Soils

Soil formation or pedogenesis typically modifies the radioelement response of the underlying bedrock or sediment, where soils have developed on transported materials (Table 2). During pedogenesis many of the primary bedrock minerals weather to secondary minerals including clays and Fe- and Al-oxyhydroxides. The nature of these secondary minerals and the rate at which they form depends on the characteristics of the bedrock and the nature of the weathering environment including geochemical, biological, hydrological and physical processes.


Table 2. Radioelement content of Australian rocks and soils. Average values in brackets (Dickson et al. 1977).

Rock type Rock Soil

K
%
U
ppm
Th
ppm
K
%
U
ppm
Th
ppm
Intrusives
granitoids 0.3 - 4.5
(2.4)
0.4 - 7.8
(3.3)
2.3 - 45
(16)
0.4 - 3.9
(2.1)
0.5 - 7.8
(2.7)
2 - 37
(13)
gneisses 2.4 - 3.8
(2.4)
2.1 - 3.6
(2.5)
18 - 55
(15)
0.7 - 1.9
(1.3)
1.6 - 3.8
(2.2)
6 - 19
(12)
pegmatite 2.6 - 5.5
(3.7)
0.3 - 1
(0.7)
0.3 - 9.6
(2)
aplites 0.6 - 4
(2.4)
1 - 8
(3.3)
3 - 20
(7)
quartz-feldspar
porphyry
1 - 5
(2.9)
1.3 - 2.9
(1.7)
6 - 14
(13)
intermediate
intrusives
0.7 - 5.6
(2.7)
0.1 - 1.2
(0.8)
0.8 - 6.1
(2.4)
0.7 - 3.4
(1.6)
1.5 - 2.3
(1.9)
2.9 - 8.4
(5.6)
mafic intrusives 0.1 - 0.8
(0.4)
0.0 - 1.1
(0.3)
0.0 - 3.1
(1.2)
Extrusives
felsic volcanics 2.0 - 4.4
(3.7)
1.4 - 13
(2.4)
13 - 28
(17)
1.8 - 3.2
(2.4)
1.3 - 2.4
(2.1)
10 - 18
(13)
intermediate
volcanics
1.8 - 4.1
(2.7)
0.9 - 5.6
(2.3)
1.5 - 15
(9)
1.0 - 2.7
(1.9)
1.2 - 3.6
(2.1)
4 - 17
(10)
low-K andesites 0.7 - 0.9
(0.8)
1.0 - 2.5
(1.6)
3 - 8
(5)
0.8 - 1.5
(1.1)
1.2 - 1.5
(1.3)
4 - 6
(5)
mafic volcanics 0.3 - 1.3
(0.9)
0.3 - 1.3
(0.7)
2.0 - 5.0
(3.0)
0.2 - 1.4
(0.7)
0.6 - 2.5
(1.6)
3.3 - 13
(7.9)
ultramafic volcanics 0.2 - 0.9
(0.4)
0.3 - 0.9
(0.6)
0.0 - 4.0
(1.2)
0.6 2.0 6
Sedimentary rocks
Archaean shales 0.4 - 1.6
(0.9)
0.3 - 1.3
(0.9)
1 - 5
(2.7)
0.8 1.2 3
other shales 0.1 - 4.0
(2.6)
1.6 - 3.8
(2.6)
10 - 55
(19)
0.7 - 3.0
(1.5)
1.2 - 5
(2.3)
6 - 19
(13)
arenites 0.0 - 5.5
(1.8)
0.7 - 5.1
(2.3)
4 - 22
(12)
0.1 - 2.4
(1.3)
1.2 - 4.4
(2.1)
7 - 18
(11)
carbonates 0.0 - 0.5
(0.2)
0.4 - 2.9
(1.6)
0 - 2.9
(1.4)

Redistributions of these secondary minerals that lead to horizon differentiation within the soil also have a direct influence on the distribution of radioelements within the profile (Figure 17). Biological process leading to the accumulation of organic matter in the topsoil, mixing due to bioturbation (e.g., termites and ants) and leaching are all important factors that need to be considered when understanding the distribution of radioelements in soils. Leaching can involve the complete removal of minerals from a soil or the removal of a mineral from one part of the soil to another, commonly referred to as eluviation (lost material) and illuviation (gained material) (Figure 18). Radioelement characteristics of regolith materials including soils are discussed in the next section).

Figure 17

Figure 17 (above). The gamma-ray response of a soil will relate to the geochemistry and mineralogy of the underlying bedrock, sediment or
wind-blown dust. However, during pedogenesis radioelements can be redistributed or removed from the developing soil. Leaching
(including and illuviation and eluviation processes), capillary movement, piping processes, organic matter recycling and
bacterial processes can all affect the distribution radioelements in the soil.


Figure 18 Figure 18 (left). Variations in K, Th and U concentrations during the weathering of basalt. Potassium is leached from the upper part of the profile whereas Th and U are concentrated with clays in a illuviation zone (Dickson and Scott 1997).