The drawdown measured in the test well and adjacent

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

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

conditions exist for 2 or 3 days.

Qw =

(C-2)

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).

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