218
1.1 Presence in Soil
The Earth’s surface layers contain, on average, 2.6–2.8 mg kg
−1
of uranium (Hu and
Gao 2008; Herring 2013). The distribution of this primordial radioactive element is
principally dictated by the underlying geology, along with other factors such as pH
and mineral composition, as well as thermodynamic parameters, such as temperature and pressure (Chabaux et al. 2003), moisture content and redox potential, and
the effects of mineral dissolution and sorption processes (Qafoku et al. 2005; Liu
et al. 2009), microbial activity and interactions with the heavy metals present in
soils. Concentrations of uranium are controlled by its physical and chemical behaviour and upon processes of diagenesis, such as crystal fractionation in igneous
rocks, hydrothermal and metamorphic processes and erosion and depositional
mechanisms forming sedimentary rocks. Once deposited, the behaviour of uranium
is predominantly governed by ion-exchange mechanisms. Transport of uranium and
thorium in soil-water occurs mainly in dissolved or suspended form, by diffusion or
mass flow. In soils, uranium is generally found in the +4 and +6 oxidation states.
1.2 Presence in Groundwater
Uranium exists in diverse physicochemical forms, commonly as free metal ions
(U
4+
or UO 2
2+
) and as complexes with inorganic ligands (like uranyl carbonate at
higher pH or uranyl phosphate below ~pH 6.5) and humic substances (like uranyl
fulvate or humate) in dissolved, colloidal and/or particulate forms (Choppin et al.
2002). Primary minerals contain U(IV) as the governing species (Smedley et al.
2006), but in oxidizing media, such as shallow groundwater or surface water, U(VI)
dominates. Since hexavalent uranium has greater degree of uptake than its tetravalent counterpart, it is more probable to be a systemic toxicant and forms relatively
soluble compounds. Uranium enters groundwater aquifers as a result of natural geochemical processes like mineral dissolution and desorption of adsorbed uranium
from mineral surfaces. The concentration of uranium in the aquifer material, the
travel time of water through the aquifer, dissolution, adsorption-desorption and
recoil processes influence the extent of uranium contamination of groundwater
sources (Davidson and Dickson 1986; Tricca et al. 2001). Moreover, factors like
ionic and organic content of stream water, chemical constituents of deposited sediments and grain size affect transport of uranium in environmental media.
2 Uranium Exposure to Humans
The natural or artificially introduction of uranium into the environment may result
in it entering the human body through inhalation, ingestion or through wounds in
the skin with air, water, soil or sediment and food. The uranium may be deposited
and retained in the human respiratory tract, the gastrointestinal tract or in various
R. Mehra and S. Kaur
1.1 Presence in Soil
The Earth’s surface layers contain, on average, 2.6–2.8 mg kg
−1
of uranium (Hu and
Gao 2008; Herring 2013). The distribution of this primordial radioactive element is
principally dictated by the underlying geology, along with other factors such as pH
and mineral composition, as well as thermodynamic parameters, such as temperature and pressure (Chabaux et al. 2003), moisture content and redox potential, and
the effects of mineral dissolution and sorption processes (Qafoku et al. 2005; Liu
et al. 2009), microbial activity and interactions with the heavy metals present in
soils. Concentrations of uranium are controlled by its physical and chemical behaviour and upon processes of diagenesis, such as crystal fractionation in igneous
rocks, hydrothermal and metamorphic processes and erosion and depositional
mechanisms forming sedimentary rocks. Once deposited, the behaviour of uranium
is predominantly governed by ion-exchange mechanisms. Transport of uranium and
thorium in soil-water occurs mainly in dissolved or suspended form, by diffusion or
mass flow. In soils, uranium is generally found in the +4 and +6 oxidation states.
1.2 Presence in Groundwater
Uranium exists in diverse physicochemical forms, commonly as free metal ions
(U
4+
or UO 2
2+
) and as complexes with inorganic ligands (like uranyl carbonate at
higher pH or uranyl phosphate below ~pH 6.5) and humic substances (like uranyl
fulvate or humate) in dissolved, colloidal and/or particulate forms (Choppin et al.
2002). Primary minerals contain U(IV) as the governing species (Smedley et al.
2006), but in oxidizing media, such as shallow groundwater or surface water, U(VI)
dominates. Since hexavalent uranium has greater degree of uptake than its tetravalent counterpart, it is more probable to be a systemic toxicant and forms relatively
soluble compounds. Uranium enters groundwater aquifers as a result of natural geochemical processes like mineral dissolution and desorption of adsorbed uranium
from mineral surfaces. The concentration of uranium in the aquifer material, the
travel time of water through the aquifer, dissolution, adsorption-desorption and
recoil processes influence the extent of uranium contamination of groundwater
sources (Davidson and Dickson 1986; Tricca et al. 2001). Moreover, factors like
ionic and organic content of stream water, chemical constituents of deposited sediments and grain size affect transport of uranium in environmental media.
2 Uranium Exposure to Humans
The natural or artificially introduction of uranium into the environment may result
in it entering the human body through inhalation, ingestion or through wounds in
the skin with air, water, soil or sediment and food. The uranium may be deposited
and retained in the human respiratory tract, the gastrointestinal tract or in various
R. Mehra and S. Kaur