Page 125 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch03_061-117.qxd 8/17/10 7:48 PM Page 87

3.12 Multiple-Well Systems 87

Similarly, for the second period:
log(t>t ) 2 log(1,200>1,080) 0.046
For third period:
log(t>t ) 3 log(960>780) 0.090

Total residual drawdown:
s (264 Q>T) [g log t>t ] 2.89 0.174 0.5 ft (0.15 m)

Solution 2 (SI System):
The problem can be decomposed into three pumping and recovery periods, with Eq. 3.26b applied
to each of the subproblems:

s¿= (0.1833 Q>T) log (t>t¿)
= (0.1833 * 1907.5>397.38) log (t>t¿)
= 0.8798 log (t>t¿)
For the first period of pumping:
t time since pumping started 1,440 min
t time since pumping stopped 1,320 min
log(t>t ) 1 log(1,440>1,320) 0.038
For the second period of pumping:
log(t>t ) 2 log(1,200>1,080) 0.046
For the third period of pumping:
log(t>t ) 3 log(960>780) 0.090
Total residual drawndown:

s¿= (0.1833 Q>T) [g log t>t¿]
= 0.8798 (0.038 + 0.046 + 0.090)
= 0.15 m

3.12 MULTIPLE-WELL SYSTEMS
Because the equations governing steady and unsteady flow are linear, the drawdown at any
point due to several wells is equal to the algebraic sum of the drawdowns caused by each
individual well, that is, for n wells in a well field:
n
s
s = a i
i=1
where s is the drawdown at the point due to the Ith well. If the location of wells, their dis-
charges, and their formation constants are known, the combined distribution of drawdown
can be determined by calculating drawdown at several points in the area of influence and
drawing contours.