Borehole Procedures
Author: Richard Beale, Managing Director of J. Gey van Pittius
When it comes to boreholes, one of the most important, and often overlooked, steps in borehole procedures is the process to follow after the hole has been drilled. Upon completion of drilling, the contractor must provide a borehole report that includes at least the following technical details:
- Depth of the borehole.
- Diameter and depth of the steel sleeve.
- Diameter of the drilled borehole.
- Depth of water strikes.
- Recommended pump depth.
- Blown yield.
The driller is the best person to determine the pump depth, as they know where the strongest water strike is located. They also know if there are any heavily fractured areas, as you do not want to install the pump below this level unless it has been stabilized. Otherwise, the structure could collapse onto the pump or cause the ingress of excessive sand and stones, which could lead to the premature failure of your pump and/or motor.
Quantity and Quality of Water
Next, it is essential to determine the amount of water available and its quality. It’s important to note that you cannot assume the blown yield is the expected yield for several reasons. The most important reason is that a blown yield test is a short-duration test, providing only a starting point for further yield testing. We have encountered boreholes that delivered much more or much less than the blown yield test suggested. We often joke that drilling contractors find it easier to get paid if they report a higher blown yield rather than saying there is little or no water.
For example, we had a case where a driller reported a blown yield of around 30,000 liters per hour. However, after completing an extended constant drawdown test, we found that the borehole yielded 30,000 liters per hour for about eight hours, and then dropped off completely. The final yield was closer to 1,200 liters per hour. The initial high yield came from a cavity, or a heavily fractured zone. Once it was depleted, the actual inflow into the aquifer was much smaller. Imagine investing in a 30,000-liter-per-hour system, only to have the pump run dry after eight hours and then needing to remove it and install a smaller unit.
Determining Borehole Yield
To determine the true yield, a longer-duration yield test is required. If you are applying for a home loan or planning a commercial project that relies on the water, we recommend completing a yield test to S.A.N.S. standards. This involves a step-drawdown test and a constant discharge test, which may require running and monitoring the pump for several days, followed by a recovery test. In such cases, we often work with a geohydrologist to analyze the data and compile a report. The cost of such a test typically starts at around R25,000, which includes a water analysis.
However, if the borehole is only for irrigation or other non-commercial purposes, we perform a less expensive six- to eight-hour maximum yield test. Based on this data, we determine a safe yield. The cost of this test, including a drinking water analysis, is approximately R7,500.
It is critically important to complete a yield test and water analysis because this information helps prevent unwanted surprises. You won’t waste money on an oversized pump system or discover—after spending money on installation—that the water quality is unsuitable for your intended purpose, resulting in additional, unforeseen water treatment expenses. With an extended yield test and water analysis, there are no surprises or unexpected costs.
For more information on borehole procedures, refer to the South African Borehole Water Association
Feel free to contact us for borehole testing, new installations, repairs, and maintenance.
