Page 180 - Water and wastewater engineering

WELLS 4-35

An analysis of the water indicates that it is corrosive
Sieve analysis of water-bearing sand

U.S. sieve no. Cumulative % passing
100 1.0
70 4.0
50 10.0
40 22.0
30 40.0
20 60.0
16 75.0
12 88.0
8 96.0
6 100.0
Water temperature 5 C

Pumping test results
3
Drawdown at observation well 300.0 m from test well pumped at 0.035 m /s
Time, min Drawdown, m
100 3.10
500 4.70
1,700 5.90

Solution:
a. Number of wells
Two wells will be provided. One will be a duplicate well and pump. Both will be of the
same design. These will be provided to meet the regulatory requirement to have one
spare available for the maximum day demand.
b. Pump size
From the estimated average day demand, select pumping rate based on rules of thumb for
a small system. In this instance, a six-hour pumping time is selected for each daily cycle.

3
190 m /d 3
Q ( 1 day) 3166 or. 32 m /h
6 h
Therefore, one well can supply the maximum day (estimated maximum day
3 3
demand (2)(190 m /d) 380 m /d) in 12 hours.
c. Well diameter
From Table 4-4 , based on the demand, select a trial diameter of 15 cm. Note this will
have to be adjusted if the actual pump selected will not fit with the recommended clear-
ances.
d. Check for potential well failure due to dewatering aquifer
From the well log and the static water level in the well, the sand layer is determined to be
an artesian aquifer. Therefore, the transient flow equations for a confined aquifer may be