INTERCEPTION AND WEATHERING

4.1.1. Mass interception of Fukushima fallout radionuclides by edible parts of herbaceous plants

Radionuclides released from the FDNPP accident were deposited onto many agricultural areas of Eastern Japan resulting in contamination of crops with radiocaesium and radioiodine. Fresh green vegetables need to be harvested when they are still tender to ensure that they are suitable for consumption in salads or for preservation by processes such as pickling. Following the deposition of radionuclides onto green vegetables, the initial radionuclide activity concentration of the vegetables is described by the interception factor, which quantifies the amounts of deposited radionuclides retained by the edible parts of the plant. Therefore, the interception factor is a key quantity needed for estimating the exposure arising from ingestion of recently contaminated fresh vegetables. Estimation of the time-dependence of radionuclide activity concentrations in green vegetables in the emergency response phase and the transition phase greatly assists the management of such crops. In combination with data from radionuclide monitoring of agricultural plants, it facilitates an optimized implementation of countermeasures and further implementation of remedial options. Retention goes beyond the discussion in this section and it is partially discussed for fruit in sub-section 4.3.3.

Interception of radionuclides by vegetation depends on the vegetation density, the contributions of dry and wet deposition to the total deposition, the chemical properties of the radionuclide and the amount of rainfall (if wet deposition contributes to the total deposition) [4.1]. In this section, mass interception data determined after the FDNPP accident are summarized and compared with those previously reported in international literature.

4.1.2. Approach used to estimate interception

To calculate the interception fraction following a deposition event, data are needed on the standing biomass (kg/m² DM) for converting the radionuclide activity per unit mass (Bq/kg DM) into the radionuclide activity per unit area (Bq/m²). For wild plants, the radionuclide activity per unit area is difficult to measure because several wild species often grow together in the same area. For vegetables, monoculture is generally used making it potentially simpler to measure the standing biomass per unit area. However, during the initial few days after an accident it is unlikely that time can be devoted to the collection of such data.

Therefore, instead of the theoretically preferable interception based on area, the mass interception fraction (𝑓 ) is more often determined which is defined as the ratio between the radionuclide activity concentration in vegetation (Bq/kg DM) and the total radionuclide activity deposited per unit area (Bq/m²) as defined in IAEA TRS 472 [4.2]. The approach avoids the need to determine the standing biomass per unit area of a plant. The normalization of the initial radionuclide activity in plants to the total deposition is especially useful if 𝑓 is determined specifically for edible parts of fresh vegetables, since it facilitates the estimation of the internal dose from the ingestion of foods.

The interception parameter is important as a starting point for estimating the time–dependence of radionuclide concentrations in plants. Whereas in TRS 472, the mass interception factor was applied to the total above-ground part of the crop, in this section the interception parameter has been applied specifically to the edible part of fresh vegetables such as broccoli and spinach. For these plants, such a simplification is appropriate because the edible parts are a major fraction of the standing biomass. Since the monitoring of fresh vegetables did not start immediately after the deposition occurred, the initial radionuclide activity concentration of green vegetables was back calculated from monitoring data.

4.1.3. Weathering half-lives for calculation of mass interception

Radionuclide activity concentration in plants that arise from interception subsequently decline due to what is commonly termed a weathering effect which encompasses all processes that lead to a loss of radionuclide contamination from the surface of plants. Biomass dilution due to plant growth can also contribute to the time dependence of radionuclide activity concentrations in crops. The decrease, due to weathering and growth dilution, is normally characterized by a single exponential function, which is quantified by determining the effective half-lives, 𝑇_eff, of radionuclides in plants, especially in the edible part of crops.

4.1.3.1.Weathering half-lives (𝑇_eff and 𝑇_eco) of ¹³¹I and radiocaesium in edible part of crops The 𝑇_eff can be used to back–calculate the radionuclide activity concentration in vegetables at the time of deposition as shown in Fig. 4.1.

FIG. 4.1. An example of back-calculating the initial ¹³⁷Cs activity concentration (red) from measured monitoring data (blue).

Farmers want to have a long harvesting period to maintain a continuous income. Therefore, planting can occur over an extended period so there are crops present with different biomasses at any one time, which is particularly important for cropping of fresh vegetables. The sampling scheme, given in Fig. 4.2, shows how the stage of development of various plants differs over the growing period. Plants are harvested when they have reached a suitable size which occurs at different times for each crop. Plants removed during the first harvest were most affected after the FDNPP accident. By the time of subsequent harvests, there had been more time since deposition, during which a greater proportion of radionuclides were lost from plant surfaces due to weathering. Additionally, the radionuclide activity concentration in the plant decreased due to the greater increase of biomass during growth after the time of deposition, which had a diluting effect.

FIG. 4.2. Schematic of growth dilution of radionuclide activity concentrations in leafy vegetables with time, after deposition occurred. Horizontal axis is time, and the vertical axis shows the size of the plants at the time of the accident, A>B>C>D (The largest plant on the vertical axis was harvested at the 1st harvest, and the smallest plant on the vertical axis was harvested at the 4th harvest).

The FDNPP accident occurred on 11 March 2011. Relatively heavy depositions of radionuclides on land occurred primarily in two periods from 15 to 16 March and from 20 to 23 March 2011. For most areas in the Kanto plain, where many data sets were collected, deposition was heaviest on 21 March. Because the number of deposition data collection sites in earlier period were limited, 20 March, at the beginning of the later period, was selected as the target date for the initial activity concentration.

In Japan, from winter to early spring, fresh vegetables are harvested over a period of several weeks. Until early June 2011, continuous monitoring of fresh vegetables (edible part, not washed) was carried out where radionuclide activity concentrations in a vegetable crop once exceeded, or were close to, the provisional limits applied in 2011 of 2000 Bq/kg FM for ¹³¹I and 500 Bq/kg FM for total radiocaesium (¹³⁴Cs+¹³⁷Cs). The measurements were performed every one or two weeks. Data sets from municipalities with continuous monitoring data (3 to 5 times in a month) for specified vegetables, such as cabbage and spinach, were used to derive the 𝑇_eff of the radionuclide activity concentration in crops.

Relevant data were obtained from food monitoring datasets of the Ministry of Health, Labour and Welfare [4.3] and from observations for wild butterbur measured by Tagami and Uchida [4.4]. The selected data complied with the following criteria:

 Crops were grown outside (not in a greenhouse);

 At least three continuous sampling data records existed for a specified crop (e.g., spinach) in a municipality;

 Sampling started in March 2011 (Because the loss of deposited radionuclides is rapid immediately after the deposition onto plants later initiation of continuous sampling

would not appropriately represent the weathering and growth dilution in the initial phase);

 Pearson's correlation p-value was lower than 0.05 from statistical fitting of data for each crop observation.

Derived 𝑇_eff data for leafy vegetables and flowering head crops are summarized in Table 4.1 (see also the detailed data Table II.1). The reported values were limited to those wherethe correlation coefficients of the statistical fits exceeded 0.88. Clearly, if sampling start dates were from 12 to 15 March 2011, then the fitting results would differ from those for datasets for which sampling started from 20 to 25 March 2011.

TABLE 4.1. SUMMARY OF EFFECTIVE 𝑇_EFF AND ECOLOGICAL 𝑇_ECO (IN

PARENTHESIS) HALF-LIVES (D) OF RADIOCAESIUM (¹³⁷CS OR ^134,137CS) AND ¹³¹I IN FRESH VEGETABLES AFTER THE FDNPP ACCIDENT

Crop type Radionuclide N^a Effective (ecological) half–life (d)

AM^b SD^c GM^d GSD^e Minimum Maximum

a the number of data points used in analysis

b arithmetic mean

c standard deviation

d geometric mean

e geometric standard deviation (unitless)

The 𝑇_eff data showed that radioactivity concentrations in fresh vegetables declined quickly.

Radiocaesium data are combined as the differing physical half-lives had a minor effect on 𝑇_eff half-lives over the time period considered. Nevertheless, to remove the physical decay effect, 𝑇_eco values were also calculated. The GM of 𝑇_eco values for radiocaesium and radioiodine in leafy vegetables were 7.4 and 8.6 days, respectively, which were not significantly different (ANOVA test). The 𝑇_eco data for flowering heads were not significantly different between leafy vegetables and flowering heads for ¹³¹I. Similarly, the data compilation of 𝑇_eco in TRS 472 [4.2]

suggested that there was little difference in weathering half-lives for ¹³⁷Cs and ¹³¹I in grass.

4.1.3.2.Weathering half-lives (𝑇_eff and 𝑇_eco) of ¹³¹I and radiocaesium in weeds

Further assessments of weathering half-life were carried out utilising data for weeds collected for the Fukushima plant-monitoring programme of the Ecological Radioactivity Monitoring centre of Fukushima [4.5]. Sampling started only 3–4 days after initial deposition occurred on 18 March 2011 initially at six sites in Iitate village (sampling site code: 2-1), Kawamata town (2-2), Tamura city (2-3), Minamisoma city (2-4), Ono town (2-5) and Iwaki city (2-6) in

Fukushima Prefecture. As for the crop data, the radionuclide activity concentrations in the weeds were affected by various weathering processes.

Data on the early time trends of ¹³¹I and ¹³⁷Cs activity concentrations in weeds for these first eight weeks after 11 March 2011 are presented in Fig. 4.3. Associated calculation of the weathering half-life was based on these data with 𝑇_eco and 𝑇_eff for ¹³¹I and 𝑇_eff for ¹³⁷Cs given in Table 4.2. GM values of 𝑇_eff for ¹³¹I and ¹³⁷Cs activity concentrations in the weeds agreed well with that for edible part of crops (Table 4.2). Similar trends for perennial grasses in Japan were reported by Fesenko et al. [4.6].

FIG. 4.3. Time trends of ¹³¹I (a, c) and ¹³⁷Cs (b, d) activity concentrations in weeds at six emergency monitoring sites near the FDNPP; the figure legends show the sampling site codes (see text).

TABLE 4. 2. EFFECTIVE (𝑇_eff) AND ECOLOGICAL (𝑇_eco) WEATHERING HALF–LIVES OF ¹³¹I AND ¹³⁷CS IN WEED SAMPLES COLLECTED AT SIX EMERGENCY

MONITORING SITES (SEE FIG. 4.3) IN FUKUSHIMA PREFECTURE FROM 18 MARCH TO 6 MAY 2011

Sampling site ^a

131I ¹³⁷Cs

N ^b 𝑇eff (d) 𝑇eco (d) R^{2 c} N 𝑇eff (d) 𝑇eco (d) R²

2-1 48 3.7 7.0 0.93 49 6.7 6.7 0.74

2-2 48 4.2 8.7 0.90 51 7.7 7.7 0.70

2-3 42 4.3 9.1 0.91 49 7.1 7.1 0.65

2-4 49 5.7 19.9 0.92 49 9.9 9.9 0.71

2-5 43 3.9 7.5 0.94 46 5.2 5.2 0.91

2-6 49 3.9 7.8 0.96 49 8.7 8.7 0.84

GM n.a.^d 4.2 9.3 n.a. n.a. 7.4 7.4 n.a.

a sampling site code, see Fig. 4.3

b the number of data points used for analysis

c the squared correlation coefficient

d not applicable

Analysis of data collected over a longer period of 300 days [4.5] suggested that root uptake became the major pathway about 100 days after the FDNPP accident occurred (Fig. 4.4).

FIG. 4.4. Long-term monitoring of ¹³⁷Cs activity concentrations in weeds collected at emergency monitoring site 2-1 near the FDNPP (see also TABLE 4.2 and FIG. 4.3).

The 𝑇_eco estimated for both ¹³¹I and ¹³⁷Cs are reasonably consistent with values derived after the Chernobyl accident. The mean 𝑇_eco value for ¹³¹I in plants after the Chernobyl accident was 8.8 days, whilst that for ¹³⁷Cs was 17.0 days with a range of 9.0 to 34 days [4.1]. The narrow ranges for 𝑇_eco after the FDNPP accident compared with those after the Chernobyl accident may be explained by the smaller size of the affected area and the lower diversity of environmental conditions [4.7].

4.1.4. Calculation of mass interception for edible part of crops

The 𝑓 is usually based on dry mass (DM), however, a fresh mass (FM) basis was used for edible crops as it was easier to use in the prevailing conditions soon after the FDNPP accident.

The concepts underpinning the calculation are shown in Fig. 4.1.

The 𝑇_eco values derived above were used for estimation of the initial radionuclide activity concentrations in crop samples at the time of deposition. Online deposition data for the relevant municipalities for the crops considered were used [4.8]. For some municipalities, deposition data were not available, so daily deposition data from neighbouring municipalities were applied.

All daily deposition data were decay corrected to 20 March 2011 and the total deposition was estimated. The mass interception fractions summarized in Table 4.3 were estimated for ¹³¹I and

134,137Cs on leafy vegetables [4.4] (see also detailed data in Table II.2).

TABLE 4.3. MASS INTERCEPTION FRACTION 𝑓 (M²/KG FM) FOR RADIOCAESIUM (¹³⁷CS OR ^134,137CS) AND ¹³¹I ESTIMATED FOR LEAFY VEGETABLES AFTER THE FDNPP ACCIDENT

Plant species Radionuclide N ^a Mass interception fraction 𝑓 (m²/kg FM)

AM ^b SD ^c GM ^d GSD ^e Minimum Maximum

a the number of data points used for analysis

b arithmetic mean

c standard deviation

d geometric mean

e geometric standard deviation (unitless)

These data were converted to a DM basis using dry matter content information collated in the Standard Tables of Food Composition in Japan [4.9] allowing comparison with the data collated in IAEA TRS 472 (Table 4.4). The applied dry/wet ratio for spinach was 0.076, and for Brassica rapa crops (kakina, kukitachina) it was 0.117. For Japanese butterbur, a dry/wet ratio of 0.15 was applied based on field data.

The estimations for the mass interception fractions based on monitoring data obtained after the FDNPP accident are consistent with findings reported after the Chernobyl accident.

Measured ^134,137Cs activity concentrations in rye and Italian ryegrass collected on 31 March 2011 in Tochigi Prefecture, 112 km south-southwest of the FDNPP were supplemented later

with radiocaesium deposition to soil (Bq/m²) allowing 𝑓 estimation [4.10]. The estimated interception fraction and mass interception fraction values are shown in Table 4.5. The values for 𝑓 are within the range of those reported by [4.11] measured in Chiba Prefecture, and also similar to the Chernobyl fallout values [4.2].

TABLE 4.4. MASS INTERCEPTION FRACTION 𝑓 (M²/KG DM) OF ¹³⁷CS AND ¹³¹I ESTIMATED FOR LEAFY VEGETABLES AFTER THE FDNPP ACCIDENT AND COMPARISON WITH DATA IN TRS-472 [4.2]

Plant species Source

a the number of data points used for analysis

b arithmetic mean

c standard deviation

d geometric mean

e geometric standard deviation (unitless)

f no data

TABLE 4.5. AVERAGE INTERCEPTION FRACTION (F) AND MASS INTERCEPTION FRACTION (FB) OF RADIOCAESIUM (^134,137CS) ON 31 MARCH 2011 IN TOCHIGI AND CHIBA PREFECTURES

Plant species N f 𝑓 (m²/kg) Reference

Rye (Secale cereale L.) 3 0.24 0.48 [4.10]

Italian ryegrass

(Lolium multiflorum Lam.) 3 0.53 0.99 [4.10]

Mugwort, giant butterbur, dandelion, Japanese dock, field horsetail, wild

onion) 6 — ^a 0.9

(Range: 0.5–1.3) [4.11]

a no data

4.1.5. Summary and limitations 4.1.5.1.Summary

Observations and analysis after both the FDNPP accident and the Chernobyl accident give similar values for 𝑓 for ¹³¹I and ¹³⁷Cs. The data are also in general agreement with those from controlled experiments reported in TRS 472 [4.2]. The similarity indicates that the mass interception factor is a robust concept for estimating initial activity concentrations is crops after deposition of radionuclides.

The initial 𝑇_eco over the first few weeks following deposition onto perennial weeds are consistent with those calculated for edible leafy vegetables.

4.1.5.2.Limitations

Interception quantifies the initial retention of radionuclides by vegetation during a specific deposition event. Following interception, radionuclides are subject to weathering which reduces the radionuclide activity on vegetation. Under field conditions in accidental situations, the simultaneous measurement of radionuclides deposited on the ground and retained by vegetation is difficult to achieve. However, availability of food monitoring and deposition data made it possible to derive the interception fraction.

Estimation of the mass interception fraction from monitoring measurements carried out under emergency conditions introduces several sources of uncertainties. Data collection after the FDNPP accident was not carried out immediately after the heavy deposition occurred, but a few days later. Simplification was necessary to enable the assumption of a single deposition event.

The initial radionuclide activity concentrations were not directly measured but were estimated by back-calculation using 𝑇_eco for ¹³⁷Cs and ¹³¹I, determined from monitoring data.

The mass interception fraction, according to the definition in IAEA TRS 472 [4.2], is estimated for the total above-ground plant parts. In the assessment after the FDNPP accident, the mass interception factor was also applied to the edible parts of plants, such as broccoli and spinach, which gave similar values to the total above-ground value. The approach is consistent with the definition of the mass interception factor, which normalizes the interception to the mass, so the consideration of a specific plant part should not be significantly different from the consideration of the total plant.

The mass interception fraction depends on the deposition mode. Plants were contaminated with both dry and wet deposition, so it was difficult to identify which deposition form contributed most to the contamination of the vegetables.

The data in TRS 472 were given on a DM basis; however, FM data would be more appropriate when considering radionuclide retention by vegetables under emergency and other situations.

Provision of conversion factors from dry-to-wet and wet-to-dry would be helpful when quantifying interception data sets in future.