Geoscience Reference
In-Depth Information
Table 1.2 Diffusion constants of CO
2
,O
2
, and N
2
in air and water and their solubility in water at
20 C. Reprinted from Paul and Clark (
1989
)
Diffusion constant (cm
2
/s)
Solubility in H
2
O (mL/L)
Air
Water
0.177 9 10
-4
CO
2
0.161
8.878
0.180 9 10
-4
O
2
0.205
0.031
0.164 9 10
-4
N
2
0.205
0.015
anaerobic zones may coexist in a porous medium even under partially saturated
conditions.
1.4 Aquifers
Aquifers form the region below the vadose zone, where the solid phase is in
contact with a flowing groundwater phase and a local chemical equilibrium
between the solid and aqueous phases has a tendency to be reached. An aquifer is
defined as a saturated geological unit that can transmit significant quantities of
water under ordinary hydraulic gradients. Within the context of aquifers, an
aquitard is defined as a less permeable geological unit that has the potential to
store water in significant quantities. In addition to geological controls of an
aquifer, groundwater composition is affected by the properties of recharge water,
which originates from the initial incoming water constituents. The properties of the
solid matrix (porous medium) were discussed in
Sect. 1.1
.
1.4.1 Aquifer Structure
The lithology, stratigraphy, and structure of a geological system control the nature
and distribution of aquifers. Lithology includes the mineral composition, grain
size, and grain aggregation of the sediments or rocks; stratigraphy depicts the
relation among the lenses, beds, and formations of geological sediments; and
structural features describe the geometry of the geological system resulting from
deformation.
According to their origins, aquifer materials may be classified as deposits of
various origins and sedimentary or metamorphic rocks. Aquifers of fluvial origin
are characterized by alluvial deposits composed of particles of gravel, silt, and clay
of various sizes that are not bound or hardened by mineral cement, pressure, or
temperature. The topography controls the deposition of sediments and their spatial
redistribution as a function of their textural properties. Aeolian deposits consisting
of sand or silt are more homogeneous than fluvial deposits. Glacial deposits,
