The basic concept of a pumping test is very simple: water is abstracted by pumping or bailing from a borehole, thus lowering the water level. The water abstraction, leveling and the pumping rate are monitored over time, along with various other parameters if possible such as water levels in observation boreholes.
The way in which the water levels respond to the pumping is then analyzed to derive information about the performance characteristics of the borehole and the hydraulic Properties of the aquifer. One can infer information about the borehole and aquifer only by observing how the water level changes in response to pumping.
Why do we undertake pumping tests?
Pumping tests can be undertaken for a wide variety of reasons, including the following:
1. To determine the reliable long-term yield or safe yield of a borehole and how many people it will be able to supply.
2. To assess the hydraulic performance of a borehole, usually in terms of its yield-drawdown characteristics. How much drawdown does it take to yield a certain amount of water?
3. To derive the hydraulic properties of the aquifer. Pumping tests are the only way to derive in situ aquifer hydraulic properties, such as Transmissivity and the storage coefficient, or to reveal the presence of any hydraulic boundaries.
4. To test the operation of the pumping and monitoring equipment, to make sure that everything is working safely and efficiently, and if applicable, to confirm that the contractors have done their job properly.
5. To determine the effects of abstraction on neighboring abstractions sometimes referred to as derogation.
6. To determine the environmental impact of the abstraction. All groundwater abstraction eventually has an impact; it is only a matter of where, when and whether or not the impact is acceptable.
7. To provide information on water quality. Is the water quality suitable for the intended use? Is it stable in the long term? Are there likely to be any problems such as drawing in saline or polluted water after extended periods of pumping?
8. To optimize operational pumping regimes including selecting the most suitable pump for long-term use, and estimating probable pumping and treatment costs.
9. To help determine the correct depth at which the permanent pump should be installed in the borehole
Main types of pumping test
There are many different types of pumping test from which to choose. The main types are as follows:
Designed to establish the short-term relationship between yield and drawdown for the borehole being tested. It consists of pumping the borehole in a series of steps, each at a different discharge rate, usually with the rate increasing with each step. The final step should approach the estimated maximum yield of the borehole.
Carried out by pumping at a constant rate for a much longer period of time than the step test, and primarily designed to provide information on the hydraulic characteristics of the aquifer. Information on the aquifer storage coefficient can be deduced only if data are available from suitable observation boreholes.
It is carried out by monitoring the recovery of water levels on cessation of pumping at the end of a constant-rate test and sometimes after a step test. It provides a useful check on the aquifer characteristics derived from the other tests but is valid only if a foot-valve is fitted to the rising main; otherwise water surges back into the borehole. These tests can be carried out singly or in combination.
A full test sequence usually starts with a step test, the results of which help to determine the pumping rate for the constant-rate test, with a recovery test completing the sequence. The test design can be adapted for use in small, medium or large boreholes, the main differences being the pumping rates, the length of test and the sophistication of the monitoring system.