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68 Chapter 3 Water Sources: Groundwater
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weight of fluid. The product Cd depends on the properties of the medium alone and is
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called the intrinsic or specific permeability of a water-bearing medium, k Cd . It has the
dimensions of area.
Hence,

K = kg>m = krg>m = kg>n (3.4)
r
n
where is the specific density and , the kinematic viscosity.
The fluid properties that affect the flow are viscosity and specific weight. The value of
n
K varies inversely as the kinematic viscosity, , of the flowing fluid. The ratio of specific
weight to viscosity is affected by changes in the temperature and salinity of groundwater.
Measurements of K are generally referred to a standard water temperature such as 60 F or
15.5 C. The necessary correction factor for field temperatures other than standard is pro-
vided by the relationship

K 1 >K = n >n 2 (3.5)
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Most groundwaters have relatively constant temperatures, and this correction is usu-
ally ignored in practice and K is stated in terms of the prevailing water temperature.
Special circumstances in which correction may be important include influent seepage into
an aquifer from a surface-water body where temperature varies seasonally.
Darcy’s law is applicable only to laminar flow, and there is no perceptible lower
limit to the validity of the law. The volume rate of flow is the product of the velocity
given by Darcy’s law and the cross-sectional area A normal to the direction of motion.
Thus,

Q = KA(dh>dl) (3.6)
and solving for K,

K = Q>[A(dh>dl)] (3.7)
Hydraulic conductivity may thus be defined as the volume of water per unit time flow-
ing through a medium of unit cross-sectional area under a unit hydraulic gradient. In the
standard coefficient used by the U.S. Geological Survey, the rate of flow is expressed in
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3
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gpd/ft (m /d/m ) under a hydraulic gradient of 1 ft/ft (m/m) at a temperature of 60 F
(15.5 C). This unit is called the meinzer. For most natural aquifer materials, values of K
fall in the range of 10 to 5,000 meinzers.
3.8 AQUIFER CHARACTERISTICS
The ability of an aquifer to transmit water is characterized by its coefficient of transmis-
sivity. It is the product of the saturated thickness of the aquifer, b, and the average value
of the hydraulic conductivities in a vertical section of the aquifer, K. The transmissivity,
T Kb, gives the rate of flow of water through a vertical strip of an aquifer 1 ft wide
extending the full saturated thickness of the aquifer under a unit of hydraulic gradient.
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It has the dimensions of (length) /time, that is, ft /d or gpd/ft. Equation 3.6 can be
rewritten as
Q = TW (dh>dl) (3.8)
where W is the width of flow.
The coefficient of storage is defined as the volume of water that a unit decline in head
releases from storage in a vertical prism of the aquifer of unit cross-sectional area (Fig. 3.3).