Water Bores-Testing Yield

Establishing a water supply bore often involves a large financial outlay. However, this expense may be small in comparison to the total cost of your water-related infrastructure. The cost of a well-designed and constructed bore is even less significant in comparison to its end worth. The viability of a whole enterprise can hinge on how reliably the bore can produce the required supply.

Proper testing of a bore's capacity to meet long-term demand is valuable insurance-protecting your investment in a scheme dependent on that bore.

Why test a bore?

One of the basic reasons for testing is to find out just how much a bore will produce in the long term. This allows you to determine if the bore is capable of supplying the requirements of your proposed scheme.

There are many other questions which arise when planning the construction and equipping of a bore.

What about a pump?
What size and type of pump is suitable for that bore?
How much pump shaft will be needed? Will the bore pump air after a while?
Are there likely to be effects from interference between neighbouring bores?
How efficient is the bore?

All these questions can be answered if you properly test your bore.

WWhat happens when you pump a bore?

There is a misconception that the water level in the bore becomes stationary shortly after pumping commences. This is not the case unless the aquifer is directly connected to a source of recharge. When you begin pumping water, the water level in the bore falls immediately so that there is a difference in the water level from outside to inside the bore. This difference allows water to move under gravity. From then on, the water level will continue to fall with time in response to friction losses as water moves through the aquifer.

The distance the water level falls below the standing water level is called the drawdown. How far the water level will fall for a given pumping rate and time, will depend on a number of factors including the:

  • areal extent of the aquifer
  • ability of the aquifer to drain water freely
  • bore diameter
  • bore design
  • pumping interference from other bores.

The effect of all of these factors can be determined by carrying out a proper test on the bore. This cannot be determined if the test consists simply of pumping the bore and noting how much it produces after the short period of pumping. Remember, it will be operating for a much longer period than the length of any test.

Types of tests

There are quite a number of tests that can be carried out on bores and each is useful in a particular application.
Some of these are:

  • bailer test-water removed from the bore in a cylindrical pipe dropped from the drilling rig mast. Useful for low yield bores
  • air test-compressed air injected into the bore to cause a mixture of air and water to rise out of the bore. Can give an indication of the amount of water available, but is not accurate
  • pumping test-pump used to extract water from the bore. There are a number of types of pumping tests carried out, but to be useful they must include a number of specific measurements
  • flow/pressure test-this is a test that is particular to artesian bores.