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64 Chapter 3 Water Sources: Groundwater
is constant from point to point over the medium. It is nonhomogeneous if the permeability
varies from point to point in the medium. Aquifers with secondary porosity are nonhomo-
geneous. Isotropy and homogeneity are often assumed in the analysis of groundwater
problems. The effects of nonhomogeneity and anisotropy can, however, be incorporated
into an analysis under certain conditions.

3.3 GROUNDWATER GEOLOGY
The geologic framework of an area provides the most valuable guide to the occurrence and
availability of groundwater. Rocks, the solid matter forming Earth’s crust, are an assem-
blage of minerals. In the geologic sense, the term rock includes both the hard, consolidated
formations and loose, unconsolidated materials. With respect to their origin, they fall into
three broad categories: igneous, metamorphic, and sedimentary.
The two classes of igneous rocks, intrusive and extrusive, differ appreciably in their
hydrologic properties. Fresh intrusive rocks are compact and, in general, not water bearing.
They have very low porosities (less than 1%) and are almost impermeable. When fractured
and jointed they may develop appreciable porosity and permeability within a few hundred
feet of the surface. Permeability produced by fracturing of unweathered rocks generally
ranges from 0.001 to 10.0 D, where D is darcy, which is unit of permeability, named after
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Henry Darcy. Extrusive or volcanic rocks can be good aquifers.
Metamorphic rocks are generally compact and highly crystalline. They are impervious
and make poor aquifers.
Rocks may be grouped into hydrologic units on the basis of their ability to store and
transmit water. An aquifer is a body of rock that acts as a hydrologic unit and is capable of
transmitting significant quantities of water. An aquiclude is a rock formation that contains
water but is not capable of transmitting it in significant amounts. Aquicludes usually form
the boundaries of aquifers, although they are seldom absolute barriers to groundwater
movement. They often contain considerable water in storage, and there is frequently some
interchange between the free groundwater above an aquiclude and the confined aquifer
below. Materials that have permeabilities intermediate between those of aquifers and
aquicludes have been termed aquitards.
The boundaries of a geologic rock unit and the dimensions of an aquifer often do
not correspond precisely. The latter are arrived at from the considerations of the
degree of hydraulic continuity and from the position and character of hydrologic
boundaries. An aquifer can thus be a geologic formation, a group of formations, or
part of a formation.
Sedimentary formations include both consolidated, hard rocks (shale, sandstone, and
limestone) and loose, unconsolidated materials (clay, gravel, and sand). Some sandstones
may be almost impermeable, and others highly pervious. The degree of cementation plays
a crucial role. Partially cemented or fractured sandstones have very high yields. Porosity of
sandstones ranges from less than 5% to a maximum of about 30%. Permeability of
medium-range sandstones generally varies from 1 to 500 mD (millidarcy).
Limestones vary widely in density, porosity, and permeability. When not deformed,
they are usually dense and impervious. From the standpoint of water yield, secondary
porosity produced as a result of fracturing and solution is more important than density and
permeability. The nonuniform distribution of interstices in limestones over even short

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One Darcy is equal to the passage of 1 cm of fluid of 1 centipose viscosity in 1 second (i.e., 1 mL/s) under a
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pressure differential of 1 atmosphere having an area of cross-section of 1 cm and a length of 1 cm.