46
In all sod-gley soils (S SL 1, S SL 2 and S SL 3 plots), there was an increase in the share
of uranium at a depth (10–25)-cm from the soil surface as compared to above and
below the underlying layers. The observed differences in the distribution of uranium
in soddy-gley soils can to be due to the different composition of the underlying
rocks on which soils are formed.
In the distribution of uranium along the vertical profile of the alluvial peaty-gley
soil (P2 plot), two maxima are observed at a depth of (10–15) and (25–30) cm. At
S SL 1, S SL 2, S SL 3, and P2, plots (0–5) and (5–10)-cm soil layers are depleted in uranium compared to (10–15)-cm soil layers, which may be due to the removal of
uranium during the flood periods, as well as with the assimilation of radionuclides
by plant organisms.
In general, the distribution of uranium along the vertical profiles of the studied
soils, as in the case of natural uranium inherited from soil-forming rocks, depends
on the genetic features and water regime of soils (Aleksahin et al. 1990, Morton
et al. 2002; Titayeva 2005; Anisova and Yakushev 2008). Along with the accumulation of uranium in the upper organic layer enriched with organic matter (SP S 1 and
SP S 2 plots), a relatively uniform vertical distribution of uranium (plot P2), an
increase in its content with depth (plot P1), and accumulation in deeper layers of soil
(S SL 1, S SL 2, and S SL 3 plots) were observed. In all cases considered, in the (0–20)-cm
layer, 46–57% of the uranium inventory in the soil profile was concentrated.
5 Speciation Uranium in Soils and K d Coefficient
5.1 Uranium in the Interstitial (Pore) Waters of Soils
During the migration of radionuclides from soil to vegetation, groundwater and
surface water, soil pore waters play an important role. Pore waters are found mainly
in small (less than 2 mm in diameter) soil capillaries and are retained by interaction
with the surface of soil particles. Only some natural soils with a high content of clay
fraction (e.g., clay soils) can contain larger closed pores, where part of the pore moisture is in a free state. In such pores, water molecules that are sufficiently far from the
surface of soil particles practically do not experience its influence, and when the
pores are opened, part of the water flows freely out of the soil under the action of
gravity. The possible presence of such free soil moisture pore waters differ from the
liquid phase of the soil. However, in practically all soils, the main part of the pore
moisture is in a bound state and forms the liquid phase of the soil complex.
Pore waters of soils are open thermodynamic systems, genetically related to the
solid phase of the soil complex and other types of natural waters. The pore moisture is
constantly in contact with the solid phase and contains elements (including uranium)
in the most mobile (migratory-active) state. In the presence of a difference in chemical
potentials, the components of soil pore solutions can pass into ground and surface
waters and thus participate in the geochemical migration of elements. Chemical elements in the forms present in the pore waters of soils can be actively included in the
G. A. Sokolik et al.
In all sod-gley soils (S SL 1, S SL 2 and S SL 3 plots), there was an increase in the share
of uranium at a depth (10–25)-cm from the soil surface as compared to above and
below the underlying layers. The observed differences in the distribution of uranium
in soddy-gley soils can to be due to the different composition of the underlying
rocks on which soils are formed.
In the distribution of uranium along the vertical profile of the alluvial peaty-gley
soil (P2 plot), two maxima are observed at a depth of (10–15) and (25–30) cm. At
S SL 1, S SL 2, S SL 3, and P2, plots (0–5) and (5–10)-cm soil layers are depleted in uranium compared to (10–15)-cm soil layers, which may be due to the removal of
uranium during the flood periods, as well as with the assimilation of radionuclides
by plant organisms.
In general, the distribution of uranium along the vertical profiles of the studied
soils, as in the case of natural uranium inherited from soil-forming rocks, depends
on the genetic features and water regime of soils (Aleksahin et al. 1990, Morton
et al. 2002; Titayeva 2005; Anisova and Yakushev 2008). Along with the accumulation of uranium in the upper organic layer enriched with organic matter (SP S 1 and
SP S 2 plots), a relatively uniform vertical distribution of uranium (plot P2), an
increase in its content with depth (plot P1), and accumulation in deeper layers of soil
(S SL 1, S SL 2, and S SL 3 plots) were observed. In all cases considered, in the (0–20)-cm
layer, 46–57% of the uranium inventory in the soil profile was concentrated.
5 Speciation Uranium in Soils and K d Coefficient
5.1 Uranium in the Interstitial (Pore) Waters of Soils
During the migration of radionuclides from soil to vegetation, groundwater and
surface water, soil pore waters play an important role. Pore waters are found mainly
in small (less than 2 mm in diameter) soil capillaries and are retained by interaction
with the surface of soil particles. Only some natural soils with a high content of clay
fraction (e.g., clay soils) can contain larger closed pores, where part of the pore moisture is in a free state. In such pores, water molecules that are sufficiently far from the
surface of soil particles practically do not experience its influence, and when the
pores are opened, part of the water flows freely out of the soil under the action of
gravity. The possible presence of such free soil moisture pore waters differ from the
liquid phase of the soil. However, in practically all soils, the main part of the pore
moisture is in a bound state and forms the liquid phase of the soil complex.
Pore waters of soils are open thermodynamic systems, genetically related to the
solid phase of the soil complex and other types of natural waters. The pore moisture is
constantly in contact with the solid phase and contains elements (including uranium)
in the most mobile (migratory-active) state. In the presence of a difference in chemical
potentials, the components of soil pore solutions can pass into ground and surface
waters and thus participate in the geochemical migration of elements. Chemical elements in the forms present in the pore waters of soils can be actively included in the
G. A. Sokolik et al.