Ministry of Agriculture and Forestry Zagreb, Croatia
3. RESULTS AND DISCUSSION
Differences in radionuclides activity observed among central and other sampled points in grids, for A to E grid are given in Table I to V respectively. Measured values, together with accompanying counting errors, are shown for central points of grids as well as mean values of differences for "°K, ^Ra, ^Ra, B8U and 137Cs. Mean of all measured values are shown as well as the means of two groups each consisting of 5 samples taken in cross scheme. First group comprehends central point and 4 nearest points (N, S, E and W) at distance of respective grid unit;
second group comprehends central point and 4 the furthest points (NE, SE, SW and NW) at distance of respective grid unit diagonally.
As expected, the highest variation of "°K activities are found in E grid: 312-573 Bq/kg, but variation in potassium concentration found in 9 samples of A grid at area of half square metre only, is unexpected. Near to all of considered means (of 9 or 5 points each) of """K activities in B and C
TABLE I. DIFFERENCES IN RADIONUCLIDES ACTIVITY (Bq/kg) OBSERVED IN A REGULAR GRID AMONG CENTRAL (0) AND OTHER POINTS IN GRID
POINT measured value ± counting error; mean value of differences; mean value ± standard deviation of 2<r; *mean of all samples in grid; **mean of central point and 4 points at d = 30 cm in cross scheme; ***mean of central point and 4 points at d = 42 cm in cross scheme
TABLE II. DIFFERENCES IN RADIONUCLIDES ACTIVITY (Bq/kg) OBSERVED IN B REGULAR GRID AMONG CENTRAL (0) AND OTHER POINTS IN GRID
POINT
(1 Yneasured value ± counting error; (2)mean value of differences; (3)mean value ± standard deviation of 2a; *mean of all samples in grid; **mean of central point and 4 points at d = 2.45 m in cross scheme; ***mean of central point and 4 points at d = 3.5 m in cross scheme
TABLE m. DIFFERENCES IN RADIONUCLIDES ACTIVITY (Bq/kg) OBSERVED IN C REGULAR GRID AMONG CENTRAL (0) AND OTHER POINTS IN GRID
POINT
(1)measured value ± counting error; (2)mean value of differences; (3)mean value ± standard deviation of 2a; *mean of all samples in grid; **mean of central point and 4 points at d = 19.5 m in cross scheme; ***mean of central point and 4 points at d = 27.6 m in cross scheme 138
TABLE IV. DIFFERENCES IN RADIONUCLIDES ACTIVITY (Bq/kg) OBSERVED IN D REGULAR GRID AMONG CENTRAL (25) AND OTHER POINTS IN GRID
POINT
(1)measured value ± counting error, (2)raean value of differences; (3)mean value ± standard deviation of 2a; *mean of all samples in grid; **mean of central point and 4 points at d = 156 m in cross scheme; ***mean of central point and 4 points at d = 220 m in cross scheme
TABLE V. DIFFERENCES IN RADIONUCLIDES ACTIVITY (Bq/kg) OBSERVED IN D REGULAR GRID AMONG CENTRAL (25) AND OTHER POINTS IN GRID
POINT
(I)measured value ± counting error; (2)mean value of differences; <3)mean value ± standard deviation of 2a; *mean of all samples in grid; **mean of central point and 4 points at d = 312 m in cross scheme; ***mean of central point and 4 points at d = 442 m in cross scheme
grids shows less aberration from A grid central point value, than mean value of A grid itself shows.
In case of potassium, it seems that composite of few different samples taken at points inside circle with more than 20 metres radius could be more representative for the centre of half square metre area (area of central sample) than the same composite taken at points inside circle with less than half metre radius. Similarly differences in 137Cs activity among central point and outer points are found in circles with 3.5 m an 220 m radius respectively. It seems that for "^K and 137Cs activity assessment in central point, composite sample acquiring in circle of some 150 metres radius could be the more important factor than sampling point position.
Relatively high range of ^Ra activities was found in E grid: 22-32 Bq/kg, and D grid:
23-32 Bq/kg. Activity of ^Ra in all of samples taken in A, B and C grid is the same practically.
Activity that was higher for more than single counting error was found in one case only. Nearly identical spatial activity distribution show ^Ra and ^U. It seems than, in case of same or similar soil types, differences of uranium activities that are higher than corresponding double counting error could be rarely found in circle up to some 200 m radius. In the case of thorium, that radius is probably about 100-150 metres, and about hundred metres or less in case of ^Ra. Maps of *°K,
^Ra, ^Ra, a8U and l37Cs spatial activities distribution (Figs 1-5) are generated, by inverse distance Kriging method — quadrant data search, on the basis of data collected in D and E grid.
The signal recorded by gamma ray spectrometer, elevated above ground at 100 metres height, are spatially averaged over more than 1.5 x 105 m2, whereas conventional geochemical core soil samples represent perhaps few hundred cm2. Signal recorded by ground gamma spectrometry when detector is elevated at 1 metre height are spatially averaged over more than thousand m2. In fact, the differences of sampling scales could be of some 6-7 or 3-4 orders of magnitude. As shown previously, the very first insight into "°K, ^Ra, ^Ra, a8U and 137Cs spatial activities distribution in soil at relatively small areas indicates that composite soil sample acquiring in cross scheme could be satisfactorily for activity assessment in central cross point up to distance of some 150 metres from cross centre. In that case, it seems that differences of sampling scales, which exist between conventional geochemical soil sampling and aerial gamma-ray surveys or ground gamma spectrometry, could be far less significant.
The second problem in correlation of data collected by aerial gamma-ray surveys or by ground gamma spectrometry and data obtained by laboratory gamma spectrometry of samples collected during conventional geochemical sampling arises from fact that conventional geochemical sampling provides homogeneous sample of the first 15 cm of soil profile. In the same time, the contribution of the first centimetre of soil profile in signal recorded by detector that is elevated at 100 metres height could be more than 12% (Fig. 6). If detector is elevated at 1 metre height, signals recorded from the first centimetre and from 15-100 cm soil depth compartment are equal (about 4%
of total signal recorded) at distance of approximately 3.5 metres from point under detector (Fig. 7).
If ground surface is plain, signal contribution from the first centimetre of soil profile could be even half of total signal received by detector elevated at 1 metre height. Contribution of signals from different soil compartments in total signal received by detector elevated at 1 m height above plain ground surface are shown in Fig. 8 in dependence on soil layer thickness as a function of distance from the point under detector. It is obviously that the first few centimetres of soil profile contribution in signal recorded by detector at 1 metre height is exceptionally high. In cases when ground surface is covered with vegetation, the first few centimetres of soil profile contribution in signal recorded by detector at 1 metre height is probably even unacceptably high.
Narrowing of the area wherefrom recorded signal originated is relatively easily achievable by lateral detector shielding. The information (signal) that originated from a first few centimetres of soil could be reduced significantly by lateral detector shielding in case of low elevated detector.
Cumulative gamma doses distribution which are received by at 1 metre elevated shielded detector (shield reduce the received information at 5 metres radius circle) is shown in Fig. 9. Figure 10 present percentage contribution of signals from different soil compartments in total signal received
140
Gaus-Kriger projection
BELICA
POINT 0 measured:
324.0 Bq/kg (mean values) A grid: 350.0 B grid: 327.1 C grid: 349.1 D grid: 411.9 E grid: 414.0y
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RESEARCH DEPARTMENT ZAGREB
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1996
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FIG. 1. *°K spatial distribution in D and E grid (°K activity in Bq/kg).
to Gaus-Kriger projection
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A grid: 22.8 B grid: 22.9 C grid: 22.5 D grid: 26.2
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1996
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FIG. 2 228Ra spatial distribution in D and E grid f28Ra activity in Bq/kg).
Gaus-Kriger projection
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23.2 Bq/kg (mean values)
A grid: 24.0 B grid: 23.2 C grid: 23.1 D grid: 26.1
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FIG. 3. 226Ra spatial distribution in D and E grid (*26Ra activity in Bq/kg).
Gaus-Kriger projection