TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
(Courtesy of UOP Johnson Division)
Figure C-6. Drawdown in an observation well versus pumping time (semilog scale).
The drawdown measured in the test well and adjacent
d. For flow from a circular source of seepage, the
observation wells or piezometers should always be
coefficient of permeability k can be computed from the
plotted versus (log) time during the test to check the
formulas for fully penetrating wells.
performance of the well and aquifer. Although the
Artesian Flow.
example presented in figure C-6 shows stabilization to
have essentially occurred after 500 minutes, it is con-
Qw =
(C-1)
ln(R/r)
sidered good practice to pump artesian wells for 12 to
24 hours and to pump test wells where gravity flow
Gravity Flow.
conditions exist for 2 or 3 days.
Qw =
(C-2)
c. The drawdown in an artesian aquifer as measured
ln(R/r)
by piezometers on a radial line from a test well is plot-
where
ted versus (log) distance from the test well in figure
Qw = flow from the well
C-7. In a homogeneous, isotropic aquifer with artesian
D = aquifer thickness
flow, the drawdown (H-h) versus (log) distance from
H = initial height of groundwa-
the test well will plot as a straight line when the flow
ter table (GWT)
in the aquifer has stabilized. The drawdown H2-h2
h = height of GWT at r
versus (log) distance will also plot as a straight line for
(H-h) or (H -h ) = drawdown at distance r
2
2
gravity flow. However, the drawdown in the well may
from well
be somewhat greater than would be indicated by a pro-
R = radius of influence
jection of this straight line to the well because of well
An example of the determination of R and k from an
entrance losses and the effect of a "free" flow surface
equilibrium pumping test is shown in figure C-7.
at gravity wells. Extension of the drawdown versus
(log) distance line to zero drawdown indicates the
e. For combined artesian-gravity flow, seepage from
a line source and a partially penetrating well, the coef-
effective source of seepage or radius of influence R, be-
ficient of permeability can be computed from well-flow
yond which no drawdown would be produced by pump-
formulas presented in chapter 4.
ing the test well (fig. C-7).
C-6
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