Engineering Geology 155 (2013) 87–93

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Engineering Geology

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Technical Note

Overview of rehabilitation schemes for farmlands contaminated with radioactivecesium released from Fukushima power plant

Masashi Nakanoa,⁎, Raymond N. Yongb,1

a The University of Tokyo, Japanb McGill University, Canada

⁎ Corresponding author at: 1-27-201 Misoracho, OtsuTel.: +81 77 574 2401.

E-mail addresses: qqwr73e9n@ever.ocn.ne.jp (M. Na(R.N. Yong).

1 Address: North Saanich, B.C., Canada.

0013-7952/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.enggeo.2012.12.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 November 2012Received in revised form 11 December 2012Accepted 16 December 2012Available online 16 January 2013

Keywords:Radioactive cesiumSoilContaminationRehabilitationEmbedmentBuried-landfill

Schemes for rehabilitation of farmlands contaminated with radioactive cesium are discussed based on the vari-ous pieces of information obtained to date froma series of validation trials in Fukushima, Japan. Decontaminationof affected farmlands is to be achieved by scraper-removal of the very highly contaminated shallow top soil layer.In areas with very low contamination, the plans are to implement in-situ burial of the scraper-removed contam-inated topsoil at depths greater than 2m—meaning that the affected areaswill be the burial sites. For disposal ofthe contaminated top-soils removed in the very highly contaminated areas, there will be a two-step procedure:(a) volume reduction and decontamination of the contaminated soil, and (b) disposal in a mulch barrier landfillsystem with encapsulating clay liners at selected sites. Volume reduction and decontamination of the contami-nated soils will involve removal of the finer soil fractions through washing with water. For implementation ofthe schemes for rehabilitation, science-based verification of safety and public acceptance of the schemes arerequired.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction power plant site. According to the Ministry of Education, Culture,

The Fukushima power plant buildings which were damaged by theMarch 11th 2011 East-Japan great earthquake and accompanyingtsunami had their roofs and upper portions destroyed as a result ofhydrogen explosions on March 12th, 14th and 15th, 2011. Radioactivenuclides consisting of Kr, Xe, I, Cs, Sr, Ru, Ce, etc. released to the atmo-sphere by the hydrogen explosions and by ventilation were carried bywind, leading thereby to contamination of a wide area of the surround-ing land surface (Chino et al., 2011; Yasunari et al., 2011; Hirano et al.,2012; Kaneyasu et al., 2012; Yamaguchi et al., 2012). The released radio-active nuclides present significant health threats not only to the inhab-itants in the affected areas, but also to food production in the farmlandscontaminated by deposition of the radioactive nuclides. Of great con-cern is radioactive cesium, i.e. 137Cs and 134Cs inasmuch as they possessstrong photon energies and long half-lives— relative to radioactive nu-clides released. Morino et al. (2011) have estimated deposition of thereleased radioactive cesium based on the chemical transport model(CMAQ) described by Byun and Schere (2006). Their calculationsshow that 22% of the 137Cs radioactive nuclides releasedwere depositedon the land surface in areas situated between 60 and 400 km from the

shi, Shigaken 520-0223, Japan.

kano), r.nyong@shaw.ca

rights reserved.

Sports, Science and Technology (MEXT), Japan, Itakura (2012), mostof the areas contaminated with high activity concentration of radioac-tive cesium greater than 300 kBq/m2 in the top 5 cm surface soillayer, i.e. the areas where rice planting is restrained, lies within an80 km-radius zone from the Fukushima power plant site.

Sampling and testing data show that radioactive cesium in the affect-ed soils tends to be retained in the top 5 cm of surface soil—with muchlesser amounts in the depths below 5 cm. To return the farmlands totheir pre-contamination state, the relative shallowness of the contami-nated soil surface layer suggests that complete removal of the affectedsoil would be an obvious solution. Accordingly, the rehabilitationschemes under consideration for highly contaminated areas are inthe spirit of traditional “dig-and-dump” techniques used for surface-contaminated sites. However, because of: (a) the nature of the contami-nants, (b) the extent of contamination in the contaminated areas, and(c) especially the lack of land available or amenable for secure “dump”sites, complications arise in seeking approval from both governingauthorities and affected citizenry for implementation of viable rehabilita-tion schemes for the contaminated areas.

Recognizing the complications described above, the discussion in thispaper presents anoverviewof: (a) surface soil contaminationby radioac-tive cesium in the affected areas, (b) the basic elements and require-ments to achieve secure disposal of the contaminated soil removedfrom the affected areas, and (c) the process of seeking approval fromgoverning-regulatory bodies and affected citizenry for implementationof a scheme for site decontamination.

Fig. 2. Activity concentration profiles of radioactive cesium in black bars for sum of134Cs and 137Cs and in white bars for 137Cs in paddy soil: (a) drawn from data mea-sured on May 24th, 2011 by Shiozawa, the University of Tokyo, and (b) drawn basedon data measured on Feb. 20th, 2012, in a research carried out by MAFF of Japan.

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2. Surface and subsurface soil contamination byradioactive cesium

2.1. Contamination of land surface

According to the estimates by Japan Atomic Energy Agency (JAEA)and the Nuclear Safety Commission of Japan, the activity of radioac-tive cesium released from the power plant was about 1.1×1016 Bqfor both 137Cs and 134Cs, at the end of March, 2011. The MEXT has re-leased a deposition map of radioactive cesium on 30th Aug. and sub-sequently on 16th Dec. 2011, using the data relevant to the activity ofradioactive cesium retained in the top 5 cm surface soil layer. Thedata used for mapping were obtained by a combination of aerial sur-vey of photon energy using a NaI scintillator, and in-situ measure-ments of activity both on the ground surface and on retrieved soilsamples using a germanium solid state detector. Taken altogether,the deposition map shows that radioactive cesium contamination ofsurface soil extends as far away as about 450 km from the powerplant — with activity concentrations of less than 10 kBq/m2. There areareas with activity concentration in the range of 10 to 100 kBq/m2

within a zone of 80 km to 250 km distant from the power plant site.The areas contaminated with radioactive cesium in activity concentra-tions larger than 300 kBq/m2 appear to be mainly in a zone within aradius of 80 km from the site of the power plant, as shown in Fig. 1. Avery hazardous area contaminated with activity concentrations largerthan 600 kBq/m2 is in the 50 km-radius zone and an extremelyhazardous band-shaped area with activity concentrations greater than3000 kBq/m2 is within the radius zone of 25 km.

It is surmised that formation of the highly contaminated areas in the80 km-radius zone shown in Fig. 1 was the result of the inability of themain contaminant plume to cross over mountains higher than about1400 m — the end result of which is expansion of the contaminatedarea in the direction along the mountainside.

2.2. Activity concentration profiles of radioactive cesium in soil

The typical activity concentration profiles of radioactive cesium inpaddy soils are shown in Fig. 2, in which activity concentrations aregiven for the sum of 134Cs and 137Cs by the black bars and for 137Cs onlyby thewhite bars. Themeasurement sites for Fig. 2(a) and (b)were locat-ed about 70 km and 40 km from the site of the power plant, respectively.

Fig. 1. Ground pollution by 134Cs and 137Cs in a zone within a radius of 80 km from thesite of the power plant (data observed in June and July, 2011 and released by MEXT onDec. 16th, 2011).

The bar chart shown in Fig. 2(a) is drawn frompersonally-communicateddata obtained from Siozawa et al. (2011), the University of Tokyo, andthat shown in Fig. 2(b) is fromdata reported in a research studyundertak-en by the Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan(2012). The shapes of the activity concentration profiles for the two loca-tions are distinctly different. In the case of the 70 km location, the veryhigh activity of radioactive cesium in the top 1 cm layer decreases sharplywith increasing depth (Figure 2(a)). In the case of the 40 km location,there is a large activity of radioactive cesium in the 3–5 cm depth belowground surface (Figure 2(b)). Overall, the total activity concentrationshown in Fig. 2(b) is higher than the one shown in Fig. 2(a)— especiallyin the subsoil below the 5 cm depth. It is noted that most of the radioac-tive cesium is accumulated in the top 5 cm layer of surface soil in thepaddy fields. Considering these observations to be valid for depositionof radioactive cesium in upland fields (Yamaguchi et al., 2012), it wouldappear that the rehabilitation of the affected areas could be achievedwith removal of at least the 5 cm top soil layer — given that the activityconcentration of radioactive cesium in the subsoil below the top 5 cmdepth is very small. However, the activity concentrations in the subsoilshould not be ignored in the rehabilitation of the affected areas becauseof the possibility that a slight amount of radioactive cesium in the subsoilcould affect the safety of the inhabitants and crops harvested afterrehabilitation.

2.3. Migration and transfer of radioactive cesium into and in soil

The information deduced from the characteristic profiles of activityconcentrations in soil lends support to the proposition that radioactivecesium migrates into the top soil layer in the form of particulates suchas aerosol and dust — adsorbing radioactive cesium. Migration ofthese particulates into soil will result in colloid transport phenomenathat are affected by the properties of the pore channels of the top soillayer such as tortuosity of pore channels and pore constriction (Saiers

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and Hornberger, 1996; Kretzschmar and Sticher, 1997; Kretzschmar etal., 1999; Kaneyasu et al., 2012). Most of the particulates may be cap-tured by the necks of soil pores andwall of pores in the top 5 cm surfacesoil layer, resulting in sharply decreasing quantities with depth. Takenin conjunction with the fact that the quantity of radioactive cesium de-posited on the soil surface is not uniformly distributed, it is expectedthat the migration depth of contaminated particulates will not be simi-lar throughout the affected areas, because of not only the quantity ofparticulates falling on the ground surface, but also the influence of soilproperties such as particle size distribution, organic matter content,pore size and channel lengths, etc. As shown in Fig. 2, the significantdepth of migration, i.e. the penetration depth under the soil surface, isseen to be less than about 15 cm. Assuming this to be representative,the total quantity of radioactive cesium contained in the layer belowthe top 5 cm layer can be assumed to be less than around 5% of the ra-dioactive cesium retained in the top 5 cm surface layer in the variouscontaminated areas in Fukushima — using the information available atthe present time.

However, it is expected that the nature of the activity concentrationprofiles will be dependent on the migration of both the contaminatedparticulates and free hydrated ions. That being said, it is likely that theconcentration of partitioned free ionized cesium ions would be verysmall. Nevertheless, a factor that needs consideration is the environ-mental mobility and subsequent partitioning (as time goes on) of freeionized radioactive cesium detached from the overlying contaminatedsoil layer by various chemical–biological reactions. While the ratesand quantity of transported ionized radioactive cesium would be verysmall, prolonged transport in the subsoil would contribute to contami-nation of groundwater and local aquifers. Determination of the trans-port rate for the contaminants can be made with a knowledge of thetransport properties of the affected soil in combination with thewell-known convection–dispersion equation (Tang and Weisebrod,2010; Yong et al., 2012). It is useful to note that a transport rate around1.0 cm/y has been reported by several researchers (Rosen et al., 1999;Forsberg et al., 2000; Kamei-Ishikawa et al., 2008; InternationalAtomic Energy Agency (IAEA), 2010).

3. Methods for rehabilitation of farmland

There are many ways in which rehabilitation of a site containingcontaminated soil can be achieved – ranging from the simple procedureof removal of the offending soil using the traditional dig-and-dumpmethod to more complex in-situ methods of treatment – dependingon the type of contaminants polluting the soil (Yong, 2001). In thecase of the contaminated farmlands in the vicinity of the power plantsite, at least two factors militate against a simple dig-and-dump solu-tion. These are (a) areal extent of the land surface contaminated bythe radioactive cesium — as seen in Table 1, and (b) the lack of sitesfor dump-disposal of the offending soil removed from the affectedareas— bearing in mind the land area of Japan and its population distri-bution. In addition to the preceding factors, a large determinant of thekinds of procedures to rehabilitate the contaminated farmlands is the

Table 1Contaminated areas and volume of scraped soils in farmlands.

Contaminatedzone

More than 300 kBq/m2 More than 600 kBq/m2

Area scrapedkm2

Volume ofscraped soil×104m3

Area scrapedkm2

Volume ofscraped soil×104m3

Maina 197 1428 145 1051Diffusiveb 275 1995 14 102Total 472 3423 159 1153

a Main shows a contaminated zone in the right-hand side of Fig. 1.b Diffusive shows a zone in the left-hand side of Fig. 1.

need to restore the contaminated farmlands to their original state toallow not only for the return of the inhabitants, but also for the agro in-dustry to return to normalcy.

3.1. Removal of contaminated surface soil

To fully secure the safety of the rehabilitated farmlands, the presentproposition is to remove the contaminated surface soil by scrapers forareas where radioactive cesium activity concentrations are larger than300 kBq/m2 as shown in Fig. 1. Assuming an average dry density of1.2 Mg/m3 for the contaminated surface soils in the affected areas,this would correspond to 5 kBq/kg for the top 5 cm surface soil layer.This activity concentration is a critical threshold level, since levelsabove 5 kBq/kgwill prohibit planting in paddy fields— to prevent accu-mulation of radioactive cesium in the rice grains. To ensure the safety ofthe inhabitants, the radiation dose on the freshly-scraped ground sur-face obtained after removal of the surface soil layer by scrapers mustnot exceed a threshold limit of 1 mSv/y.

The thickness of the surface layer to be scraped is a factor that im-pacts on the selection of disposal sites for the scraper-removed soil.The larger the activity concentration of radioactive cesium in the sub-soil, the largerwill be the volume of scraped soil to be removed— to en-sure that the radiation dose on the newly exposed sub-surface soilmeets the acceptable limits. In view of the relatively shallow depth ofpenetration of radioactive cesium into the surface soil, and from the re-sults shown in Fig. 2, it would appear that the thickness of soil layerneeded to be scraper-removed should be at least 3 cm or 5 cm.

In the 80 km zone from the power plant site, the sum total of areasof contaminated farmlands that show radioactive cesium activity con-centrations larger than 300 kBq/m2 is about 472 km2. According tothe results of validation trials reported by MAFF (2012) of Japan on31st Aug., the volume of the contaminated-removed soil amounted to7.25×104 m3 per unit square kilometer of farmland including paddyfields, upland fields and others. Using this result, the volume of contam-inated soil to be removed in the 80 km zone is estimated to be3423×104 m3 — as shown in Table 1. This quantity is considered tobe much too large for disposal at a particular location.

For the contaminated spots thatmight still remain after scraping be-cause of local high activity concentration of radioactive cesium in thesubsoil layer, one has the option of implementing localized removal ofthe highly contaminated soil spots or application of soil dressing tomit-igate radiation hazard. The latter option, i.e. soil dressing, can decreaseradiation dose on the ground because of radiation absorption by thesoil used to dress the ground surface, and furthermore will be usefulfor the growth of crops.

3.2. In-situ embedment of removed low-level contaminated soil

In situations where suitable disposal sites outside the affected areaare not available, disposal of the large volume of contaminated soil is amajor challenge. Under such circumstances, a proposed solution couldbe to embed scraper-removed low-level contaminated soil in the affect-ed area itself. This proposed disposal scheme is, in effect, a variation ofthe traditional dig-and-dump scheme with burial (embedment) in aconstructed dump site located within the contaminated areas. Thein-situ embedment scheme proposed here recommends embedment ofthe scraper-removed contaminated surface soil in-situ at sufficientdepth and in a contained manner as shown in Fig. 3, in the cultivatedfields. This procedure contrasts with the traditional dig-and-dumpscheme where the offending soil is most often transported to landfillsites prepared outside the affected areas. Taking into account thelow-level concentration of radioactive cesium in the contaminated soil,it is envisaged that the embedded contaminated soil (embedded soil)will not require an overlying multiple-barrier combination of a clayliner and granular layer — so long as the factors listed hereafter aretaken into account.

Fig. 3. Schematic designs for in-situ embedment of scraper-removed contaminated surface soil.

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These factors, which are key elements in assurance of radiationsafety above-ground, include: (a) low-level activity concentration ofless than 600 kBq/m2 for the embedded soil, (b) selection of a locationwithout shallow groundwater andwithout a surface fault, (c) mixing ofabsorbents to sorb radioactive cesium in the embedded soil and, (d) useof pre-determined thickness of fresh surface soil layer covering the topground surface— first, to prevent uptake of radioactive cesium by plant,i.e. to avoid invasion of vegetation roots into the embedded soil layer(Jarvis et al., 2010) and second, to minimize infiltration of water intothe embedded soil and dispersion/diffusion of radioactive cesium tothe neighboring areas and to the ground surface. The use of a slopingtop surface for the embedded contaminated soil, as shown in Fig. 3,will assist in channeling infiltration rain water.

Common techniques include: (a) construction of an embeddedsoil layer with a dry density higher than that of the adjacent naturalsoil and (b) provision of an embedding-free region devoid of any em-bedded soil. These techniques will catch rain water on the top surfaceof the embedded soil layer and ensure the proper transfer of rainwater into the deeper soil region as funnel flow at the end of the slop-ing embedded layer (Kung, 1990). For sloping areas, placement of theembedded soil layer parallel to ground surface aids in diverting rainwater to the lower end of the embedded layer (see Figure 3(b)).

For this scheme, monitoring of radioactive dosage on the groundsurface is a requirement. Radiation absorption of the top soil layer de-pends on the dry density of the soil, the nature of the soil particles,soil water, soil air, and thickness of the top soil layer. Surface accumula-tion of radioactive cesium will be negligible over the long term if thedepth of embedded soil is more than two times that of the root zoneof the indigenous plants. This is because water uptake by plants in thetop soil comes from at least twice the depth of their root zone. In thecase of gramineous plants, for example, the thickness of the top soilneeds to be larger than 2 m to prevent the uptake of radioactive cesiumsince their root zone can reach or even exceed 1 m in depth. The top soilshould be backfilled using soil in situ with the same dry densities andhydraulic conductivities as that of the natural soil.

4. Methods for disposal of scraped contaminated soil by landfilling

4.1. Decontamination and volume reduction of removed soil

Scraper-removed contaminated soil with high activity concentra-tions of radioactive cesium requires decontamination and volume re-duction (of the removed soil) before disposal in a prepared landfillsite. An ideal treatment for decontamination and volume reduction ofremoved contaminated soil is to remove the fine soil particles fromthe contaminated soil by mechanical washing with water in a chamberusing ultrasonic waves, i.e. ultrasonic cleaning of soil. The fine soil par-ticles such as clay minerals, other inorganic and organic materials and

their aggregates retain most of the radioactive cesium by strong chem-ical forces (Anderson et al., 1999). Ultrasonic waves can detach the finesoil particles and aggregates from the coarse grains of a soil and breakthe aggregates into smaller soil particles (clay minerals, inorganic com-pounds) and organic materials. The fine soil particles suspended in thewash water after ultrasonic treatment, need to be collected using floc-culants. These must be disposed in a hazardous waste disposal site. Toavoid dissolution of radioactive cesium in thewashwater, no dispersingagents should be added to the wash water. In cases where radioactivecesium is ionized and dissolves inwater in the process ofwashing, tech-niques using ion-exchange agents are needed to chemically removethem from the wash-water. Radioactive cesium adsorbed onto theseparated coarse particles such as sand and other granular materialsas particulates or exchangeable ions, must be detached or desorbed bychemical agents designed for dissociation of radioactive cesium, i.e.ammonium solution or other types of chemical agents designed for dis-sociation of radioactive cesium, based on the properties of the contam-inated soil (see Szabo, 1997; Kim et al., 2007). Ionized radioactivecesium or their compounds in the wash-water should be removed byfilters following the addition of a chemical adsorbent. This treatmentmust be performed in sealed chambers in facilities that have themeans for safe decontamination and disposal of contaminated water.Following these treatments, it will be possible to dispose of the removedfine soil particles or their compounds in the limited space of prepareddisposal sites. The decontamination rates and the extent of volume re-duction can be estimated with knowledge of the ratio of fine particlemass to the total scraped soil mass.

4.2. Disposal of high-level concentration contaminated soil and remnantsafter decontamination

Soils and/or remnants after decontamination and volume reduction,that are contaminated with high-level concentrations of radioactivecesium – with activity concentrations larger than 600 kBq/m2 – requiredisposal by landfilling in a prepared site.

Fig. 4 portrays the design of an embedded landfill system containingsoil contaminated with high concentrations of radioactive cesium. Asshown in the diagram, the contaminated soil is placed in the form of adisposal layer in a buried landfill. For identification and discussion pur-poses, this proposed procedure is known as the landfill scheme. The dif-ferences between this buried landfill scheme and in-situ embedmentscheme are summarized briefly in Fig. 5. The system is similar to thewell-known encapsulating-engineered multi-barrier systems such asthose utilized in containment of hazardous solid wastes in secure land-fills (Yong et al., 2010). The system consists of the following elements:(a) clay liners that encapsulate the disposal layer for complete isolationof the disposal layer – i.e. an upper clay liner on the top of the disposallayer and a lower clay liner at the bottom as well as at the sides – and

Fig. 4. Schematic design of landfill at suitable disposal sites.

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this clay liner should be constructed using compacted smectite-richclays such as bentonite, (b) a coarse grain material layer with drain-pipes inserted at set intervals above the upper clay liner to drain offwater percolating from the ground surface, (c) an embedment-freesoil zone to allow for drainage of water infiltrating in the coarse grainlayer, (d) a top soil layer with considerable thickness to prevent the de-struction of the upper clay liner from invading plant roots and soil ani-mals (Jarvis et al., 2010), and (e) vegetation and ditch on the groundsurface to prevent surface soil erosion by rain and wind. For placementof contaminated soil in the embedded layer, solidification techniquesusing binders should be used to produce bricks or blocks with densitiesof about 2.0 Mg/m3 as a pretreatment. The range of mixing binders in-cludes clays such as zeolite, synthetic binders and other cementingagents. Since the dry density of the affected soil in nature is around1.0 Mg/m3, production of bricks or blocks would not only result inabout one-half volume decrement, but would also allow for greaterfacility in placement operation of the contaminated soil at the disposalsite.

4.3. Selection of embedded landfill site

From a practical point of view, the most appropriate location for anembedded landfill for the contaminated soil containing high concentra-tions of radioactive cesium would be abandoned mine areas and/orindustrial waste disposal sites — preferably on a valley slope. Thepreference for an embedded landfill located in a slope is because surfacedrainagewould be facilitated— thusminimizing the potential for deliv-ery of embedded radioactive contaminants to the subsoil. As with mostcountries, the criterion for selection of a suitable site is to avoid populat-ed regions, ecological sensitive sites, sites of historical significance, and

Fig. 5. Characteristics of three kinds of methods for disposal of scraper-removed con-taminated soils.

sites containing hereditary structures. Additional considerations for siteselection include: (a) location devoid of surface fault and landslides his-torically triggered by earthquakes, and (b) stability of contiguous areasagainst ground subsidence, landslides or slope failure initiated by localsevere rains and seismic ground motion. Landfilling in shallow sea,lake and marshy areas should be avoided in Fukushima because of thenegative impact of landfills on the benthic environment, marine lifeand terrestrial landscape— the end result of which is the disappearanceof precious ecological species from the area. In cases where thescraper-removed soil includes nuclides with high radiation energyand extremely long half-life periods, such as 239Pu and 242Pu withhalf-lives of 2.4×104 y and 3.7×105 y respectively, deep geological dis-posal should be implemented— followingmany of the procedures usedin containment of high level radioactive waste material (Yong et al.,2010; Pusch et al., 2011).

5. Policy and process for implementation of rehabilitation project

5.1. Science based verification of safety

Rehabilitation of contaminated areas requires science-based veri-fication of radiation safety of the affected and neighboring areas, i.e.accurate determination of the thickness of contaminated soil layerto be removed, and accurate estimation of radiation dose on theground surface after rehabilitation. This verification should beobtained from data obtained in the field, with due account given tothe variability of activity concentration in the field. For constructionof embedment and landfill facilities, verification of safety must beobtained from theoretical analyses, first for stability of the naturalground and landfills against seismic ground motion and secondly,for the transport and fate of radioactive cesium in the layered soil, es-pecially in the layered subsoil — in addition to the transfer of ground-water (Yong et al., 2012). This is because soil is layered in nature andembedment will make the layered soil. The analyses should be under-taken using data pertaining to the properties of soil in natural groundand landfills. Long-termmonitoring will be required for: (a) radiationdosage on the ground surface, (b) quantity of radioactive cesium re-leased from the embedded contaminated soil and (c) concentrationof radioactive cesium in ground water.

5.2. Public approval and acceptance to a scheme and implementation

To undertake rehabilitation construction activities, the public ap-proval and acceptance of the schemes and implementation are re-quired. Public approval and acceptance will be reached when thenumerous requirements and factors articulated by the public are

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satisfied. In addition to verification of radiation safety by science-based methods, the requirements and factors to be considered willfall into two categories: (a) indemnity systems and (b) constructionactivities. In the case of indemnity systems, the systems must be setin place for (a) unforeseen future accidents and potential riskresulting from embedment of the contaminated soil and its constructionactivities, and (b)mental anxiety and anguish arising from acceptance ofrehabilitation schemes. In regard to construction activities, the safeimplementation of construction is to be assured. The assurances forsafe implementation of construction will be obtained from the existenceof: (a) an inspection system for operations underway, (b) a channel forroutine information transfer to the public and concerned bodies onon-going operations, (c) a safety management system for construction,(d) a crisis management system for unforeseen future accidents, (e) anaccident reporting system that promptly transmits the occurrence, rele-vant situation and remedial actions, and (f) a long termmonitoring sys-tem that operates during the construction and for a long time thereafter.

The existence of these systemswill relieve theminds of the public inrespect to anxiety over unforeseen accidents and mishaps—while pro-viding comfort and assurance to the public of a safe rehabilitation of theaffected farmlands and secure disposal of the contaminated soil at a par-ticular location. Leadership by government and political offices in thevarious processes will be required if public approval and acceptance ofthe chosen rehabilitation schemes are to be obtained.

6. Discussions and further consideration

The theoretical estimation for the thickness of the top soil layer to bescraper-removed is a challenge. While some papers have been pub-lished in regard to the kinetics of colloid particle movement in soils(Rosen et al., 1999; Forsberg et al., 2000; IAEA, 2010), this phenomenonhas yet to be fully understood. There will be a distribution of the thick-ness of the contaminated soil layer to be scraped in every field and inevery cultivated piece of land—with the likelihood that the areas closerto the power plant would show higher activity concentrations of radio-active cesium in deeper depths than that recognized in this paper. Therehave been reports in verification trials by MAFF of Japan of severe con-tamination of the top 10–15 cm soil layer by radioactive cesium in a fewfields where the surfaces have been disturbed by cultivation and ani-mals. The possibility of the existence of hot spots, i.e. highly contaminat-ed spots, outside the 80 km radius zone has not been taken into accountin this paper. Accordingly, it would be prudent to consider the volumeof scraper-removed soil to be somewhat more than that reported inthis paper. In the final analysis, exact measurements of activity concen-tration profiles at various locations in the contaminated fields should bethe governing criterion.

It is realized that in practice, it would be virtually impossible to con-trol and maintain a scraper-removal controlled thickness of 3 cm or5 cm soil layer. The results of validation trials by MAFF (2012) showedthat the plans for scraper-removal of the top 3 cm or 5 cm soil layerwere not fully realized. The actual field trials themselves producedthicknesses of the removed contaminated top soil layers of about5 cm or 7 cm, respectively, in some cultivated pieces of farmland, thatis, a 60% or 40% increase, respectively. In the event that local hot spotsare encountered, it is suggested that removal of the top soil layer shouldbe at least 10 cm — as a universal standard.

Selection of disposal methods for scraper-removed soils will dependon the activity concentration of radioactive cesium in the top 5 cm soillayer. In this paper, the concentration threshold allowing for in-situembedment as a means for disposal of the scraper-removed soil isproposed to be less than 600 kBq/m2. This limit is predicated on thereasoning that the activity of radioactive cesium in the contaminatedsoils will decrease to half of its present state in a relatively shorttime — taking note of the fact that almost equal amounts of 137Csand 134Cs were deposited on the ground at fall-out and that thehalf-life of 134Cs (2.065 y) is relatively short in comparison to that

of 137Cs (30.167 y). In addition, it is expected that a decrease in theamount of radioactive cesium in the embedded layer will occur due todispersion and diffusion of radioactive cesium to the surrounding soil.In cases where the threshold limit of 600 kBq/m2 is adopted, thevolume of contaminated soil to be disposed by landfilling in a prepareddisposal site is estimated to be less than 1153×104 m3 as shown inTable 1, because of the treatment for volume reduction of the contami-nated soil.

Preliminary tests for the appropriate ammonium solution or chemi-cal compound used in the soil washing process for removal of the finesoil particles and radioactive cesium from the scraper-removed con-taminated soils should be conducted in relation to the properties ofclay minerals and soil organic matters attached on the coarse particlesof soil (Szabo et al., 1997; Kim et al., 2007). The soil remaining aftersoil washing will consist of coarse-grained materials. This relativelylarge amount of coarse-grained materials remaining after soil washingcan be disposed by spreading them on the cultivated land where soildressing and soil amendments will be used as aids for cultivation andespecially for the growth of crops.

In cases where the in-situ embedment scheme is used for disposalof low-level contaminated soil, for areas where heavy rainfall fre-quently occurs and for areas where severe dry periods occur, thisscheme would require one to place a granular soil layer and clayliner on top of the embedded soil as a means of intercepting infiltrat-ing rain water and as a means of minimizing surface accumulation ofradioactive cesium, respectively.

To ensure safer embedment in-situ and landfilling aswell as radiationsafety in neighboring areas, dispersion/diffusion rate profiles of radioac-tive cesium in the subsoil need to be theoretically calculated. The calcu-lations can be made if a proper knowledge of the physical and chemicalproperties of subsoil, and the appropriate transfer-parameters for thedeeper soil layer are available (Yong et al., 2012).

Public acceptance for implementation of rehabilitation projects willrequire communication and discussions with the public through vari-ous processes and through proper documentation of all the requireddisposal schemes and systems. One should note that the public affectedby rehabilitation activities will not be limited to the inhabitants in thecontaminated areas. The impacts will be felt by the habitants in areasdistant from the rehabilitated areas through future changes of environ-ment from agents such as undergroundwater flow, river discharge, andsea water and air currents extending over a wide area. While the im-pacts may be extremely small, it is important to establish an officiallong term monitoring system and long term health care system forthe inhabitants in the areas distant from rehabilitated areas. In addition,a long term ecosystemmonitoring systemmay be required to seek na-tional public consent and acceptance of a disposal plan and procedurefor implementation of a rehabilitation scheme.

7. Conclusions

1. To return the farmlands to their pre-contamination state, the rela-tive shallowness of the contaminated soil surface layer suggeststhat complete removal of the affected soil would be an obvious so-lution. The thickness of the surface layer to be scraped should beexamined by taking into account the distribution of thickness ofthe contaminated soil layer to be scraped in every field and inevery cultivated piece of land. In the event that local hot spotsare encountered, it is suggested that removal of the top soil layershould be at least 10 cm — as a universal standard.

2. In situations where suitable disposal sites outside the affected areaare unavailable, the proposed disposal scheme for low-levelcontaminated-removed soil is to embed the contaminated soilin-situ in the affected area itself. Embedment of the removedlow-level concentration contaminated soil through burial in-situwill require an overlying uncontaminated top soil layer of 2 m in

93M. Nakano, R.N. Yong / Engineering Geology 155 (2013) 87–93

thickness, using soil in situ with the same dry densities and hy-draulic conductivities as that of the natural soil.

3. Construction of an embedded (subsurface) landfill system, similar tothe encapsulating-engineered multi-barrier systems presentlyemployed for hazardous solid waste disposal, is required for disposalof the high-level contaminated-removed soil. Decontamination andvolume reduction of the high-level contaminated soil are the re-quired systems before disposal in the underground landfill.

4. Public approval and acceptance will be reached when the numer-ous requirements and factors articulated by the public are satis-fied. The public involved include those in the affected areas andoutside the areas. Communication with the public should includediscussions, appropriate documentation of all the required dispos-al processes and systems, and science-based verification reportsrelating to the radiation safety of the outcome of the rehabilitationproject.

Acknowledgments

The authors gratefully acknowledge Professor S. Shiozawa, theUniversity of Tokyo, for allowing us to reproduce data, ProfessorT. Nishimura, the University of Tokyo, for literature retrieval, theMinistry of Agriculture, Forestry and Fishers of Japan for permission torecast the data, and Executive Director T. Komae, the Japanese Societyof Irrigation, Drainage and Rural Engineering for encouragement anddiscussion.

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