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137Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident
Journal of Environmental Radioactivity 128 (2014) 9e14
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137
Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident Takahiro Nakanishi*, Takeshi Matsunaga, Jun Koarashi, Mariko Atarashi-Andoh Research Group for Environmental Science, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2013 Received in revised form 22 October 2013 Accepted 23 October 2013 Available online
The large amount of 137Cs deposited on the forest floor because of the Fukushima Dai-ichi Nuclear Power Plant accident represents a major potential long-term source for mobile 137Cs. To investigate 137Cs mobility in forest soils, we investigated the vertical migration of 137Cs through seepage water, using a lysimetric method. The study was conducted in a deciduous forest soil over a period spanning 2 month to 2 y after the Fukushima nuclear accident. Our observations demonstrated that the major part of 137Cs in the litter layer moved into the mineral soil within one year after the accident. On the other hand, the topsoil prevented migration of 137Cs, and only 2% of 137Cs in the leachate from litter and humus layer penetrated below a 10 cm depth. The annual migration below a 10 cm depth accounted for 0.1% of the total 137Cs inventory. Therefore, the migration of 137Cs by seepage water comprised only a very small part of the total 137Cs inventory in the mineral soil, which was undetectable from the vertical distribution of 137 Cs in the soil profile. In the present and immediate future, most of the 137Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable 137 Cs exists in forest surface soil. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs Fukushima Dai-ichi Nuclear Power Plant accident Mobility Forest soil Lysimeter Temporal change
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant accident (Fukushima NPP accident) that was triggered by a catastrophic earthquake (M9.0) and the resulting tsunami on 11 March 2011, released a substantial amount of radionuclides to the atmosphere (Chino et al., 2011). Various terrestrial ecosystems in the wider area were affected by the Fukushima NPP accident. Of the radionuclides emitted from the Fukushima NPP accident, 137Cs with a physical half-life of 30.1 y, is the largest source of concern, because of its potential impact on humans and ecosystems over the coming decades. In particular, the vertical migration of 137Cs in soils is one of the essential factors in determining long-term external dose to humans. The internal dose to humans may also be influenced through changes in plant uptake, as radionuclides become fixed in the soil or move out of the active root zone. In earlier studies conducted after the Chernobyl NPP accident that occurred in 1986, various measures of migration and retention in soil were used, such as the fraction of radionuclide content found within a certain depth or median depth, and migration rates, which were calculated using various models. After the Fukushima NPP
* Corresponding author. Tel.: þ81 29 282 6433; fax: þ81 29 282 6760. E-mail address: [email protected] (T. Nakanishi). 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.10.019
accident, Matsunaga et al. (2013) showed, using a relaxation parameter, that the 137Cs vertical profiles in undisturbed soils were almost unchanged between points in time spanning the first rainy season. This indicated the overall immobility of the deposited 137Cs even after only three months, and the difficulty in quantifying its mobility in soils by such methods for the solid soil phase. Hence, there is a strong need to parameterize the 137Cs migration process in soils by direct in situ observation of 137Cs in seepage water through the soil profile. Koarashi et al. (2012a) demonstrated that 137Cs deposited on the forest floor by the Fukushima NPP accident was observed in topsoils (including within the litter layer or the O horizon) and has become a major potential long-term source for mobile 137Cs. They found that 137Cs penetrated deeper into the profile at forest sites, where it reached the mineral soil horizons, than at other sites such as meadows. It has also been reported that 2.1e12.8% of Fukushima-fallout 137Cs in the topmost mineral soils is retained as easily exchangeable ions by abiotic components in forest sites (Koarashi et al., 2012b). Forest ecosystems occupy 66% of the area that was heavily contaminated by the Fukushima NPP accident (>134, 137Cs 1 MBq m2) (Hashimoto et al., 2012). Therefore, it is critical for assessing the impact of the Fukushima NPP to elucidate 137 Cs mobility in forest ecosystems. The dominant forest types in the heavily contaminated area were deciduous broadleaf forests and evergreen needleleaf forests
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(Hashimoto et al., 2012). The trees in deciduous forests did not have leaves in March 2011, so the majority of radionuclides delivered to the forests were directly deposited on the forest floor. The fact that additional input of 137Cs onto the forest floor through processes such as litter fall and stem flow is negligible in a deciduous forest affords a better opportunity to explore the migration of Fukushimaaccident-derived 137Cs in forest surface soils, particularly during the early stage after deposition. Here, by use of lysimeters, we report the first direct observations of vertical migration of 137Cs in a litter-mineral soil system at a deciduous forest affected by the Fukushima NPP accident. Lysimeters were installed in a deciduous broadleaf forest soil within the 70-km zone of the Fukushima Dai-ichi NPP. The study was conducted over a period spanning 2 month to 2 y after the accident. We quantified the amounts of 137Cs migrated and retained within the litter and surface mineral soil layers, and revealed temporal changes in the rate of 137Cs migration over the first 2 y after the Fukushima NPP accident. 2. Materials and methods 2.1. Site description The study site, the Ogawa forest, is located on the undulating plateau at the southern edge of the Abukuma mountain region (36 560 N, 140 350 E; 655 m a.s.l.), w67 km southwest of the Fukushima Dai-ichi NPP (Fig. 1). The mean annual air temperature and precipitation are 10.7 C and 1910 mm, respectively (Mizoguchi et al., 2002). Monthly precipitation exceeds 100 mm, except during January, February, and December. Snow cover occurs occasionally in winter, up to a depth of about 50 cm. The study site is a deciduous broadleaf forest, dominated by Japanese beech (Fagus crenata) and Japanese oak (Quercus crispula), with no understory. The forest floor consists mostly of a litter and a thin humus layer (L layer), indicating rapid decomposition of litter at this site (mull type). Late Quaternary volcanic ash has been widely deposited on the site. There is a heterogeneous and mosaic-style pattern of distribution of Cambisols and Andosols on the site (Yoshinaga et al., 2002). Soil physicochemical properties in the sampling point are given in Table 1. The loam soil (Cambisols) showed low bulk density and was acidic. Cation exchange capacity (CEC) and organic carbon content were relatively high in the topsoil and decreased with depth. On 3 March 2011 (8 d before the Fukushima NPP accident), the forest floor was covered with <10 cm of snow and the trees did not have leaves. 2.2. Sampling and analyses Seepage water was collected from the study site using PVC zerotension lysimeters (80 cm2; Fig. 2). In a lysimeter, water is allowed to drain freely through the soil under gravity alone. Three lysimeters were set up at each of two depths (5 and 10 cm) within a 2 2 m square (total of six lysimeters). Intact soil cores were collected by pounding a pipe with 10 cm inside diameter into the soil to the prescribed depth and filled in lysimeters without further compaction. Seepage water was collected in 1-L borosilicate glass bottles, which were placed below ground to keep the samples in cool and dark conditions. Sampling of seepage water was carried out monthly in winter and biweekly in other seasons, during the period May 2011 to March 2013. From April 2012, seepage water under the L layer was also collected by installing an additional three lysimeters for this layer. We defined the period from May 2011 to April 2012 as the first year (1Y) and that from April 2012 to March 2013 as the second year (2Y), respectively. Water volume (lysimeter solutions) and pH were determined in the laboratory immediately
Fig. 1. Location of the study site.
after collection. Water volume was used for calculation of water fluxes. Samples were filtered through prewashed 0.45-mm-pore cellulose acetate membrane filter units (NalgeneÒ). An aliquot for analysis of dissolved organic carbon (DOC) was acidified to pH 3 with HCl. DOC measurements were conducted within 3 d of sampling, using a total organic carbon analyzer (TOC-L CPH, Shimadzu, Kyoto, Japan). A minimum of three measurements was taken for each sample, and the analytical precision was typically less than 1%. With a single monthly or biweekly collection, we were not able to obtain sufficient seepage water samples for subsequent radiocesium analysis (see below). Two or more seepage water samples collected (and stored at 5 C) over a certain period of time were combined for analysis. Soil samples were collected in May and December 2011, and August 2012. After sampling the L layer (30 30 cm square), three soil cores (10 cm in diameter and 10 cm in depth) were collected close to the lysimeters. The soil cores were subdivided into the depths of 0e5 cm and 5e10 cm, composited according to the sections, and then sieved through a 2-mm mesh. The soil samples were dried at room temperature for analyses of 137Cs activity. The litter samples were dried at room temperature, and then finely chopped using a mixer to obtain homogenized samples. For radiocesium analysis, seepage water samples were concentrated using a rotary evaporator and then lyophilized to a powder. These powdered samples, dried soil samples, and litter samples were filled into plastic tubes, and analyzed for 137Cs and 134Cs using
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
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Table 1 Physicochemical properties of soil. Bulk density (g cm3) Soil texturea (%)
Depth
0e5 cm 0.47 5e10 cm 0.50 10e20 cm 0.62 a
Sand
Silt
Clay
57.0 63.8 62.9
28.4 24.3 24.8
14.6 11.9 12.3
pH (H2O) CEC (cmol kg1) Exchangeable cation (cmol kg1)
4.8 5.0 5.3
28.7 18.9 16.8
Mg
K
Na
2.2 0.5 0.3
0.9 0.2 0.1
0.5 0.2 0.1
0.1 0.1 0.1
12.9 5.3 3.6
106 63 50
Sand, 0.02e2 mm; Silt, 0.002e0.02 mm; Clay,<0.002 mm.
a well type Ge detector (GWL-120230, ORTEC, Atlanta, USA). The detector was calibrated with a standard gamma-ray source with uncertainty of w5% (MX033U8PP, Japan Radioisotope Association, Tokyo) and a standard reference material (NBS 4353, National Bureau of Standards, Washington). The measurement times were from 10,000 to 260,000 s. Activity concentrations of 137Cs and 134Cs were decay-corrected to the date and time of sampling. Relative uncertainties due to propagation of errors are low, i.e., about 5%. The measured 137Cs and 134Cs showed a similar pattern all of the seepage water, and therefore, only the 137Cs results were discussed in this study for simplicity. The ratio of 137Cs to 134Cs concentrations for the seepage water samples was generally 1.0e1.1 which was decay-corrected to 11 March 2011. Cesium-137 flux from each horizon was calculated by multiplying the water fluxes by the activity concentrations of 137Cs in the lysimeter solutions. The soil temperatures at a depth of 5 cm were measured using thermorecorders (TL3633 and CB3911; As One Co., Ltd., Tokyo, Japan). Rainfall was recorded at hourly intervals using a tippingbucket rain gauge (No. 34-T, Ota Keiki Seisakusho, Tokyo, Japan) with the data logger (RF-3, T&D Co. Ltd., Nagano, Japan) placed close to the lysimeters. 3. Results 3.1.
Ca
Base saturation (%) Organic carbon (g kg1)
137
Cs inventory in litter and soil profile
The temporal change in 137Cs in the soil profile is summarized in Table 2. The total inventory of 137Cs in investigated soil (for the sampling depth of 10 cm) ranged from 16.4 to 21.8 kBq m2, with an average of 19.4 kBq m2 and a standard deviation of 2.7 kBq m2. The inventoried 137Cs resulted mostly from the Fukushima NPP accident, with a minor contribution from atmospheric nuclear weapon testing fallout (estimated to be w3 kBq m2). A striking decrease in the inventory of the L layer with time in 1Y was noted in the present study. The 137Cs in the L layer occupied 67% of the total inventory (down to 10 cm) in May 2011. This decreased drastically to 13% in December 2011, and then remained
Lysimeter
at almost the same level after December 2011 (Table 2). Contrastingly, 137Cs in the top 5 cm of mineral soil significantly increased between May 2011 (26%) and December 2011 (80%). This trend indicated that the major part of 137Cs in the L layer moved into the mineral soil within a short period of time, within just 1 y after the accident. However, 93% of the deposited 137Cs remained in a combined layer of L layer and top 5 cm of the mineral soil layer over a period of one and a half years following the accident (Table 2). 3.2. Downward fluxes of
137
Cs by seepage water
Temporal variations in 137Cs fluxes at different depths caused by seepage water are shown in Fig. 3, together with soil temperature and precipitation records. Several downpours were observed from August to September 2011, and from May to June 2012 (Fig. 3a). Cesium-137 fluxes through water seepage along the mineral soil layers decreased remarkably during the first year after the accident (1Y), with decreasing 137Cs in the L layer (Table 2 and Fig. 3c and d). Cesium-137 fluxes showed a reduced pace of downward migration during the second year after the accident (2Y, from April 2012 to March 2013). The total fluxes of 137Cs decreased from 96 Bq m2 in 1Y to 27 Bq m2 in 2Y at the 5 cm depth, and from 35 Bq m2 to 14 Bq m2 at the 10 cm depth, respectively. The leaching of 137Cs from the L layer in 2Y was significantly larger (511 Bq m2, see Fig. 3b) than the flux at 5 cm depth, indicating a predominant adsorption (95 3%) of litter-leachate 137Cs within the first 5 cm of mineral soil (one standard deviation). Only 2 2% of 137Cs leaving the L layer penetrated below a 10 cm depth. The annual 137Cs migration below 5 cm and 10 cm depths in 1Y accounted for only 0.51 0.16% and 0.16 0.07% of the total inventory in the upper layer, respectively (Table 2 and Fig. 3). The 137Cs migration in 2Y was also quite small compared to the inventory (0.14 0.07% in 5 cm and 0.07 0.05% in 10 cm, respectively), although the leaching of 137 Cs from the L layer during 2Y reached about 20% of the total amount within the L layer. 3.3. Relationship between environmental factors
137
Cs in seepage water and
The 137Cs fluxes during 2Y at all depths were significantly affected by the amount of precipitation (Fig. 3). The correlation coefficients between the average precipitation (mm d1) and 137Cs fluxes in 2Y were r ¼ 0.95 (p < 0.01) in litter leachate, r ¼ 0.93
Litter
Polypropylene meshes Glass balls
Table 2 Depth distributions of Depth
Soil Drain
Glass bottle
Litter and humus 0e5 cm 5e10 cm Total 137
Fig. 2. Depiction of the lysimeter set-up in the field.
137
Cs inventory in the soil profiles.
17 May 2011
14 December 2011
23 August 2012
Inventory (kBq m2)
%
Inventory (kBq m2)
%
Inventory (kBq m2)
%
11.0 0.34
67
2.65 0.10
13
2.74 0.10
13
4.28 0.21 1.11 0.15 16.4 0.43
26 7
16.0 0.68 1.36 0.14 20.1 0.70
80 7
17.5 0.70 1.51 0.12 21.8 0.72
80 7
Cs inventories were decay-corrected to the date and time of sampling. Errors are combined uncertainties of measurements.
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
(a) Soil temperature (ºC)
30
1Y
2Y
15
20
10
10
5
0
M J J A S O N D J F M A M J J A S O N D J F M
2011
2012
Precipitation (mm d−1)
12
0
2013
2Y
(b) 4
511 Bq m
4. Discussion −2
4.1. Vertical migration of
137Cs flux (Bq m−2 d −1)
3 2
1 0
A M J J A S O N D J F M
2012
2013
1Y
(c) 1
96 Bq
2Y m−2
27 Bq m−2
flux d−1)
0.4
(Bq
m−2
137Cs
0.8 0.6
0.2 0
M J J A S O N D J F M A M J J A S O N D J F M
2011
(d)
137Cs flux (Bq m−2 d−1)
0.3
(p < 0.01) at 5 cm depth, and r ¼ 0.94 (p < 0.01) at 10 cm depth, respectively. In other respects, the 137Cs activity concentration in seepage water showed positive temperature dependence in 2Y. The correlation coefficients between the average soil temperature and 137 Cs activity concentrations were r ¼ 0.78 (p < 0.05) in litter leachate, r ¼ 0.89 (p < 0.05) at 5 cm depth, and r ¼ 0.84 (p < 0.05) at 10 cm depth, respectively (Fig. 4a and b). On the one hand, 137Cs activity concentration in litter leachate correlated with DOC concentration in litter leachate (r ¼ 0.78, p < 0.05; Fig. 4c), while 137Cs activity concentration in soil solution did not correlate with DOC concentration in soil solution (Fig. 4d). There is no indication that 137 Cs activity concentration correlated with the pH of seepage water (Figs. 4e and f).
2012
2013
1Y
2Y
35 Bq m−2
14 Bq m−2
0.2
137
The lysimetric method used in the present study successfully quantified an imperceptible migration of 137Cs in the litter-mineral soil system soon after the accident; this migration had never been previously detected by the vertical distribution of Fukushimaaccident-derived 137Cs in the soil profile. Our data demonstrated that annual 137Cs migration below 5 cm and 10 cm depths accounted for only 0.1e0.5% and 0.1e0.2%, respectively, of the total inventory in the upper layer (Table 2 and Fig. 3). These results correspond with the result showing that 93% of the total 137Cs inventory remained in the L layer and in the top 5 cm of the mineral soil (Table 2). The migration of 137Cs by seepage water was already limited during the very early period after the Fukushima NPP accident. Such high retention on the topsoil and low mobility are consistent with the results of 137Cs depth profiles in forest soils in Fukushima City in 2011 (Koarashi et al., 2012a; Matsunaga et al., 2013) and with earlier studies on the Chernobyl accident (Comans et al., 1991; Schimmack et al., 1994; Bunzl et al., 1998). There are only a few studies on 137Cs in seepage water in soils under natural conditions; they showed a comparable annual migration of 0.1e0.15% below 5 cm depth three years after the Chernobyl NPP accident (Kliashtorin et al., 1994; Tegen and Dörr, 1996). These results suggest that the 137Cs mobility in the mineral soil was practically lost within several months in our study site. Moreover, a considerable adsorption of litter-leachate 137Cs in the 0e5 cm layer occurred through 2Y (95 3%, see Fig. 3), suggesting that the cesium adsorption capacity of the topsoil in the study site was significant and that, in the future, a large proportion of 137Cs deposited on the forest floor will probably remain in the topsoil. On the other hand, it was found that 2 2% of 137Cs in seepage water from the L layer penetrated below a 10 cm depth along the water movement through the soil profile. Identifying the source, dynamics, and fate of these highly mobile 137Cs in the soil profile is the key to evaluating and predicting the impact of Fukushimaaccident-derived 137Cs on forest ecosystems. 4.2. Precipitation effects on
0.1
0
M J J A S O N D J F M A M J J A S O N D J F M
2011
2012
2013
Fig. 3. Temporal data on (a) soil temperature (solid line) and precipitation (gray bar), and 137Cs fluxes (b) in the litter leachate, (c) at 5 cm depth and (d) at 10 cm depth, respectively. The arrows in the figures represent periods of soil sampling. The values
Cs in the litter-mineral soil system
137
Cs flux
The 137Cs flux responded to precipitation, implying that the movement of 137Cs to deeper layers may occur mainly via preferential flow paths. The temporal variation in the 137Cs flux at each depth strongly correlated with the seasonal variation in the amount of precipitation (Fig. 3). During and following a rain event, new water bypasses the soil matrix through preferential flow paths, and/or
on figures indicate total fluxes over the periods indicated. Errors are combined uncertainties of measurements.
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
13
Fig. 4. Relation between 137Cs activity concentration in seepage water during the second year (2Y) and (a, b) soil temperature, (c, d) DOC concentration, (e, f) pH of seepage water. Errors are combined uncertainties of measurements.
displaced old water flows through the soil matrix because of slow convective flux (Legout et al., 2009). Bundt et al. (2000) suggested the importance of preferential flow for the transport of strongly adsorbing radionuclides with dissolved or colloidal ligands, showing that 137Cs was enriched in the preferential flow paths by a factor of up to 3.5 in a forest soil. However, as mentioned in the previous Section (4.1), the amount of 137Cs moving into the deep layer was quite small compared with the amount of 137Cs adsorbed in the topsoil. 4.3. Release of
137
Cs from litter layer
The remarkable decrease in 137Cs fluxes with time observed at depths of 5 cm and 10 cm was concurrent with the transfer of the majority of 137Cs from the L layer to mineral soil in 1Y (Table 2 and Fig. 3). The 137Cs flux (511 Bq m2) from the L layer in 2Y was equivalent to about 20% of 137Cs inventory within the L layer. Although no observation was conducted for the litter-leachate 137Cs in 1Y, the large (w8400 Bq m2 or 76%) decrease in the 137Cs inventory within the L layer between May and December 2011 (Table 2) suggests a faster release of 137Cs from the 137Cs-contaminated litter materials in 1Y than in 2Y. The rate of 137Cs loss from the L layer in 1Y was greater than that of litter mass loss (w40% y1) observed for beech and oak leaves at the study site (Ono et al., 2013). Sauras et al. (1994), using an incubation experiment of holm oak leaves treated with aerosol 134Cs, showed that around 70% of the initial 134Cs was transferred to the underlying layers during the first year, while litter mass loss was about 30% of the initial weight. It is very likely that 137Cs derived from the Fukushima NPP accident was deposited directly on the litter surface in the forest floor at our study site. The result of this study thus suggests that 137Cs weakly trapped in the L layer was mobilized by December 2011, and that 137Cs which remained in the L layer in 2Y was more difficult to desorb by processes such as incorporation into the litter structure and absorption onto recalcitrant organic compounds.
The decomposition of 137Cs-contaminated litter on the forest floor appeared to be a process contributing strongly to the migration of 137Cs in the forest soil in 2Y. The activity concentration of 137 Cs in seepage water showed positive temperature dependence in 2Y (Fig. 4a and b), although the 137Cs flux also strongly correlated with precipitation, as mentioned above (4.2). In general, litter decomposition on the forest floor increases with increasing temperature (Fierer et al., 2005), suggesting that the process of 137Cs loss from the L layer in 2Y was closely related with that of litter decomposition. Tegen and Dörr (1996) showed that 137Cs activity concentrations in an organic-rich forest soil strongly correlated with DOC concentrations, and both were highly correlated with soil temperature and thus microbial activity. In this study, a positive relation was found between the 137Cs and DOC concentrations in litter leachate (Fig. 4c), but there was no correlation between the two in soil solutions collected at two depths of the mineral soil (Fig. 4d). Therefore, the microbial decomposition of soil organic matter would not contribute to the 137Cs migration in the mineral soil at present. Moreover, because there is no indication that 137Cs activity concentrations correlated with the pH of seepage water (Figs. 4e and f), the mobilization of 137Cs in mineral soils by the ion exchange processes at the observed pH range (4.8e5.4) is also small compared to the release of 137Cs by litter decomposition process. 4.4. Prediction of
137
Cs transfer from soil solution to plant
The 137Cs activity concentration in the soil solution represents a quantitative characteristic of bioavailability for uptake by roots, as well as the vertical migration in the soil profile (Konoplev et al., 1993). It has been found that the concentration factor for 137Cs uptake by plants (defined as the ratio of 137Cs activity concentration in plant, Bq kg-DW1, to that in soil solution, Bq L1) directly correlates with the activity concentration of 137Cs in the soil solution (Absalom et al., 1999; Sanchez et al., 1999). Almost all of the forest
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herbs and most trees take up their nutrients from the uppermost 10 cm of soil, in which most 137Cs is deposited over the long term (Table 2). Although the concentration factor could not be observed in this study, 137Cs activity concentration in seepage water observed here, and reported concentration factors (4e1 104 L kg-DW1, Sombré et al., 1994; Gommers et al., 2005), allow us to estimate 137 Cs activity concentration in tree species to be 0.02e240 Bq kgDW1 in our study site. Because 137Cs activity concentration in the litter layer in August 2012 was about 3000 Bq kg-DW1, the additional input of 137Cs by new litter fall would be less important as a source of subsequent contamination on the forest floor. However, the evidence for the small but certain amount of bioavailable (soil solution) 137Cs in forest surface soils indicates that the uptake by plant and recycling of 137Cs, particularly in heavily contaminated forests, can cause significant external and internal (through consumption of food products) radiation exposure to humans over the long term. Further observations are necessary to identify the processes controlling the mobility (and thus bioavailability) of 137Cs in forest surface environments. 5. Conclusions Our lysimetric method captured the migration of 137Cs in a deciduous forest soil following the Fukushima NPP accident. The migration of 137Cs by seepage water was characteristic for a very small part of the total 137Cs inventory in the soil profile. The majority of 137Cs in seepage water originated from the litter layer, and the leaching from the surface mineral soil to the deeper layer was considered to be slower and of a considerably smaller quantity. The major proportion of 137Cs in the litter layer moved into the mineral soil within a year after deposition. Most of the 137Cs in seepage water from the litter layer was adsorbed in the topsoil and only a small percentage of 137Cs penetrated to deeper layers. The annual amount of 137Cs migrated below a 10 cm depth was only about 0.1% of total amount of 137Cs deposited onto the forest floor. A reduced rate of downward migration was observed in the second year after the accident. Therefore, most of the 137Cs deposited on the forest floor probably remains in the topsoil for a long time, although the litter layer will be notable as a source of highly mobile 137Cs in the soil system. In future, when most of 137Cs in the litter layer transfers to the mineral soil, 137Cs migration will be controlled by several physical, chemical, and environmental factors such as pH, cation exchange capacity, organic carbon content, animal activity, soil texture, and type of clay minerals. Since a proportion of 137Cs in the soil solid phase is not available for exchange with the solution phase, because of its irreversible adsorption by minerals (Rosén et al., 1999), 137Cs in seepage water (and thus, the bioavailability of 137Cs in soil) may decrease with time. Therefore, we will continuously observe 137Cs migration by seepage water in order to improve our understanding of the post-deposition early-stage dynamics of this mobile 137Cs in forest ecosystems. Acknowledgments We thank the Ibaraki District Forest Office for permission to use the Ogawa forest site. We also thank Drs. Takuya Kobayashi, Katsunori Tsuduki, and Takashi Suzuki of JAEA and Mr. Koichi Moriya of Nagoya University for their technical assistance with sampling, and Dr. Shigeyoshi Otosaka for his support with laboratory work. Finally, we thank Drs. Akira Endo and Haruyasu Nagai of JAEA, and
Prof. Seiya Nagao of Kanazawa University for providing helpful comments. References Absalom, J.P., Young, S.D., Crout, N.M.J., Nisbet, A.F., Woodman, R.F.M., Smolders, E., Gillet, A.G., 1999. Predicting soil to plant transfer of radiocesium using soil characteristics. Environ. Sci. Technol. 33, 1218e1223. Bundt, M., Albrecht, A., Froidevaux, P., Blaser, P., Flühler, H., 2000. Impact of preferential flow on radionuclide distribution in soil. Environ. Sci. Technol. 34, 3895e3899. Bunzl, K., Kracke, W., Schimmack, W., Zelles, L., 1998. Forms of fallout 137Cs and 239þ240 Pu in successive horizons of a forest soil. J. Environ. Radioact 39, 55e68. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G., Yamazawa, H., 2011. Preliminary estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima Daiichi nuclear power plant into the atmosphere. J. Nucl. Sci. Technol. 48, 1129e1134. Comans, R.N.J., Haller, M., De Preter, P., 1991. Sorption of cesium on illite: nonequilibrium behavior and reversibility. Geochim. Cosmochim. Acta 55, 433e 440. Fierer, N., Craine, J.M., McLauchlan, K., Schimel, J.P., 2005. Litter quality and the temperature sensitivity of decomposition. Ecology 86, 320e326. Gommers, A., Gäfvert, T., Smolders, E., Merckx, R., Vandenhove, H., 2005. Radiocaesium soil-to-wood transfer in commercial willow short rotation coppice on contaminated farm land. J. Environ. Radioact. 78, 267e287. Hashimoto, S., Ugawa, S., Nanko, K., Shichi, K., 2012. The total amounts of radioactively contaminated materials in forests in Fukushima, Japan. Sci. Rep.. http:// www.nature.com/srep/2012/120525/srep00416/full/srep00416.html. Kliashtorin, A.L., Tikhomirov, F.A., Shcheglov, A.I., 1994. Vertical radionuclide transfer by infiltration water in forest soils in the 30-km Chernobyl accident zone. Sci. Total Environ. 157, 285e288. Koarashi, J., Atarashi-Andoh, M., Matsunaga, T., Sato, T., Nagao, S., Nagai, H., 2012a. Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions. Sci. Total Environ. 431, 392e 401. Koarashi, J., Moriya, K., Atarashi-Andoh, M., Matsunaga, T., Fujita, H., Nagaoka, M., 2012b. Retention of potentially mobile radiocesium in forest surface soils affected by the Fukushima nuclear accident. Sci. Rep.. http://www.nature.com/ srep/2012/121219/srep01005/full/srep01005.html. Konoplev, A.V., Viktorova, N.V., Virchenko, E.P., Popov, V.E., Bulgakov, A.A., Desmet, G.M., 1993. Influence of agricultural countermeasures on the ratio of different chemical forms of radionuclides in soil and soil solution. Sci. Total Environ. 137, 147e162. Legout, A., Legout, C., Nys, C., Dambrine, E., 2009. Preferential flow and slow convective chloride transport through the soil of a forested landscape (Fougères, France). Geoderma 151, 179e190. Matsunaga, T., Koarashi, J., Atarashi-Andoh, M., Nagao, S., Sato, T., Nagai, H., 2013. Comparison of the vertical distributions of Fukushima nuclear accident radiocesium in soil before and after the first rainy season, with physicochemical and mineralogical interpretations. Sci. Total Environ. 447, 301e314. Mizoguchi, Y., Morisawa, T., Ohtani, Y., 2002. Climate in Ogawa forest Reserve. In: Nakashizuka, T., Matsumoto, Y. (Eds.), Ecological Studies, Diversity and Interaction in a Temperate Forest Community: Ogawa Forest Reserve of Japan, vol. 158. Springer-Verlag, Tokyo, pp. 11e18. Ono, K., Hiradate, S., Morita, S., Hirai, K., 2013. Fate of organic carbon during decomposition of different litter types in Japan. Biogeochemistry 112, 7e21. Rosén, K., Öborn, I., Lönsjö, H., 1999. Migration of radiocaesium in Swedish soil profiles after the Chernobyl accident, 1987e1995. J. Environ. Radioact 46, 45e66. Sanchez, A.L., Wright, S.M., Smolders, E., Naylor, C., Steven, P.A., Kennedy, V.H., Dodd, B.A., Singleton, D.L., Barnett, C.L., 1999. High plant uptake of radiocesium from organic soils due to Cs mobility and low soil K content. Environ. Sci. Technol. 33, 2752e2756. Sauras, T., Roca, M.C., Tent, J., Llauradö, M., Vidal, M., Rauret, G., Vallejo, V.R., 1994. Migration study of radionuclides in a Mediterranean forest soil using synthetic aerosols. Sci. Total Environ. 157, 231e238. Schimmack, W., Bunzl, K., Dietl, F., Klotz, D., 1994. Infiltration of radionuclides with low mobility (137Cs and 60Co) into a forest soil. Effect of the irrigation intensity. J. Environ. Radioact. 24, 53e63. Sombré, L., Vanhouche, M., de Brouwer, S., Ronneau, C., Lambotte, J.M., Myttenaere, C., 1994. Long-term radiocesium behaviour in spruce and oak forests. Sci. Total Environ. 157, 59e71. Tegen, I., Dörr, H., 1996. Mobilization of cesium in organic rich soils: correlation with production of dissolved organic carbon. Water Air Soil Pollut. 88, 133e144. Yoshinaga, S., Takahashi, M., Aizawa, S., 2002. Landforms and soil characteristics in Ogawa Forest Reserve. In: Nakashizuka, T., Matsumoto, Y. (Eds.), Ecological Studies, Diversity and Interaction in a Temperate Forest Community: Ogawa Forest Reserve of Japan, vol. 158. Springer-Verlag, Tokyo, pp. 19e26.
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137
Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident Takahiro Nakanishi*, Takeshi Matsunaga, Jun Koarashi, Mariko Atarashi-Andoh Research Group for Environmental Science, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2013 Received in revised form 22 October 2013 Accepted 23 October 2013 Available online
The large amount of 137Cs deposited on the forest floor because of the Fukushima Dai-ichi Nuclear Power Plant accident represents a major potential long-term source for mobile 137Cs. To investigate 137Cs mobility in forest soils, we investigated the vertical migration of 137Cs through seepage water, using a lysimetric method. The study was conducted in a deciduous forest soil over a period spanning 2 month to 2 y after the Fukushima nuclear accident. Our observations demonstrated that the major part of 137Cs in the litter layer moved into the mineral soil within one year after the accident. On the other hand, the topsoil prevented migration of 137Cs, and only 2% of 137Cs in the leachate from litter and humus layer penetrated below a 10 cm depth. The annual migration below a 10 cm depth accounted for 0.1% of the total 137Cs inventory. Therefore, the migration of 137Cs by seepage water comprised only a very small part of the total 137Cs inventory in the mineral soil, which was undetectable from the vertical distribution of 137 Cs in the soil profile. In the present and immediate future, most of the 137Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable 137 Cs exists in forest surface soil. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs Fukushima Dai-ichi Nuclear Power Plant accident Mobility Forest soil Lysimeter Temporal change
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant accident (Fukushima NPP accident) that was triggered by a catastrophic earthquake (M9.0) and the resulting tsunami on 11 March 2011, released a substantial amount of radionuclides to the atmosphere (Chino et al., 2011). Various terrestrial ecosystems in the wider area were affected by the Fukushima NPP accident. Of the radionuclides emitted from the Fukushima NPP accident, 137Cs with a physical half-life of 30.1 y, is the largest source of concern, because of its potential impact on humans and ecosystems over the coming decades. In particular, the vertical migration of 137Cs in soils is one of the essential factors in determining long-term external dose to humans. The internal dose to humans may also be influenced through changes in plant uptake, as radionuclides become fixed in the soil or move out of the active root zone. In earlier studies conducted after the Chernobyl NPP accident that occurred in 1986, various measures of migration and retention in soil were used, such as the fraction of radionuclide content found within a certain depth or median depth, and migration rates, which were calculated using various models. After the Fukushima NPP
* Corresponding author. Tel.: þ81 29 282 6433; fax: þ81 29 282 6760. E-mail address: [email protected] (T. Nakanishi). 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.10.019
accident, Matsunaga et al. (2013) showed, using a relaxation parameter, that the 137Cs vertical profiles in undisturbed soils were almost unchanged between points in time spanning the first rainy season. This indicated the overall immobility of the deposited 137Cs even after only three months, and the difficulty in quantifying its mobility in soils by such methods for the solid soil phase. Hence, there is a strong need to parameterize the 137Cs migration process in soils by direct in situ observation of 137Cs in seepage water through the soil profile. Koarashi et al. (2012a) demonstrated that 137Cs deposited on the forest floor by the Fukushima NPP accident was observed in topsoils (including within the litter layer or the O horizon) and has become a major potential long-term source for mobile 137Cs. They found that 137Cs penetrated deeper into the profile at forest sites, where it reached the mineral soil horizons, than at other sites such as meadows. It has also been reported that 2.1e12.8% of Fukushima-fallout 137Cs in the topmost mineral soils is retained as easily exchangeable ions by abiotic components in forest sites (Koarashi et al., 2012b). Forest ecosystems occupy 66% of the area that was heavily contaminated by the Fukushima NPP accident (>134, 137Cs 1 MBq m2) (Hashimoto et al., 2012). Therefore, it is critical for assessing the impact of the Fukushima NPP to elucidate 137 Cs mobility in forest ecosystems. The dominant forest types in the heavily contaminated area were deciduous broadleaf forests and evergreen needleleaf forests
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(Hashimoto et al., 2012). The trees in deciduous forests did not have leaves in March 2011, so the majority of radionuclides delivered to the forests were directly deposited on the forest floor. The fact that additional input of 137Cs onto the forest floor through processes such as litter fall and stem flow is negligible in a deciduous forest affords a better opportunity to explore the migration of Fukushimaaccident-derived 137Cs in forest surface soils, particularly during the early stage after deposition. Here, by use of lysimeters, we report the first direct observations of vertical migration of 137Cs in a litter-mineral soil system at a deciduous forest affected by the Fukushima NPP accident. Lysimeters were installed in a deciduous broadleaf forest soil within the 70-km zone of the Fukushima Dai-ichi NPP. The study was conducted over a period spanning 2 month to 2 y after the accident. We quantified the amounts of 137Cs migrated and retained within the litter and surface mineral soil layers, and revealed temporal changes in the rate of 137Cs migration over the first 2 y after the Fukushima NPP accident. 2. Materials and methods 2.1. Site description The study site, the Ogawa forest, is located on the undulating plateau at the southern edge of the Abukuma mountain region (36 560 N, 140 350 E; 655 m a.s.l.), w67 km southwest of the Fukushima Dai-ichi NPP (Fig. 1). The mean annual air temperature and precipitation are 10.7 C and 1910 mm, respectively (Mizoguchi et al., 2002). Monthly precipitation exceeds 100 mm, except during January, February, and December. Snow cover occurs occasionally in winter, up to a depth of about 50 cm. The study site is a deciduous broadleaf forest, dominated by Japanese beech (Fagus crenata) and Japanese oak (Quercus crispula), with no understory. The forest floor consists mostly of a litter and a thin humus layer (L layer), indicating rapid decomposition of litter at this site (mull type). Late Quaternary volcanic ash has been widely deposited on the site. There is a heterogeneous and mosaic-style pattern of distribution of Cambisols and Andosols on the site (Yoshinaga et al., 2002). Soil physicochemical properties in the sampling point are given in Table 1. The loam soil (Cambisols) showed low bulk density and was acidic. Cation exchange capacity (CEC) and organic carbon content were relatively high in the topsoil and decreased with depth. On 3 March 2011 (8 d before the Fukushima NPP accident), the forest floor was covered with <10 cm of snow and the trees did not have leaves. 2.2. Sampling and analyses Seepage water was collected from the study site using PVC zerotension lysimeters (80 cm2; Fig. 2). In a lysimeter, water is allowed to drain freely through the soil under gravity alone. Three lysimeters were set up at each of two depths (5 and 10 cm) within a 2 2 m square (total of six lysimeters). Intact soil cores were collected by pounding a pipe with 10 cm inside diameter into the soil to the prescribed depth and filled in lysimeters without further compaction. Seepage water was collected in 1-L borosilicate glass bottles, which were placed below ground to keep the samples in cool and dark conditions. Sampling of seepage water was carried out monthly in winter and biweekly in other seasons, during the period May 2011 to March 2013. From April 2012, seepage water under the L layer was also collected by installing an additional three lysimeters for this layer. We defined the period from May 2011 to April 2012 as the first year (1Y) and that from April 2012 to March 2013 as the second year (2Y), respectively. Water volume (lysimeter solutions) and pH were determined in the laboratory immediately
Fig. 1. Location of the study site.
after collection. Water volume was used for calculation of water fluxes. Samples were filtered through prewashed 0.45-mm-pore cellulose acetate membrane filter units (NalgeneÒ). An aliquot for analysis of dissolved organic carbon (DOC) was acidified to pH 3 with HCl. DOC measurements were conducted within 3 d of sampling, using a total organic carbon analyzer (TOC-L CPH, Shimadzu, Kyoto, Japan). A minimum of three measurements was taken for each sample, and the analytical precision was typically less than 1%. With a single monthly or biweekly collection, we were not able to obtain sufficient seepage water samples for subsequent radiocesium analysis (see below). Two or more seepage water samples collected (and stored at 5 C) over a certain period of time were combined for analysis. Soil samples were collected in May and December 2011, and August 2012. After sampling the L layer (30 30 cm square), three soil cores (10 cm in diameter and 10 cm in depth) were collected close to the lysimeters. The soil cores were subdivided into the depths of 0e5 cm and 5e10 cm, composited according to the sections, and then sieved through a 2-mm mesh. The soil samples were dried at room temperature for analyses of 137Cs activity. The litter samples were dried at room temperature, and then finely chopped using a mixer to obtain homogenized samples. For radiocesium analysis, seepage water samples were concentrated using a rotary evaporator and then lyophilized to a powder. These powdered samples, dried soil samples, and litter samples were filled into plastic tubes, and analyzed for 137Cs and 134Cs using
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
11
Table 1 Physicochemical properties of soil. Bulk density (g cm3) Soil texturea (%)
Depth
0e5 cm 0.47 5e10 cm 0.50 10e20 cm 0.62 a
Sand
Silt
Clay
57.0 63.8 62.9
28.4 24.3 24.8
14.6 11.9 12.3
pH (H2O) CEC (cmol kg1) Exchangeable cation (cmol kg1)
4.8 5.0 5.3
28.7 18.9 16.8
Mg
K
Na
2.2 0.5 0.3
0.9 0.2 0.1
0.5 0.2 0.1
0.1 0.1 0.1
12.9 5.3 3.6
106 63 50
Sand, 0.02e2 mm; Silt, 0.002e0.02 mm; Clay,<0.002 mm.
a well type Ge detector (GWL-120230, ORTEC, Atlanta, USA). The detector was calibrated with a standard gamma-ray source with uncertainty of w5% (MX033U8PP, Japan Radioisotope Association, Tokyo) and a standard reference material (NBS 4353, National Bureau of Standards, Washington). The measurement times were from 10,000 to 260,000 s. Activity concentrations of 137Cs and 134Cs were decay-corrected to the date and time of sampling. Relative uncertainties due to propagation of errors are low, i.e., about 5%. The measured 137Cs and 134Cs showed a similar pattern all of the seepage water, and therefore, only the 137Cs results were discussed in this study for simplicity. The ratio of 137Cs to 134Cs concentrations for the seepage water samples was generally 1.0e1.1 which was decay-corrected to 11 March 2011. Cesium-137 flux from each horizon was calculated by multiplying the water fluxes by the activity concentrations of 137Cs in the lysimeter solutions. The soil temperatures at a depth of 5 cm were measured using thermorecorders (TL3633 and CB3911; As One Co., Ltd., Tokyo, Japan). Rainfall was recorded at hourly intervals using a tippingbucket rain gauge (No. 34-T, Ota Keiki Seisakusho, Tokyo, Japan) with the data logger (RF-3, T&D Co. Ltd., Nagano, Japan) placed close to the lysimeters. 3. Results 3.1.
Ca
Base saturation (%) Organic carbon (g kg1)
137
Cs inventory in litter and soil profile
The temporal change in 137Cs in the soil profile is summarized in Table 2. The total inventory of 137Cs in investigated soil (for the sampling depth of 10 cm) ranged from 16.4 to 21.8 kBq m2, with an average of 19.4 kBq m2 and a standard deviation of 2.7 kBq m2. The inventoried 137Cs resulted mostly from the Fukushima NPP accident, with a minor contribution from atmospheric nuclear weapon testing fallout (estimated to be w3 kBq m2). A striking decrease in the inventory of the L layer with time in 1Y was noted in the present study. The 137Cs in the L layer occupied 67% of the total inventory (down to 10 cm) in May 2011. This decreased drastically to 13% in December 2011, and then remained
Lysimeter
at almost the same level after December 2011 (Table 2). Contrastingly, 137Cs in the top 5 cm of mineral soil significantly increased between May 2011 (26%) and December 2011 (80%). This trend indicated that the major part of 137Cs in the L layer moved into the mineral soil within a short period of time, within just 1 y after the accident. However, 93% of the deposited 137Cs remained in a combined layer of L layer and top 5 cm of the mineral soil layer over a period of one and a half years following the accident (Table 2). 3.2. Downward fluxes of
137
Cs by seepage water
Temporal variations in 137Cs fluxes at different depths caused by seepage water are shown in Fig. 3, together with soil temperature and precipitation records. Several downpours were observed from August to September 2011, and from May to June 2012 (Fig. 3a). Cesium-137 fluxes through water seepage along the mineral soil layers decreased remarkably during the first year after the accident (1Y), with decreasing 137Cs in the L layer (Table 2 and Fig. 3c and d). Cesium-137 fluxes showed a reduced pace of downward migration during the second year after the accident (2Y, from April 2012 to March 2013). The total fluxes of 137Cs decreased from 96 Bq m2 in 1Y to 27 Bq m2 in 2Y at the 5 cm depth, and from 35 Bq m2 to 14 Bq m2 at the 10 cm depth, respectively. The leaching of 137Cs from the L layer in 2Y was significantly larger (511 Bq m2, see Fig. 3b) than the flux at 5 cm depth, indicating a predominant adsorption (95 3%) of litter-leachate 137Cs within the first 5 cm of mineral soil (one standard deviation). Only 2 2% of 137Cs leaving the L layer penetrated below a 10 cm depth. The annual 137Cs migration below 5 cm and 10 cm depths in 1Y accounted for only 0.51 0.16% and 0.16 0.07% of the total inventory in the upper layer, respectively (Table 2 and Fig. 3). The 137Cs migration in 2Y was also quite small compared to the inventory (0.14 0.07% in 5 cm and 0.07 0.05% in 10 cm, respectively), although the leaching of 137 Cs from the L layer during 2Y reached about 20% of the total amount within the L layer. 3.3. Relationship between environmental factors
137
Cs in seepage water and
The 137Cs fluxes during 2Y at all depths were significantly affected by the amount of precipitation (Fig. 3). The correlation coefficients between the average precipitation (mm d1) and 137Cs fluxes in 2Y were r ¼ 0.95 (p < 0.01) in litter leachate, r ¼ 0.93
Litter
Polypropylene meshes Glass balls
Table 2 Depth distributions of Depth
Soil Drain
Glass bottle
Litter and humus 0e5 cm 5e10 cm Total 137
Fig. 2. Depiction of the lysimeter set-up in the field.
137
Cs inventory in the soil profiles.
17 May 2011
14 December 2011
23 August 2012
Inventory (kBq m2)
%
Inventory (kBq m2)
%
Inventory (kBq m2)
%
11.0 0.34
67
2.65 0.10
13
2.74 0.10
13
4.28 0.21 1.11 0.15 16.4 0.43
26 7
16.0 0.68 1.36 0.14 20.1 0.70
80 7
17.5 0.70 1.51 0.12 21.8 0.72
80 7
Cs inventories were decay-corrected to the date and time of sampling. Errors are combined uncertainties of measurements.
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
(a) Soil temperature (ºC)
30
1Y
2Y
15
20
10
10
5
0
M J J A S O N D J F M A M J J A S O N D J F M
2011
2012
Precipitation (mm d−1)
12
0
2013
2Y
(b) 4
511 Bq m
4. Discussion −2
4.1. Vertical migration of
137Cs flux (Bq m−2 d −1)
3 2
1 0
A M J J A S O N D J F M
2012
2013
1Y
(c) 1
96 Bq
2Y m−2
27 Bq m−2
flux d−1)
0.4
(Bq
m−2
137Cs
0.8 0.6
0.2 0
M J J A S O N D J F M A M J J A S O N D J F M
2011
(d)
137Cs flux (Bq m−2 d−1)
0.3
(p < 0.01) at 5 cm depth, and r ¼ 0.94 (p < 0.01) at 10 cm depth, respectively. In other respects, the 137Cs activity concentration in seepage water showed positive temperature dependence in 2Y. The correlation coefficients between the average soil temperature and 137 Cs activity concentrations were r ¼ 0.78 (p < 0.05) in litter leachate, r ¼ 0.89 (p < 0.05) at 5 cm depth, and r ¼ 0.84 (p < 0.05) at 10 cm depth, respectively (Fig. 4a and b). On the one hand, 137Cs activity concentration in litter leachate correlated with DOC concentration in litter leachate (r ¼ 0.78, p < 0.05; Fig. 4c), while 137Cs activity concentration in soil solution did not correlate with DOC concentration in soil solution (Fig. 4d). There is no indication that 137 Cs activity concentration correlated with the pH of seepage water (Figs. 4e and f).
2012
2013
1Y
2Y
35 Bq m−2
14 Bq m−2
0.2
137
The lysimetric method used in the present study successfully quantified an imperceptible migration of 137Cs in the litter-mineral soil system soon after the accident; this migration had never been previously detected by the vertical distribution of Fukushimaaccident-derived 137Cs in the soil profile. Our data demonstrated that annual 137Cs migration below 5 cm and 10 cm depths accounted for only 0.1e0.5% and 0.1e0.2%, respectively, of the total inventory in the upper layer (Table 2 and Fig. 3). These results correspond with the result showing that 93% of the total 137Cs inventory remained in the L layer and in the top 5 cm of the mineral soil (Table 2). The migration of 137Cs by seepage water was already limited during the very early period after the Fukushima NPP accident. Such high retention on the topsoil and low mobility are consistent with the results of 137Cs depth profiles in forest soils in Fukushima City in 2011 (Koarashi et al., 2012a; Matsunaga et al., 2013) and with earlier studies on the Chernobyl accident (Comans et al., 1991; Schimmack et al., 1994; Bunzl et al., 1998). There are only a few studies on 137Cs in seepage water in soils under natural conditions; they showed a comparable annual migration of 0.1e0.15% below 5 cm depth three years after the Chernobyl NPP accident (Kliashtorin et al., 1994; Tegen and Dörr, 1996). These results suggest that the 137Cs mobility in the mineral soil was practically lost within several months in our study site. Moreover, a considerable adsorption of litter-leachate 137Cs in the 0e5 cm layer occurred through 2Y (95 3%, see Fig. 3), suggesting that the cesium adsorption capacity of the topsoil in the study site was significant and that, in the future, a large proportion of 137Cs deposited on the forest floor will probably remain in the topsoil. On the other hand, it was found that 2 2% of 137Cs in seepage water from the L layer penetrated below a 10 cm depth along the water movement through the soil profile. Identifying the source, dynamics, and fate of these highly mobile 137Cs in the soil profile is the key to evaluating and predicting the impact of Fukushimaaccident-derived 137Cs on forest ecosystems. 4.2. Precipitation effects on
0.1
0
M J J A S O N D J F M A M J J A S O N D J F M
2011
2012
2013
Fig. 3. Temporal data on (a) soil temperature (solid line) and precipitation (gray bar), and 137Cs fluxes (b) in the litter leachate, (c) at 5 cm depth and (d) at 10 cm depth, respectively. The arrows in the figures represent periods of soil sampling. The values
Cs in the litter-mineral soil system
137
Cs flux
The 137Cs flux responded to precipitation, implying that the movement of 137Cs to deeper layers may occur mainly via preferential flow paths. The temporal variation in the 137Cs flux at each depth strongly correlated with the seasonal variation in the amount of precipitation (Fig. 3). During and following a rain event, new water bypasses the soil matrix through preferential flow paths, and/or
on figures indicate total fluxes over the periods indicated. Errors are combined uncertainties of measurements.
T. Nakanishi et al. / Journal of Environmental Radioactivity 128 (2014) 9e14
13
Fig. 4. Relation between 137Cs activity concentration in seepage water during the second year (2Y) and (a, b) soil temperature, (c, d) DOC concentration, (e, f) pH of seepage water. Errors are combined uncertainties of measurements.
displaced old water flows through the soil matrix because of slow convective flux (Legout et al., 2009). Bundt et al. (2000) suggested the importance of preferential flow for the transport of strongly adsorbing radionuclides with dissolved or colloidal ligands, showing that 137Cs was enriched in the preferential flow paths by a factor of up to 3.5 in a forest soil. However, as mentioned in the previous Section (4.1), the amount of 137Cs moving into the deep layer was quite small compared with the amount of 137Cs adsorbed in the topsoil. 4.3. Release of
137
Cs from litter layer
The remarkable decrease in 137Cs fluxes with time observed at depths of 5 cm and 10 cm was concurrent with the transfer of the majority of 137Cs from the L layer to mineral soil in 1Y (Table 2 and Fig. 3). The 137Cs flux (511 Bq m2) from the L layer in 2Y was equivalent to about 20% of 137Cs inventory within the L layer. Although no observation was conducted for the litter-leachate 137Cs in 1Y, the large (w8400 Bq m2 or 76%) decrease in the 137Cs inventory within the L layer between May and December 2011 (Table 2) suggests a faster release of 137Cs from the 137Cs-contaminated litter materials in 1Y than in 2Y. The rate of 137Cs loss from the L layer in 1Y was greater than that of litter mass loss (w40% y1) observed for beech and oak leaves at the study site (Ono et al., 2013). Sauras et al. (1994), using an incubation experiment of holm oak leaves treated with aerosol 134Cs, showed that around 70% of the initial 134Cs was transferred to the underlying layers during the first year, while litter mass loss was about 30% of the initial weight. It is very likely that 137Cs derived from the Fukushima NPP accident was deposited directly on the litter surface in the forest floor at our study site. The result of this study thus suggests that 137Cs weakly trapped in the L layer was mobilized by December 2011, and that 137Cs which remained in the L layer in 2Y was more difficult to desorb by processes such as incorporation into the litter structure and absorption onto recalcitrant organic compounds.
The decomposition of 137Cs-contaminated litter on the forest floor appeared to be a process contributing strongly to the migration of 137Cs in the forest soil in 2Y. The activity concentration of 137 Cs in seepage water showed positive temperature dependence in 2Y (Fig. 4a and b), although the 137Cs flux also strongly correlated with precipitation, as mentioned above (4.2). In general, litter decomposition on the forest floor increases with increasing temperature (Fierer et al., 2005), suggesting that the process of 137Cs loss from the L layer in 2Y was closely related with that of litter decomposition. Tegen and Dörr (1996) showed that 137Cs activity concentrations in an organic-rich forest soil strongly correlated with DOC concentrations, and both were highly correlated with soil temperature and thus microbial activity. In this study, a positive relation was found between the 137Cs and DOC concentrations in litter leachate (Fig. 4c), but there was no correlation between the two in soil solutions collected at two depths of the mineral soil (Fig. 4d). Therefore, the microbial decomposition of soil organic matter would not contribute to the 137Cs migration in the mineral soil at present. Moreover, because there is no indication that 137Cs activity concentrations correlated with the pH of seepage water (Figs. 4e and f), the mobilization of 137Cs in mineral soils by the ion exchange processes at the observed pH range (4.8e5.4) is also small compared to the release of 137Cs by litter decomposition process. 4.4. Prediction of
137
Cs transfer from soil solution to plant
The 137Cs activity concentration in the soil solution represents a quantitative characteristic of bioavailability for uptake by roots, as well as the vertical migration in the soil profile (Konoplev et al., 1993). It has been found that the concentration factor for 137Cs uptake by plants (defined as the ratio of 137Cs activity concentration in plant, Bq kg-DW1, to that in soil solution, Bq L1) directly correlates with the activity concentration of 137Cs in the soil solution (Absalom et al., 1999; Sanchez et al., 1999). Almost all of the forest
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herbs and most trees take up their nutrients from the uppermost 10 cm of soil, in which most 137Cs is deposited over the long term (Table 2). Although the concentration factor could not be observed in this study, 137Cs activity concentration in seepage water observed here, and reported concentration factors (4e1 104 L kg-DW1, Sombré et al., 1994; Gommers et al., 2005), allow us to estimate 137 Cs activity concentration in tree species to be 0.02e240 Bq kgDW1 in our study site. Because 137Cs activity concentration in the litter layer in August 2012 was about 3000 Bq kg-DW1, the additional input of 137Cs by new litter fall would be less important as a source of subsequent contamination on the forest floor. However, the evidence for the small but certain amount of bioavailable (soil solution) 137Cs in forest surface soils indicates that the uptake by plant and recycling of 137Cs, particularly in heavily contaminated forests, can cause significant external and internal (through consumption of food products) radiation exposure to humans over the long term. Further observations are necessary to identify the processes controlling the mobility (and thus bioavailability) of 137Cs in forest surface environments. 5. Conclusions Our lysimetric method captured the migration of 137Cs in a deciduous forest soil following the Fukushima NPP accident. The migration of 137Cs by seepage water was characteristic for a very small part of the total 137Cs inventory in the soil profile. The majority of 137Cs in seepage water originated from the litter layer, and the leaching from the surface mineral soil to the deeper layer was considered to be slower and of a considerably smaller quantity. The major proportion of 137Cs in the litter layer moved into the mineral soil within a year after deposition. Most of the 137Cs in seepage water from the litter layer was adsorbed in the topsoil and only a small percentage of 137Cs penetrated to deeper layers. The annual amount of 137Cs migrated below a 10 cm depth was only about 0.1% of total amount of 137Cs deposited onto the forest floor. A reduced rate of downward migration was observed in the second year after the accident. Therefore, most of the 137Cs deposited on the forest floor probably remains in the topsoil for a long time, although the litter layer will be notable as a source of highly mobile 137Cs in the soil system. In future, when most of 137Cs in the litter layer transfers to the mineral soil, 137Cs migration will be controlled by several physical, chemical, and environmental factors such as pH, cation exchange capacity, organic carbon content, animal activity, soil texture, and type of clay minerals. Since a proportion of 137Cs in the soil solid phase is not available for exchange with the solution phase, because of its irreversible adsorption by minerals (Rosén et al., 1999), 137Cs in seepage water (and thus, the bioavailability of 137Cs in soil) may decrease with time. Therefore, we will continuously observe 137Cs migration by seepage water in order to improve our understanding of the post-deposition early-stage dynamics of this mobile 137Cs in forest ecosystems. Acknowledgments We thank the Ibaraki District Forest Office for permission to use the Ogawa forest site. We also thank Drs. Takuya Kobayashi, Katsunori Tsuduki, and Takashi Suzuki of JAEA and Mr. Koichi Moriya of Nagoya University for their technical assistance with sampling, and Dr. Shigeyoshi Otosaka for his support with laboratory work. Finally, we thank Drs. Akira Endo and Haruyasu Nagai of JAEA, and
Prof. Seiya Nagao of Kanazawa University for providing helpful comments. References Absalom, J.P., Young, S.D., Crout, N.M.J., Nisbet, A.F., Woodman, R.F.M., Smolders, E., Gillet, A.G., 1999. Predicting soil to plant transfer of radiocesium using soil characteristics. Environ. Sci. Technol. 33, 1218e1223. Bundt, M., Albrecht, A., Froidevaux, P., Blaser, P., Flühler, H., 2000. Impact of preferential flow on radionuclide distribution in soil. Environ. Sci. Technol. 34, 3895e3899. Bunzl, K., Kracke, W., Schimmack, W., Zelles, L., 1998. Forms of fallout 137Cs and 239þ240 Pu in successive horizons of a forest soil. J. Environ. Radioact 39, 55e68. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G., Yamazawa, H., 2011. Preliminary estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima Daiichi nuclear power plant into the atmosphere. J. Nucl. Sci. Technol. 48, 1129e1134. Comans, R.N.J., Haller, M., De Preter, P., 1991. Sorption of cesium on illite: nonequilibrium behavior and reversibility. Geochim. Cosmochim. Acta 55, 433e 440. Fierer, N., Craine, J.M., McLauchlan, K., Schimel, J.P., 2005. Litter quality and the temperature sensitivity of decomposition. Ecology 86, 320e326. Gommers, A., Gäfvert, T., Smolders, E., Merckx, R., Vandenhove, H., 2005. Radiocaesium soil-to-wood transfer in commercial willow short rotation coppice on contaminated farm land. J. Environ. Radioact. 78, 267e287. Hashimoto, S., Ugawa, S., Nanko, K., Shichi, K., 2012. The total amounts of radioactively contaminated materials in forests in Fukushima, Japan. Sci. Rep.. http:// www.nature.com/srep/2012/120525/srep00416/full/srep00416.html. Kliashtorin, A.L., Tikhomirov, F.A., Shcheglov, A.I., 1994. Vertical radionuclide transfer by infiltration water in forest soils in the 30-km Chernobyl accident zone. Sci. Total Environ. 157, 285e288. Koarashi, J., Atarashi-Andoh, M., Matsunaga, T., Sato, T., Nagao, S., Nagai, H., 2012a. Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions. Sci. Total Environ. 431, 392e 401. Koarashi, J., Moriya, K., Atarashi-Andoh, M., Matsunaga, T., Fujita, H., Nagaoka, M., 2012b. Retention of potentially mobile radiocesium in forest surface soils affected by the Fukushima nuclear accident. Sci. Rep.. http://www.nature.com/ srep/2012/121219/srep01005/full/srep01005.html. Konoplev, A.V., Viktorova, N.V., Virchenko, E.P., Popov, V.E., Bulgakov, A.A., Desmet, G.M., 1993. Influence of agricultural countermeasures on the ratio of different chemical forms of radionuclides in soil and soil solution. Sci. Total Environ. 137, 147e162. Legout, A., Legout, C., Nys, C., Dambrine, E., 2009. Preferential flow and slow convective chloride transport through the soil of a forested landscape (Fougères, France). Geoderma 151, 179e190. Matsunaga, T., Koarashi, J., Atarashi-Andoh, M., Nagao, S., Sato, T., Nagai, H., 2013. Comparison of the vertical distributions of Fukushima nuclear accident radiocesium in soil before and after the first rainy season, with physicochemical and mineralogical interpretations. Sci. Total Environ. 447, 301e314. Mizoguchi, Y., Morisawa, T., Ohtani, Y., 2002. Climate in Ogawa forest Reserve. In: Nakashizuka, T., Matsumoto, Y. (Eds.), Ecological Studies, Diversity and Interaction in a Temperate Forest Community: Ogawa Forest Reserve of Japan, vol. 158. Springer-Verlag, Tokyo, pp. 11e18. Ono, K., Hiradate, S., Morita, S., Hirai, K., 2013. Fate of organic carbon during decomposition of different litter types in Japan. Biogeochemistry 112, 7e21. Rosén, K., Öborn, I., Lönsjö, H., 1999. Migration of radiocaesium in Swedish soil profiles after the Chernobyl accident, 1987e1995. J. Environ. Radioact 46, 45e66. Sanchez, A.L., Wright, S.M., Smolders, E., Naylor, C., Steven, P.A., Kennedy, V.H., Dodd, B.A., Singleton, D.L., Barnett, C.L., 1999. High plant uptake of radiocesium from organic soils due to Cs mobility and low soil K content. Environ. Sci. Technol. 33, 2752e2756. Sauras, T., Roca, M.C., Tent, J., Llauradö, M., Vidal, M., Rauret, G., Vallejo, V.R., 1994. Migration study of radionuclides in a Mediterranean forest soil using synthetic aerosols. Sci. Total Environ. 157, 231e238. Schimmack, W., Bunzl, K., Dietl, F., Klotz, D., 1994. Infiltration of radionuclides with low mobility (137Cs and 60Co) into a forest soil. Effect of the irrigation intensity. J. Environ. Radioact. 24, 53e63. Sombré, L., Vanhouche, M., de Brouwer, S., Ronneau, C., Lambotte, J.M., Myttenaere, C., 1994. Long-term radiocesium behaviour in spruce and oak forests. Sci. Total Environ. 157, 59e71. Tegen, I., Dörr, H., 1996. Mobilization of cesium in organic rich soils: correlation with production of dissolved organic carbon. Water Air Soil Pollut. 88, 133e144. Yoshinaga, S., Takahashi, M., Aizawa, S., 2002. Landforms and soil characteristics in Ogawa Forest Reserve. In: Nakashizuka, T., Matsumoto, Y. (Eds.), Ecological Studies, Diversity and Interaction in a Temperate Forest Community: Ogawa Forest Reserve of Japan, vol. 158. Springer-Verlag, Tokyo, pp. 19e26.