Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 54

Figure 2.9 shows the three phases (gaseous, aqueous, and solids) of soils and the vari-
ous kinds of constituents that make up a soil. The various constituents in the solid phases
are generally identiied as soil fractions since each type of constituent is a fraction of the
soil solids that comprise the total soil itself. The various soil fractions combine to form the
natural soils that one sees at any one site. The soils may have been transported and depos-
ited as sedimentary deposits of alluvial, luvial, or marine action. The soils derived from
these actions are appropriately called luvial soils, alluvium, and marine soils.
Figure 2.10 shows an idealized schematic of the various soil fractions grouped into a soil
unit. Not all soils have all the various fractions shown in Figure 2.9. How the proportions
and distributions of the different soil fractions occur will depend on not only the geologi-
cal origin of the soil but also the regional controls and weathering processes existent at the
soil location. At least ive factors and four different processes are involved in the produc-
tion of individual soil fractions. The ive factors are:
1. Parent rock material: Composition and texture are important. The inluence of these
features depends on where weathering occurs. In extreme humid conditions and
temperatures, the inluence of composition and texture are short lived. However, in
arctic and arid regions, the inluence of composition and texture of the parent rock
material are long-lived and can even remain indeinitely. Alkali and alkaline earth
cations are important factors in determining the weathering products. Thus, for
example, rocks containing no alkali can only produce kaolinite or lateritic soils as
weathering products Meanwhile, weathering of igneous rocks, shales, slates, schists,
and argillaceous carbonates will produce a large variety of weathering products
because of the presence of alkalis, alkaline earth cations, alumina, silica, etc.
2. Climate: Temperature and rainfall are important climatic factors. Warm and
humid climates encourage rapid weathering of the minerals of the parent rock
material. Decaying vegetative products and organic acids contribute signiicantly
to the weathering process.
3. Topography: This affects how water iniltrates into the ground. The greater the
residence time of water, for example, found at low-lying areas that impound water,
the greater is the reactions between the solutes in water and the soil material.
4. Vegetation: Decaying vegetation is a signiicant factor since this reacts with the
parent silicate minerals.
5. Time: This is an important factor in situations where reaction rates are slow.
The four processes that are inluential in the weathering sequences include:
1. Hydrolysis: This is the reaction between the H + and OH − ions of water and other
mineral ions, particularly for the rock-forming silicates.
2. Hydration: This is important for the formation of hydrous compounds with the min-
erals in the rocks such as the silicates, oxides of iron and aluminum, and the sulfates.
3. Oxidation: Since most rocks carry iron in the form of sulides or oxides, oxidation
of the Fe to FeS, FeS 2 could easily occur in the presence of moisture since this pro-
motes the process of oxidation.
4. Carbonation: The interaction or reaction of carbonic acid with bases will yield car-
bonates. The process of carbonation in silicates is accompanied by the liberation of
silica. The silica may remain as quartz or may be removed as colloidal silica.