accurate test. For a fairly comprehensive test of a well, the pump should
be operated first at a rate that will lower the water in the well about one-
third of the maximum drawdown possible, or at about one-third of the
capacity of the pump. Continue pumping at this rate until the pumping level
remains about constant. This will require from one to four hours in most
cases. After making the necessary measurements, change the rate of pumping
to produce two-thirds of the capacity of the pump. Repeat the measurements
when the pumping level becomes about stable. Increase the rate of pumping
to produce the maximum drawdown; or increase it to the capacity of the pump
and make measurements a third time when the pumping level becomes stable
again.
b. The drawdown observed during a well test is the difference in feet
between the pumping level and the static water level before pumping was
started.
This drawdown can be expressed as a percent of the well's total
capacity (total depth of water).
In other words, 100 percent drawdown
equals the depth of water in the well before pumping. Remember, never draw
down an Army well below 50 percent.
As an example, assume a well is 600
feet deep, with a static level of 100 feet and that the well is under a
nonartesian condition. This means that there is 500 feet of water in the
well. After test pumping at 16 GPM (one-third of the capacity of the pump)
for four hours, the water level dropped 100 feet and stabilized (drawdown).
Subtracting 100 from 500 leaves 400 feet of water in the well. Dividing the
100-foot drawdown by the original 500 feet of water gives 20 percent
drawdown. This shows that pumping at 16 GPM uses 20 percent of the water
available in this well. Now suppose the same well is pumped at 32 GPM (two-
thirds of the capacity of the pump) for four hours and the water level
dropped 200 feet and stabilized. Subtracting 200 from 500 leaves 300 feet
of water in the well.
Dividing the 200-foot drawdown by the original 500
feet of water gives 40 percent drawdown. This shows that pumping at 32 GPM
uses 40 percent of the water available in this well. To continue, the same
well is pumped at 50 GPM (total capacity of the pump) for four hours and the
water level dropped 300 feet and stabilized.
Subtracting 300 from 500
leaves 200 feet of water in the well. Dividing the 300-foot drawdown by the
original 500 feet of water gives a 60 percent drawdown.
This shows that
pumping at 50 GPM uses 60 percent of the water available in this well.
Since this is greater than the 50 percent allowed for drawdown from an Army
well, this well cannot be pumped at maximum (50 GPM) pump capacity.
c. Stop pumping operations by closing the 2-inch gate valve on the
wellhead. Then push the "STOP" button on the distribution box. Next turn
the ON/OFF switch to "OFF." Shut down the generator according to the TM and
perform after-operation PMCS.
QM4912
4-8
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