Procedure for Conducting Pumping Tests part (1)

Procedure for Conducting Pumping Tests part (1)

What is a pumping test ?
A pumping test consists of pumping groundwater from a well, usually at a constant rate, and measuring water levels in the pumped well and any nearby wells (observation wells) or surface water bodies during and after pumping.

A pumping test is a practical, reliable method of estimating well performance, well yield, the zone of influence of the well and aquifer characteristics (i.e., the aquifer’s ability to store and transmit water, aquifer extent, presence of boundary conditions and possible hydraulic connection to surface water).

Aquifer test and aquifer performance test (APT) are alternate designations for a pumping test. In petroleum engineering, a pumping test is referred to as a drawdown test.

Purpose of conducting Aquifer Tests:
Hydrogeological studies include determination of aquifer parameters by conducting pumping tests on dug / bore / tube wells and analysis of pumping test data.

Basically, pumping tests are conducted for a wide variety of reasons, including the following:

1) To determine the reliable long-term yield (or ‘safe’ yield) of a borehole.
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.
4) Pumping tests are the classic (and perhaps 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.
5) To test the operation of the pumping and monitoring equipment,
6) To determine the effects of abstraction on neighbouring abstractions (sometimes referred to as derogation).
7) To determine the environmental impact of the abstraction.
8) To provide information on water quality. Is the water quality suitable for the intended use? Are there likely to be any problems such as drawing in saline or polluted water after extended periods of pumping?
9) To optimize operational pumping regimes.
10) To help determine the correct depth at which the permanent pump should be installed in the borehole.

Common types of pumping tests
The common types of pumping tests conducted include the following:

Constant-rate tests:
In this test it is necessary to maintain pumping at the control well at a constant rate. This is the most commonly used pumping test method for obtaining estimates of aquifer properties. These tests are 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.

Step-drawdown tests :
These tests proceed through a sequence of constant-rate steps at the control well to determine well performance characteristics such as well loss and well efficiency. Step tests are 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

Recovery tests :
These tests use water-level (residual drawdown) measurements after the termination of pumping. Although often interpreted separately, a recovery test is an integral part of any pumping test. Recovery test are 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.

Preliminary studies:
When planning a pumping test, it is useful to gather together all the information that can be found about the aquifer and the borehole itself.

Basic geology:
Are the rocks crystalline basement, volcanic, consolidated sediments or unconsolidated sediments? Groundwater occurs in these rocks in different ways, and behaves in different ways.
Aquifer configuration:
Is the aquifer confined, unconfined or leaky?
Borehole construction:
How deep is the borehole, and of what diameter?
Has solid casing, screen or gravel pack been installed?
Installed equipment:
If a pump is already installed in the borehole, what are its type and capacity, and at what depth is the pump’s intake? Can the pumping rate be varied?
Historical or background water levels:
Information about the historical behaviour of the groundwater level is very useful.
Does the water level vary much from wet season to dry season?
In the period before the test takes place, is the water level already falling or rising or is it stable? What is the current water level?
Local knowledge:
Residents often have a surprisingly good understanding of how the groundwater in the area behaves. For example, how does the water level respond to rainfall?
Can borehole yields be maintained?
Is the water safe for drinking, and does the water quality change over time?


END OF PART 1