1. Behaviour and significance of radioactive substances released into agricultural, forestry and fisheries ecosystems

Contents - Previous - Next

1.1. Radiation in the environment

1.2 Radioecology, behaviour, significance, models

1.3 Chernobyl and other accidents

1.4. Some international implications

1.5 Summary

1.1. Radiation in the environment

1.1.1 Natural levels
1.1.2 Manmade (anthropogenic) sources
1.1.3 Radioactive wastes and discharges
1.1.4 Comparative radiation exposures under normal conditions
1.1.5 Accident sites and accident potential

1.1.1 Natural levels

Radioactive substances are natural components of the entire biosphere, and some are constantly formed de nova in the atmosphere as a result of cosmic radiation (6) from outer space (especially carbon-14 and tritium or hydrogen-3).

Since all life on this planet has evolved and, evidently, flourishes in this radioactive environment it is useful to quantify these natural levels of radioactivity. This not only indicates "natural background levels", which must be known before trying to detect and measure anthropogenic increases, but it also indicates radiation levels which must surely be regarded as "acceptable". Indeed, like the air one breathes, there is no known alternative to acceptance on this planet.

Before the testing of nuclear weapons in the atmosphere, the latter contained radioactive carbon-14 (as carbon dioxide) equivalent to approx. 0,25 Bq g^-1 carbon. With the testing of nuclear weapons in the atmosphere in the '60s the level reached a peak of approx. 0.45 Bq g^-1 carbon but it later declined (7; 8; for units and terminology see Section 4).

Typical soils have been indicated to contain approx. 3 x 10⁵ Bq m^-2 of the natural radioactive isotope potassium-40 to a "rooting" depth of 20 cm (2). Seawater contains approx. 10 Bq 1^-1 of potassium-40 (2, 9). Food, feed, fruit and vegetables have been reported to contain approx. 50 - 150 Bq kg^-1 which is in line with the known potassium-40 content of natural potassium of 30 Bq g^-1 (see also Section 2.3).

Rocks (and soils) and, therefore, derived building materials can contain certain natural "primordial'' (as distinct from "cosmogenic") radionuclides of uranium, thorium, and actinium which show complex radioactive decay patterns with the formation of radioactive "daughters" such as radium-226 and the chemically-inert but radioactive gas radon-222. Levels up to approx. 10⁴ Bq m^-2 of radium in soils (to rooting depth) have been reported (2). Levels of natural radium-226 in food and water have been indicated in the following ranges (5):

Levels, distribution patterns and anthropogenic changes in the environment are reviewed elsewhere (6), briefly discussed below, and in more detail in Part 2 of this review.

1.1.2 Manmade (anthropogenic) sources

Since 1945 there has been a large increase in the production of artificial radionuclides, originally with electrical sub-atomic particle accelerators (e.g., the cyclotron), with small radium-beryllium neutron sources, but, above all as a result of uranium fission and by neutron irradiation by the core of research or power-plant nuclear reactors.

In the context of accidental releases there is no doubt that the fuel elements of a stationary or mobile (ship or submarine) nuclear reactor represent the greatest potential source of unintended contamination of the environment. There is, additionally, the possibility of an accidental release of fissile material and fission products as a result of an accident involving nuclear weaponry, which is outside the scope of this apraisal, as is the possible release of radioactive substances as a result of war or terrorism. These considerations, however, do illustrate the urgency and importance of extending and implementing fully the "Non-Proliferation Treaty (NPT)" in which IAEA plays a central role (10).

The range and quantities of radionuclides which accumulate in the operational reactor core, or in structural or circulating media (e.g., cooling water) as a result of incidental neutron irradiation are well known. The fission products consist largely of elements of mass numbers lying in the range 80 - 160. Neutron activation products are those elements capable of effective "neutron capture" and which are temporarily or permanently resident in the reactor's containment structure, plumbing, circulating media, etc. These activation products include carbon-14 (from nitrogenous impurities in carbon dioxide-cooled reactors), manganese-56, cobalt-60, zinc-65 and the "transuranium" elements such as the plutonium isotopes-238 to 242 (2; 6; 11, 12) which are present within the reactor fuel.

While fission and radioactive products are produced at a constant rate under constant operational conditions some radioactive products reach a "steady-state" level. This level is reached when the rate of their radioactive decay balances their rate of production. Long half-life radionuclides, however, tend to accumulate continuously with the operational life of the reactor fuel elements. For these reasons the radiochemical composition of the reactor eve changes with time, as well therefore, as the range and levels of radionuclides to be expected in a post-accident release or fallout event.

The radionuclides most likely to appear in the environment as a result of an accidental or explosive release from a nuclear reactor are listed in Table I. The radionuclides of high potential significance to agriculture, food, feed, forestry and fisheries are marked. The selection takes into account earlier studies (2; 5; 11; 12; 13; 46) and some of the many ongoing ones following "Chernobyl" (14 - 18).

1.1.3 Radioactive wastes and discharges

Attention has been drawn to accumulation and production of radionuclides in reactors and their distribution for various uses. This implies local additions to the natural radiation levels of the human environment and releases of some radioactive wastes or by-products into the environment. However, because of their potential biological significance (see Section 2.4.1), the processing and transport of nuclear fuels and wastes, and the distribution of radioisotopes for use in medicine, research, and industry have probably been studied, and better controlled than any other kind of potential physical or chemical threat to human health. Likewise, a great deal of attention has in most cases been given to the siting of nuclear reactors, safety measures, and to the methods and routes for the transport, storage and disposal of radioactive wastes (23-30; 31; 32; 214).

Of importance when considering wastes is some understanding of the "nuclear fuel cycle" and the logistics for radioactive materials from the initial uranium ore mining operations, enrichment, fuel assembly, removal of spent fuel, reprocessing, etc. Descriptions of the "nuclear fuel cycle" in relatively non-technical language are available (29; 33; 34).

Table I
Radionuclides that may be released in a nuclear accident and with potential significance for agriculture, forestry and fisheries following a nuclear reactor accident

- FP denotes Fission products. AP denotes Activation products.
- "Daughter" isotopes of fission products, etc. not included.
- ²³⁹Np and ²³⁸Pu are Transuranium activation products or 'actinides'.

- The radioactive half-life (T_R) indicates the time needed for the level of radioactivity to decrease by one half as a result of radioactive decay alone (e.g., in a sealed sample of soil or water).

- The radioactivity of any radionuclide by decay alone is reduced to less than 0.1 % after ten half-lives. The reduction will be less than 10 % in 24 hr. for any radionuclide with a radioactive half-life of one week or more.

- The effective half-lives (T_E) of many critical radionuclides in livestock, as in man, may be very much less than the radioactive half-life as implied by the equation below. Thus, in the cow the effective half-life of 137_CS is of the order of 20 days, and 2 - 3 days in the goat (see ref. 5; p. 107). It follows, for example, that if a cow grazes on 137Cs-contaminated pasture for a limited period (e.g., for a month when the 137_CS may have had time to move into the soil where it becomes relatively unavailable to the roots of growing crops or grass) the 137_CS will tend to leave the cow on the basis of the effective half-life of 20 days. Therefore say, 3 months (approx. 4.5 effective half-lives) the level of 137_CS in the meat or milk would be down to approx. 6 2 of its initial peak value and down to less than 0.5 % after 6 months. However, if the cow continue to be exposed to 137_CS contaminated pasture or contaminated stored feed the 137_CS level will tend to reach a "steady-mate level'. when the rate of intake will be balanced by the effective rate of elimination.

T_E = T_R * T_B/(T_R + T_B)

where T_B is the biological half-life (see ref. 19, pp. 33-34).

1.1.4 Comparative radiation exposures under normal conditions

The protection of human health is, clearly, the main purpose of radiological protection. Adverse effects of ionizing radiation must be due to radiation absorbed from an external (i.e., to the body) source such as environmental radionuclides, an X-ray machine, or from an internal radionuclide absorbed through the skin, inspired by the lungs, received in food and drink, or deliberately inserted for medical purposes (e.g., for the treatment of a malignant growth).

All human beings, either individually or collectively, are now exposed to ionizing radiation from anthropogenic sources as, indeed, they are also exposed to environmental chemicals: Dusts from mining, construction, demolition, the chemicals used in agriculture, for food preservation, emissions from motorized transport, fossil-carbon based heating, industrial effluents, etc. (35; 36; 37).

The significant question here is: What proportions of the total average population dose of radiation are respectively due to natural sources and the various anthropogenic ones ? This question has been studied and answered quantitatively, notably by UNSCEAR (13; 38) and several excellent publications in non-technical language are available (39; 40). The results are briefly illustrated in Table II.

TABLE II
Approximate relative contributions to the average annual effective dose equivalent from all sources.

Total exposure is equivalent to approx. 2.5 mSv yr^-1.

An interesting result of more recent studies of exposure to natural radiation is the variation according to location, social habits (e.g., frequent flying), and that poorly ventilated spaces may accumulate significant increases in radon gas (²²²Rn) from ground and building materials (38). Clearly, individual exposure to man-made sources will also depend on behaviour, health, occupation, and proximity to routine and accidental releases and this may involve large individual variations.

An important factor in exposure after a nuclear accident would be through contaminated food and drink - even at great distances from the accident site and across international boundaries. Therefore, monitoring the agricultural, forestry, and fisheries environment and restriction of the movement and export of possibly contaminated products, whether food, feed, components of drink, fibre products such as cotton and paper, will be important. This, in turn, can affect (as did the Chernobyl accident) the land, practices, and welfare of the dependent communities. These important problems will be discussed further (see Sections 1.4.2 and 1.4.9).

1.1.5 Accident sites and accident potential

This appraisal is concerned only with accidents under peacetime conditions of potential significance to agriculture, forestry and fisheries and to their dependent communities.

The relatively small quantities of radioactive substances involved, and generally strict controls on account of occupational hazards, suggest that accidents from industrial, medical and scientific research applications could only be of local significance, and unlikely to affect agriculture. There could be localized accidents affecting inland fisheries but these would not likely be on a scale which could not readily be contained.

The available information and experience to date including that of "Chernobyl" indicate that accidents of any potential significance to agriculture, etc., in order of importance would be:

(a) Land-based nuclear power reactors

(b) Mobile marine nuclear power reactors and air or space-vehicles carrying nuclear facilities

(c) Reactors used for research, teaching, and radioisotope production

(d) Fuel processing plants, waste discharges, etc. (especially into aquatic or marine ecosystems)

(e) Mining, storage and transport of radioactive materials as part of the nuclear fuel cycle (excluding (d))

(f) Nuclear-medical, industrial irradiation, and research facilities.

Clearly, the significance of any accident for agriculture, etc. will depend upon location, nature and magnitude of the release, and upon subsequent weather conditions. This highlights a need for a global map pinpointing the sites of all reactors and nuclear installations also showing the proximity of vulnerable agricultural, forestry and fisheries areas within, say, a 150 km radius of each site. Such a need in connexion with possible accidents of relatively local significance had been earlier identified (193, p. 51). Much of the necessary information and data for such a global map are already available (56; 83; 84; 85). Such maps would also be useful to 'non-nuclear' states which could, nevertheless, become exposed to radioactive fallout from a remote trans-boundary source as a result of weather-dependent atmospheric transport and deposition.

The location of nuclear powered ships or submarines could not be indicated by a static map and stresses the vital importance of early notification. A major step forward has been achieved since "Chernobyl" by the recent international Agreement on "Early Notification" in Vienna (41; 42; 43).

An important difference between a land and marine accident is that most of a radioactive release into the sea or into an inland water body would not lend itself to independent detection at a distance by the monitoring networks already established in many countries. On the other hand, the radioactivity would be largely contained, and move relatively slowly. This would delay and, probably prevent any significant contamination affecting agriculture, etc. (71; 210; see also Section 1.2).

1.2 Radioecology, behaviour, significance, models

1.2.1 Significance
1.2.2 Radioecology
1.2.3 Models
1.2.4 Anticipatory- and countermeasures

1.2.1 Significance

Given an accidental release, at least three questions arise: What is the behaviour of the radionuclides within the ecosystem ? What is their significance to human and environmental health ? What can be done about it ?

While its radioactivity enables a radionuclide to be detected, measured, and "traced" or followed within the ecosystem its chemical behaviour will be effectively identical with that of the normal non-radioactive element, or at least similar to the closely-related chemical elements naturally present. Thus, radioactive iodine - 131 behaves as the normal element iodine. Radioactive cesium - 137 behaves approximately like potassium. There are some minor "isotope effects", for example those due to transmutations on radioactive decay (see Section 1.5.2): Except for tritium, the small difference in mass number between the radioactive and stable nuclide can only very slightly affect the speed of chemical and enzymic reactions (47). However, such very small "isotope effects" are of no significance here. In short, while health effects are entirely due to radioactive decay and the emitted radiation, the ecological behaviour of the radionuclide will be entirely due to its chemistry (or biochemistry within a living organism).

As in the context of human health, radionuclides such as tritium and carbon-14, could become incorporated into the DNA structure of living organisms and, theoretically at least, have genetic or somatic effects on animals or plants of the ecosystem (48, 49). However, these radionuclides and their possible transmutation effects would not be significant in the context of effects on agriculture, etc. following an accident.

The chemical weights of fallout fission products and actinides will be so small as to be of no significance in the context of chemical toxicology (or ecotoxicology). This has important implications when considering decontamination problems (see Section 1.2.4). However, because of the relatively high weights produced in the reactor core (34), its potential as a fuel in future "fast-breeder" reactors (33; 50) and its high radiotoxicity (51), plutonium-239 could, conceivably, be an exception in a future serious accident (see also Section 2.4.2). On the other band it does not appear to be transferred rapidly through the food chain to man (52; 53).

The radiation of a contaminated ecosystem, at sufficiently high levels, can be directly injurious to microscopic life, plants and animals. Many studies have been made of these effects, e.g., on the biota of aquatic ecosystems (21). "Fish eggs have been considered one of the most sensitive components of marine biota" (see Nishiaki, et al. in the second part of ref. 21, pp. 195 - 209). These observations were made with aquatic ecosystems deliberately contaminated with radionuclides in the range of approx. 40,000 400,000 Bq1-1 "No positive correlation between mortality and hatching rate of the eggs (Rainbow trout) and the radiation doses received" was observed. The doses ranged up to 0,07 Gy for cesium - 137 and up to 0,37 Gy for ruthenium 106 in a 30-day period of the experiment.

All these studies (see especially refs. 77; 183; 213) indicate that radiation levels so high in a contaminated ecosystem as to affect significantly any or all levels of wildlife, crops, livestock etc. would represent such a relatively serious threat to human health that direct effects on wildlife would have to be ignored in emergencies (54; 55). See also Section 2.4.2 for a more detailed discussion of these aspects.

1.2.2 Radioecology

Radioecology is the study of the behaviour of radionuclides in the entire biosphere, or in some particular ecosystem which could be a small experimental fish-tank, green house, or a 10000--hectare artificial ecosystem such as large hill sheep farm or cereal growing area. Radionuclides may be deliberately added for such studies or may have been the result of fall-out from the testing of nuclear weapons in the atmosphere or the result of a nuclear accident. The steady expansion of the nuclear power industry (56) has sustained an ever growing body of radioecological research and publication (refs. 1; 2; 3-5; 18; 20; 21; 22; 46; 53; 57; 58-62; 63; 64; 65; 66-75; 76; 77). Any detailed discussion of these wide-ranging studies is beyond the scope, or needs, of this appraisal. Instead, some of the significant findings will be indicated briefly.

The principal pathways and sources of internal or external irradiation following an accidental release are illustrated diagrammatically in Figs 1, 2 and 3 which are self-explanatory. Figs 1 and 2 illustrate the role of agriculture and fisheries in the transfer of radioactive contaminants of agriculture, etc. to man. Fig. 3 illustrates the special problem of external radiation for the farming community who spend a great proportion of time working in the open. They may also be especially exposed to external irradiation from harvested and stored products, such as hay (16). The possible transfer of external radiation sources to urban communities in the form of non-food products (e.g. wool, cotton, pulp for paper and news print, timber etc.) is not illustrated. Likewise, 'the generally minor' pathway (124, p. 44; see Section 2.2.2 (d)) of radionuclide transfer by atmospheric re-suspension is not shown in Fig. 1.

Fig.1 Principal pathways by which radioactive contaminants reach man from the air.

Fig.2 Principal pathways by which radioactive contaminants reach man from water.

Fig.3 External irradiation on farm from outside building structures and from barn-stored hay harvested after fall-out.

There is some evidence that fallout radionuclides over pine forests tend to be intercepted by the foliage (14) which will become ground "litter" and not, therefore, enter timber or pulp. Nevertheless, this potential problem may justify investigation.

Despite the range of potentially significant radionuclides (see Table I), in the experience of fall out studies and accidental releases only a few radionuclides have assumed major significance. These arc strontium - 89, strontium - 90 (long term), ruthenium - 106, iodine - 131 (short-term), cesium - 134, cesium - 137 (long-term), and the plutonium isotopes, as largely anticipated three decades ago (53, p. 61; see also Table I and footnote).

The significance of iodine - 131, is due to its rapid transfer to milk and its accumulation in the thyroid gland. On the other hand, its relatively short radioactive half-life preempts the need for agricultural countermeasures beyond some 8 weeks after a single fall out episode.

Freshly deposited cesium - 137 (or, of course, cesium - 134) undergoes relatively rapid transfer from pasture into livestock, meat and milk. May harvested after a fall out episode and later used as feed can result in a delayed or new rise in the cesium - 137 levels of the resulting meat. Cesium 137 tends to become widely distributed in human muscle at the end of the food chain. Strontium - 90 behaves like calcium and is a bone seeker in man. Fallout over mature fruit or vegetable crops ready for harvest represents the highest probability of dietary intake in the absence of suitable intervention (see Section 3). Movement of deposited cesium - 137 and strontium - 90 down the soil profile under conditions of natural precipitation or irrigation tends to be very slow - a mere centimetre or so within months. Both radionuclides can be taken up through the roots of later crops but the availability of soil cesium - 137 is far less (about 1/10) than that of strontium - 90 (see also Sections 2.5 and 2.7). Both radionuclides can appear in harvested cereals after fall out over agriculture land. Milling and "high extraction" reduce their levels in bread.

Only very small fractions of the plutonium isotopes in soil tend to be taken up by crops (dry weight concentrations in the crop less than 1 % of those in the soil).

The extremely low chemical concentrations of fallout radionuclides do not favour direct translocation into the plant from foliar deposits at least not in the very early phase. However, while cesium - 137 in soil is taken up by crops less readily than strontium - 90, the reverse situation applies to their direct absorption from foliar deposits.

Precipitation is a major factor in the transfer of atmospheric borne radionuclides to land and water surfaces. Radionuclides released, intentionally (as waste discharges) or accidentally, into marine or fresh water fisheries become confined relatively locally. However fish and especially aquatic micro-flora may "bioconcentrate" the radionuclides by factors of 1,000 or more especially in oligotrophic lakes. Fortunately, again because of their low chemical concentrations they tend to become strongly adsorbed on particulate suspensions and sediment accordingly (e.g., see ref. 210). This in turn, fortunately, has hindered transport in rivers across international boundaries (71).

1.2.3 Models

Many mathematical models and simple equations have been developed for quantifying the movement and behaviour of radionuclides released into the environment (78; 79). This provides for the very important task of quantifying the relationships between the levels of deposited radionuclides expected in different parts of the ecosystem, times involved, and the overall transfer in the food chain to man. Also in the study and control of routine radioactive discharges (24, 80). "Market basket" and dietary data are, clearly, of vital importance in these estimates, and in the final estimate of radiation dose-equivalent rates or of commitments as a result of diet (81; see also Section 2.5). Computer-based models have provided for the rapid storage, retrieval and handling of the ever-increasing range of radio-ecological data (e.g., 59, pp. 125 - 134; 77, Vol. I, pp. 13 14, Vol. IV, p. 75; 80; 178; 187; 188). For an indication of the units and terminology used, principles of radiation dose estimates, limiting parameters for occupational and public health protection under emergency conditions, see Sections 2.4.3; 2.5.3; 3.1 - 3.3 and 4 (also refs. 40; 44; 81; 182; 205; 211).

In emergencies following accidental releases, models are increasingly being used for predicting likely pathways and, therefore, as guides to monitoring and sampling. They have also provided useful integrated descriptive scenarios in retrospect, e.g., after "Chernobyl" (18). However, because of the immense complexity and variability in any natural or agricultural ecosystem, the vagaries of weather, precipitation, and the unknown factors of any nuclear accident in the very early stages it is obvious that there is no substitute for constant vigilance and emergency onsite monitoring of "critical pathways" (see below), especially of air and precipitation, for radionuclide levels and composition as well as external radiation levels and corresponding dose equivalent rates. It must be on this basis that emergency intervention and countermeasures for agriculture, forestry and fisheries are implemented for the protection of public health.

There are, of course, infinite possibilities for sampling and monitoring for radionuclides appearing within the ecosystem and its complexity of foodwebs following accidental or routine releases into an aquatic system or as fallout over agricultural land. The "critical pathway" approach (e.g., see ref. 22 and 64, pp. 573 - 582 based on radioecological studies identifies the most important radionuclides and their pathways leading to human exposure and, indeed, to exposure of the "critical organs". Thus, "Chernobyl" confirmed the importance of the early monitoring of precipitation, pasture, green vegetables near harvest time, and milk for iodine-131. Its measurement in irrigation water for use on later crops would have been of relatively low priority. In short, the "critical pathway.' approach is important for optimizing the deployment of vital monitoring resources, especially under emergency conditions.

The problems of radioactive fallout over agricultural, forestry, and fisheries ecosystems are very briefly compared below. Those of fallout over cropped soils in particular would represent a much larger scale of potential threat to public health following a major nuclear power plant accident and are dealt with in part 2. For authoritative and detailed comparative studies, references should be made to the comprehensive but pre-Chernobyl work of Coughtrey, Thorne and their colleagues (77), and to the classical work of Scott Russell and his colleagues (53).

The different impacts of the same radioactive fallout episode over soil and aquatic ecosystems have long been recognized (53, pp. 47-50). Firstly, relatively mature crops, tree fruits, etc. could soon lead to dietary intake after harvest, and carry relatively high levels of the deposited radionuclides (see Part 2). However, fallout over a water body must first undergo some vertical distribution, and will tend to be adsorbed on suspended or sedimenting particulates, microbiota, etc. (71; 210), and time will be needed to undergo movement through the aquatic foodchain before bioconcentration or direct absorption by edible fish. In any event, inland fisheries usually represent a minor contribution to the diet of populations generally (ref. 53, pp. 63-82). For these and other reasons, aquatic food chains were earlier considered to be a relatively "minor source" of human exposure to radioactive fallout (53, p. 49). For these same reasons, time could diminish or enhance the radiological significance of dietary fish (53, pp. 385-392) and, once again, underlines the importance of post-accident onsite monitoring (see above). However, as already mentioned radionuclides can undergo high bioconcentration under oligotrophic conditions (21, pp. 473-481) as confirmed by post-Chernobyl experience with certain inland fisheries in Sweden (82). This suggests that the problems of inland fisheries exposed to post-accident fallout will justify further study and monitoring in future despite earlier viewpoints (see also Part 2).

Radioactive fallout as "wet" or "dry" deposition over agriculture, forestry or fisheries is illustrated diagrammatically in Fig. 4 and summarized in the footnotes (see also part 2).

FIG.4. Diagrammatic illustration of 'wet' and 'dry' deposition of radionuclides over agriculture, forestry, and fisheries.

A - Closed forest, dense tree cover, high foliar interception, soil protection by bound litter, exposed wildlife

B - Fruit trees, relatively high foliar- low fruit-interception.

C - Crazed pasture, high livestock intake after 'spike' deposition.

D - Greenhouse, potted plants only exposed to 'dry' deposition through ventilation.

E - High interception by mature green vegetable cover, potentially rapid entry into food basket. Potential soil protective cover.

F - Bare fallow after harvest and ploughing, maximal exposure of soil to subsequent deposition. Rapid vertical dispersion down the soil profile to ploughing depth after prior deposition.

G - Delayed but high bioconcentration by fish in oligotrophic water bodies. effective removal of radionuclides by particulate adsorption and sedimention but bottom feeders relatively more exposed.

Contents - Previous - Next