Accuracy of the data produced by the sampling programmes was extremely important to determine the remediation strategies. A few years after the accident, the sampling programme for farmland was improved based on the prior sampling
experience, which allowed the sampling programmes to be optimized. The data of the first surveys were widely used to determine sampling plans for further surveys, and sometimes they were used to assess contamination levels of areas that had not been surveyed, for whatever reason. For example, data on ratios between 137Cs contamination densities and dose rates on both cultivated and non-cultivated land, obtained after the accident, were widely used to assess agricultural land that had not been surveyed to identify hot spots and to determine remedial actions.
Experience showed that detailed maps of contamination for each field were required rather than averaged data derived from large scale maps. Other important data collected included the accumulation of radionuclides by agricultural plants and soil properties — parameters specific to each field. Individual field surveys began in 1986 and continued until 1993 by means of dose rate measurements and soil and vegetation sampling.
The sampling units were marked and numbered (see Fig. 8.10). Standard schematic maps with a scale of 1:10 000 and 1:25 000 were used, while maps with a scale of 1:5000 to 10 000 were used on irrigated (drained) land. The cartograms of contamination of fields in each farm were thus created, which allowed the most contaminated land to be excluding from agricultural use. Based on the cartograms of terrestrial contamination density, the agricultural fields were divided into groups shown in Table 8.2.
8.1.4.2. Arable land
The dose rate measurements were performed by personnel walking along 50–100 m routes. One composite sample was taken from one field during one crop rotation. Soil sampling was carried out using samplers (drills) with a known sampling area (typically 4–5 cm in diameter). Soil samples of arable land that were cultivated after the accident were taken from the ploughed layer (≥20 cm) and unploughed layer (10 cm). Composite samples (1500–3000 cm3) comprised several subsamples collected from each sampling unit. The number of subsamples (individual samples) depended on the diameter of the soil corer but was not fewer than five samples. In the absence of dose rate measurements, one composite sample was taken from 100 ha.
Ten or more individual soil samples were taken from each 5–25 ha arable land sampling unit before spring cultivation. Samples were taken at equal distances on diagonals and in the centre of the site. At each point, the external dose rate was measured at a height of 1 m and 2–4 cm above the surface. If the dose rates measured at 1 m differed to that near the surface, the sampling point was excluded from the sampling schema. From each sampling site, the individual soil samples were pooled to form a 2 kg composite sample.
FIG. 8.10. Location of sampling units on (a) the farm and (b) cartograms on 137 Cs soil contamination.
Vegetation samples were collected at the same locations as the soil samples. For a composite sample of vegetation with a fresh weight of 0.5–1 kg, 8–10 sampling points were selected. The sample could contain all above ground parts or certain parts of the plant (e.g. stems, leaves, fruits, grains or roots). The part of grass above ground was cut with a sharp knife or scissors, removing soil particles, and then it was placed into a plastic bag or paper wrap and labelled.
Labels were made of cardboard or paper and bore information on the crop, phase of growth, region, district, division, agriculture, crop rotation number, selected part of plant, date of sampling and name of the person. The underside of the plants was usually contaminated with soil. In this case, the sample was taken above the contaminated parts of a plant, or the sampled material was washed with clean water prior to packaging.
Homogeneously contaminated fields or some parts of the fields where only one culture was grown were selected as sampling units. Such sites were chosen to be representative of the affected area according to soil types, relief elements and type of agricultural use.
Every sample selected comprised ten or more individual samples, collected uniformly from the entire sampling unit area. When sampling with a borer, the number of puncture samples required depended on the diameter of the borer.
When sampling with a spade, the sample was taken to the depth of the cultivated or ploughed layer. All samples collected in the sampling unit were homogenized;
and with the quartering method, a sample (≥2 kg) was selected from the total mass of the composite sample, placed into a plastic bag, which was placed into a second plastic bag and then wrapped in hard paper. Each sample was accompanied by a chain of custody form placed between the plastic bag and the paper cover.
At fruit and berry sites, samples collected from the whole garden or from the whole crop were quartered, with every part representing a sampling unit.
The individual samples were collected from the entire depth interval of primary cultivation, and the composite sample was prepared as describe above.
8.1.4.3. Uncultivated farmland
The objective of sampling uncultivated farmland (e.g. meadows, pasture and grassland) is to evaluate its farming potential. The collection of representative samples was similar to that used in the settlements:
— External dose rate measurements were taken along the perimeter and in the diagonals of the monitored site.
— Soil sampling was conducted with the rings at five points of the sampling unit.
On grassland and pasture, the sampling unit size was approximately 10 ha.
In the first five years after the accident, soil samples were collected from the top 5 cm of a mown plot. The sampling depth was then increased to 10 cm to account for migration within the soil profile. At locations with potentially elevated migration, such as peat soils, the sample depths were taken separately from two soil layers (0–10 cm and 10–20 cm). Where necessary, soil samples at the central point of a sampling unit were collected at depth intervals of 0–2, 2–5, 5–10 and 10–20 cm to determine their vertical distribution. Identification of virgin areas (i.e. where the soil had not been ploughed since the accident) was made based on prior measurements of the external gamma dose rate, which were then compared with those from neighbouring arable land. Sites were preliminary assigned as virgin soil if the exposure dose rate was 1.5–2 times higher than that of arable soil.
The number of individual samples taken at each sampling unit depended on the sampling equipment: there were 24 individual samples for the 50 mm diameter drill and over 30 individual samples for the 40 mm diameter drill. The total weight of the samples was 8 kg. After a thorough mixing and homogenization of the composite sample, one sixth was taken for the sequent measurements; the rest was discarded.
Vegetation sampling was conducted along with soil sampling. At a sampling site, 8–10 isolated 1 m × 1 m or 2 m × 2 m areas were chosen, arranged diagonally.
Grass (hay) was cut with a scythe, sickle or other cutting tool at 3–5 cm above the ground. Green vegetation from all points or areas were collected, mixed thoroughly and spread into an even layer to form a composite sample. From this, a 1.5–2 kg composite sample of 150–200 g portions from different places was prepared for analysis.
For grass, a sample was obtained from a 1 m2 plot at the sampling point, 3 cm above the surface. Where grass yield was low, the sampling area was increased to obtain a mixed 2 kg sample. Composite samples were wrapped in hard paper and labelled. The number of grass samples was the same as for soil samples. Information on the culture, vegetative stage, sampling area (for grass and cereal) and the number of the collected plants was recorded in the field notebook.