Porous Aquifers and Groundwater Circulation - Hydrogeology: Objectives, Methods, Applications - page 42

The results of this experiment can be applied to “horizontal” fl ow
within an aquifer (Figure 17).
Darcy's equation: Q = K . A . Δh /L.
By dividing both sides by A, one obtains: Q/A = K . i
Q/A = U, the fi ltration velocity.
The inconvenience of U is that it applies to the entire cross-section of
the aquifer, as though it were an open channel of fl owing water without
a solid skeleton. As a result, this minimalist velocity is not supported by
tracer test results (Figure 18).
The velocity given by tracing tests is the actual velocity of water
molecules in a porous medium. This actual velocity is equal to the Darcy's
fl ux velocity divided by the porosity. So, for example, in the pores of a
sandstone with a porosity of 10%, the actual velocity will be 10 times greater
than the Darcy's fl ux velocity.
Figure 18 Darcy's fl ux velocity and actual velocity of water.
3 HYDRODYNAMIC PARAMETERS OF AN AQUIFER
The permeability K of a reservoir describes the ease with which water
passes through it under the infl uence of a hydraulic gradient.
It is a coeffi cient, defi ned by Darcy's law, corresponding to the discharge
Q (en m 3 /s) fl owing through a section unit A (in m 2 ), under a unitary
(of one unit) hydraulic gradient (i).
Given by the equation K = Q/A . i, it corresponds to a fi ltration velocity
and is expressed in meters per second (m·s -1 ). It ranges in values from 10 -1