What is a borehole yield test?
A yield test involves a test to see the balance between the maximum amount of water that can be pumped out of the borehole and the amount of water that recharges back from the surrounding groundwater source.
There are a number of aspects that need to be taken into consideration when testing a borehole. There are two important rules that need to be kept in mind when determining the sustainable yield i.e. The total abstraction from the borehole should be less than the natural groundwater recharge, secondly, a borehole should be pumped in such a manner that the water level never reaches the position of the main water strike (normally associated with a fracture). Should this happen the yield will inevitably be affected and the borehole will eventually dry up.
Test pumping boreholes is normally carried out to meet two main objectives: To establish borehole potential. To estimate the sustainable yield and hydraulic performance of individual boreholes for water supplies. To establish aquifer potential. To assess the hydraulic characteristics of the aquifer to determine the groundwater resources.
During this test the pump rate is increased in steps at regular intervals. For example, a borehole may be pumped at a rate of 1000l/hr for a period of 1 hour and increased thereafter to a rate of say 2000l/hr for the next hour and so on for several more steps. This type of test is particularly useful to determine the effectiveness of the borehole, but not useful in determining the long term sustainable yield of a borehole. In this regard, the constant rate test is more useful.
Constant Rate Test
In the Constant Rate Test (CTR), the borehole is pumped at a constant discharge rate over a period ranging from 8 – 48 hours (or longer) – the length of the test is normally proportional to the expected yield and importance of the borehole. The discharge is kept constant for the duration of the test and water levels are recorded in the pumping boreholes as well as observation boreholes (if any). The time-drawdown data obtained from the CRT is then analyzed. The analysis provides useful input to assess the sustainable yield of individual boreholes and the potential of aquifers. Hydro-geologists are trained to utilize different mathematical equations to estimate a sustainable yield.
In this test, recovering water levels are measured in the pumping borehole immediately after the CRT, when the pump is switched off. This recovery test is very useful in assessing the pumping effect and possible dewatering of the aquifers that may result due to the limited extent of an aquifer. Furthermore, the recovery test will indicate the level to which the aquifer is actually dewatered by measuring the residual drawdown after the borehole was allowed to recover.