After development and preliminary tests, a long-duration well test is carried out. Well tests have to be carried out on all newly-completed wells, because not only does this give an indication of the success of the drilling, design and development, but it also yields formation on aquifer parameters which are vital to the hydrogeologist.
A well test consists of pumping a well from a measured static water level (SWL) at a known or measured yield, and recording the rate and pattern by which the water level within the well changes. Once a dynamic water level (DWL) is reached, rate of inflow to the well equals the rate of pumping. Today, it is a common practice to carry out a ‘step drawdown test’, in which the yield is increased stepwise. Each step is continued until hydraulic equilibrium is reached after which the yield is increased with 50 to 100%.Towards the end of the test a water sample of 2 liters is collected for chemical analysis.
The duration of the test is 24 hours, with a further 8 hours for a recovery test (during which the rate of recovery to SWL I recorded). The results of the test enables the hydro geologist to calculate the best pumping rate, the installation depth, and the drawdown for a given discharge rate.