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Groundwater investigation techniques 183

small in relation to the saturated aquifer thickness,
Leaky, unconfined and bounded aquifer systems
then good approximations are possible. Where draw-
The above solution methods for the non-equilibrium downs are larger, the assumption that water released
equation of radial flow apply to ideal, confined aqui- from storage is discharged instantaneously with a
fers but for leaky, unconfined and bounded aquifers decline in head is frequently not met. As shown in
variations of the curve matching technique must be Fig. 5.36c, the drawdown response of unconfined
applied. A summary of aquifer responses is provided aquifers typically resembles an S-curve with three
here but for a further treatment with worked exam- distinct sections. At early time, following switching
ples of solution methods, including the case of par- on the pump, water is released from storage due to
tially penetrating wells, the reader is referred to compression of the aquifer matrix and expansion of
Kruseman and de Ridder (1990). The determination the water in an analogous way to a confined aquifer
of aquifer parameters from large-diameter dug well (see Section 2.11.2). A Theis type-curve matched
pumping tests is presented by Herbert and Kitching to this early data would give a value for storage
(1981). Once familiar with the various techniques for coefficient comparable to a confined aquifer. As
the analysis of pumping test data in different hydro- pumping continues and the water table is lowered,
geological situations, it is then possible to use com- gravity drainage of water from the unsaturated zone
puter programs for the ease of estimating aquifer in the developing cone of depression contributes
properties. delayed yield at a variable rate.
In the case of a leaky, or semiconfined, aquifer, The pattern of drawdown in an unconfined aquifer
when water is pumped from the aquifer, water is also depends on the vertical and horizontal hydraulic
drawn from the saturated portion of the overlying conductivity and the thickness of the aquifer. Once
aquitard. By lowering the piezometric head in the delayed yield begins, the drawdown curve appears to
aquifer by pumping, a hydraulic gradient is created flatten (Fig. 5.36c) compared with the ideal, confined
across the aquitard that enables groundwater to aquifer response. The drawdown is less than expected
flow vertically downwards. From a consideration of and resembles the response of a leaky aquifer. At later
Darcy’s law (eq. 2.9) and the sketch in Fig. 5.36b, the time, the contribution of delayed yield declines
amount of downward flow is inversely proportional and groundwater flow in the aquifer is mainly radial
to the thickness of the aquitard (b′) and directly pro- producing a response that can be matched to a Theis
portional to both the hydraulic conductivity of the type-curve. Values of storage coefficient calculated
aquitard (K′) and the difference between the water for this third segment of the curve provide a value
table in the upper aquifer unit and the potentiometric for the specific yield, S , of the aquifer (see Section
y
head in the lower aquifer unit. Compared with an 2.11.3). Graphical methods for interpreting pumping
ideal, confined aquifer (Fig. 5.36a), the effect of a test data in unconfined aquifers which account for the
leaky aquifer condition on the drawdown response differing aquifer responses are provided by Boulton
measured at an observation well is to slow the rate (1963) and Neuman (1975).
of drawdown until a true steady-state situation is When a well is pumped close to an aquifer bound-
reached where the amount of water pumped is ary, for example an influent river or impermeable
exactly balanced by the amount of recharge through geological fault, the assumption that the aquifer is
the aquitard, assuming the water table remains con- of infinite areal extent is no longer true and the
stant (Fig. 5.36b). Methods of solution for the situ- drawdown response is of the type shown in Fig.
ation of steady-state and non-equilibrium conditions 5.36d. As shown in Fig. 5.37a, where the boundary is a
in a leaky aquifer with or without storage in the constant head, for example a surface water body such
aquitard layer are provided by Hantush (1956) and as the sea, a river or lake, the drawdown around the
Walton (1960). pumping well is less than expected compared with
Methods of pumping test analysis for confined the ideal, confined aquifer of infinite extent, eventu-
aquifers can be applied to unconfined aquifers provid- ally reaching a steady-state condition with the amount
ing that the basic assumptions of the Theis solution of water pumped balanced by the water recharg-
are mostly satisfied. In general, if the drawdown is ing from the constant head boundary. Where the