Water Supply Boreholes must be Test Pumped

This past drought (summer of 2016/17) in the Western Cape has resulted in a number of boreholes “failing” and desperate farmers calling for more boreholes to be drilled. A closer look shows that many, if not most, of these boreholes were tested by the long-discredited “Maximum drawdown-yield at end of 48 hours x 60% = yield” method. A prime example was a borehole drilled and tested by the “old” method in 1983. This borehole was the main borehole supplying a stud horse farming operation. The borehole was equipped with a large capacity pump set at depth for paddock irrigation, plus a low capacity pump set above for drinking water supply. Using the existing main pump the author carried out a step-test in 2012. The borehole appeared to be sustainable. When re-tested in the middle of the drought of 2017 it hit pump- suction in 8 hours, i.e. it is not sustainable. Two radical examples of water supply boreholes are examined: a borehole with air-lift yield of 10 to 15 L/sec for which the sustainable yield was determined to be 0.5 L/sec, and a borehole with an air-lift yield of 0.5 to 0.7 L/sec for which the sustainable yield was determined to be 7.5 L/sec. Conclusion: In order to determine the sustainable yield of a borehole, especially in the fractured rock environment of Southern Africa, do not rely on the driller’s report of air- lift yield, and use the proper method of test-pumping a borehole. If not you may be in trouble.