2.5.6 Application of models to soil-food chain transfer

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Of the many models developed for predicting the atmospheric transport of a radioactive release, fallout patterns, etc. (e.g., see ref. 188), the "SPADE" model for soil-food chain transfer (79) is of particular interest here. In one exercise two fallout situations were considered: An initial "spike" fallout episode leading to a ground deposit of 30 MBq m^-2 or a "continuous uniform" deposition equivalent to total ground deposition at the continuous rate of 30 MBq m^-2 year^-1. The model further provided for separate crop-growing and -dormant (winter) phases, effects of harvest, ploughing, etc.

Following "spike" deposition over pasture the model indicated an initial rapid buildup of cesium-137 in milk within 10 days to a level of approx. 3.2 MBq 1^-1 which declined to less than 0.5 MBq 1^-1 after 100 days. Under conditions of "continuous" deposition the achievement of an effective steady level of 0.5 MBq 1^-1 was indicated after about l year. Some comparative data for predicted levels of cesium-137 in various foodstuffs are illustrated in Table XVII from the same report (79, p. 151). As indicated earlier the model also shows relatively very much lower uptake of the actinides or transuranium isotopes.

TABLE XVI
Migration of radionuclides down the soil profile.
Comparative parameters for top 9-cm horizon of cambisole: pH - 4.4; organic carbon - 2.4 % (from data of ref. 72)

TABLE XVII
Cesium-137 contamination levels (CL-fresh weight basis) in foodstuffs indicated by SPADE model for different input scenarios (from ref. 79)

It will be of interest to compare such predicted levels (as fractions of the "spike" deposit) with post-Chernobyl experience in Europe (146). As indicated elsewhere (125), considerable time may have to elapse before a full picture emerges of the behaviour of 'Chernobyl' fallout over soils.

2.6. Detection and measurement

2.6.1 Importance of early notification
2.6.2 Sampling and monitoring

2.6.1 Importance of early notification

The 'Chernobyl' experience certainly demonstrated the importance of early notification of unintended radioactive releases. Prompt notification and communication to the levels of national, agricultural extension service and local monitoring centres (e.g., those of agricultural or environmental research and teaching institutes with monitoring facilities) could, in future, contribute a great deal to improved preparedness and attenuation of some of the consequences of fallout over agriculture, forestry or inland fisheries. Moreover, as indicated earlier, the timely dissemination of information coupled with an improved use of local weather forecasting could better provide for some anticipatory countermeasures (shelter of grazing livestock).

Timely information at the right level would also provide for the rapid mobilization of monitoring facilities and for the earliest detection of fallout over crops and soil.

2.6.2 Sampling and monitoring

Sufficient here to note that in addition to routine monitoring, e.g., for national atomic energy programmes (189; 190) there is a need for improved local soil and crop monitoring facilities worldwide (where necessary these could be based as a mobile unit on some suitable national centre) - especially for dry and/or wet deposition. Those would be the best and quickest indicators of likely soil, crop, and livestock exposure levels (see also Section 1.4.4). Post-Chernobyl experience has shown the value of grass sampling and monitoring as a fallout indicator because of its ubiquitous distribution and relative accessibility in both urban and rural areas (163).

The importance of representative soil sampling, and the needs for both 'objective' (random) and 'subjective' (e.g., of suspected 'hot-spot" areas see Section 2.3.4) will be no different from those of conventional soil sampling and analysis (e.g., 191).

In consultation with other UN Agencies (FAO, UNSCEAR, WHO and WMO) IAEA has initiated a new programme on Fallout monitoring in Environment and Food. This has led to the elaboration of an up-to-date and concise report on laboratory organization, sampling, and radiochemical analysis (192). This authoritative report should be consulted as a first step for further information on sampling and radiochemical analysis for fallout levels in air, water, soil, crops, etc.

Summaries and technical descriptions of the basic principles of sampling and monitoring for radionuclides, which have not changed during the last decade or so, are available (e.g., see 59, p. 487; 190, pp. 115185).

2.7. Countermeasures, reclamation and use of contaminated soils

2.7.1 General considerations
2.7.2 The time factor
2.7.3 Ploughing, irrigation, leaching, harvest
2.7.4 Fertilizer and other amendments
2.7.5 Choice of crops; safety checks
2.7.6 Possible problems from non-reactor accidents

2.7.1 General considerations

As indicated earlier (section 1.2.1), a significant radioactive fallout episode represents an immediate challenge to radio-ecotoxicological knowledge and its application to the implementation of any necessary countermeasures. The global scale of radioactive fallout from the atmospheric testing of nuclear weapons in the '50s and '60s prompted much of the now classical work on the problems of soil-crop contamination (e.g., by Scott-Russell and his colleagues - see ref. 59, pp. 3-12, 79-88 & 657-662; ref. 53).

An important difference between fallout from the atmospheric testing of nuclear weapons and that from a serious nuclear reactor release such as that from Chernobyl can now be recognized. The earlier tests injected radioactive debris high into the stratosphere resulting in widespread and persisting fallout (180). A mean stratospheric residence time of the order of one year has been indicated (59, p. 5). The deposition of finely divided radioactive material has continued at detectable levels for many years (see Section 2.3.3. This, in turn, resulted in direct crop contamination by interception so that even later contamination levels of crops were due more to interception than by uptake from the soil (59, p. 6).

The "explosive" emission of radioactive debris from the Chernobyl reactor, on the other hand, reached a height of approx. 1 km before 'horizontal transport began" (46, p. 84). Therefore, most of this radioactive material would be expected to remain in the troposphere and deposition to be associated with precipitation - as indeed was observed. This also implies that contamination of post-fallout sown crops will be largely the result of soil uptake or soil surface contamination.

At worst, topsoil (and floral cover) was so heavily contaminated by longer-lived radionuclides that reclamation for agricultural use would be neither economic nor acceptable in the context of local community or public health. This situation evidently occurred near the Chernobyl site and prompted the physical removal of topsoil from some areas within the critical 30-km zone for disposal at special sites (46, pp. 82-83). However, such high levels were relatively localized.

While it is difficult to assign quantitative parameters to the initial fallout pattern some indication can be made on the basis of available data. An assumed initial angular plume of 10° (194; 188) extending to 60 km from Chernobyl (46) would correspond to a segment fallout area of approx. 3 x 10⁸ m². If 2 % of the reactor's total radioactive content i.e., one half of the total emission (46, p. 81) had been deposited uniformly over this area it would result in a deposition of some 5 x 10⁸ Bq m^-2 of significant radionuclides (iodine, cesium, strontium, ruthenium, plutonium). However, if the remaining half of the total emission were uniformly deposited over the total affected area of Europe of some 10¹³ m² this would correspond to an average deposit of the order of 10⁴ Bq m^-2. In fact, of course, the deposition was very far from uniform (see Section 2.3.4) but, evidently, there were no levels so high as to indicate the need to evacuate local populations or to remove topsoils. Levels deposited in fact were, of course, also reduced by the radioactive decay of shorter-lived radionuclides such as iodine-131 during atmospheric transport and after deposition. Moreover, the fraction deposited over mainland Europe outside the U.S.S.R. must have been considerably less than one half because of its widespread detection beyond Europe. A fraction of one quarter has been suggested (104).

Given levels which do not indicate drastic intervention such as physical removal of topsoil or the temporary evacuation of local communities; then what are the prospects for continuing, normal use, limited or modified use, or for the application of effective countermeasures ? These questions are briefly addressed on the basis of existing information in the sections below (see also section 1.2.4 and ref. 53, pp. 509-518).

It is important to recall that, in principle, no remedial measure should be undertaken at farm, extension or advisory service level without the guidance of the appropriate radiological protection or public health authority (2, p. 15). There could, conceivably and otherwise, be a possible hazard to the personnel involved or, indeed, be implementation of a completely unnecessary or unsuitable countermeasure. The IAEA has provided machinery for assistance and guidance on the problems of 'off-site response to a nuclear accident (98; 193).

Finally, monitoring may indicate sufficiently low levels that, on the basis of national or recognized intervention levels for harvested products, and on the basis of crop uptake to be expected in future crops, no action of any kind is needed and agricultural use can continue unchanged.

2.7.2 The time factor

The timely harvest of exposed crops, cut grass, etc. and their removal would, of course, eliminate that proportion of intercepted fallout (see Section 2.5.2 and 2.7.3). In the absence of further fallout episodes or any significant continuing, fallout, the potential for soil-crop transfer will diminish with time. Firstly, as a result of radioactive decay, especially of the shorter-lived radionuclides (e.g., iodine-131, ruthenium--103, strontium-89) and, secondly, as a result of movement down the soil profile by leaching,, mechanical disturbance (cultivation practice, movement of soil mesofauna, etc.) and by effective mobilization by microbiological action (76).

The decrease in soil--plant transfer has been quantitatively studied (e.g., 75, pp. 260264). The decrease may not be sufficiently rapid to result in a corresponding removal of all possible constraints. For example, a later new seasonal growth of fresh pasture may lead to a second smaller yet significant transfer into the new grass and rise in the radionuclide intake by grazing livestock, as recently warned for some hill-farming areas in North Wales for the post-Chernobyl spring of 1987 (195).

A later rise in dairy cattle, etc. could also result when livestock are moved to winter shelter and fed silage or hay which had intercepted fallout or undergone soil uptake before harvest and before any significant decline in soil levels (88, p. 14). For an illustration see Section 3.3.

The relative significance of soil surface deposit and crop-uptake by interception or from the soil through the crop roots as contributors to the absorbed radiation dose will also vary with time. For example, as contributors to the total dose received by farm personnel "living off the land".

This can be illustrated as follows: The "conservative" scenario (Section 2.5.3) for committed dose equivalent as a result of dietary intake for one post-accident year is indicated to be less than 400 m Sv. The integrated dose from external radiation for a worker on the same diet who spent 8 hours each day for 6 days in each week on the unploughed or uncultivated land and, assuming conservatively, that the effective deposit remained on the surface at 10,000 Bq m^-2 of cesium-137 (i.e., neglecting radioactive decay, soil penetration, crop removal, etc.) will be (Eq. 1):- 52 (weeks) x 6 (days week^-1) x 8 (hours day^-1) x 0.01 (SD_M) x 0.95 (CF for cesium-137) = 24 m Sv approx. (i.e., 6% of the dose equivalent committed through dietary intake).

Now consider the same worker in the first critical 10 weeks (Intermediate Phase) after the fallout episode Intake from local inland lake fish will be lower than indicated in Table XVI, or even negligible, because of the time needed for movement of the radionuclide in the aquatic food chain and its bioconcentration in fish (Kornberg & Davis in ref. 53, pp. 383418). However, if the same farm worker consumed his local freshly exposed vegetables and cow's milk (of cattle grazing on freshly exposed pasture), contamination levels (CL) by interception of the order of at least 1,000 Bq kg^-1 fresh weight of green vegetables might be expected. Assuming an intake of 10kg fresh weight of green vegetables in the 10-week period (i.e., until completion of the exposed harvest) with 150 kg of drinking water (assumed to be contaminated as initially at approx. 2 Bq kg^-1) and 40 kg of cow's milk, then a dose of the order 500 m Sv would be committed. The external dose on the same basis would be approx. 5 m Sv i.e., about 1% of the dietary-based commitment.

It follows that the external dose contribution may not decline so rapidly as that from the diet but, on the basis of a 50-yr. integrated projection for external dose (when soil penetration and radioactive decay will become significant), it will remain by far the lesser radiation dose contributor. However, it could become relatively more significant if dietary intake become based on products from an unexposed area, or effective soil-plant transfer factors declined more rapidly than soil surface radiation emission (196). The continuing accumulation of post-Chernobyl data will surely clarify this situation in due course (124).

2.7.3 Ploughing, irrigation, leaching, harvest

As indicated (see Section 2.5.1) ploughing will extend the vertical distribution of an initial deposit, reduce the level of above-surface radiation, and the potential of transport by erosion and run-off. However, leaching tends to be very slow because of the cationic nature of the radionuclides, and because of the immensely low chemical concentrations likely to be involved, at least until isotopic dilution by any available naturally present element, or by cationic displacement by chemically related cations.

Effects of ploughing on soil-plant transfer are not large. Early work indicated a reduction of only 25 % by ploughing-in a surface deposit to 28 cm (58). Likewise, the uptake of actinides was relatively little affected by ploughing (77, Vol. I, p. 18) which, however, tends to be low in any event (see Section 2.5.2).

The effects of different tillage on the profile distribution of surface strontium-89/90 have been quantified (77, Vol. I, pp. 97-98). Almost all the added radionuclides remained in the top 15 cm of untilled soils while deep ploughing (to 30 cm) resulted in 30% moving into the 10 to 30-cm profile.

The use of "solvents" has been mentioned in connexion with the "fixation" of radionuclides in the highly contaminated soil around Chernobyl (46, p. 83). This suggests the admixture of ion exchange resin supensions or irrigation water containing added cationic solutions of (e.g., of calcium, potassium and strontium salts) which would be expected either to "fix" the radionuclides or reduce the effective soil-plant transfer.

It has ions been recognized that the removal and safe disposal of foliar-intercepted fallout can greatly reduce the level of soil contamination (89; 193). An obvious limitation is that the presence of such an effective cover-crop will depend very much on location, growth status, and weather at the times of fallout. The most vulnerable time would, equally obviously, be that of the common bare fallow periods of modern agricultural practice.

Interception by woodland or 'closed forest' areas will also be important in the protection of underlying soils but, in this case, without harvest and removal. Intercepted fallout would be largely retained by the foliage or in ground surface litter as a result of post--fallout abscission (see Section 2.5.2). Near the Chernobyl site forest fire prevention was recognized as an important factor in the role of the forest as "long term accumulators of radioactive substances" (46, p.82).

Should eventual clearance of woodland or forest for crop cultivation or pasture be undertaken, removal and suitable disposal of the litter mat might be indicated as desirable by predisposal monitoring.

The effects of irrigation, whether part of normal practice or as a deliberate measure to accelerate movement of the radioactive deposit from crop and soil surface into the soil profile, will clearly depend upon the quality of the irrigation water. Other things being equal, it will especially depend on pH, cationic species, and their concentrations in the irrigation water for the reasons briefly indicated earlier. In practice many other factors would have to be considered such as standing crop needs, soil structure, water table, evapotranspiration, etc. as reviewed elsewhere in relation to "leaching fractions" (198). In any event the downward migration of radionuclides tends to be relatively slow in relation to water infiltration.

2.7.4 Fertilizer and other amendments

Increased availability of isotopic or some chemically-related elements can reduce the soil-plant transfer (98; 58). For this reason the uptake of strontium-89/90 tends to lessen with liming and increased pH (77, Vol. I, p. 107) and that of cesium-134/137 by the addition of K-P fertilizer (77, Vol. I, p. 331; 91).

In this connexion the benefit versus cost would need careful consideration unless the amendments were consistent with normal practice. For example, soil amendments with natural cesium salts would probably be far more expensive and less effective than, say, the use of a crop from the untreated soil as animal feed but amending this feed with the commercially available (e.g., see ref. 197) cesium iodide. However, while that would be expected to reduce the appearance of cesium134/137 and iodine-131 in meat it could increase the fraction of total iodine-131 appearing in the milk of lactating cattle (see Section 1.2.4). The use of iodine compounds to reduce possible transfer of iodine-131 through milk into the human thyroid is well recognized as an emergency prophylactic (199, pp. 174-178) and so applied with success around Chernobyl (46; 124).

2.7.5 Choice of crops; safety checks

Choice of crop would only be a post-fallout option. The low frequency of accidental releases of any significance to agricultural soils suggests that modified practices purely on account of proximity to some nuclear installation would not be justified unless choice had no economic implications anyway. In that event, vegetables, for example, would be wiser (high probability of interception and economic removal and disposal) than viticulture or cash crops for export (because of the potential for international constraints whether technically justified or not). However, the high interception potential of conifer stands (see Section 2.5.2), their relative insignificance to the human food chain and absence of human habitation suggest their suitability as the immediate reactor site environment. This possibility was not, apparently, considered in earlier discussions of reactor siting (see Section 1.1.3).

Changed cropping practice might be indicted under some circumstances including the time factor. While only surface contamination obtained, "minimum tillage followed by the planting of a deep-rooted crop should reduce the uptake" of radionuclides, compared with that of shallow-rooted crops (199, p. 273).

Safety checks. At some point after the imposition of possible emergency constraints i.e., at a stage in the Recovery Phase (see Section 2.4.1), it might be desirable to check the soil-plant transfer potential of the earlier exposed soils before re-sowing or committing a new programme of cultivation. For this purpose a radiobio-assay using a fast growing plant under temporary glass or plastic cover (to accelerate growth but using the undisturbed soil) would be useful. A plant species with a high TF value for the residual soil radionuclides should be chosen. Such values for a wide range of plant species have been documented (e.g., 77; 178).

2.7.6 Possible problems from non-reactor accidents

This review has been concerned with the problems of soil contamination by fallout as a result of an unintended nuclear reactor release into the atmosphere. However, other kinds of accidents are possible as a result of some failure in the nuclear fuel cycle or in some other application of nuclear science. For example, a release of cobalt-60 as a result of fire or explosion at a gamma-irradiation facility.

Other kinds of accident have, indeed, occurred. According to reports, a military aircraft carrying "thermonuclear weapons" crashed and exploded on the ground near the Spanish village of Palorames in January 1966 resulting in the release of plutonium (200). The destruction of local tomato crops was ordered and some 500 truckloads of soil were removed. Despite the obviously serious nature of this accident the effects evidently remained localized.

It seems highly probable that the effects of a nuclear accident other than a major reactor release would also be relatively localized and be effectively contained by the national radiological and public health authorities concerned. However, global fallout of plutonium-238 has been recorded as a result of at least one 'burn-up' episode in the upper atmosphere of a space-programme satellite containing a plutonium generator. This occurred in the Southern hemisphere in April, 1964 (201); but the resulting levels of plutonium-238 found in soils have been some two orders lower than those of plutonium-239 + 240 as a result of nuclear weapons testing in the atmosphere (145).

A large number of studies have been, and are being, made on the behaviour and significance of routine radioactive releases into the agriculture (and fisheries) environment (e.g., 62; 74; 75; 77; 170; 202; 214; 215). The evidence suggests that these releases are effectively controlled and represent no significant threat to agricultural soils or to the health and welfare of the dependent communities.

2.8. Concluding comments; some questions and needs

Notwithstanding its tragic human consequences at the local community level, the Chernobyl accident in April, 1986 has provided some new opportunities. Firstly, for obtaining answers to many important and relatively neglected practical questions on the basis of the newly emerging data. Secondly, to provide for much better preparedness for the impact of future accidents upon agriculture and upon the dependent communities.

There will, surely, be accidents in future. The problem now appears to be that of improved protection of exposed agricultural (forestry and fisheries) resources and the welfare of their dependent communities. This aspect of nuclear accident emergencies seems to have been relatively neglected. Moreover, unlike other technology-based accidents, a major release of radioactive material into the atmosphere can have wide-ranging consequences as a result of fallout over agricultural soils. However, these consequences could be largely contained or attenuated, given the necessary international cooperation and prompt communication.

Despite the massive range of reports and publications on the behaviour and effects of radionuclides in the soil-plant system, it remains difficult to identify useful translation into practical and simple language for guidance and clarification at agricultural advisory service or farmer level. This could result in serious post-accident problems in future; especially in countries which may lack a nuclear science or monitoring infra-structure but which might, nevertheless, be significantly exposed to fallout from an out-of-state release. For these same reasons 'Chernobyl' prompted a great deal of unnecessary fear and suspicion at farm community level, even within industrialised countries with their own nuclear power programmes.

Against this background, some unanswered questions and related needs have been identified (see also Sections 1.4 and 1.5).

- Once deposited, radionuclides tend to move relatively slowly down the soil profile and may sustain crop uptake for many seasons after the initial deposit. Fortunately, soil-plant transfer, while variable is usually very fractional, especially for the more radiotoxic actinides (see Section 2.5.2). Soil-plant transfer tends to decline slowly with time for various reasons. There is a need to quantify the benefit-cost ratio of countermeasures involving ploughing, chemical or special irrigation amendments over and above those of normal agricultural practice - all, of course, as a function of the initial level of deposit. Would it be more effective to accept crops from untreated soils as livestock feed but amend this feed with suitable chemicals to reduce animal absorption of the radionuclides (see Section 2.7.4)? Alternatively, what might the prospects be for diverting the contaminated cropto alternative use? E.g., wheat for fuel alcohol production and animal feed as already developed for wheat surpluses in Sweden (203). Would such diversion lead to any significant occupational radiation hazard?

- Existing standing crops, grass, etc. clearly represent a potentially important fallout-intercepting and consequent soil-protective cover. Emerging post-Chernobyl data should be specifically studied in that context to quantify its potential value as a protective measure in future (see Sections 2.5.2 and 2.7.3) and for the development of internationally-available guidelines.

- There is a need to study the most effective ways of harvesting post-fallout crops, which have successfully intercepted fallout, in order to determine practicable methods of disposal. For example, by composting or burning and returning the compost or ash later to the soil or retaining the compost of ash at a protected site? Would the radionuclides of returned ash or aged compost be as available for future soil-crop transfer as radionuclides directly deposited on the unprotected soil? Radionuclides in the ash might have undergone more effective isotopic or chemical dilution. In any event the treatment of ash would be far more economic than treatment of the original soil areas.

- Precipitation and weather evidently played a critical role in post-Chernobyl fallout (see Section 2.3.4). This indicates the need for a feasibility study for advance warning at farmer level of possible or probable fallout episodes. Such warning could be of practical value. For example, with warning a farmer might usefully, and even critically, delay the harvest of, say, mature vegetable crops and effectively protect otherwise fully-exposed soils (see Sections 2.7.2 and 2.7.3).

- There is, evidently, scope for the development of improved freshly harvested food crop decontamination procedures by simple washing or soaking in suitable 'carrier' salt solutions (see Sections 1.2.4 and 2.4.2.

The U.N. Agencies (especially IAEA, FAO, UNSCEAR, WHO, WHO) and other relevant international organizations (especially CEC, OECD-NEA, ICRP, IUR) have an important opportunity for cooperation and for the elimination of some of the duplication and confusion in this subject area - even at the scientific level. An opportunity for the joint preparation and publication of simple explanations and guidelines on anticipatory- and counter-measures for agriculture and fisheries in the event of a future serious accidental release of radioactive material into the environment. Likewise international agreement on the principles of insurance and compensation, and appropriate derived intervention levels for soils, crops, and for products moving in international trade.

Finally, it is important to recognize that fallout radionuclides are only part of the range of chemical contaminants of agricultural and forestry soils worldwide as a result of agrochemical usage, irrigation, industrial, and transport emissions from remote sources (e.g., as acid rain). This underlines the importance of considering any particular soil contaminant in a comparative and integrated context (209).

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