Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 156

￿ instantaneous drawdown at the start of pumping and at each restart
following a pause, with a magnitude of a few meters to a few tens of
meters. This rapid decrease in the water level of the pumping well
(generally over a time period of less than a day) corresponds to the
establishment of a new permanent drainage pattern for the aquifer
through the well, and represents the capacitative effect of the well and
the losses of head in the perforated tubing and the fractures connecting
the well with the karst conduit;
￿ a period of slow, regular decrease in the water level (from 1 to 20 cm
per day), resulting from the slow emptying of the aquifer in the absence
of any pluviometric infl uence;
￿ a more or less rapid rise in the piezometric surface once pumping is
stopped. The residual drawdown represents the portion of the aquifer
extracted by the test.
Figure 86 Typical curve for an emptying test (from Collignon, 1986).
The slow emptying phase is linear, indicating that the karst unit under
consideration has the same behavior as a cylindrical reservoir perfectly
connected to the well, with an impermeable envelope and constant
capacitative properties over depth (at least in the slice of terrain affected
by the test). The proportional relationship between the drawdown and the
extracted volume enables an easy calculation of the specifi c volume Vs of
the aquifer, which is equal to the volume of water extracted per meter of
drawdown. In the example of Figure 87, the specifi c volume Vs is given by
the relationship ΔV/Δs and has a value of 220,000 m 3 ·m -1 .