MATERIALS AND METHODS 1. Study area

The study sites are located at Santa Rosa farm, an Experiment Station of the Universidad Austral de Chile located in Valdivia, Chile (39º 46'S, 73º 14'W). Two of the main soils for crop production in south central Chile occur on this farm: Umbric Andosol and Dystric Fluvisol. The latter is flooded during autumn, winter, and spring. The mineralogy of the Umbric Andosol is dominated by allophane in the upper horizon, with an increase of gibbsite and metahallosysic clays in deeper horizons [4]. The mean annual rainfall in the area is about 2300 mm y^-1, with the heaviest rates between April and

October (autumn-winter period). Figure 1 illustrates the mean monthly rainfall estimated for the Experiment Station during the years 1960 to 2002 [5].

FIG. 1. Mean monthly precipitation at Santa Rosa Experiment Station (39º46'S, 73º14'W).

2.2. Preliminary trial

A preliminary field experiment was performed during the growing period 1999–2000 in the Umbric Andosol to estimate the amount of radiocaesium to be incorporated to the soil that would not cause the concentration in the plants exceed the derived intervention level of 1500 Bq kg^-1 given for green vegetables in Chile [6]. The time-course of the radiocaesium TF soil-to-Swiss chard (whole plants), with and without K-fertilization, was also observed.

2.3. Plot installation

At the end of 2000, plots and check plots (without radiocaesium-labelling) of 6.3 m x 1.2 m were installed for all radiocaesium treatments in both soils. The upper 0 – 20 cm soil layer was removed from each plot and sieved. The plots were contaminated with 100 kBq ¹³⁴Cs m^-2 (∼800 Bq ¹³⁴Cs kg^-1):

the Umbric Andosol on November 14 2000 and the Dystric Fluvisol on January 3 2001. To obtain an almost homogeneous distribution of the radionuclide in the soil, the ¹³⁴Cs was incorporated down to 20 cm by pouring 1/10 of the total amount of CsCl in aqueous solution^a over each 2 cm layer with a watering can. To minimize the adhesion of radionuclides to plant leaves [7,8], the upper 2 cm soil layer was not contaminated.

Similar plots and check plots of 1.0 m x 1.2 m (according to the amount of radiostrontium available for the contamination of the plots) were installed for Sr and Sr-Ca. During two consecutive years (2001 and 2002), as soon as the rainy season ended, the plots were contaminated with approximately 100 kBq ⁸⁵Sr m^-2 (∼800 Bq ⁸⁵Sr kg^-1) using SrCl₂ in 0.5 M HCl^b.

Every spring, the plots were prepared for cultivation using the local agricultural practices and procedures. The soil was manually inverted, treated with weed killers and fertilized before planting or seeding. The soil surface was covered again with a 2 cm deep uncontaminated soil layer to avoid the deposition of contaminated soil on plant leaves.

2.4. Soil fertilization

P and N were added to the soil, in order to improve plant growth under the natural low nutrient status of the selected soils, as observed in a preliminary trial. K-fertilizer was added to half of the Cs

a Provided by Amersham Pharmacia Biotech UK Limited, Amersham Place, Little Chalfont, Buckinghamshire HP7 9NA, UK

b Provided by New England Nuclear GmbH, Spandlgasse 58, 1220 Vienna, Austria

0 100 200 300 400 500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Mean precipitation rate (mm month-1)

Triple superphosphate (all plots) (300 kg ha^-1) Sodium nitrate (all plots) (200 kg ha^-1)

Potassium sulphate (only for plots with ¹³⁴Cs-K treatment) (200 kg ha^-1 ≈ 90 kg K ha^-1) Soprocal (only for plots with ⁸⁵Sr-Ca treatment) (400 kg ha^-1 ≈ 100 kg Ca ha^-1) 2.5. Plants selected

Swiss chard (Beta vulgaris L. var. Cicla L.) was selected for observing the time course of the TF, because it is harvested several times per season. This hardy biennial plant is grown exclusively for the large, fleshy and glossy leaves. It has a taproot with many deep secondary roots down to 0.20 m, exploring a great soil volume. It produces abundant basal large green leaves during spring, summer and part of autumn, becoming smaller and less numerous through winter. After wintering, chards produce stalks with caulinary leaves which are still edible, and also useful for foliar analyses.

The Swiss chard plants were transplanted avoiding the contamination of leaves by leaving the upper soil layer (2 cm) without Cs or Sr; the plants were irrigated according to needs and weather conditions.

The species were rotated annually simulating the conventional regional practises.

Sweet corn was selected because it is widely grown and generally accepted by Chilean consumers. In addition, there is ample knowledge of TFs to maize for other soil types to permit appropriate comparisons.

Cabbage was cultivated during the last growing period, because it is often classified in a separate crop group [9, 10].

2.6. Sample collection

For observing the change with time of TFs to Swiss chard, plants were harvested sequentially, every 1–2 months, from late summer to the end of winter in the Umbric Andosol and to the beginning of the flooding season (autumn) in the Dystric Fluvisol. For the Cs treatment, in most cases there was enough material for three replicates (each from 1/3 of the plot), separated into external and internal leaves and external and internal stalks.

The sweet corn and cabbage were harvested when they were ripe for human consumption. They were harvested earlier in the Dystric Fluvisol when a flood shortened the growing season.

Three replicate soil samples were collected every year at the end of the harvest period along the plant rows from the upper 20 cm soil layer for measurement of ¹³⁷Cs and ⁸⁵Sr. Incremental 2 cm soil samples were also collected down to 30 cm to check for possible vertical migration of Cs below the treated layer. Additionally, soil bulk samples were collected annually from the upper 20 cm layer to determine the physical and chemical soil properties.

2.7. Gamma analyses

Swiss chard and cabbage leaves were washed before performing gamma analysis. The radionuclide concentration in plants was determined in the fresh material. Afterwards, the vegetables were dried at room temperature and later at 105ºC. The ¹³⁴Cs concentration in soil and vegetable samples was measured using a Canberra GC2518 gamma detector, in 500 mL Marinelli beakers and in 81.3 mL Petri dishes, depending on the available sample volume. The efficiency was determined for the Marinelli beaker and Petri dish geometries, for several soil and vegetable matrices of different density

using a gamma standard solution supplied by PTB^c. Spectra were analysed using Accuspec B software from Nuclear Data^d. The counting time for each sample was set between 6 and 20 h, depending on the sample activity. Under these conditions the detection limit for ¹³⁴Cs was about 0.2 Bq kg^-1 in soil (Marinelli beaker), and ranged from 0.3 Bq kg^-1 (Marinelli beaker) to 0.8 Bq kg^-1 (Petri dish) for fresh vegetable samples. The detection limit for ⁸⁵Sr concentration in fresh vegetables varied from 0.5 Bq kg^-1 in the Marinelli beaker to about 2.0 Bq kg^-1 in the Petri dish.

3. RESULTS