Page 69 - Water Engineering Hydraulics, Distribution and Treatment

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3.3 Groundwater Geology
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3.2 PERMEABILITY
named after Henry Darcy. Extrusive or volcanic rocks can
be good aquifers.
The permeability or perviousness of a rock is its capacity
Metamorphic rocks are generally compact and highly
for transmitting a fluid under the influence of a hydraulic
crystalline. They are impervious and make poor aquifers.
gradient. An important factor affecting the permeability is
Rocks may be grouped into hydrologic units on the basis
the geometry of the pore spaces and of the rock particles.
of their ability to store and transmit water. An aquifer is a
The nature of the system of pores, rather than their relative
body of rock that acts as a hydrologic unit and is capable
volume, determines the resistance to flow at given velocities.
of transmitting significant quantities of water. An aquiclude
There is no simple and direct relationship between perme-
is a rock formation that contains water but is not capable
ability and porosity. Clays with porosities of 50% or more
of transmitting it in significant amounts. Aquicludes usually
have extremely low permeability; sandstones with porosities
form the boundaries of aquifers, although they are seldom
of 15% or less may be quite pervious.
absolute barriers to groundwater movement. They often con-
A standard unit of intrinsic permeability, dependent only
tain considerable water in storage, and there is frequently
on the properties of the medium, is darcy, D. It is expressed
some interchange between the free groundwater above an
as flow, in cubic centimeters per second, of a fluid of one cen- aquiclude and the confined aquifer below. Materials that have
2
tipoise viscosity, through a cross-sectional area of 1 cm of
permeabilities intermediate between those of aquifers and
the porous medium under a pressure gradient of 1 atm/cm. It aquicludes have been termed aquitards.
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2
2
is equivalent to a water flow of 18.2 gpd/ft (0.743 m /d/m )
The boundaries of a geologic rock unit and the dimen-
under a hydraulic gradient of 1 ft/ft (1 m/m) at a temperature sions of an aquifer often do not correspond precisely. The
◦
◦
of 60.0 F (15.53 C).
latter are arrived at from the considerations of the degree of
The homogeneity and isotropy of a medium refer to
hydraulic continuity and from the position and character of
the spatial distribution of permeability. A porous medium
hydrologic boundaries. An aquifer can thus be a geologic
is isotropic if its permeability is the same in all directions. It
formation, a group of formations, or part of a formation.
is called anisotropic if the permeability varies with the direc-
Sedimentary formations include both consolidated, hard
tion. Anisotropy is common in sedimentary deposits where
rocks (shale, sandstone, and limestone) and loose, unconsol-
the permeability across the bedding plane may be only a
idated materials (clay, gravel, and sand). Some sandstones
fraction of that parallel to the bedding plane. The medium
may be almost impermeable, and others highly pervious. The
is homogeneous if the permeability is constant from point to
degree of cementation plays a crucial role. Partially cemented
point over the medium. It is nonhomogeneous if the perme-
or fractured sandstones have very high yields. Porosity of
ability varies from point to point in the medium. Aquifers
sandstones ranges from less than 5% to a maximum of about
with secondary porosity are nonhomogeneous. Isotropy and
30%. Permeability of medium-range sandstones generally
homogeneity are often assumed in the analysis of groundwa-
varies from 1 to 500 mD (millidarcy).
ter problems. The effects of nonhomogeneity and anisotropy
Limestones vary widely in density, porosity, and per-
can, however, be incorporated into an analysis under certain
meability. When not deformed, they are usually dense and
conditions.
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 nonuni-
3.3 GROUNDWATER GEOLOGY
form distribution of interstices in limestones over even short
The geologic framework of an area provides the most valu- distances results because of marked differences in secondary
able guide to the occurrence and availability of groundwa- porosity, which depends on local conditions. They are second
ter. Rocks, the solid matter forming Earth’s crust, are an only to sandstones as a source of groundwater. Limestones
assemblage of minerals. In the geologic sense, the term rock are prolific producers under suitable conditions.
includes both the hard, consolidated formations and loose, Although consolidated rocks are important sources of
unconsolidated materials. With respect to their origin, they water, the areas served by them in the United States are
fall into three broad categories: igneous, metamorphic, and relatively small. Most developments lie in granular, uncon-
sedimentary. solidated sediments. Unconsolidated, sedimentary aquifers
The two classes of igneous rocks, intrusive and extru- include (a) marine deposits, (b) river valleys, (c) alluvial
sive, differ appreciably in their hydrologic properties. Fresh fans, (d) coastal plains, (e) glacial outwash, and, to a much
intrusive rocks are compact and, in general, not water bear- smaller degree, (f) dune sand. Materials deposited in seas are
ing. They have very low porosities (less than 1%) and are often extensive; sediments deposited on land by streams, ice,
almost impermeable. When fractured and jointed they may and wind are less extensive and are usually discontinuous.
develop appreciable porosity and permeability within a few
hundred feet of the surface. Permeability produced by frac- 1 One darcy is equal to the passage of 1 cm of fluid of 1 centipose viscosity
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turing of unweathered rocks generally ranges from 0.001 to in 1 second (i.e., 1 mL/s) under a pressure differential of 1 atmosphere
10.0 D, where D is darcy, which is the unit of permeability, having an area of cross-section of 1 cm and a length of 1 cm.
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