Sea and they assumed the mechanism was ion diffusion through the inert layer on the surface of the sediment particles. Although the similar time dependency of Kd-90Sr and Kd-137Cs with the 10-y intervals were found, from the Kd value difference between 90Sr and 137Cs, we concluded that much less 90Sr should be in the immobile fraction of the sediment compared to the 137Cs amount in the immobile fraction.
Log (K )
4
2.0 p<0.001
1.0
0.0 N
K - Sr d
90
K - Sr d
N
K - Cs d
Kd
137
Cs
Fig. 3. Comparison of Kd (L kg1) values of stable elements (NSr and NCs) and global fallout radionuclides (90Sr and 137Cs).
Table 2 Sediment-seawater distribution coefficient (Kd-sediment) values for stable Sr and Cs in Japanese coastal areas. Element
Data
N
Median
Sr
All data Without outliers All Without outliers
386 360 383 376
4.7 4.6 1.6 1.7
Cs
100 100 103 103
GM 5.9 5.3 1.6 1.7
100 100 103 103
GSD
Min.
2.4 2.0 2.5 2.2
1.0 1.0 2.1 1.4
Max. 100 100 101 102
7.7 3.6 1.0 7.2
5% 101 101 105 103
1.1 1.4 2.6 3.6
95%
100 100 102 102
3.4 2.0 9.6 7.7
101 101 103 103
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
3.0
4.0
d
3.0
2.0
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Ratio of exchangeable fraction to total in sediment
d
K-
137
Cs
1.0
N
for K - Cs d
Fig. 5. Effect of changing the ratio of the exchangeable fraction to the total concentration in sediment to estimate radiocesium Kd using Kd-NCs. Highlighted areas shows ranges of Kd-137Cs (1.5 IQR of lower quartile and þ1.5 IQR of upper quartile: light yellow, lower and upper quartiles: yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
correction factor of 1.0, and the result was 0.07, which was lower than the assumed correction factor by IAEA (2004). In a preliminary test we reported previously (Yamamoto et al., 2015), we contaminated three soil samples collected in Japan with deionized water containing 137Cs at a soil: water ratio of 1:10 and then these soil samples were contacted with seawater for 10 days in an end-over-end shaker and we found that the extractability did not change for 10 days. Although we did not use sediment, this result suggested that 137Cs fixed in soil were not in a form that was readily extractable with seawater, probably due to Cs fixation in soil particles (Lujaniene et al., 2010). After a longer contacting time, the 137Cs in sediment became less exchangeable as was also reported by Oughton et al. (1997). From these facts, we could conclude that a factor of less than 0.1 would be appropriate for NCs to estimate Kd for radioactive Cs, for the case of Japanese coastal areas. For the other areas, Kd-137Cs of 1.4 102 to 9.5 102 L kg1 were reported (Nyffeler et al., 1984; Carroll et al., 1999; Rudjord et al., 2001), which were 3.5e24% of the recommended value in IAEA report (2004). Sediment characteristics affected to the Cs fixation to the marine sediment (Rudjord et al., 2001; Lujaniene et al., 2010), however, more data collection is necessary to discuss the Kd-NCs and Kd-137Cs in details.
d
Log (K )
2.0
5.0
Log (K )
thought it was likely that applying a constant exchangeable fraction percentage to all elements would sometimes overestimate and other times, underestimate, the real values. Then the ratio between the exchangeable fraction to the total amount was changed from 1.0 to 0.1 to identify the suitable exchangeable fraction portions for Sr and Cs. For Kd-NSr (Fig. 4), if we set all NSr in sediment as being exchangeable, that is 1.0, the Kd-NSr values (GM ¼ 2.6 102 L kg1) came within the range of Kd-90Sr values. Interestingly, however, even the correction factor was changed to 1.0, Kd-NSr was statistically lower than Kd-90Sr, probably due to the different Sr sources for NSr and 90Sr in marine environment. Although NSr is continuously discharged to the marine environment via rivers from the terrestrial environment, the amount of 90Sr from the terrestrial environments decreases with time, which causes a lower 90Sr activity concentration in seawater. Such different mechanisms may cause different Kd-NSr values, however, a more detailed study is necessary to clarify the difference. At least, for Sr, applying a factor of 0.2 to any types of coastal sediments would not be appropriate, because the Sr exchangeable fraction may vary widely with location; indeed, for the case of Japanese coastal areas, the exchangeable fraction would be close to 1.0 on average. For Sr in sediment, probably due to different (biogenic) carbonate concentration and sedimentation rates (Shumilin et al., 2002; Yang et al., 2002) would affect the Sr concentrations in sediments. Although the methods to extract the exchangeable fraction were different, it was reported that 9e18% was exchangeable in the Mississippi River mixing zone in the USA (Xu and Marcantonio, 2004) and 43e89% at Daya Bay in China (Gao et al., 2010). Fig. 5 shows the stable Kd-NCs values obtained by changing the exchangeable fraction factor, as was done for Sr. The result was different from that of Sr; when 1.0 was applied, the GM of Kd-NCs (8.3 103 L kg1) was about two orders of magnitude higher than the geometric mean of Kd-137Cs. When 0.2 was applied as an exchangeable fraction factor, the Kd-NCs was still higher than Kd-137Cs. Thus the exchangeable fraction for Cs could be overestimated when a factor of 0.2 to the total Cs in sediment was applied. The exchangeable fraction of Cs would be roughly estimated by dividing the GM of Kd-137Cs by the GM of Kd-NCs with a
5
1.0 4. Conclusions
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Ratio of exchangeable fraction to total in sediment
d
90
-1.0
K - Sr
0.0
N
for K - Sr d
Fig. 4. Effect of changing the ratio of the exchangeable fraction to the total concentration in sediment to estimate radiostrontium Kd using Kd-NSr. Highlighted areas shows ranges of Kd-90Sr (1.5 IQR of lower quartile and þ1.5 IQR of upper quartile: light blue, lower and upper quartiles: blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Stable element data have been used to estimate radionuclide Kds in the marine environment (IAEA TRS-422). In the ocean margins, 20% of total element concentration in sediment has been assumed to be the exchangeable fraction for all elements (except carbon) and thus a factor of 0.2 has been uniformly applied to the sediment concentrations of all elements. However, from the present study, it was clear that no additional factor is necessary to estimate Kd-90Sr using Kd-NSr, and the factor would be less than 0.1 to estimate Kd-137Cs using Kd-NCs, for the case of Japanese coastal areas. For important radionuclides in dose assessment, it is necessary to have more realistic Kd values in coastal areas. Therefore, further studies should be undertaken to clarify whether or not the factor of 0.2 is applicable to estimation of Kd using stable elements.
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
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S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
Acknowledgement We thank Dr. Hyoe Takata, Marine Ecology Research Institute, for his support in collecting stable element Kds. This work was partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry, Japan. References Assinder, D.J., Kelly, M., Aston, S.R., 1985. Tidal variation in dissolved and particulate phase radionuclide activities in the Esk estuary, England, and their distribution coefficients and particulate activity fractions. J. Environ. Radioact. 2, 1e22. Børretzen, P., Salbu, B., 2002. Fixation of Cs to marine sediments estimated by a stochastic modelling approach. J. Environ. Radioact. 61, 1e20. Carroll, J., Boisson, F., Teyssie, J.L., King, S.E., Krosshavn, M., Carroll, M.L., Fowler, S.W., Povinec, P.P., Baxter, M.S., 1999. Distribution coefficients (Kds) for use in risk assessment models of the Kara Sea. Appl. Radiat. Isot. 51, 121e129. CRC Press, 2012. In: Haynes, W.M. (Ed.), CRC Handbook of Chemistry and Physics 93rd Edition. CRC Press, Boca Raton. Gao, X., Chen, C.T.A., Wang, G., Xue, Q., Tang, C., Chen, S., 2010. Environmental status of Daya Bay surface sediments inferred from a sequential extraction technique. Estuar. Coast. Shelf Sci. 86, 369e378. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 2010. Geochemical Map of Sea and Land of Japan (abstract in English). AIST, Tsukuba. IAEA (International Atomic Energy Agency), 2004. Sediment Distribution Coefficients and Concentration Factors for Biota in the Marine Environment. Technical Report Series No. 422. IAEA, Vienna. Kasamatsu, F., Inatomi, N., 1998. Effective environmental half-lives of 90Sr and 137Cs in the coastal seawater of Japan. J. Geophys. Res. 103, 1209e1217. Lujaniene, G., Benes, P., Stamberg, K., Joksas, K., Vopalka, D., Radziute, E., Silobritiene, B., Sapolaite, J., 2010. Experimental study and modelling of 137Cs sorption behavior in the baltic sea and the Curonian Lagoon. J. Radioanal. Nucl. Chem. 286, 361e366. NRA (Nuclear Regulation Authority), 2016. Environmental Radiation Database. http://search.kankyo-hoshano.go.jp/servlet/search.top (Accessed 29 July 2016). Nyffeler, U.P., Li, Y.-H., Sanschi, P.H., 1984. A kinetic approach to describe trace-
element distribution between particle and solution in natural aquatic systems. Geochim. Cosmochim. Acta 48, 1513e1522. Oughton, D.H., Børretzen, P., Salbu, B., Tronstad, E., 1997. Mobilization of 137Cs and 90 Sr from sediments; potential sources to arctic waters. Sci. Total Environ. 202, 155e165. Povinec, P.P., Aarkrog, A., Buesseler, K.O., Delfanti, R., Hirose, K., Hong, G.H., Ito, T., Livingston, H.D., Nies, H., Noshkin, V.E., Shima, S., Togawa, O., 2005. 90Sr, 137Cs and 239,240Pu concentration surface water time series in the Pacific and Indian Oceans - WOMARS results. J. Environ. Radioact. 81, 63e87. Rudjord, A.L., Oughton, D., Bergan, T.D., Christensen, G., 2001. Radionuclides in marine sedimentsddistribution and processes. In: Palsson, S.E. (Ed.), Marine radioecology Final Reports from Sub-projects within the Nordic Nuclear Safety Research Project EKO-1. Nordisk Kernesikkerhedsforskning, Roskilde. Shumilin, E.N., Carriquiry, J.D., Camacho-Ibar, V.F., Sapozhnikov, D., Kalmykov, S., nchez, A., Aguîniga-Garîa, S., Sapozhnikov, Y.A., 2002. Spatial and vertical Sa distributions of elements in sediments of the Colorado river delta and upper Gulf of California. Mar. Chem. 79, 113e131. Skipperud, L., Oughton, D.H., Salbu, B., 2000. The impact of plutonium speciation on the distribution coefficients in a sediment-sea water system, and radiological assessment of doses to humans. Health Phys. 79, 147e153. Tagami, K., Uchida, S., 2013. Sediment-seawater distribution coefficient for radionuclides and estimation of radionuclide desorption ratio from soil in seawater (Abstract in English). Bunsekikagaku 62, 527e533. Takata, H., Aono, T., Tagami, K., Uchida, S., 2016. A new approach to evaluate factors controlling elemental sediment-seawater distribution coefficients (Kd) in coastal regions, Japan. Sci. Total Environ. 543, 315e325. Terada, K., Hamano, M., Komura, K., Matsumoto, K., 1986. The distribution and characterization of 137Cs in shallow water sediments. Bull. Chem. Soc. Jpn. 59, 1863e1868. Xu, Y.F., Marcantonio, F., 2004. Speciation of strontium in particulates and sediments from the Mississippi River mixing zone. Geochim. Cosmochim. Acta 68, 2649e2657. Yamamoto, K., Tagami, K., Uchida, S., Ishii, N., 2015. Model estimation of 137Cs concentration change with time in seawater and sediment around the Fukushima Daiichi Nuclear Power Plant site considering fast and slow reactions in the seawater-sediment systems. J. Radioanal. Nucl. Chem. 304, 867e881. Yang, H.S., Hwang, D.W., Lee, H.P., Kim, G., 2002. Distribution of 90Sr in coastal seawater, sediments and organisms off two atomic power stations in Korea. J. Environ. Radioact. 59, 105e112.
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Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs Shigeo Uchida*, Keiko Tagami Biospheric Assessment for Waste Disposal Team, National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 12 December 2016 Accepted 26 December 2016 Available online xxx
Values of the sediment-seawater distribution coefficient (Kd) in Japanese coastal areas were compared for stable and radioactive Sr and Cs, respectively, in order to clarify whether or not the constant exchangeable fraction in sediment of 20% for the ocean margin (the IAEA recommended method) for all elements was reasonable. Global fallout origin 90Sr and 137Cs concentrations in sediment and seawater were obtained from open data sources as collected in 1964e2010 and 319 and 1506 Kd values were calculated, respectively. When the values were classified into about 10 y intervals from the 1960's to 2000's, Kd values of the two most recent decades did not show any difference for 90Sr and 137Cs. The geometric means of the most recent Kd values in 2000e2010 were 1.2 102 L kg1 for 90Sr and 5.8 102 L kg1 for 137Cs. The stable Sr and Cs concentration data in sediment and seawater samples collected in 2000e2011 were used to calculate stable Sr and Cs Kd values based on the IAEA's recommended method and values were compared. It was found that the stable Sr Kd (geometric mean ¼ 5.3) was much less than that of Kd-90Sr, while stable Cs Kd (geometric mean ¼ 1.7 103) was higher than that of Kd-137Cs. Thus, it was judged to be unsuitable to estimate the Kds of these radionuclides by apply a factor of 0.2 to the total concentration of Sr and Cs in Japanese coastal sediment, assuming 20% was the exchangeable fraction. It is likely that applying the constant exchangeable fraction percentage to all elements will sometimes lead to overestimated or underestimated values. © 2016 Elsevier Ltd. All rights reserved.
Editorial handling by Prof. M. Kersten. Keywords: Marine environment Radiocesium Radiostrontium Exchangeable fraction Stable element
1. Introduction The distribution coefficient (Kd, L kg1) of radionuclides in sediment-seawater systems is of importance in estimating radionuclide concentrations in sediment and seawater after their anthropogenic release to a marine environment. Such values are useful to estimate migration of the radionuclides in sediment, their transfer in sediment-seawater-seafood-human systems, and the radiation doses to marine organisms from the radionuclides in sediment (external dose) and from the radionuclides in their bodies (internal dose). However, unfortunately, Kd values of many radionuclides in the natural marine environment are not available because it is impossible to measure all the radionuclides in seawater and sediment, only some radionuclides originating from global fallout (including hydrogen bomb tests) and nuclear fuel recycling facilities have been measured (Assinder et al., 1985;
* Corresponding author. E-mail address: [email protected] (S. Uchida).
Oughton et al., 1997; Carroll et al., 1999; Skipperud et al., 2000; Tagami and Uchida, 2013). It is necessary to consider more radionuclides which can be potentially discharged to the marine environment. Thus, to estimate the Kd values, IAEA (2004) employed stable element information to derive Kd values for many elements. Other analogue approaches to derive marine Kd values are applying the Kd values obtained from another element (e.g., K and Cs) and freshwater Kd values for the same element. However, further studies are needed to use these analogue approaches to the marine environments. For example, for the former case, distribution of K and Cs in coastal sediment were different (Terada et al., 1986) so that the Kd values would be different; and for the latter case, salinity is generally high in the marine environments than that in the freshwater environments and thus the different salinity conditions should affect the Kd values for many elements. Although Kd is simply explained as a ratio between concentration of a radionuclide in sediment (Bq kg1-dry) and that in water (Bq L1), the concept of Kd translates an equilibrium ratio between radionuclides sorbed onto solid phase and dissolved in liquid
http://dx.doi.org/10.1016/j.apgeochem.2016.12.023 0883-2927/© 2016 Elsevier Ltd. All rights reserved.
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phase. Therefore, setting the percentage of the exchangeable fraction is the key parameter to determine reliable Kd values by measuring concentrations of stable elements. IAEA (2004) adopted the assumption that 20% of the total element concentration in pelitic coastal sediment (clays and silts) represented the exchangeable phase components for all elements (except carbon) in the ocean margins. The current assumption is that the percentage that is exchangeable is similar for all areas of the world. In the long-term, the interaction dynamics may provoke that the exchangeable fraction of stable and radioactive elements become similar, but each element may have its own exchangeable fraction percentage. It was not clear whether the value was truly applicable in real environments for a long time because 20% was an assumed value. Indeed, the following comments were made in IAEA (2004): “The value of 20% is intended to take account both the varying proportions of coarse material (which is not generally involved in exchange processes) in coastal sediments and the proportion of the element associated with politic fractions available for exchange with the aqueous phase”. In this study, therefore, using coastal area data in Japan we tested the suitability of the estimated exchangeable fraction as the assumed value for stable Cs and Sr (expressed as NSr and NCs in this study) and global fallout 137Cs and 90Sr data in seawater and sediment to provide appropriate radiocesium and radiostrontium marine Kd values. We focused on these two elements because they
Kd
1
L kg
2. Materials and method Concentration data of global fallout 137Cs and 90Sr in coastal sediment and seawater were collected in 1964e2010 (NRA, 2016) off 24 prefectures in Japan. Seawater and sediment data sets were selected by checking the sampling site as well as the sampling date; the sites and dates in the respective sets should be the same, however, for the sampling collection dates, a difference within two days was accepted. If the sampling site information indicated large areas (e.g., off Fukushima), then the data were not used; the present survey needed to identify the exact sampling place. Using these data criteria, the data sets we selected for 1984 to 1994 were not the same as reported by Kasamatsu and Inatomi (1998) reported for 12
Kd
1
L kg
To obtain Kd for stable elements, we used two data sources. The first one was our recent co-authored paper on Kd derived from stable element data in sediment and seawater concentrations in Japanese coastal areas (Takata et al., 2016). In that study, sediment and seawater were collected from 19 coastal sites from 2007 to 2011 (shallower than 61 m) and 86 datasets for NSr and 83 datasets for NCs were used for Kd calculation. The second one was a report by Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ-AIST, 2010), including element concentration data in about 5000 Japanese coastal sediment samples. We selected sediment data collected from the similar sampling depth and year of the first data source (Takata et al., 2016), that is, samples collected shallower than 61 m in 2000e2007; finally, the total number of selected data was 300 for each element. Unfortunately, seawater concentration data for NSr and NCs were not available in the GSJ-AIST report (2010), therefore, we applied the world average data in seawater (CRC Press, 2012) for NSr (7.9 mg L1) and NCs (3 104 mg L1) although these values may vary widely with location. We used the factor 0.2 in the following equation recommended by IAEA (2004) to deduce Kd values in ocean margins from measurements of stable elements.
Concentration per unit mass of sediment mg kg1 dry weight ¼ Concentration per unit mass of seawater mg L1
have been considered for dose assessment for planed releases from nuclear power plants and nuclear waste disposal facilities; in Japan, all nuclear power stations locate on the coast.
and seawater (Bq L1).
(2)
However, to test the equation, we changed the factor from 0 to 1.0 by 0.1 increments and compared these results with the Kd values of global fallout 137Cs and 90Sr. To compare the obtained Kd values, t-test (for two datasets) and ANOVA test (for three or more datasets) were carried out using KaleidaGraph software (Synergy Software, version 4.1.4). 3. Results and discussion 3.1. Global fallout
90
Sr
Before comparison of the Kd values, we looked at activity concentration changes of 90Sr in sediment and seawater samples from Japanese coastal environments from 1964 to 2010; a plot of the changes in shown in Fig. 1. The 90Sr concentrations in seawater and
Concentration per unit mass of sediment Bq kg1 dry weight ¼ Concentration per unit mass of seawater Bq L1
regions in Japan. Using the selected data for each radionuclide, best fit exponential trend lines in sediment and seawater were computed using KaleidaGraph software (Synergy Software, version 4.1.4). We used the selected data sets and simply calculated the Kd (L kg1) as the concentration ratio between sediment (Bq kg1-dry)
0:2
(1)
sediment samples decreased with time exponentially. Thus effective half-life (Teff) values were calculated for 90Sr and the values were 15.4 ± 0.2 y in seawater and 20.8 ± 1.7 y in sediment anthropogenic. Povinec et al. (2005) reported that the Teffs of 90Sr in North Pacific seawater were 10e16 y; Kasamatsu and Inatomi (1998) reported that the value in the coastal seawater of Japan
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
-1
Concentration, Bq kg -dry or Bq L
-1
S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-3
90
Sr-seawater, Bq L
90
Sr-sediment, Bq kg -dry
equilibrium conditions. Indeed, for the experiment with added 85Sr, the radionuclide in sediment was readily exchangeable form; however, global fallout 90Sr in sediment was less mobile (Oughton et al., 1997). From the results, we could assume that after a long contact period in sediment, 90Sr partially became less mobile. Because of the presence of immobile 90Sr in sediment, Kd values after the 70's would be expected to increase because of dilution of these radionuclides in seawater; Kd values increased in the 80's, but it became constant afterwards. Probably migration of 90Sr to a deeper layer, sedimentation of less contaminated particles onto the contaminated sediment surface, physical and/or biological mixing might affect the dilution of 90Sr in sediment. Further research is necessary to discuss the Kd change processes in details.
-1 -1
-4
10 1960
1970
1980
1990
2000
2010
Sampling date 90
Fig. 1. Global fallout Sr activity concentration change in sediment and seawater collected in coastal areas of Japan from 1964 to 2010.
ranged from 12.7 to 21.0 years based on data from the 1984-1994 observation period. Thus the present data agreed well with those reported previously. Kd values of 90Sr (Kd-90Sr) were then calculated using equation (1); a total number of 319 Kd-90Sr values were obtained. The geometric mean (GM) of Kd-90Sr, 9.0 101 L kg1, was within the range of previously reported values of 1.8 100e1.35 102 L kg1 by Nyffeler et al. (1984), Carroll et al. (1999) and Rudjord et al. (2001). We also calculated Kd-90Sr using the global fallout 90Sr data in Korean coastal areas reported by Yang et al. (2002), and the values were 4.6 102e4.9 102 L kg1, within the Kd-90Sr range of the Japanese coastal area. Then the Japanese Kd-90Sr values were summarized by year for 1964e1969 (60's), 1970e1979 (70's), 1980e1989 (80's), 1990e1999 (90's) and 2000e2010 (00's) as shown in Table 1. The results showed a log-normal distribution or close to a log-normal distribution (data not shown). Thus, logarithms of the data were chosen for statistical analysis. According to the Teff data, a slight Kd-90Sr increase with time was expected; however, statistically, the data obtained in the 80's, 90's and 00's decades showed no difference, although the data in the 70's showed statistically lower values (ANOVA, p < 0.01). The GM Kd-90Sr values (L kg1) in the 60's, 70's, 80's, 90's and 000 were 9.8 101, 7.2 101, 1.2 102, 1.3 102, and 1.2 102, respectively. For the Kd-90Sr values in the 60's, because the amount of global fallout was the highest among the 10 y intervals, it may affect the distributions of 90Sr in sediment and seawater samples. After 60's, less 90Sr deposited to the marine surface, therefore, the calculated Kd-90Sr values would taken under
Table 1 Sediment-seawater distribution coefficient (Kd) values for
90
Sr and
Element
Year
N
Median
90
All 1964e1969 1970e1979 1980e1989 1990e1999 2000e2010 All 1964e1969 1970e1979 1980e1989 1990e1999 2000e2010
319 64 155 31 36 33 1506 80 445 352 313 316
9.0 9.0 7.5 1.2 1.2 1.2 4.5 7.3 2.6 3.3 4.9 5.8
Sr
137
Cs
101 101 101 102 102 102 102 102 102 102 102 102
3
3.2. Global fallout
137
Cs
Similar to 90Sr, we looked at global fallout 137Cs activity concentration changes in seawater and sediment in the coastal areas of Japan, and the results are shown in Fig. 2. The calculated Teff in seawater was 14.9 ± 0.1 y, and that was almost the same as for 90Sr, while the calculated Teff in sediment was 15.8 ± 0.4 y, and that was slightly shorter than that for 90Sr. Teffs of 137Cs in seawater reported here were the same as previously reported values, that is, 10.6e16.5 y by Povinec et al. (2005) and 13.4e31.2 y by Kasamatsu and Inatomi (1998). Using the 137Cs data, we obtained a total number of 1506 data for Kd of 137Cs (Kd-137Cs), and the summarized results from the 60's to 00's data are shown in Table 1. The Kd values (L kg1) showed a
Fig. 2. Global fallout 137Cs activity concentration change in sediment and seawater collected in coastal areas of Japan from 1964 to 2010.
137
Cs in Japanese coastal areas. GM 9.0 9.8 7.2 1.2 1.3 1.2 4.3 6.8 3.3 3.5 4.9 5.8
101 101 101 102 102 102 102 102 102 102 102 102
GSD
Min.
1.9 1.7 1.9 1.6 1.7 1.9 2.5 2.0 2.7 2.5 2.3 2.1
6.0 2.7 6.0 3.5 5.8 1.4 1.3 8.0 4.6 4.0 1.3 2.7
Max. 100 101 100 101 101 101 101 101 101 101 101 101
6.1 6.1 2.3 2.7 4.1 3.5 5.5 5.5 4.0 4.6 3.9 4.2
5% 102 102 102 102 102 102 103 103 103 103 103 103
2.6 3.4 2.1 5.1 4.7 3.3 6.9 1.7 4.6 5.7 9.5 1.3
95%
101 101 101 101 101 101 101 102 101 101 101 102
3.1 2.8 2.5 2.9 3.6 4.3 2.6 2.6 2.4 2.2 2.5 2.5
102 102 102 102 102 102 103 103 103 103 103 103
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S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
3.3. Comparison of stable element and global fallout radionuclide Kd In order to compare the Kd values of stable Sr (Kd-NSr) and Kd-90Sr, and also for stable Cs (Kd-NCs) and Kd-137Cs, we adopted Kd-90Sr and Kd-137Cs values collected in the 00's which would be comparable to the sediment and seawater sampling years for stable elements as shown in the materials and method section. We calculated Kd-NSr and Kd-NCs using equation (2) and the results for all data are summarized in Table 2. Similar to Kd-90Sr and Kd-137Cs, Kd-NSr and Kd-NCs values were close to log-normal distributions (data not shown). The Kd-NSr and Kd-NCs ranged 1.0e7.7 101 L kg1 and 2.1 101e1.0 105 L kg1, respectively. For comparison, recommended Kd values for Sr and Cs by IAEA (2004) were 8 and 4 103 L kg1, respectively. We also calculated the Kd-NSr and Kd-NCs using data in sediments of the Upper Gulf of California (Shumilin et al., 2002) and the world average seawater data (CRC press, 2012), and the obtained Kd-NSr and Kd-NCs were 4.4 and 3.5 103 L kg1, respectively, which were within the range of the present results. The Kd-NCs values observed in the Japanese coastal areas varied four orders of magnitude; to compare the Kd values of stable and radioactive isotopes, it would be better to have
reasonably narrow range datasets. Then, to decrease the Kd-NCs range, the data in logarithm for both NSr and NCs were treated by removing outliers; 26 and 7 data were removed from the Kd-NSr and Kd-NCs datasets. The results of datasets without outliers are also summarized in Table 2. The GM values of Kd-NSr and Kd-NCs before and after removal of outliers did not changed much, for example, from 5.9 to 5.3 L kg1 for Kd-NSr, and 1.6 103 to 1.7 103 L kg1 for Kd-NCs, however, geometric standard deviation (GSD) decreased by 0.3e0.4 for both elements. Thus, in this study, the datasets without outliers were used for further discussion. Comparisons between Kd-NSr and Kd-90Sr and between Kd-NCs and Kd-137Cs are shown in Fig. 3. It was clear that considering 20% of total Sr or Cs was exchangeable, the obtained Kd-NSr and Kd-NCs values were statistically different from Kd-90Sr and Kd-137Cs (t-test, p < 0.001). More than one order of magnitude difference was found between the geometric mean (GM) values of Kd-90Sr (GM ¼ 1.2 102 L kg1) and Kd-NSr (GM ¼ 5.3 L kg1); GM of Kd-NSr was smaller than that recommended in IAEA report (2004) (KdSr ¼ 8 L kg1). For the case of Kd-NCs, the GM value was 1.7 103 L kg1, while that for Kd-137Cs was 5.8 102 L kg1. The recommended value in IAEA report (2004) is 4 103 L kg1 for Cs in the ocean margins. The 5e95% confidence intervals for Kd-NSr and Kd-NCs were 1.4e20 L kg1 and 3.6 102e7.7 103 L kg1, respectively, and the recommended values by IAEA (2004) were within these ranges. Thus, the lower GM values of Kd-NSr and Kd-NCs than the recommended values for Sr and Cs in IAEA report (2004) were possibly due to the lower concentrations of these elements in coastal sediments in Japan. Although the GM values of Kd-NSr and Kd-NCs were slightly different from the IAEA report values, larger differences were found between Kd values of stable and radioactive isotopes of Sr and Cs. Previously, we also found a difference between Kd values of stable and radioactive Co; the GM values of Kd of stable Co was 3 105 L kg1 in IAEA report (2004) and the GM of Kd of radioactive Co in Japanese coastal areas was 1.2 103 L kg1 (Tagami and Uchida, 2013). From these results, we
4.0
3.0 p<0.001 d
log-normal distribution or close to a log-normal distribution (data not shown). The GM Kd-137Cs values in the 60's, 70's, 80's, 90's and 00's were 6.8 102, 3.3 102, 3.5 102, 4.9 102, and 5.8 102 L kg1, respectively. Previously reported Kd values for Cs ranged from 1.4 102 to 9.5 102 L kg1 (Nyffeler et al., 1984; Carroll et al., 1999; Rudjord et al., 2001), thus the data presented in this study were within the reported range. Compared to the recommended value in IAEA report (2004) (Kd ¼ 4 103 L kg1), the values reported in the present study were 5e12 times lower. When collection period differences were examined based on the logarithms of the Kd values, Kd-137Cs values were lower in the 70's and 80's compared to the 60's, 90's and 00's (ANOVA, p < 0.01). We saw that in the 60's, Kd-137Cs values were the highest among all periods, but the values decreased once in the 70's and then increased again. Thus, interestingly, similar to Kd-90Sr, the respective GM value of Kd-137Cs was the lowest in the 70's. According to laboratory experiments (Oughton et al., 1997; Børretzen and Salbu, 2002), Kd of 137Cs increased with time in a one-year observation period and the sequential extraction results showed that the 137Cs fraction in irreversible sites also increased. Thus, similar to 90Sr, 137 Cs partially became less mobile after a long contact period in sediment, but for the Kd-137Cs values in the 60's, probably these would be affected by global fallout. Except the 60's results, indeed, GM values indicated a continuous increase, which is in agreement with Cs interaction mechanisms. Lujaniene et al. (2010) reported different sorption behaviors of 137Cs for different particle sizes of sediment collected from the Baltic Sea and they assumed the mechanism was ion diffusion through the inert layer on the surface of the sediment particles. Although the similar time dependency of Kd-90Sr and Kd-137Cs with the 10-y intervals were found, from the Kd value difference between 90Sr and 137Cs, we concluded that much less 90Sr should be in the immobile fraction of the sediment compared to the 137Cs amount in the immobile fraction.
Log (K )
4
2.0 p<0.001
1.0
0.0 N
K - Sr d
90
K - Sr d
N
K - Cs d
Kd
137
Cs
Fig. 3. Comparison of Kd (L kg1) values of stable elements (NSr and NCs) and global fallout radionuclides (90Sr and 137Cs).
Table 2 Sediment-seawater distribution coefficient (Kd-sediment) values for stable Sr and Cs in Japanese coastal areas. Element
Data
N
Median
Sr
All data Without outliers All Without outliers
386 360 383 376
4.7 4.6 1.6 1.7
Cs
100 100 103 103
GM 5.9 5.3 1.6 1.7
100 100 103 103
GSD
Min.
2.4 2.0 2.5 2.2
1.0 1.0 2.1 1.4
Max. 100 100 101 102
7.7 3.6 1.0 7.2
5% 101 101 105 103
1.1 1.4 2.6 3.6
95%
100 100 102 102
3.4 2.0 9.6 7.7
101 101 103 103
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S. Uchida, K. Tagami / Applied Geochemistry xxx (2016) 1e6
3.0
4.0
d
3.0
2.0
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Ratio of exchangeable fraction to total in sediment
d
K-
137
Cs
1.0
N
for K - Cs d
Fig. 5. Effect of changing the ratio of the exchangeable fraction to the total concentration in sediment to estimate radiocesium Kd using Kd-NCs. Highlighted areas shows ranges of Kd-137Cs (1.5 IQR of lower quartile and þ1.5 IQR of upper quartile: light yellow, lower and upper quartiles: yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
correction factor of 1.0, and the result was 0.07, which was lower than the assumed correction factor by IAEA (2004). In a preliminary test we reported previously (Yamamoto et al., 2015), we contaminated three soil samples collected in Japan with deionized water containing 137Cs at a soil: water ratio of 1:10 and then these soil samples were contacted with seawater for 10 days in an end-over-end shaker and we found that the extractability did not change for 10 days. Although we did not use sediment, this result suggested that 137Cs fixed in soil were not in a form that was readily extractable with seawater, probably due to Cs fixation in soil particles (Lujaniene et al., 2010). After a longer contacting time, the 137Cs in sediment became less exchangeable as was also reported by Oughton et al. (1997). From these facts, we could conclude that a factor of less than 0.1 would be appropriate for NCs to estimate Kd for radioactive Cs, for the case of Japanese coastal areas. For the other areas, Kd-137Cs of 1.4 102 to 9.5 102 L kg1 were reported (Nyffeler et al., 1984; Carroll et al., 1999; Rudjord et al., 2001), which were 3.5e24% of the recommended value in IAEA report (2004). Sediment characteristics affected to the Cs fixation to the marine sediment (Rudjord et al., 2001; Lujaniene et al., 2010), however, more data collection is necessary to discuss the Kd-NCs and Kd-137Cs in details.
d
Log (K )
2.0
5.0
Log (K )
thought it was likely that applying a constant exchangeable fraction percentage to all elements would sometimes overestimate and other times, underestimate, the real values. Then the ratio between the exchangeable fraction to the total amount was changed from 1.0 to 0.1 to identify the suitable exchangeable fraction portions for Sr and Cs. For Kd-NSr (Fig. 4), if we set all NSr in sediment as being exchangeable, that is 1.0, the Kd-NSr values (GM ¼ 2.6 102 L kg1) came within the range of Kd-90Sr values. Interestingly, however, even the correction factor was changed to 1.0, Kd-NSr was statistically lower than Kd-90Sr, probably due to the different Sr sources for NSr and 90Sr in marine environment. Although NSr is continuously discharged to the marine environment via rivers from the terrestrial environment, the amount of 90Sr from the terrestrial environments decreases with time, which causes a lower 90Sr activity concentration in seawater. Such different mechanisms may cause different Kd-NSr values, however, a more detailed study is necessary to clarify the difference. At least, for Sr, applying a factor of 0.2 to any types of coastal sediments would not be appropriate, because the Sr exchangeable fraction may vary widely with location; indeed, for the case of Japanese coastal areas, the exchangeable fraction would be close to 1.0 on average. For Sr in sediment, probably due to different (biogenic) carbonate concentration and sedimentation rates (Shumilin et al., 2002; Yang et al., 2002) would affect the Sr concentrations in sediments. Although the methods to extract the exchangeable fraction were different, it was reported that 9e18% was exchangeable in the Mississippi River mixing zone in the USA (Xu and Marcantonio, 2004) and 43e89% at Daya Bay in China (Gao et al., 2010). Fig. 5 shows the stable Kd-NCs values obtained by changing the exchangeable fraction factor, as was done for Sr. The result was different from that of Sr; when 1.0 was applied, the GM of Kd-NCs (8.3 103 L kg1) was about two orders of magnitude higher than the geometric mean of Kd-137Cs. When 0.2 was applied as an exchangeable fraction factor, the Kd-NCs was still higher than Kd-137Cs. Thus the exchangeable fraction for Cs could be overestimated when a factor of 0.2 to the total Cs in sediment was applied. The exchangeable fraction of Cs would be roughly estimated by dividing the GM of Kd-137Cs by the GM of Kd-NCs with a
5
1.0 4. Conclusions
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Ratio of exchangeable fraction to total in sediment
d
90
-1.0
K - Sr
0.0
N
for K - Sr d
Fig. 4. Effect of changing the ratio of the exchangeable fraction to the total concentration in sediment to estimate radiostrontium Kd using Kd-NSr. Highlighted areas shows ranges of Kd-90Sr (1.5 IQR of lower quartile and þ1.5 IQR of upper quartile: light blue, lower and upper quartiles: blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Stable element data have been used to estimate radionuclide Kds in the marine environment (IAEA TRS-422). In the ocean margins, 20% of total element concentration in sediment has been assumed to be the exchangeable fraction for all elements (except carbon) and thus a factor of 0.2 has been uniformly applied to the sediment concentrations of all elements. However, from the present study, it was clear that no additional factor is necessary to estimate Kd-90Sr using Kd-NSr, and the factor would be less than 0.1 to estimate Kd-137Cs using Kd-NCs, for the case of Japanese coastal areas. For important radionuclides in dose assessment, it is necessary to have more realistic Kd values in coastal areas. Therefore, further studies should be undertaken to clarify whether or not the factor of 0.2 is applicable to estimation of Kd using stable elements.
Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023
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Please cite this article in press as: Uchida, S., Tagami, K., Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs, Applied Geochemistry (2016), http://dx.doi.org/10.1016/j.apgeochem.2016.12.023