Geology Reference
In-Depth Information
drawdown
in m
Figure 87 Example of long-term monitoring of a pumping well (from Collignon, 1988).
This test also enables the estimation of the permanent available reserve
(equal to the volume of water stored in the aquifer at average low water),
by taking the product of the specifi c volume and the usable height of the
pumping well.
The emptying of the aquifer by pumping, in non infl uenced periods,
is compensated by recharge of the aquifer by effective rainfall. The
quantifi cation of this recharge is represented graphically by the distance
along the abscissa between two emptying lines, or by taking the product
of the specifi c volume Vs and the recharge height on the Δs ordinate. In the
example of Figure 87, the recharge volume for 1986 reached 1.1 million m 3 .
In addition, the alternation over time of induced discharge and
pluviometric recharge allows the evaluation of the renewable reserve, which
is equal to the volume of water fl owing into the aquifer over the course of
one year, due to direct and indirect precipitation effects.
Finally, this type of test can reveal changes in the emptying curve over
time, indicating variations within the aquifer or distant environmental
effects. A steepening slope, for example, characteristic of a specifi c volume
limit, can result from decreased permeability of the reservoir at a certain
depth, or from loss of a lateral infl ux (stream losses, exchanges with a
neighboring aquifer). Conversely, a decreasing slope indicates an increase
in the specifi c volume and can be explained by a more permeable section
of the aquifer, by a connection with a lateral reservoir, or by a decrease in
losses due to leakage (drying of a spring).
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