Borehole Yield Test
Borehole Yield testing is the process whereby a borehole is subjected to pumping under controlled test conditions in order to determine its performance characteristics. A borehole yield is the volume of water that can be abstracted from a borehole. The borehole yield test is done primarily to determine the sustainable flow or quantity of water moving from the surrounding aquifer, or breaks, into the borehole. The borehole yield test gives us important information about the borehole and how it should be equipped. When we arrive at the borehole to do the yield test we measure the height of the water before pumping. This is called the static water level. Once we start abstracting the water the from the borehole we will reach a water level where the water flowing into the borehole, from the surrounding aquifer/breaks, that is the borehole recharge, is equal to the flow of water being pumped out of the borehole. This is called the dynamic water level. We then calculate how much water is being pumped out of the borehole without it running dry. This is called the yield of the borehole.
In order to pump the borehole at a sustainable rate, which will also accommodate the dry seasons, it is recommended that the borehole be pumped at approximately 70% of its yield. Hence, if a borehole is producing 1000 litres per hour then we will want to abstract the borehole water at 700 litres per hour. One may be tempted to pump the borehole to its maximum capacity but this is never a good idea. In many cases the continuous over pumping of a borehole will lead to the eventual complete failure of the borehole. Sands and silts are drawn into the borehole ultimately clogging up the borehole and rendering it useless. To ensure the sustainability of the borehole, and to prevent it from running dry, one would ideally want to pump at a rate of 70% of this dynamic level. So, for example, if the borehole yields 1 000 litres per hour, one would pump the borehole at 700 litres per hour.
A borehole yield test is highly recommended after the borehole has been drilled. When the drilling contractor completes the borehole he will provide the client with an estimated yield. This estimation should not be used as a substitute for a more accurate yield test. There are many factors which influence the volume of water that is blown out out of the borehole with a compressor and only a yield test will be able to give you more accurate information.
In order to make the correct pump selection for your particular borehole, it is important to determine the yield of the borehole. The incorrect pump selection can have a detrimental impact on the borehole. It can lead to the borehole running dry and ultimately render it non-viable. Pumping a borehole at a sustainable rate will go a long way to ensuring the sustainability of your borehole and the pumping equipment.