Procedure

Procedure

An excellent procedure for anyone to follow in well testing which will answer both the contractor's needs and provide some geological data is as follows:

During preliminary well pumping, establish the maximum rate that can be pumped from the well with the equipment employed.

For example, assume a well can be pumped at 100 gallons per minute for several hours without breaking suction with the test pump or dewatering the aquifer below the screen.

By use of a discharge valve (such as a gate valve) throttle the pump to about 80 percent of the maximum capacity.

Stop the pump and allow the well to recover to its static level. Normally this requires an overnight shutdown.

Measure (prior to testing) the static level of the well.

Noting the time, commence pumping at the established flow rate.

Measure the drawdown every minute for the first ten minutes, thence every five minutes for the first two hours of pumping being careful to maintain exactly the same pumping rate by adjusting the gate valve.

Measurements of pumping levels after the initial two hours can then be spaced every thirty minutes as the test continues.

What will be observed is that the water level will fall rapidly during the early period of pumping (thus the reason for frequent measurements) then less rapidly as the test continues.

What you will be observing is the general behavior of most wells drilled in artesian basins.

Subject to the accuracy of the measurements and to special conditions found in some wells you will observe that the change in pumping levels during the first ten minutes of pumping will be nearly the same as that observed from ten to 100 minutes.

Or the change from five minutes to fifty minutes will also be the same as that observed from 10 to 100 minutes and from 4 to 40 or 8 to 80, etc.

More importantly, if this value is for example 25 feet, then one might anticipate that the well will continue to lower another 25 feet in the time period measured from 100 to 1000 minutes, and another 25 feet in the time period from 1000 to 10,000 minutes, etc.

In more meaningful terms we can say that the pumping level would lower 25 feet in the first 10 minutes, 50 feet after 1 hour and 40 minutes, 75 feet after 16 hours 40 minutes and to 100 feet after nearly seven days of continuous pumping.

The test should be conducted for at least 10 hours and preferably 24 hours.

This will convince any well tester that he stands on dangerous grounds if he attempts to base pump selections on tests conducted after short periods of pumping.

A discussion of the procedures for testing wells to obtain data for calculating the principal factors of aquifer performance is beyond the scope of this chapter.

In such tests, the variations of drawdown with time of pumping as discussed above together with the measurement of one or more observation wells is required.

Instructions on how to perform these tests are discussed in Chapter 5 of "Ground Waters and Wells" (Edward E. Johnson, Inc. 1966).

If convenient, plot the test data on semi-log graph paper which is available from most stationary supply houses. You will observe that the points will fall on nearly a straight line.

Calculate the specific capacity of the well based upon the measured or calculated pumping level at 1440 minutes (24 hours).

The desired pumping level for other flow rates lower than the tested capacity may be safely calculated from the specific capacity obtained.

With an understanding of how rapidly the water level is dropping during the test a more meaningful selection of pumping equipment can be made for the well.

Conducting this "Constant Rate Pump Test" a few times will provide valuable experience in well evaluation.