Geoscience Reference
In-Depth Information
21.2.2 s
peCiFiC
y
ield
(s
torage
C
oeFFiCient
)
Specific yield is the percentage of water that is free to drain from the aquifer under the influence
of gravity. It is not equal to porosity because the molecular and surface tension forces in the pore
spaces keep some of the water in the voids. Specific yield reflects the amount of water available for
development (Davis and Cornwell, 1991). Specific yield and the storage coefficient may be used
interchangeably for unconfined aquifers.
21.2.3 p
ermeability
Permeability (
K
) is the property of an aquifer that is a measure of its ability to transmit water under
a sloping piezometric surface. It is defined as the discharge that occurs through a unit cross-section
of aquifer.
21.2.4 t
Transmissivity
Transmissivity (
T
) describes the capacity of an aquifer to transmit water. It is the product of hydrau-
lic conductivity (permeability) and the aquifer's saturated thickness:
T
=
K
×
b
(21.2)
where
T = Transmissivity of an aquifer (gpd/ft, m
2
/d).
K
= Permeability (gpd/ft
2
, m
3
/(d⋅m)).
b
= Thickness of aquifer (ft, m).
A rough estimation of
T
can be obtained by multiplying specific capacity by 2000 (USEPA, 1994).
■
EXAMPLE 21.1
Problem
: If an aquifer's thickness is 60 ft, estimate the permeability of the aquifer with transmis-
sivity of 30,000 gpm/ft.
Solution
: Rearranging Equation 21.2,
K = T/b
= (30,000 gpm/ft)/60 ft = 500 gpm/ft
2
21.2.5 h
ydrauliC
g
radient
and
h
ead
The height of the potentiometric surface at any point in an aquifer is the hydraulic gradient. Stated
differently, the hydraulic gradient is the slope of the piezometric surface. The difference in elevation
from one point to another along the hydraulic gradient is a measure of pressure. This elevation dif-
ference is called pressure head. We usually express the amount of mechanical energy that ground-
water contains in terms of the hydraulic head (
h
), the total mechanical energy per unit weight. The
hydraulic head is conveniently measured as the elevation to which water will rise in an open pipe,
relative to some reference level. The hydraulic head therefore has units of length and is the elevation
to which water will rise.
Two main components contribute to the mechanical energy or the hydraulic head of ground-
water: potential energy due to gravity and pressure that is exerted on the water. Kinetic energy, a
third energy, is due to the movement of water and is very small compared to the other two energies
because groundwater flows very slowly and can therefore be neglected. In terms of hydraulic head,
the potential energy is expressed as the elevation head (
z
) or simply the elevation of the point of
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