Recharge systems for protecting and enhancing groundwate

334
TOPIC 3
Modelling aspects and groundwater hydraulics
both within and outside r e . However it should be noted that segments 2 and 3 occur at progressively greater elapsed
times as the distance between the monitoring borehole and the pumping well increases. Inside the effective radius,
Warren and Root (1963) note that r e must be used instead of the value r, whilst r e may be determined from distance
drawdown analysis (Figure 3).
DOUBLE POROSITY ANALYSIS OF LEAKY HYDRAULICS
Providing data is taken from the third hydraulic segment from an observation borehole located outside r e , other
conventional leaky or boundary hydraulic analytical methods, e.g. the leaky method of Hantush and Jacob (1955),
can also be used to determine aquifer hydraulic parameters. This is feasible because the response of a double porosity
aquifer at late pumping times approximates to the
response of a homogeneous porous aquifer (Kazemi et al.,
1969). Selection of an observation borehole just outside r e ,
will maximise the period of data that can be used. The leaky
response of the pumping test at Ladywell Fields was therefore
best analysed using data from the Catford Town Hall
borehole (Figure 4). This figure shows it is difficult to
obtain a definitive log-log leaky response fit. However this
problem can be overcome if the start time of third segment
is determined from a semi-log graph, and only the third segment
data values are then fitted on the log-log response.
Respectively transmissivity, storage and leakance values
of 1,590 m 2 /day, 1 x 10 –3 and 5,500 m were obtained, all of
which were consistent with the double porosity analysis.
W(u,r/B)
Hantush
10 1
10 0
10 -1 10 0 10 1 10 2 10 3
10 -1
10 -2
Figure 4. Analysis of the leaky
time-drawdown response
in the Catford Town Hall OBH
CONTOUR MAP
ANALYSIS
Contour maps of drawdown in the
study area are another useful
method of analysing double porosity
aquifers. Fracture systems will
often be better developed in the
main fracture direction creating
marked azimuthal variation in
transmissivity. This was a feature
of all of the pumping tests carried
out in the SLARS area (Streatham,
Ladywell Fields and Bell Green).
Figure 5 shows the drawdown map
for the Streatham test. Here the
drawdown is not circular but elliptical,
with the long axis showing
the orientation of maximum transmissivity.
Contour maps from the
Bell Green and Ladywell Fields
tests produced ellipses with the
Figure 5. Drawdown contours from the Streatham tests
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin