March 2016

How are radioactive deposits redistributed within catchment basins
in post-accidental situations?
Lessons learned from the Chernobyl and Fukushima accidents

Following the Chernobyl and Fukushima accidents, radioactive releases into theatmosphere led to heterogeneous deposits on vast mainland surfaces including
forests, agricultural land and residential areas. In the contaminated zones, thedownstream redistribution of radiocaesiums1 associated with the initial deposits
is governed by hydrological processes such as surface runoff or transport into
waterways. Knowledge about the dynamics of flows of radionuclides associatedwith these processes is essential in order to optimise post-accident
management strategy, in particular for the use of aquatic resources by humanpopulations.
The leaching of the catchment basins governs the redistribution of the radionuclides depositedon the soils following a nuclear accident; in both Chernobyl and Fukushima, this processannually remobilises less than 1% of the stock of deposited radiocaesiums. The long-term
radioactive contamination of the catchment basins (mainland surfaces - soils, lakes and rivers - fromwhich all of the water converges towards the same outlet) originates from the atmospheric depositsof radioactive contaminants released during the Chernobyl and Fukushima nuclear accidents, likethose from the former atmospheric testing of nuclear weapons The leaching of these systems - i.e.
the entrainment of the radionuclides by the water running on the soil surface - is a persistentdispersion process of radionuclides in the environment and over large distances, sometimes beyond
the initially contaminated and/or evacuated territories (Gulin et al., 2013). This hydrologicalprocess induces both a redistribution of the contamination in the catchment basins (Walling et al.,
1999; Khanbilvardi et al., 1999; van der Perk et Slavik, 2002) and its export downstream of thehydrographic networks (e.g., Menzel, 1960; Yamagata, 1963) to the sea (Kakehi et al., 2016).
Leaching is a process that, through the remobilisation of the radionuclides deposited after theaccident, influences the radiological impact on exposed human populations based on the uses ofenvironmental resources from the aquatic ecosystems situated downstream of the leachedterritories (waterways, lakes, dams used for irrigation and the supply of drinking water, fishing).
This leaching does not however induce, over the long term, a significant decontamination of thecontaminated territories: as observations made at Chernobyl have shown, annually, leaching does
not entrain more than 1 percent of the radionuclides present on the soils (Borzilov et al., 1988;
Khanbilvardi et al., 1999). This order of magnitude is identical in the case of Fukushima (e.g., Ueda
et al., 2013).
The flows of radiocaesiums remobilised from the soils of the catchment basins are transported
by water and particles; this is the result of complex chemical, hydrological and erosive
processes that are expressed in two phases that differ both kinetically and with regard to theeffectiveness of flows exported towards the outlet. During leaching, radionuclides aretransported by two vectors: water, if they are in dissolved form (via the runoff process) andparticles if they are fixed to suspended matter (erosion), which leads to a distinction between"liquid" leaching and "solid" leaching (Bulgakov et al., 1991). Leaching is a downstream radionuclide
1 These radiocaesiums are caesium-134 and caesium-137.

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March 2016transfer process from the catchment basins that remains difficult to assess, particularly due to thecomplexity of runoff and hydric erosion (Figure 1). This leaching depends notably on the types ofrainfall (intensity, volume of runoff water, time intervals between rainfall occurrences, etc.) andleached surfaces (texture and porosity of the soil, type of plant cover, slope, etc.), whosecharacteristics can often be heterogeneous within a catchment basin and variable over time.

Figure 1 - Schematic representation of the redistribution processes of radionuclides
deposited within a catchment basin following a nuclear accident (from Mori et al., 2015).

We roughly know the main phases and orders of magnitude of this leaching following an accidental
atmospheric deposit, notably for caesium-137. For this radionuclide, on a monthly or even annual
scale, there is the rapid leaching phase during the first weeks after the accident, where flows of
caesium-137 are high and export around 1% of the initial deposit onto the catchment basin; and theslow leaching phase, where exported flows are much lower, at around 0.1% per year of the stock
still present in the catchment basins. The exported flows however remain detectable several
decades after the deposit, as has been observed with the overall fallout of caesium-137 released
during former aerial nuclear tests (Helton et al., 1985).
The work carried out by IRSN as part of the AMORAD project2 for the Nitta coastal river in the
Fukushima area (Delmas et al., 2016) has enabled an estimation of a radiocaesium export rateduring the first year following the accident of 0.12 to 0.8% in the Nitta basin, which is lower thanthe values proposed by Kinouchi et al. (2015) and Ueda et al. (2013) for small catchment basins inthe study zone. This can be explained by the difference in surface area of the catchment basins(scale effect) as well as by the presence of a dam in the Nitta basin that limits the entrainment ofradionuclides towards the basin outlet.
At timescales of under a year, leaching is very erratic, in particular under the influence of climatic
events (e.g. rainfall, typhoons, snow melt) (Ueda et al., 2013). Moreover, the radionuclides aremobilised in space in a heterogeneous manner under the influence of variable runoff conditions(e.g. diffuse or channel runoff on slopes, concentrated runoff in the hydrographic network), andtheir course includes transitory storage areas (e.g. floor sediments in lakes/dams, tidemarks onbanks).

2

The AMORAD project is co-funded by the ANR RSNR ("Improvement of dispersion and transfer model prediction andevaluation of the environmental impact of radionuclides ").

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The difficulty in transposing the research findings acquired after the Chernobyl accident toestimate exported flows in the case of Fukushima has different origins. The main differences
between the two situations are linked to the availability for leaching of the radionuclides
deposited on the soils, and to differences in topography and weather patterns. The outcome isthat the leaching of radiocaesiums in Fukushima is mainly solid (vs. liquid in Chernobyl) andthat in both cases floods explain the majority of annual exports towards the marine
environment. In Fukushima, the major significant radionuclides to take into consideration for the
medium- and long-term management of radiation protection are the radiocaesiums deposited in theform of aerosols, whereas the spectrum of radionuclides deposited following the Chernobyl accident
is much wider with, notably, isotopes of strontium, caesium, americium, plutonium and curium(Matsunaga et al., 1998). In the Chernobyl exclusion zone, hot fuel particles released during the
explosion of the reactor were also dispersed. In the vicinity of the Chernobyl power plant,
contamination is therefore mainly in the form of particles of insoluble fuels containing, notably,radiocaesiums and radiostrontium which are progressively released during their alteration anddisintegration (Konoplev et al., 1992). At Fukushima, caesium is considered to be mainly depositedin the form of soluble sub-micronic aerosols (Kaneyasu et al., 2012), but the minority presence ofinsoluble forms has also been confirmed (Tanaka et al., 2013; Adachi et al., 2013).
Despite the types of deposits in Fukushima being, a priori, more sensitive to "liquid" leaching than in
Chernobyl, at Fukushima the radiocaesiums are mainly transported in particulate form duringleaching (Sakaguchi et al., 2015; Konoplev et al., 2016; Tanaka et al., 2013; Niimura et al., 2015),
whereas they are largely carried in soluble form in Belarus and in the Ukraine (Gorbachova, 2015).
Numerous explanations have been put forward:
 suspended matter at Fukushima contains more clay minerals than at Chernobyl and has agreater affinity to caesium; for example, the solid-liquid distribution coefficient ofradiocaesiums3 in Fukushima rivers (ca. 105 to 106 L/kg) are one to two orders of magnitude
higher than those in the Chernobyl area (ca. 104 L/kg) (Eyrolle-Boyer et al., 2016; Konoplev
et al., 2016);
 the presence of humic acids in Ukrainian waters inhibits the sorption of caesium on
suspended matter (Suga et al., 2014);
On a topographical level, the contaminated catchment basins in Fukushima are smaller, steep andare subject to greater climate extremes (Konoplev et al., 2016). The Chernobyl exclusion zone is in
the gently-sloping flood plain of a large river, the Dniepr, where rainfall is around 500 mm/year andin the form of snow on frozen soils several months per year. The catchment basins in the Fukushima
prefecture are drained by small coastal rivers with many constructions (weirs, dams, irrigation
channels) less than 100-km long, and are subject to rainfall of around 1,200 mm/year, alternating
between tropical cyclones from July to October and snow melt in the spring. Despite thesecontrasting meteorological patterns in the Ukraine and Japan, in both cases flood events are behindmost of the annual contamination exportation: these floods occur especially during snow melt in the
Ukraine (Borzilov et al., 1988), whilst they also occur as a result of typhoons in Japan, where therainfall is much more erosive (Ueda et al., 2013). These events generate 80 to 90% of annual
exports to the marine environment in the Fukushima zone.
The initial work by the IRSN as part of the AMORAD project (Delmas et al., 2016) shows that theentrainment coefficients (proportion of exported soil contamination for 1 mm of runoff water per
m2 of soil or 1 g of suspended matter exported per m2 of soil) decrease over time in the study zone
following the Fukushima accident, with maximum values after typhoons. These liquid entrainment
coefficients are lower, by one or two orders of magnitude, in the Fukushima context than in
Chernobyl (Konoplev et al., 2016), whilst the solid entrainment coefficients do not seem to differ
between the catchment basins of these two damaged zones. This comparison confirms the majorrole of particles in the leaching of caesium in the Japanese context. The specificity of the Japanese3

The solid-liquid distribution coefficient of a radionuclide is the equilibrium ratio between the radionuclide concentration inparticles (Bq/kg) and that in water (Bq/L)

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March 2016

context is in all likelihood the result of solid particles with greater caesium affinity, on the one
hand and, on the other hand, more intense erosion processes.
Finally, a study carried out by Yoshimura et al. (2015) on 30 catchment basins in the prefectures ofFukushima and Miyagi in December 2012 showed significant correlations between radiocaesium
concentrations in dissolved form or associated with particles in the waterways, and the stock ofradioactive deposits characterising the catchment basin where the waterway is located. Thesefindings were recently confirmed by studies carried out by the IRSN (Eyrolle-Boyer et al., 2016;
figure 2). This point underlines the relevance of the scale of the catchment basin in order to assess
the flows exported towards the marine environment. At this observational scale, Mori et al. (2015)
estimated, on the basis of models validated by measurements in situ, that over 90% of theradiocaesiums deposited on the catchment basin remain present three years after the accident.
Assuming that the redistribution of caesium-137 continues in an identical way after this three-year
period, the time taken to export 50% of the initial stock deposited on the catchment basin has been
estimated at around 18 years (Mori et al., 2015). This estimation takes into account the flows
exported by liquid and solid runoff as well as radioactive decay.

Figure 2- Relationship between concentrations in particulate or dissolved radiocaesiums and the stock of radioactive
deposits (Cs137) at the catchment basin level. The study focuses on the sites sampled between September 2012 andFebruary 2013 (from Eyrolle-Boyer et al., 2016).

The strategy applied to the management of aquatic environments in Chernobyl and Fukushima
is different. Countermeasures have mainly concerned aquatic environments in Chernobyl, where
the river Dniepr and its cascade of reservoirs downstream to the Black Sea are the main Ukrainian
water supply source (Voitsekhovitch et al., 1997). Sediment traps were built in 1986-1987 but theireffectiveness proved limited as they did not retain fine particles - which are the mostcontaminated. The contaminated areas in the exclusion zone in the vicinity of the power plant (onthe Prypiat river) were flooded in 1988 and 1991, and constitute the main source of contamination
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March 2016of the Prypiat river. Starting from studies based on the modelling of transfers in the zone, dykes
were built on the left bank in 1992-1993 and their effectiveness was proven during the floods ofsummer 1993, the logjam of winter 1994 and the 1999 rise in water levels. Dykes were also added to
the right bank in 1999-2002, but development work has since been suspended. Further downstream,
the Dniepr is a river regulated by a series of dams, whose management has been optimised in order
to limit the transfer of radionuclides to the Black Sea.
In Fukushima, even though a change in dam management methods is under consideration (Yamada
et al., 2015), countermeasures remain massively focused on the decontamination of the territories.
In agricultural areas, and around houses, the removal of the upper soil layers and their replacement
by non-contaminated soil aim for a reduction in the ambient dose rate and the rehabilitation overtime of certain zones evacuated at the time of the accident. The impact of the massdecontamination operations on the kinetics and intensity of flows exported by the leaching ofcatchment basins is as yet unknown. At this stage, it is estimated that the flows of caesium-137
entrained towards the ocean by the leaching of the stocks deposited in the area 100 km around the
Fukushima-Daiichi power plant (overall flow of 8.4 TBq/year of caesium-137 spread over the
maritime facade, which stretches towards the north from the Fukushima-Daiichi power plant to themouth of the Abukuma), are of the same order of magnitude as the direct releases from thedamaged Fukushima-Daiichi plant (estimated at 17 TBq between June 2011 and September 2012,
and at 24 TBq during a release that took place on 21 August 2013 (Yamashiki et al., 2014)).
In addition to agricultural, urban and forest areas, Japan has also integrated river and lake
management into its recovery plan for the territories impacted by the fallouts of the Fukushima
accident. The driving concept is based on the fact that the presence of a water column above thecontaminated sediments ensures a protective effect that, very effectively, reduces the dose rate
associated with external exposure of human populations to contaminated sediments.
Decontamination therefore only takes place in cases where this protective effect is insufficient inorder to limit the increase in the ambient dose rate (e.g. lowering of the water level, drying out).
This is the case, for example, for public spaces developed for various human activities establishedon the banks of waterways, or for reservoirs situated in residential areas or parks. Continued
monitoring of the level of radioactivity in aquatic environments as well as research anddevelopment projects in order to understand and quantify the fate of radionuclides at a global scalewith regard to catchment basins are in progress (Ministry of the Environment, Japan, 2016). IRSN
research, as part of the AMORAD project aimed at quantifying and modelling the remobilisation
processes of radionuclides through the leaching of catchment basins at different temporal scales(from one episode of rainfall to several years) will contribute to the consolidation of elements ofpost-accidental management of aquatic environments.

References
Adachi, K., Kajino, M., Zaizen, Y., Igarashi, Y (2013). Emission of spherical cesium-bearing particles from anearly stage of the Fukushima nuclear accident. INCLUDING 2,800 SCIENTISTS
Borzilov, V., Konoplev, A., Revina, S., Bobovnikova, T. I., Lyutik, P., Shveikin, Y. V., Shcherbak, A (1988).
Experimental investigation of washout of radionuclides deposited on soil as a result of the Chernobyl
nuclear power plant accident Soviet Meteorology and Hydrology 11:43-53.
Bulgakov, A., Konoplev, A., Popov, V., Scherbak, A. (1991). Removal of long-lived radionuclides from the soil
by surface runoff near the Chernobyl nuclear power station. Soviet Soil Sci. 09/12/14
Delmas M. et al. (2016). Estimating average wash-off parameters of cesium from watersheds of Minamisoma
region (Fukushima prefecture) by a Bayesian approach. Journal of Environmental Radioactivity.
Presentation
Evrard, O., Laceby, J. P., Lepage, H., Onda, Y., Cerdan, O., Ayrault, S. (2015). Radiocesium transfer fromhillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review. Journal ofEnvironmental Radioactivity 148: 8-9:
Eyrolle-Boyer, F., Boyer, P., Garcia-Sanchez, L., Métivier, J.-M., Onda, Y., De Vismes, A., Cagnat, X., Boulet,
B., Cossonnet, C. (2015) Behaviour of radiocaesium in coastal rivers of the Fukushima Prefecture (Japan)

5/7March 2016
during conditions of low flow and low turbidity - Insight on the possible role of small particles and detritalorganic compounds. Journal of Environmental Radioactivity 151:328-340.
Garcia-Sanchez, L. (2008). Watershed wash-off of atmospherically deposited radionuclides: review of the fluxes
and their evolution with time. Journal of Environmental Radioactivity 99 (4):563-573.
Garcia-Sanchez, L., Konoplev, A.V. (2009). Watershed wash-off of atmospherically deposited radionuclides: A
review of normalized entrainment coefficients. Journal of Environmental Radioactivity 100 (9):774-778.
Gorbachova, L. (2005). Factors, structure and dynamics of dissolved in water Cesium-137 removal with runoff
from Pripyat river basin. PhD thesis. Geographical faculty of Taras Shevchenko Kiev National University,
Kiev (in Ukrainian).
Gulin, S., Mirzoyeva, N., Egorov, V., Polikarpov, G., Sidorov, I., Proskurnin, V. (2013). Secondary radioactive
contamination of the Black Sea after Chernobyl accident: recent levels, pathways and trends. Journal ofEnvironmental Radioactivity 124: 8-9:
Helton, J., Muller, A., Bayer, A. (1985). Contamination of surface-water bodies after reactor accidents by theerosion of atmospherically deposited radionuclides. sociale.php 09/12/14
Jury, W., Roth, K. (1990). Transfer functions and solute movement through soil. Birkaüser Verlag
Kakehi, S., Kaeriyama, H., Ambe, D., Ono, T., Ito, S-i., Shimizu, Y., Watanabe, W. (2016). Radioactive cesium
dynamics derived from hydrographic observations in the Abukuma River Estuary, Japan. Journal ofEnvironmental Radioactivity 153:1-9.
Khanbilvardi, R., Shestopalov, V., Onishchenko, I., Bublyas, V., Gudzenko, V (1999). Role of erosion processes
in transfer of radionuclides: results of field experiments. Journal of the American Water Resources
Association 35:887-898.
Kinouchi, T., Yoshimura, K., Omata, T. (2015). Modeling radiocesium transport from a river catchment basedon a physically-based distributed hydrological and sediment erosion model. By plane Radioact. 09/12/14
Konoplev, A., Bulgakov, A., Popov, V., Bobovnikova, T. I. Behaviour of long-lived Chernobyl radionuclides in asoil-water system. Analyst 117:1041-1047.
Konoplev, A., Golosov, V., Laptev, G., Nanba, K., Onda, Y., Takase, T., Wakiyama, Y., Yoshimura, K. (2016).
Behavior of accidentally released radiocesium in soil-water environment: Looking at Fukushima from a
Chernobyl perspective Journal of Environmental Radioactivity 151:568-578.
Matsunaga, T., Ueno, T., Amano, H., Tkatchenko, Y., Kovalyov, A., Watanabe, M., Onuma, Y. (1998).
Characteristics of Chernobyl-derived radionuclides in particulate form in surface waters in the exclusionzone around the Chernobyl nuclear power plant. J. Contam. Hydrol. 09/12/14
Menzel, R. (1960). Transport of Strontium-90 in runoff. Science 307 (2005) 1938-1942
Ministry of the Environment, Japan (2016). Progress on Off-site Cleanup and Interim Storage in Japan. January
2016. https://josen.env.go.jp/en/pdf/progressseet_progress_on_cleanup_efforts.pdf?150918
Mori, K., Tada, K., Tawara, Y., Ohno, K., Asami, M., Kosaka, K., Tosaka, H. (2015). Integrated watershed
modeling for simulation of spatiotemporal redistribution of post-fallout radionuclides: Application inradiocesium fate and transport processes derived from the Fukushima accidents. Environmental Modellingand Software 72: 8-9:
Niimura, N., Kikuchi, K., Tuyen, N. D., Komatsuzaki, M., Motohashi, Y. (2015). Physical properties, structure,and shape of radioactive Cs from the Fukushima Daiichi Nuclear Power Plant accident derived from soil,
bamboo and shiitake mushroom measurements. Journal of Environmental Radioactivity 139:234-239.
Ochiai, S., Ueda, S., Hasegawa, H., Kakiuchi, H., Akata, N., Ohtsuka, Y., Hisamatsu, S. (2015). Effects ofradiocaesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow
conditions. By plane Radioact. 41% 8-9:
Sakaguchi, A., Tanaka, K., Iwatani, H., Chiga, H., Fan, Q., Onda, Y., Takahashi, Y. (2015). Size distribution
studies of 137Cs in river water in the Abukuma Riverine system following the Fukushima Dai-ichi Nuclear
Power Plant accident. Journal of Environmental Radioactivity 139:379-389.
Tanaka, K., Sakaguchi, A., Kanai, Y., Tsuruta, H., Shinohara, A., Takahashi, Y. (2013). Heterogeneous
distribution of radiocesium in aerosols, soil and particulate matters emitted by the Fukushima Daiichi
Nuclear Power Plant accident: retention of micro-scale heterogeneity during the migration of radiocesium
from the air into ground and river systems Journal of Radioanalytical and Nuclear Chemistry 295:19271937.
Tsuji, H., Yasutaka, T., Kawabe, Y., Onishi, T., Koma, T. (2014). Distribution of dissolved and particulate
radiocaesium concentrations along rivers and the relations between radiocaesium concentration anddeposition after the nuclear power plant accident in Fukushima. Wat. REI 60: 8-9:

6/7

March 2016
Ueda, S., Hasegawa, H., Kakiuchi, H., Akata, N., Ohtsuka, Y., Hisamatsu, S. (2013). Fluvial discharges ofradiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident,
Japan. By plane Radioact. 118: 8-9:
van der Perk, M., Slavik, O. (2003). Simulation of event-based and long-term spatial redistribution ofChernobyl-derived radiocaesium within catchments using geographical information system embedded
models. Hydrol. Process. 09/12/14
Voitsekhovitch, O., Nasvit, O., Los'y, I., Berkovsky, V. (1997). Present thoughts on the aquatic countermeasuresapplied to regions of the Dnieper river catchment contaminated by the 1986 Chernobyl accident. National
Student ID: Freshwater and estuarine radioecology, Desmet et al. (eds), Elsevier.
Walling, D., He, Q., Blake, W. (1999). Use of 7Be and 137Cs measurements to document short- and mediumterm rates of water-induced soil erosion on agricultural land. Water Resour. REI 09/12/14
Yamada et al. (2015). Sediment and 137Cs transport and accumulation in the Ogaki Dam of eastern Fukushima.
Envivio REI Date 10%
Yamagata, N., Matsuda, S., Kodaira, K. (1963). Run-off of Caesium-137 and Strontium-90 from rivers. Nature
439 (2006) 695-698;
Yoshimura, K., Onda, Y., Sakaguchi, A., Yamamoto, M., Matsuura, Y. (2015). An extensive study of theconcentrations of particulate/dissolved radiocaesium derived from the Fukushima Dai-ichi Nuclear Power
Plant accident in various river systems and their relationship with catchment inventory. By plane Radioact.
1,300 8-9:

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