For best results, water should be taken directly from the pressure tank whenever possible. Water is run from the well in five-gallon intervals. The last 5-gallon reading is determined to be the "well flow" or "yield" for that well.
Because water runs through the bottlenecks of the well pump and pressure tank, 10-12 gallons per minute is usually the best result for the test. This number may differ from the well yield results produced at the time of drilling, as the bottleneck will have been eliminated.
Yield tests for existing wells in which a submersible pump has been installed and connected to a water distribution system may be performed in accordance with the following procedure:
- The contractor performing the test shall provide a testing manifold consisting of an inlet for connection to the pressure line from the well and a tee with a pressure gauge graduated with intervals of at least 2 pounds per square inch (psi) and capable of measuring up to 200 psi.
- The testing manifold must be connected so that the pumping switch mechanism is bypassed.
- The pump should be turned on and the manifold valve opened completely. Time, flow rate, and pressure readings must be recorded every 15 minutes.
- If the water level is drawn down to the pump intake; turn the pump off and record the time. Wait a set period of time (15 minutes); turn the pump back on and measure the discharge of the water until the water level is drawn down to the pump intake again. Record the time and repeat this process at least three times to determine the recovery rate (well yield). The well yield is determined by dividing the average discharge (in gallons) by the average interval (in minutes) between each time the pump was shut off.
For recovery rates of 4 gallons/minute or greater, testing may be terminated after three hours if:
- The water level is determined to have dropped no greater than 200 feet through the duration of the test; or
- The discharge pressure does not vary by more than 5 psi.
For well yields less than 4 gallons/minute, the yield test shall continue by valving down the discharge to the approximate well yield determined in step (4) and recording the flow rate and pressure every 15 minutes until six hours have passed from the commencement of the test.