Artesian boreholes – which spontaneously discharge water without being pumped – are a common feature worldwide. However, the hydraulic testing of these boreholes, and estimating of aquifer properties (where groundwater occurs in reasonable quantities, it is referred to as an aquifer) are still challenging for hydrogeologists.

Cape Town water and earth sciences consultancy Umvoto Africa is addressing this challenge with the development of a specially designed and constructed wellhead for the installation of the pump and monitoring equipment prior to the hydraulic test.

The project where this wellhead has been tested is a strong artesian borehole in the Blossoms Wellfield, south of Oudtshoorn in the Western Cape. Umvoto has designed and managed a groundwater project for the Oudtshoorn Municipality for over 10 years.

Umvoto’s findings were discussed by Dr Kornelius Riemann (principal hydrogeologist) and Dr Chris Hartnady (technical director) of Umvoto Africa,  at the recent Groundwater Conference in Durban during September.

Common hydraulic tests, such as step-drawdown or constant discharge rate tests, require a static water level at the start of the test, and the measurement of drawdown (increasing over time) and abstraction rate (fixed for a period of time).

Usually, when undertaking a pumping test in an artesian borehole, the drawdown is measured from ground level and the drop in hydraulic head between static pressure and ground level is often ignored. This also implies that the starting time of the test is not at the static water level. A constant head test, set at ground level, is the other option. However, the decrease in flow rate is not only dependent on the hydraulic properties of the aquifer, but also masked by pipe hydraulic effects within the well. This kind of test would also limit the available drawdown to be used for the test.

Umvoto has developed a method for undertaking hydraulic tests in strong artesian boreholes allowing for the drawdown to fluctuate between above and below ground and avoiding the pitfalls described. The solution is a specially designed and constructed wellhead for the installation of the pump and monitoring equipment prior to the hydraulic test. The standard tests are slightly modified and are only carried out after sealing the wellhead and reaching static hydraulic pressure.

Umvoto has already applied these principles and designs for production boreholes on another  project  in the Hemel en Aarde Valley, north of Hermanus, where five boreholes show artesian or semi-artesian conditions.