Geoscience Reference
In-Depth Information
Table 1.3 Range of hydrologic properties of lower Yakima basalt flows and interbeds (Freeze
and Cherry
1979
)
Hydraulic conductivity (m/s)
Porosity (%)
10
-11
-10
-8
Dense basalt
0.1-1
10
-9
-10
-8
Vesicular basalt
5
10
-9
-10
-5
Fractured, weathered, or brecciated basalt
10
10
-8
-10
-5
Interbeds
20
Igneous and metamorphic rocks, which exhibit a low porosity generally smaller
than 2 %, are characterized by a minute permeability. As result of changes in
stress conditions occurring during geological periods, wide fractures are formed,
which lead to a substantial increase in the permeability of igneous and crystalline
meta-morphic rocks. Dissolution of siliceous rocks increases the widths of frac-
tures and consequently increases their permeability. However, the permeability of
crystalline rocks usually decreases with depth. Volcanic rocks, for example, are
formed by solidification of magma, and their hydraulic properties are determined
by the cooling regime of magma reflected in an anisotropic permeability. The
variation of properties in a basalt is presented in Table
1.3
.
1.4.2 Groundwater Composition
The groundwater composition is controlled by chemical and biochemical inter-
actions with the geological materials through which water flows as well as by the
chemistry of incoming water. Because inorganic components dissolved in
groundwater mainly are in an ionic form, groundwater may be considered an
electrolyte solution with a conductance ranging from tens of microsiemens (a
value close to that of rainwater) to hundreds of thousands of microsiemens (for
brines in sedimentary basins). Dissolved inorganic constituents are grouped in
three classes: major constituents, with concentrations greater than 5 mg/L; minor
constituents, with a concentration between 0.01 and 5.0 mg/L; and trace constit-
uents, having a concentration of less than 0.01 mg/L. Table
1.4
gives examples of
constituents included in these groups.
Organic constituents may be found in groundwater in dissolved forms, asso-
ciated with ligands, or adsorbed on colloidal materials. Natural products, such as
humic and fulvic acids, have a low aqueous solubility but may serve as ligands for
inorganic trace components. The majority of organic pollutants—including
petroleum hydrocarbons, solvents, and toxic organic chemicals of industrial origin
known as NAPLs (nonaqueous phase liquids)—exhibit a limited solubility in
aqueous solutions, with usually very low concentrations. However, it should be
recognized that even low solubility limits (and the corresponding concentration)
usually are several orders of magnitude higher than the maximal allowable con-
centrations for potable water. As a consequence (NAPLs), as a mixture or as
