Page 166 - Water and wastewater engineering

WELLS 4-21

t
o
0.0
Nosleep
Well Field
Well No. 007
1.0

Drawdown, m 2.0

3.0 Q 2.21 10 m /s
3
2
Readings at observation
well, r 68.58 m
4.0

5.0
0.1 1.0 10.0 100.0
Time, min
FIGURE 4-9
Pumping test results.
( Source: Davis and Cornwell, 2008.)

Cooper and Jacob showed that an extrapolation of the straight-line portion of the plot to the
point where s 0 yields a “virtual” (imaginary) starting time ( t 0 ). At this virtual time, Equation
4-3 may be solved for the storage coefficient, S, as follows:

225 Tt 0
.
S 2 (4-6)
r
The calculus implies that the distance to the observation well ( r ) may be as little as the radius of
the pumping well itself. This means that although it is preferable to have a second observation
well to use for drawdown measurements, drawdown measured in the pumping well may be used
as a source of data.

Example 4-3. Determine the transmissivity and storage coefficient for the Nosleep’s well field
based on the pumping test data plotted in Figure 4-9 .

Solution. Using Figure 4-9 , we find s 1 0.49 m at t 1 1.0 min, and at t 2 100.0 min, we find
s 2 2.37 m. Thus,
2 3
221 10 m /s 100..0
.
T ln
(
.
4 3 14 2 37 m 0 49. m) . 10
)
(
.
− 4 3 2

( . 9 35 10 )( . 4 61 ) . 4 3110 m /s