2.5.2 Uptake by crops

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Except in cases of very high contamination by fallout the significance of soil contamination will be almost entirely owing to the associated or resulting contamination of crops, pasture, and, thereby, possible movement of radionuclides into foodwebs and the human diet.

Two mechanisms need consideration:
- Direct interception of the fallout by aerial biomass of the plant;
- Uptake from the soil through the plant root system.

Above ground interception

The interception factor (r) is defined as the proportion of total (wet + dry) deposition retained by the aerial biomass as follows:

(Eq. 2)

where m = the "absorption coefficient" expressed in m² kg^-1 dry weight of aerial biomass while Y_V= dry weight density of aerial biomass expressed in kg m^-2 of soil area.

Equation (2) implies that r must lie between 0 and 1. Clearly, the absorption coefficient will be higher for leafy vegetable (e.g., cabbage) than sparsely-leaved crops (e.g., tomato or onion). Moreover, it is important to note that while most of freshly harvested vegetables might be consumed, only the fruits of tomato plants, nut and pome trees, etc. would be consumed.

The value of r tends to vary as follows:

Wet deposition > dry deposition
Dry deposition on wet grass > dry deposition on dry grass
Small particulate deposition > large particulate deposition

The interception factor has been especially applied to pasture grass where yields in the range of 0.25-1.0 kg dry weight m^-2 might be expected and m has been assigned the value of 2.8 m² kg^-1 (77, Vol. I, p. 21; 13, p. 331).

It follows that these grass yields respectively imply corresponding interception factors of 0.5 and 0.9. In short, at high yields there would be high interception. However, it is obvious that weather conditions could also play an important role in practice. For example, a fallout episode followed by continuous but uncontaminated rainfall would move a higher proportion into the soil that one followed by dry weather.

The important implication of interception is that the harvest of recently exposed vegetable, fruit crops, etc. could lead to a relatively high dietary intake. This, indeed, accounted for extensive constraints across Europe, soon after 'Chernobyl', on the consumption of fruit and vegetables, and upon the use and movement of cattle grazing over contaminated pasture (174; 175; various post-accident press reports, etc.).

Forest cover would also be associated with high interception (77, Vol. I, p. 23) but relatively small proportions would be likely to become absorbed by fruit, nuts, etc. Much of the fallout intercepted by conifers was expected to become ground litter after 'Chernobyl' (14). The possibility of high interception of longer--lived gamma-emitting radionuclides (e.g., cesium-134 and 137) by timber stands for paper pulp, and, possibly, become a source of external irradiation to readers of the printed paper does not appear to have been investigated.

There is evidence that under conditions of prolonged fallout most of the activity of herbage would be due to foliar uptake rather than from the soil (77, Vol. I, p. 23; 59, pp. 5-6).

Root uptake. Uptake through the root system has also been the subject of wide ranging studies which illustrate enormous variability (77, Vol. I, pp. 137-150 & p. 331; 176) and, therefore, the difficulty of establishing reliable models. This notwithstanding, "it now seems to be standard practice during the estimation of the extent of radionuclide uptake by plants to adopt a single transfer ratio of plant activity/soil activity" (77, Vol. I, p. 26; 177, pp. 295-318).

The "standard transfer factor" (TF_S) has been defined as:

(Eq. 3)

as measured under 'standard conditions' involving the top 10 cm of soil under grass and top 20 cm for other crops, organic matter content of 10% for grass, 4% for other crops, soil pH of 6, total contamination period of 2 years.

The actual TF value (TF_A) has been related to TF_S as follows:

(Eq 4)

where OM is organic matter-%, D is soil depth-cm, y is contamination period-yr. The 'correction factors' a, b, c and d vary according to the radionuclide and class of crop and have been tabulated elsewhere (178).

Some values are illustrated below:

Some correction factors for equation - 4 (from 178):

Equation (4) provides for adjustment of TF on the basis of differences between the conditions of their measurement or application and those defined as "standard". A useful summary of "standardized" TF values has been prepared for cesium-134/137 (146; see also Table XII).

Combined mechanisms of uptake. Crop contamination may involve simultaneous uptake from the soil via the root system and from continuing fallout by interception. Soil-plant TF values can be measured under sheltered conditions to eliminate interception. Alternatively, the difference in isotopic ratio (see Section 2.3.3) between the aged radionuclide in the soil and the corresponding ratio of the newly intercepted fallout may be a sufficient indicator (e.g., 59, p. 6). Recognition of the combined effects can be important. Thus, apparent TF values for plutonium-239 under the conditions of continuing fallout (e.g., in the proximity of a nuclear processing plant) were of the order of 2 x 10^-2. However, when uptake only occurred from the soil TF values of 5 x 10^-4 were obtained (77, Vol. IV, p. 34).

There is a large body of data on transfer factors based on studies with soils contaminated by the earlier nuclear weapons test fallout or by the experimental addition and mixing of known levels of radionuclides in the test soils. A few data are illustrated in Table XIII (based on ref. 144) and Table XIV (based on ref. 179).

TF values are usually expressed as ratios on a dry weight basis for soil and plant. However, it is often useful to convert TF values for the wet weight of product as would be harvested, retailed fresh, or consumed, while still retaining the dry weight basis for the soil (146). For this purpose note:

(Eq.5)

where MC is the %-moisture content of the plant product.

A wide range of TF values under 'standard conditions' for the radionuclides Am, Co, Cs, Mn, Np, Pu, Ra, Sr, Th, U. Tc, and Zn for sandy, loamy and clay soils have been tabulated (178); likewise an extremely comprehensive collection of related data (77, Vol. VI).

Some TF values under standard conditions (from ref. 178) are compared in Table XII below:

TABLE XII
Some 'standard' soil-plant transfer factors (TF_S) (from ref. 178, pp. 11 & 12)

Note: See Tables XIII and XIV for additional data on Pu isotopes. See equations 3 and 4 for adjusting to non-standard soil conditions. Use equation 5 for calculating TF values on 'wet weight' basis; for this purpose a moisture content of about 20% can be assumed for harvested cereals, 75% for other crops.

TABLE XIII
Transfer factors determined with soils (dry wt. basis), plants (fresh wt.) in U.K. contaminated by fallout from nuclear weapons testing (from 144)

Note: Moisture content of wheat 20-25%, potato 70-80% (for Eq. 5).

TABLE XIV
Transfer factors determined with experimentally contaminated soils typical of EC countries (from 179)

Note: OC = Organic carbon, OM = organic matter. Moisture contents of freshly harvested cabbage or potato would be expected to be in the range of 70-80% (re equation 5).

The many plant uptake studies (see especially 62; 73-77; 59) suggest some general conclusions. For example:

Cesium-137 tends to accumulate in certain but not all species of fungi, including edible species. Lichens, mosses, etc. tend to accumulate and to retain relatively high levels of fallout radionuclides as a result of their shallow soil or equivalent matrices with corresponding higher movement into the dependent fauna, e.g., grazing, reindeer (184).

The plant uptake of the relatively radiotoxic actinides tends to be very low (e.g., see Tables XI to XIV).

It can be assumed that approx. 10% of iodine-131 in soil is available for plant uptake but has limited radiological significance because of its short radioactive half-life (77, Vol. III, p. 324).

The stage of plant growth can be very important in a fallout episode. E.g., "Cereals will be most affected if there is deposition between ear emergence and harvest as floral contamination can then occur" (59, p. 82).

Many of the soil-plant transfer data show great variation, including instances where TF values may greatly exceed unity and suggest bioconcentration in the plant (e.g., of radiostrontium - see ref. 77, Vol. I, pp. 137-150). They also indicate some possibly important anomalies, e.g., higher TF values determined after fallout deposition than those determined under experimental conditions (176).

As would be expected, uptake depends upon species and variety - other things being equal. For example, more strontium-89/90 is taken up by legumes than by fruit or cereal crops (77, Vol. I, p. 126).

2.5.3 Scenario for movement into food

TF values (see above and Section 2.5.4 below) have been used for some conservative estimates (i.e., conservative on the basis of a given deposit), by the writer, of dietary uptake of cesium-137 in the immediate years after "Chernobyl" on the basis of earlier data and assumptions (e.g., 88 and associated appendix) as follows:

(a) Effective total soil deposit (SD) from fallout = 10,000 Bq m^-2 (cf. Section 2.7.1);

(b) Effective weight of dry soil (W) in crop root zone - 250 kg m^-2; or SO kg m^-2 for pasture.

(c) Effective soil contamination level in root zone (SC) = (SD)/(W) = 40 Bq kg^-1 or 200 Bq kg^-1 for pasture.

(d) Average "food basket" for "central Europe" applicable;

(e) Lake or reservoir water contamination level (WC) at effective average weight of 5,000 kg water m^-2 = 2 Bq kg^-1, i.e., 10,000/5,000 (it is likely to be lower because of adsorption and sedimentation factors, but, in the case of cesium-137, possibly not very much lower (see ref. 77, Vol. I, pp. 389 et seq.).

The contamination level (CL) of basket commodity is given by the equation (Cf. equations 3, 4 & 5 above):

CL = TF (fresh weight basis) x SC

Additionally, the following relationships have been adopted (see ref. 88, appendix and Section 1.2.2 of Part 1 for fish/water bioconcentration factor):

CL (cow's milk) = 0.6 x CL (grass or fodder)
CL (beef) = 1.5 x CL (grass or fodder)
CL (lamb meat) = 15 x CL (grass or fodder)
CL (goat's milk) = 3 x CL (grass or fodder)
CL (goat meat) = 7 x CL (grass or fodder)
CL (lake fish) = 1,000 x (WC)
CL (drinking water) = WC (Contamination level of reservoir- derived drinking-water and that of exposed fisheries assumed to be identical, i.e., 2 Bq kg^-1. That of the ICRP "standard man" is assumed below for the annual intake of drinking water - see ref. 19.

Results of estimates based on the above assumptions and upon the higher TF-values variously reported are indicated in Table XV below. Comparably-related orders of contamination levels following a specified deposit of cesium-137 have been indicated elsewhere (123, p. 170).

More sophisticated models have been used for developing dietary intake scenarios following a single "spike" fallout episode or under the conditions of "continuous uniform" fallout (79; see also Section 2.5.6).

TABLE XV
Conservative scenario for poet-' Chernobyl ' dietary intake of cesium-137 in 'Central Europe' (estimate by author)

Application of the appropriate radiation dose conversion factors in terms Sv Bq^-1(e.g., see refs. 181; 182; also Section 3.1) indicates the average effective dose equivalent (EDE) commitment. Thus, the total Cs-137 intake indicated above corresponds to an EDE of less than 0.4 mSv for that year of intake on the basis of a radiation dose conversion factor of 1.5 x 10^-8 Sv Bq^-1 (182, Part 1. Suppl., pp. 236 & 237). The relatively high potential contributions from lake fish and drinking-water should be noted. It should also be noted that the estimates above neither take into account pre-Chernobyl base-line levels of soil Cs-137 (see above), nor the possibility of some post-'Chernobyl' direct crop interception of further fallout or dry deposition of contaminated soil particulates. The scenario above illustrates the nature of the problems involved when estimating dose equivalent commitments as a result of radionuclide intake as food and drink (see also Sections 2.5.4; 3 and 4).

Finally, while such projected EDE values can indicate the suitability of a soil for continuing or modified agriculture or, likewise, the suitability of water for fisheries - such indications are no substitute for on-site monitoring in emergency situations as already stressed (Section 1.2.3).

In relation to the scenario depicted in Table XV above it may be noted that a ground deposit of 50,000 Bq m^-2, for example, would have implied an average dose commitment of 2 mSv for that year of intake. On the basis of the "long-term" post-Chernobyl dose intervention level (see Section 3) of 1 mSv adopted in Sweden (82) this would, then, have indicated the need for intervention by constraint on fish consumption. Moreover, if the higher dose conversion factor of 10-7 Sv Bq^-1 were adopted, as was the case in Sweden (ref. 82, p. 8) on the basis of 1-year old infants being the critical population group, such constraint at lower levels of ground deposit would, accordingly, have been indicated (see Section 3.1).

2.5.4 Movement in foodwebs

As indicated earlier (Section 2.4.2), when human health is adequately protected, radiation effects on soil-dependent wildlife may, indeed, be of interest but unlikely to be of concern under the conditions of an accidental release (183; 213).

The movement of radionuclides in foodwebs per se does not lie within the scope of this review but certain aspects are relevant here. A classic and still useful review of this subject was prepared many years ago by Scott Russell and his colleagues (53).

The significance of plant or grazing livestock uptake by direct interception or indirectly from the soil through crop, pasture or livestock drinking water will depend upon time, and upon post-harvest and beyond-farm gate treatment. Thus, food processing may involve washing, milling and extraction of cereals, dilution with uncontaminated products, etc. Drying and concentration processes, on the other hand, (e.g., for milk powder and concentrates) may involve a corresponding concentration of the radionuclide which may or may not be reversed before human consumption (see Table IV and Sections 3.1 and 3.3). The time factor will largely eliminate the significance of short-lived radionuclides (e.g., iodine-131, manganese-56) in processed or stored foods, even when prepared from products harvested immediately after fallout. Likewise, each stage of the food chain or web will also be likely to result in reduction on transfer. This reduction can be very high. Thus, the movement of ruthenium-106 through the pasture-cow-milk-man chain was very low, less than 0.1% being absorbed through the rumen of the lactating cow and less than 0.002% being secreted in the milk (77, Vol. I, p. 297). In any event, these problems would be the responsibility of the food industry and public health authorities. However, in aquatic foodwebs, transfer may involve bioconcentration (see below).

There may be problems for local communities living more directly "off the land", and for local communities in 'developing' countries where land might be exposed to the fallout from an out-of-state release and where there may be no technical infra-structure for the detection and monitoring of fallout and its possible health significance (see also Section 1.1.5 in connexion with 'maps').

In Sweden, wildlife (fish, reindeer)-dependent communities were seriously affected by 'Chernobyl' fallout on account of high movement into reindeer and bioconcentration by fish of inland lakes (184). Here, however, a monitoring infrastructure existed and was rapidly mobilized (82). The movement of strontium-90 and cesium-137 in the food chain lichen-reindeer or caribou-man has been especially studied in relation to Lapp communities (59, p. 145).

Of interest here is a post-'Chernobyl' report (185) that pickling in brine of Cs-137 contaminated meat successfully reduced levels in the meat proper from 570 Bq kg^-1 to 70 Bq kg^-1 (i.e., by more than 85%) after three weeks.

The term "transfer coefficient" (F_m) has been introduced (186) for relating radionuclide concentration in milk (Bq kg^-1) to the daily feed intake of a lactating cow. Using the terms defined above (section 2.5.3):

A mean steady state value of 0.004 was assigned (186) to F_m for cows fed alfalfa contaminated by fallout cesium-137. Since 50 kg day^-1 would be a high (wet weight) intake rate (Comer in ref. 53, pp. 127-156), the above equation implies a factor of 0.2 in the relation used on in Section 2.5.3 and confirms the 'conservative' nature of the factor 0.6 adopted for the 'scenario' above.

The Fm concept was developed as a less variable transfer coefficient for radionuclides in the feed - milk step of the food chain over the longer periods of time when milk and tissue radionuclide concentrations had "equilibrated". Reference has been made to some of the problems of decontaminating freshly harvested fruit and vegetables, and to the effects of food processing "beyond the farm gate" in Section 1.2.4.

Contamination of inland fisheries would result from direct fallout. There could also be transfer from contaminated surface soils by erosion and run-off in river and lake catchments. The accumulation and bioconcentration by fish can be high. For example, of cesium-137 in oligotrophic lakes (77, Vol. 1, pp. 389 et seq.). Bioconcentration of cesium-137 decreased with increasing concentration of natural potassium in water. It was concluded that a bioconcentration factor of 1,000 for cesium should generally be assumed for the food chain freshwater lake-fish-man (21, pp. 473-481).

The possible transfer from catchment soils to inland fisheries will, clearly, depend upon erosion, post-fallout precipitation and run-off. Radioactive particulates will, therefore, tend to have become exposed to water before reaching the water body and retain contamination until sedimentation (71).

Finally, in this Section, if there be a short period of high radionuclide intake by a local community as a result of using freshly harvested exposed vegetables, etc. this is likely to represent a single input. Under these circumstances the "effective half-life" of the absorbed radionuclide (see footnote to Table I) would be very much less than the radioactive half-life. Thus, the effective half-life of cesium-137 (radioactive half-life - 30 yr.) in man would be expected to lie between 45 and 135 days for "most individuals" after the cessation of input (77, Vol. I, p. 375; see also footnotes to Table I).

2.5.5 Leaching

While any biological effects of a radionuclide will be due to the emitted radiation, leaching down the soil profile will be a matter of its physical-chemical form. The quantitative study and prediction of radionuclide behaviour parallels that of chemical solutes, e.g., of fertilizer-derived nitrate (92). However, the study of radionuclide leaching is facilitated by the ease and sensitivity by which it can be instrumentally followed.

Methods for the quantitative study and modelling of radionuclide migration down the soil profile under conditions of leaching have been usefully discussed and compared by Bachhuber et al. (72) as briefly summarized below:

The critical parameter in the migration of a radionuclide (or any solute) down the soil profile with water infiltration is the "retardation factor", R (reciprocal function of the Rf value in partition chromatographic analysis), defined as:

R = W/N (Eq. 6)

where W is the average net downward migration velocity of the pore water, and N that of the radionuclide.

In the dynamic method, R is determined experimentally with columns of soil. In the static method, R is calculated on the assumption that the chromatographic type partition of radionuclide between mobile water phase and solid soil phase (i.e., by reversible sorption) is applicable. The chromatographic type "distribution coefficient", K, is defined as:

K (cm³ g^-1) = Bq g^-1 soil Bq cm^-3 pore water (Eq.7)

and:

R = 1 + d.K/V (Eq. 8)

where d is the bulk density of dry soil and V the volumetric water content. Measured radioactivities in Bq are, of course, adjusted for significant radioactive decay between measurements.

In the fallout method R is calculated by applying a conventional model to the distribution observed in the soil profile after the known period of time since the effective deposition by fallout.

These three methods were applied to 'ranker, podsol, and cambisole soils (FAO classification) to obtain the comparative values of K for the different horizons of each soil. For the dynamic and static methods, carrier-free strontium-85 and high-specific-radioactivity cesium-137 were used in the presence of tritium-labelled water as a water-front and--movement tracer. The fallout method was applied in 1978 to the residual strontium-90 and cesium-137 in the same kinds of soil exposed to the much earlier nuclear weapons testing fallout.

A few data for the cambisole are illustrated in Table XVI (from 72). The respective average migration rates (estimated from the original data by the writer) for strontium and cesium roughly parallel the respective values of 1.7 and 1.0 cm per year given by the original authors (72) by their fallout method and imply an average effective (or excess) rainfall in respect to infiltration of cat 40 cm per year in the top 9-cm horizon under the field conditions after fallout. These investigations confirm the more rapid migration of strontium than cesium observed by many workers but also suggest limitations of the static method.

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