Page 191 - Hydrogeology Principles and Practice

HYDC05 12/5/05 5:35 PM Page 174

174 Chapter Five

be monitored in one or more observation wells in
close proximity to the pumped well (Fig. 5.31a). The
constant discharge test programme has three parts:
pre-test observations; pumping test; and observations
during potentiometric recovery after the pumping
has stopped. Prior to the start of the test, the initial
water levels relative to a local datum must be meas-
ured and monitored for effects external to the pump-
ing test, for example tidal fluctuations and barometric
variations (Section 5.2), and the details of the site
hydrogeology recorded, for example well depths
and diameters, strata penetrated and the location
of nearby streams that could act as recharge bound-
aries. From the start of the test, the pumping rate
is monitored and paired values of drawdown and
time are measured in the pumped and observation
wells at specific time intervals that increase as the
test progresses (Fig. 5.31b). Initially, the cone of
depression expands rapidly but the expansion slows
logarithmically with time as the volume of aquifer
contributing to the pumped well increases. Hence,
the measurement time interval can also increase
Fig. 5.31 (a) Suggested cruciform borehole array and (b)
approximately logarithmically as the cone of depres-
diagrammatic representation of the drawdown response in an
sion grows. Eventually, a state of quasi- or actual
observation well for a constant discharge test followed by a
equilibrium conditions may be reached, when the recovery test. By siting observation boreholes along two radii at
rate of recharge to the borehole catchment balances right angles to each other, this enables the aquifer characteristics
the rate of abstraction. to be measured and an indication of aquifer geometry and
anisotropy to be obtained (Toynton 1983). As a minimum, there
The last stage of the constant discharge test is the
should be at least one observation borehole in order to obtain
recovery phase after the pump has been switched off.
reliable pumping test data. The recommended minimum period
On cessation of pumping, groundwater levels will of a constant discharge test where observation wells are more than
recover to a static water level following a drawdown 100 m from the pumped well, or in an unconfined aquifer where
versus time curve that is approximately the converse response time is slow, is 3 days. A 5-day test may be advised in
situations where derogation effects on neighbouring wells need
of the drawdown curve (Fig. 5.31b). The ground-
to be measured. Constant discharge tests involving more than one
water levels should be measured from the time the
pumping well (a group test) are used to test wide areas of aquifer
pump is switched off with a similar, logarithmically and require extended periods of pumping (Clark 1988).
increasing time interval as in the pumping test. In
theory, the length of the recovery test is the same as
the pumping test but, in practice, the recovery is
monitored until the water level is within about 10 cm results. To avoid this problem, the pumped water
of the original static water level (Clark 1988). should be abstracted to a point beyond the range of
For further details on the requirements of a pump- influence of the test. The following sections provide
ing test, the reader is referred to the code of practice a description of the common methods of pumping
produced by the British Standards Institution (BSI test analysis that are applied to confined aquifers
1992) and the procedures given by Walton (1987), and, where conditions allow, unconfined aquifers.
Clark (1988) and Brassington (1998). One important For a fuller treatment of the various solutions and
consideration is the disposal of the discharged water. techniques for more complex aquifer conditions,
In tests of shallow aquifers, this water may infiltrate the reader is directed to the specialist handbook of
back into the aquifer and interfere with the test Kruseman and de Ridder (1990).