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3.2 Permeability 63
3.1 POROSITY AND EFFECTIVE POROSITY

The amount of groundwater stored in saturated materials depends on the material’s poros-
ity, the ratio of the aggregate volume of interstices in a rock or soil to its total volume. It is
usually expressed as a percentage. The concept of porosity involves all types of interstices,
both primary (original) and secondary. Primary interstices were created at the time of the
rock’s origin. In granular unconsolidated sediments, they coincide with intergranular
spaces. In volcanic rocks, they include tubular and vesicular openings. Secondary inter-
stices result from the action of geologic, mechanical, and chemical forces on the original
rock. They include joints, faults, fissures, solution channels, and bedding planes in hard
rocks. The extent of fracturing and intensity of weathering exert a profound influence on
the distribution of larger interstices. The importance of secondary porosity in determining
the amount of water that can be obtained from a formation is often great in those hard
rocks that lack intergrain porosity. This type of porosity is dependent on local conditions
and gives water-bearing formations a heterogeneous character. The distribution of second-
ary porosity varies markedly with depth.
Porosity is a static quality of rocks and soils. It is not itself a measure of pervious-
ness or permeability, which are dynamic quantities controlling the flow. Not all the water
stored in a saturated material is available for movement; only the interconnected inter-
stices can participate in flow. Water in isolated openings is held immobile. Furthermore,
water in a part of the interconnected pore space is held in place by molecular and surface
tension forces. This is the dead storage and is called specific retention. Thus not all the
water stored in a geologic formation can be withdrawn by normal engineering opera-
tions. Accordingly, there is a difference between total storage and useful storage. That
portion of the pore space in which flow takes place is called effective porosity, or specific
yield of the material, defined as the proportion of water in the pores that is free to drain
away or be withdrawn under the influence of gravity. Specific yields vary from zero for
plastic clays to 30% or more for uniform sands and gravels. Most aquifers have yields of
10% to 20%.

3.2 PERMEABILITY
The permeability or perviousness of a rock is its capacity for transmitting a fluid under the
influence of a hydraulic gradient. An important factor affecting the permeability is the
geometry of the pore spaces and of the rock particles. The nature of the system of pores,
rather than their relative volume, determines the resistance to flow at given velocities.
There is no simple and direct relationship between permeability and porosity. Clays with
porosities of 50% or more have extremely low permeability; sandstones with porosities of
15% or less may be quite pervious.
A standard unit of intrinsic permeability, dependent only on the properties of the
medium, is the darcy, D. It is expressed as flow, in cubic centimeters per second, of a
2
fluid of one centipoise viscosity, through a cross-sectional area of 1 cm of the porous
medium under a pressure gradient of 1 atm/cm. It is equivalent to a water flow of 18.2 gpd/ft 2
3
2
(0.743 m /d/m ) under a hydraulic gradient of 1 ft/ft (1 m/m) at a temperature of 60 F
(15.5 C).
The homogeneity and isotropy of a medium refer to the spatial distribution of perme-
ability. A porous medium is isotropic if its permeability is the same in all directions. It is
called anisotropic if the permeability varies with the direction. Anisotropy is common in
sedimentary deposits where the permeability across the bedding plane may be only a frac-
tion of that parallel to the bedding plane. The medium is homogeneous if the permeability