Environmental Engineering FOURTH EDITION

114 ENVIRONMENTAL ENGINEERING

Extraction well

Observation

Well 1 Well 2

t 50rndl

t 100rn

Figure 6-6. lbo monitoring wells define the extent of drawdown during extraction.

EXAMPLE 6.3. A well is 0.2 m in diameter and pumps from an unconfined aquifer 30 m
deep at an equilibrium (steady-state) rate of 1000m3 per day. lho observation wells
are located at distances 50 and 100m, and they have been drawn down by 0.2 and
0.3 m, respectively. What is the coefficient of permeability and estimated drawdown
at the well?

= 37.1 m3/m2-day.

Qln(rilr2) - 1000 ln(100/50)

K= -

JC (hf - hi) JC [(29.8)2 - (29.7)2]

Now if the radius of the well is assumed to be 0.2/2 = 0.1 m, this can be plugged into

the same equation, as

nK(hf - hi) - - JC x 1.97 x [(272) - hi] = 1000

Q= W1/r2) ln(50/0.1)

and solving for h2,

h2 = 28.8 m.

Since the aquifer is 30m deep, the drawdown at the well is 30 - 28.8 = 1.2 m.

Multiple wells in an aquifer can interfere with each other and cause excessive

drawdown. Consider the situation in Fig. 6-7 where first a single well creates a cone

of depression. If a second extraction well is installed, the cones will overlap, causing