Geoscience Reference
In-Depth Information
Chapter 1
Characterization of the Subsurface
Environment
The zone between land surface and the water table, which forms the upper
boundary of the groundwater region, is known as the vadose zone. This zone is
mostly unsaturated—or more precisely, partially saturated—but it may contain a
saturated fraction in the vicinity of the water table due to fluctuations in water
levels or capillary rise above the water table. The near-surface layer of this zone—
the soil—is generally partially saturated, although it can exhibit periods of full
saturation. Soil acts as a buffer that controls the flow of water among atmosphere,
land, and sea and functions as a sink for anthropogenic contaminants.
Soil forms from the disintegration or decomposition of parent rock material,
due to weathering processes and decomposition of organic materials. The mineral
fraction of soil is formed by physical weathering of rock, resulting from thermal
expansion of minerals along crystallographic axes or expansion of water in rock
fissures. Stable weathering products from mineral decomposition may consist of
ionic or neutral organic and inorganic components, solid organic residues, and
newly formed silicates, oxides, hydroxides, carbonates, sulfides, and other solids
(Greenland and Hayes
1978
). The organic fraction of soil is an heterogeneous
mixture of products resulting from microbial and chemical transformation of
nonliving organic residues, plant roots, and living microorganisms.
Natural subsurface zones of the earth are characterized by heterogeneous
assemblies of materials, forming a porous medium where solid, liquid, and gaseous
phases are present. The open boundaries between the solid, liquid, and gaseous
phases, as well as the activity of microorganisms, create conditions suitable for
developing processes of chemical and biological origin, which dynamically affect
the properties of subsurface environments. The porosity of the near surface (soil
layer) is controlled mainly by the association of mineral and organic parts, with solid
particles tending to be molded into aggregates, or peds, by biologically induced
exudates, plant roots, fungi, soil microorganisms, and animals. The porosity of this
region may be changed either by a shrink-swell phenomenon under wetting and
drying-freezing conditions or by a redistribution of the colloidal fraction under
water transport from the land surface to the water table. Over time, activity between
the solid and liquid phases also can affect the stability of the porous medium itself
and subsequently change the physicochemical properties of the solid phase.
