2. Soil and crop contamination by radioactive fallout

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2.1. Agricultural and forestry soils as a vital global resource

2.2. Soil in the nutrient cycle

2.3. Sources and nature of radioactive contamination

2.4 Biological significance of contaminated soils

2.5. Behaviour in soils and movement into foodwebs

2.6. Detection and measurement

2.7. Countermeasures, reclamation and use of contaminated soils

2.8. Concluding comments; some questions and needs

2.1. Agricultural and forestry soils as a vital global resource

2.1.1 Soil as a limited resource
2.1.2 Radioactive contaminants and Pollution

2.1.1 Soil as a limited resource

Escalating technology, virtual consumer saturation, and apparently abundant food supplies in the spreading, 'super-markets' of the industrialized nations tend to obscure the fact that food, unlike all other human acquisitions or products, is literally vital to human survival. Food, in turn, critically depends upon the soil and water resources of the finite human environment. Moreover, food supplies for the present world population can now only be sustained by the application of increasingly intensive agricultural practices to effectively diminishing soil and water resources per capita (135; 136).

2.1.2 Radioactive contaminants and Pollution

The serious nuclear accident at the Chernobyl power plant on 26 April 1986 in the U.S.S.R. demonstrated dramatically the vulnerability of agricultural resources to radioactive contamination from atmospheric fallout. This affected large areas across Europe up to thousands of miles from the accident site. Except for people exposed to the radiation occupationally or on account of proximity to Chernobyl, public health was effectively protected. Within the Soviet Union prompt action by the authorities which involved a large scale evacuation had clearly greatly contained threats to human health (14; 46).

Across Europe as a whole, however, public health protection involved some serious constraints upon agricultural and farm practices, harvest, and food movement. These, in turn, created problems of finance, compensation, and psychological stress for many of the dependent communities. For example, an understandable fear among some exposed agricultural land owners that post-"Chernobyl" fallout would adversely affect the market value of their farm properties (212).

All soils of the world are radioactive as a result of cosmogenic and/or primordial radionuclides naturally present. These, respectively, include traces of radioactive carbon (carbon-14) and radioactive hydrogen "hydrogen-3 or tritium) formed by the action of cosmic rays in the atmosphere and which reach the soil through soil-borne photosynthesis and water absorption, and the naturally radioactive elements of potassium (potassium-40), uranium, thorium and their 'daughters'. The latter are present in the geological precursors of many soils.

Since the effective start of the 'atomic energy era' with the construction of the first nuclear reactor in Chicago in 1942 there has been a series of additions to soil radiation levels as a result of human activities: From the testing of nuclear weapons, their military use in the final stage of 'World War-II', as a result of controlled releases as radioactive wastes from the peaceful uses of atomic energy, and as a result of nuclear accidents which include those of Windscale (U.K.), Three-Mile Island (U.S.A.), the crash of a U.S. military aircraft evidently carrying thermonuclear bombs containing plutonium (in Spain), and Chernobyl (U.S.S.R.). Fortunately, most of these additions to the levels of soil radionuclides have not involved levels of any public health significance.

In general, it is important to distinguish between the mere presence of a chemical 'contaminant' of soil (whether radioactive or not) and one which becomes a 'pollutant' as a result of location, concentration and adverse biological or toxic effects.

2.2. Soil in the nutrient cycle

2.2.1 Composition and variability of soils
2.2.2 Agricultural and forestry ecosystems
2.2.3 Cycling of plant nutrients and fertilization

2.2.1 Composition and variability of soils

Soils are the relatively thin film of heterogeneous particulate and fibrous matter which constitutes much of the land-atmosphere interface. They are a complex mixture of minerals, carbonaceous matter and water, in which live the vast range of soil micro- and macro-flora and fauna.

Composition, classification and distribution of soils have been authoritatively reviewed elsewhere (e.g., 137; 138; 139; 140). Certain features are especially relevant to the problems of radioactively contaminated soils, especially the great variability of the soil-based (as distinct from aquatic-based) ecosystem in terms of chemical composition and of physical structure, and of environmental and exposure factors.

Composition varies especially in organic matter (both humus and undecomposed plant residues), inorganic matter, pH, and the availability of plant-essential elements (N. P. S. Na, K, Mg, Ca, Cl, B. Co, Cu. Fe, Mn, Mo, and Zn); of elements taken up and which may have a plant-biofunctional role or an essential role for grazing animals or human food (Cr. Ni, Se, Si, V, Br, F. and I); and in contaminants (e.g., pesticide residues, radionuclides, etc.). Plant C, H. and O are, of course, derived by photosynthesis and from atmospheric and irrigation water. Physical structure also varies in terms of the soil profile, soil water capacity, porosity, ground water level, topography, etc.

Environmental factors vary in terms of climate and weather; floral cover and dependent fauna; agricultural and/or forestry practice. These, in turn, vary with time, e.g. mature crop ready for harvest, post harvest bare fallow, or tend to remain in a 'steady state' like dense permanent cover of the tropical rain forest, permanent pasture, etc. Location, especially in relation to potential nuclear accident sites and radioactive fallout areas, is perhaps, the critical variable here. Clearly, such variability will influence decisively the levels, behaviour and significance of radioactive contaminants reaching a particular soil-based ecosystem as well as the kind of countermeasures which may be needed.

2.2.2 Agricultural and forestry ecosystems

Of particular importance will be the degree of soil exposure at the time of radioactive fallout (in precipitation or as 'dry deposition'). Soils under bare fallow would obviously be most exposed while those under dense vegetation would be least. For these and other reasons it would be neither useful nor realistic to consider the radiological protection of soil in abiotic isolation (see Fig. 4 in Part 1 for illustration).

As will be discussed further, there are three important aspects of a contaminated soil-plant system:

(a) Surface radiation level which may directly irradiate local personnel occupying or working on the land;

(b) The presence of radionuclides directly intercepted by the crop (or pasture) during fallout which might, very soon, become food or feed;

(c) Transfer of the radionuclides by later crop uptake from the soil and, thereby, to future food intake by local or distant populations, or transfer by surface erosion to other soils, crops or water.

(d) Some transfer of radionuclides, as with agrochemical residues, could occur as a result of "re-suspension" by surface-eroding wind or water but this would represent a relatively "minor pathway" within the exposed ecosystem (see also last part of Section 2.5.4).

It is in relation to (c) that an appreciation of nutrient cycling becomes important, as discussed below.

2.2.3 Cycling of plant nutrients and fertilization

Under constant cropping and removal of the entire crop there is a constant drain of soil nutrients as illustrated in Table IV (based on 92; 141).

TABLE IV
Removal of elements from soil by the annual harvest of 4 tonnes of wheat grain dry matter per hectare. (only gross removal with no return via manure etc.)

Note: In U.K. farming generally, it has been indicated that removals from soil of the trace elements selenium (Se), iodine (I), and molybdenum (Mo) lay within the range of approx. 0.000025-0.0125 kg ha^-1 yr^-1 (141).

The cycling of mineral nutrients and plant uptake in agroecosystems have been authoritatively reviewed and tabulated elsewhere (139; 141; 142). The agricultural significance of these removals will clearly depend upon the rate of natural replacement (e.g., by biofixation, dry deposition), by the mineral content of irrigation water, and by fertilizer applications. The significance here is owing to the fact that radionuclides tend to behave as their non-radioactive chemical relatives. Thus, cesium-137 often behaves like potassium, strontium-90 as calcium. When the radionuclide is an isotope of the natural soil element then its behaviour will be effectively identical (subject, of course, to any necessary chemical modification to the plant-available form), thus iodine-131 or manganese-56 will respectively behave as the natural elements. Plutonium-239, on the other hand, is an artificial nuclear reactor activation product and will behave uniquely. These considerations have important implications for the use and reclamation of contaminated soils (see Section 2.7).

2.3. Sources and nature of radioactive contamination

2.3.1 Naturally occurring radionuclides
2.3.2 Anthropogenic radionuclides
2.3.3 Trends since 1945
2.3.4 Post-'Chernobyl' fallout over soils

2.3.1 Naturally occurring radionuclides

For the purpose of detecting and monitoring for any increase or changes in soil radioactivity as a result of an accidental or planned release, of time, or of deliberate counter measures, the pre-event or effective 'base-line' levels must, of course, be known as indicated in Part 1. This level will be due to cosmogenic and primordial radionuclides naturally present (natural background level) and to anthropogenic additions during the 'atomic energy era'. These levels will depend upon the nature of the soil and to exposure to fallout, etc.

The 'base line' level must not be confused with the instrumental 'background' level of the particular detector and counting assembly used. This 'background' level will be the level indicated in the presence of a 'dummy' sample devoid of detectable radioactivity. This background of 'extraneous counts' will be due to possible electronic noise, the traces of radionuclides inevitably present in the structure of the detector, lead 'castle' or other radiation screen, incidental incoming cosmic rays, etc. (143).

Potassium-40 is the most significant of the natural radionuclides in 'average' soils (6). The specific radioactivity of potassium-40 in natural potassium is 31 Bq g^-1 (6). Assuming a content of 2% of elemental potassium in 'average' mineral soil (139) of bulk density of 1.5 g cm^-3 to a rooting depth of 20 cm, this corresponds to a level of potassium-40 in soil of the order of 2 x 10⁵ Bq m^-2. Likewise, on the basis of 2.5% biogenic elementary carbon in the soil and the known cosmogenic carbon-14 content of approx. 0.2 Bq g^-1 natural non-fossil carbon (13) there will be approx. 10 Bq m^-2 of carbon-14.

Natural tritium (hydrogen-3) will be, effectively, in steady-state equilibrium with the hydrogen of surface soil and water (13). Assuming the necessary soil 'field capacity' and average water content equivalent to 6 cm rainfall and known cosmogenic tritium level of 0.5 Bq/1 of atmospheric water (13) the tritium content of the soil was estimated. All these estimates, together with other data (6), are summarized in Table V below.

Radon-222 (an isotope of the chemically-inert gas radon) is also included in Table V because in the confined air spaces of soil it will tend to accumulate to a steady-state level as a result of uranium-238 decay.

TABLE V

Estimated levels of radionuclides naturally present in mineral soil to rooting depth of 20 cm and of bulk density of 1.5 g cm^-3

2.3.2 Anthropogenic radionuclides

Significant additions of radionuclides to the environment commenced with the construction and operation of nuclear reactors from 1942 onwards. Above all, from the military use of two atomic bombs (based on U235 fission) in 1945, and the subsequent testing of nuclear weapons in the atmosphere. There have also been continuing discharges of relatively small amounts of radioactive wastes from the nuclear power industry, use of research reactors, and from the uses of radioactive substances in medicine, research and technology. Finally, there has been a number of relatively minor nuclear accidents (see Sections 1.3.2 and 2.7.6 below) involving the release of radionuclides into the atmosphere with subsequent fallout or release episodes into agriculture, forestry and fisheries.

Tables I and V summarize and characterize the more important natural and anthropogenic radionuclides which might be expected to be found in contemporary soil samples. Levels and relative proportions will vary greatly according to:

- Geologic origin and composition of the soil;

- History of exposure to radioactive fallout;
- Post-exposure history in terms of time, agricultural practices, erosion, leaching, and countermeasures designed to reduce contamination levels.

Fertilizer practice can affect the levels of naturally occurring radionuclides, especially the use of compound 'N-P-K' fertilizers. For example, the application of approx. 130 kg nitrogen, 45 kg phosphorus, and 155 kg potassium per hectare of potato crop would be typical in parts of Europe (137). This would add some 500 Bq m^-2 of potassium-40 to the soil surface, together with traces of uranium--238, radium-226, and thorium-232 if the phosphate component had been derived from the commonly used 'phosphate rock' mineral resources (8).

Only significant cosmogenic radionuclides are included in Table V. Anthropogenic and cosmogenic radionuclides now present in the environment have been fully tabulated elsewhere (6).

2.3.3 Trends since 1945

There have been some continuous and sporadic additions of anthropogenic radionuclides since the first atomic bomb test in New Mexico (U.S.A.) in 1945. This means that effective soil 'base--line' levels have varied upwards (with additions) and downwards (as a result of radioactive decay, erosion, leaching, etc.) but always above the pre-1945 'natural background' level. Moreover, the 'base-line' levels of soils must remain significantly above the natural 'background' level indefinitely because of long radioactive half-life and relative immobility in soils (e.g., of plutonium-239).

Environmental trends since 1945, together with their implied population radiation dose commitments, have been extensively studied and documented by UNSCEAR (13; 8). Of interest is that, before 'Chernobyl', fallout from the earlier testing of nuclear weapons in the atmosphere was continuing at detectable levels. For example, 1982-monitoring data (144) indicated the annual fallout over areas of the U.K. illustrated in Table VI which also show the influence of rainfall. Some accumulated 'base-line' levels in soil as estimated by the writer on the basis of data in the same report (144) are illustrated in Table VII. Accumulated levels of plutonium-239 and -240 in soils of North--West Italy, comparable with those of Table VII for the U.K., have recently been reported; likewise, higher levels in areas with higher annual rainfall (145). Similar levels for cesium-137 have also been reported for the Netherlands (146).

TABLE VI
Continuing fallout over the U.K. during 1982 from earlier nuclear weapons testing in the atmosphere (144)

TABLE VII
Accumulated 'base-line' levels in soils of the U.K. by 1982 (based on data of ref. 144)

The age of a particular fallout episode can be indicated by the changing ratio of two radionuclide activities when they are isotopes of the same chemical element. Their respective soil chemistry must, effectively, be identical so that slight changes as a result of chemical or physical fractionation can be neglected. Therefore, in the absence of significant interim additions the change in isotopic ratio will be a function of their different radioactive decay rates and of the lapse of time involved. The changing ratios for cesium and strontium radionuclides are indicated in Table VIII. The Cs-134/137 ratio has been used to distinguish between deposition from the 'Chernobyl' release and those of earlier releases (147; 146).

Table VIII
Change in radioisotopic ratios of fallout as time - origin indicator

There now appears to be a need to determine and/or collect (possibly existing) data on 'base-line' levels of radionuclides in agricultural and forestry soils worldwide. At present, the apparently limited published or available data appear mainly to be those of countries with relatively sophisticated atomic energy programmes. However, as demonstrated by 'Chernobyl', a major release of radioactive material into the atmosphere can result in fallout on a major international, if not global scale. Under no circumstances is this observation intended to imply that fallout detected in future would, necessarily, have any agricultural or public health significance whatever.

2.3.4 Post-'Chernobyl' fallout over soils

Understandably, the accident prompted an immediate activation of the full environmental radioactivity monitoring potential worldwide and a still-continuing stream of reports and publications relating to national (e.g. 147; 149-155) and international (e.g. 88, 156; 158) fallout and radiation levels. As the first post-accident year drew to a close a study of these reports indicated, very briefly, the following conclusions (see also Part 1):

Within days of the accident fallout episodes over soils in many countries across Europe were recorded. In many areas levels were considered sufficiently high to prompt intervention at national and regional levels to obviate the possibilities of exposed and contaminated standing crops and livestock entering public food supplies. Fallout was detected up to many thousands of kilometers from the Chernobyl accident site, e.g. in Japan (153), U.S.A. (101) and in the Republic of China where it was estimated that up to 0.6% of the total Chernobyl iodine-131 emission had fallen (159). Cesium-134 and -137 were detected in snow as far north as Greenland (160). Strontium-89 and -90 were detected in fallout and at significant levels on pasture in Yugoslavia. Attention was drawn to the paucity of post-Chernobyl fallout data on these radionuclides, evidently due to the 'time-consuming radiochemical isolation' needed for their reliable assay (161). Detectable fallout was not evidently reported from the 'Southern hemisphere' (124).

Despite the wide range of fission and radioactivation products certainly emitted, two radionuclides have attracted conspicuous attention. These are iodine131 and cesium-137. The short radioactive half-life of iodine-131 (8 days) eliminated its significance as a soil contaminant within weeks of deposition. Cesium-137 (half-life 30 years), on the other hand, will effectively remain in the soil for centuries and, according to its availability to crops or pasture (see Section 2.5.2), may present problems in some local areas for some years into the future. Many other radionuclides were, indeed, detected and determined, incidentally, in the fallout spectrum such as silver-110m, but not at levels likely to be of radiological significance (15; 162).

Accumulated cesium-137 deposited over Europe in the first post-accident week varied from zero (e.g., southern Spain, Sicily), extensively in the range of 1,00010,000 Bq m^-2 over western Europe, and in the range 10,000-100,000 Bq m^-2 and more in the U.S.S.R. Soil deposition outside the U.S.S.R. was, not surprisingly, highest in the nearer countries in the path of atmospheric drift. Levels reached 200,000 Bq m^-2 in some relatively local areas of Sweden (82).

The physico-chemical form of a radionuclide in fallout depends on the nature and magnitude of the accident, the particular radionuclide, atmospheric conditions during transport, local weather on deposition, and time between emission and deposition. Studies of atmospheric samples collected in Switzerland (152) showed that iodine-131 was mainly in the gas (vapour) phase while cesium-137 and ruthenium103 were apparently ejected and persisted as particles, later becoming attached to aerosols which could grow by coagulation. The latter could account for the presence of distant 'hot particles', e.g., of ruthenium-103 collected in Austria (115). However, the explosive emission at Chernobyl of radioactive debris evidently also included physically-persistent 'micron-size uranium oxide fuel particles' carrying both fission and activation products (163).

Emission of the chemically-inert radioactive gases krypton-85 and xenon-133 would, of course, remain in the gaseous form and would not appear in the soil deposition. Most of the fallout on soil was associated with rainfall as 'wet deposition' (164) although cases of 'dry deposition' had been observed (88). Relatively soluble vapours, if not already chemically combined, would tend to be dissolved in the water of rain or snow (see Fig. 4 in Part 1).

Above all, Chernobyl confirmed that a major accidental release into the atmosphere can lead to significant radioactive fallout on ail international and even global scale which, fortunately, can be detected unambiguously with enormous sensitivity at levels far below those of any conceivable biological significance. Post-accident meteorological and local weather conditions played a critical role in location and level of fallout. Observed deposition was closely related to precipitation events and to the well-known local variability of rainfall, e.g., between valley and hill. Fallout in Japan was found largely in rainfall (153), and the highest fallout report in the U.K. (>6,000 Bq m^-2) was associated with 'exceptionally heavy rainfall' (147) during an unusually 'wet' year (165). Likewise, high fallout occurred during heavy thunderstorms in Europe (150). As stressed earlier these observations strongly indicate scope for a considerably improved and coordinated weather forecasting and monitoring network for giving some advance warning for agriculture of accident fallout episodes in future (see Section 1.2.4). The local variability of precipitation would also account for 'hotspot' deposition (not to be confused with 'hot particles' - see above) above local average levels as observed at various locations in Europe (158).

2.4 Biological significance of contaminated soils

2.4.1 General considerations
2.4.2 Radio-ecotoxicology
2.4.3 Occupational and public health implications

2.4.1 General considerations

International authorities have identified three phases in relation to the accidental release of radioactive material: The 'early phase' which includes the short time before the accident when a clear threat of a significant release has been recognized at the site; the 'intermediate phase' or critical post-release period which may dictate off site intervention in the interests of public health, etc.; and a final 'recovery phase' or the extended time to the point when a return to 'normal' conditions can be assumed (44; 205; 90; 81).

These phases have very important implications in relation to soil contamination and to the protection of soil-dependent products and communities. These are briefly indicated seriatim below. (They can be compared with the natural disaster 'warning', 'crisis', and 'rehabilitation' phases as a result of floods, hurricanes, drought, etc. recognized by FAO (Personal communication, M.F. Purnell, Land and Water Development Division, FAO, Rome).

Early Phase : It is here that, on the basis of responsible and prompt notification nationally or internationally, agricultural authorities could be alerted to fallout possibilities - especially in areas whose juxtaposition with nuclear installations indicated potentially high fallout (see Sections 1.1.5; 1.2.4 and 1.4.3).

Intermediate Phase: On the basis of an improved communication network extending to farm community level, coupled with an internationally coordinated nuclear and meteorological assessment, to give warning of likely fallout episodes. This would, surely, provide for possibly valuable anticipatory measures as earlier indicated (Section 1.2.4). Such measures would, of course, depend on appropriateness, practicability, and upon economic considerations.

Recovery Phase: At farm level this will involve local monitoring of soils, crops, and livestock so that, on the basis of established radioecology, any necessary extension of countermeasures could be planned and implemented.

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