Science of Water : Concepts and Applications

All about Water 39
DROP PIPE
The drop pipe or riser is the line leading from the pump to the wellhead. It ensures adequate support
so that an aboveground pump does not move and a submersible pump is not lost down the well. This
pipe is either made of steel or PVC. Steel is the most desirable.
MISCELLANEOUS WELL COMPONENTS
These include:
Gauge and air line Measures the water level of the well.
Check valve Located immediately after the well. It prevents system water from returning to the
well. It must be located above the ground and be protected from freezing.
Flowmeter Required to monitor the total amount of water withdrawn from the well, including
any water blown off.
Control switches Controls for well pump operation.
Blowoff Valved and located between the well and the storage tank; used to fl ush the well of sediment
or turbid or super-chlorinated water.
Sample taps (a) Raw water sample tap—located before any storage or treatment to permit
sampling of the water directly from the well. (b) Entry point sample tap—located after
treatment.
Control valves Isolates the well for testing or maintenance or used to control water fl ow.
WELL EVALUATION
After a well is developed, a pump test must be conducted to determine if the well can supply the
required amount of water. The well is generally pumped for at least 6 h (many states require a 48-h
yield and drawdown test) at a rate equal to or greater than the desired yield.
Yield is the volume or quantity of water per unit of time discharged from a well (GPM, ft3 /sec).
Regulations usually require that a well produce a minimum of 0.5 gal/min/residential connection.
Drawdown is the difference between the static water level (level of the water in the well when
it has not been used for some time and has stabilized) and the pumping water level in a well.
Drawdown is measured by using an airline and pressure gauge to monitor the water level during the
48 h of pumping.
The procedure calls for the airline to be suspended inside the casing down into the water. At
the other end are the pressure gauge and a small pump. Air is pumped into the line (displacing the
water) until the pressure stops increasing. The gauge’s highest-pressure reading is recorded.
During the 48 h of pumping, the yield and drawdown are monitored more frequently during the
beginning of the testing period because the most dramatic changes in fl ow and water level usually
occur then.
The original static level should be recovered within 24 h after pumping stops.
Testing is accomplished on a bacteriological sample for analysis by the MPN method every half
hour during the last 10 h of testing. The results are used to determine if chlorination is required or if
chlorination alone will be suffi cient to treat the water. Chemical, physical, and radiological samples
are collected for analyses at the end of the test period to determine if treatment other than chlorination
may be required.
√ Note: Recovery from the well should be monitored at the same frequency as during the yield and
drawdown testing and for at least the fi rst 8 h, or until 90% of the observed drawdown is obtained.
Specifi c capacity (often called productivity index) is a test method for determining the relative
adequacy of a well, and over a period of time is a valuable tool in evaluating the well production.
Specifi c capacity is expressed as a measure of well yield per unit of drawdown (yield divided by