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www.thevillagenews.co.za
11 March 2020
MY ENVIRONMENT
Hermanus water: Quo Vadis? By Dr Anina Lee
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capacity audience of over 100 people attended a recent public talk at Whale Coast Conservation. They came to get the low-down on the water situation in Hermanus from Dylan Blake, principal geologist at Umvoto Africa, the consultants to the Overstrand Municipality on groundwater. We all know that De Bos Dam is not a reliable source of water. Climate change brings less rain and higher temperatures, and farmers in the valley abstract more and more water before it even reaches the dam. The health of the Onrus estuary depends on the release of a specified amount of water (the ecological reserve) from the dam, but in drought years this does not happen as the water has to be diverted for human use. The alien trees along the banks of the Onrus River suck up more water. This severely impacts the estuary, and its recreational use. With commendable foresight, the Overstrand Municipality started planning for groundwater extraction three decades ago. The idea was not entirely new, as a wellfield (i.e. a series of boreholes) was installed on the golf course in the 1950s, and supplied Hermanus in the 1960s – 1970s, until De Bos Dam was built. We now have three wellfields – at Gateway, Volmoed and Camphill – a total of 10 boreholes (soon to be 12), all tapping into the fractured Peninsula Aquifer of the Table Mountain Group (TMG). But what exactly are aquifers?
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Aquifers like the Peninsula Aquifer are underground layers of rock or sediment that are saturated with water that comes to the surface through natural springs or can be brought to the surface by pumping. (There are other types of aquifers that are more like underground rivers, but these occur mainly in limestone.) Fractured hard rock aquifers are often associated with geological faults. These faults are caused by tectonic movements of the earth’s crust. The faults along SA’s west and south coasts were formed and/or re-activated when Gondwana broke up and the land masses between the plates were pulled apart, cracked and fractured. As a result, rocks were brecciated, broken and crushed along fault lines, resulting in permeable material. Hermanus has several fault lines. The most obvious ones are the Attakwaskloof Fault within the Hemel-en-Aarde Valley where the Volmoed and Camphill Wellfields are. The Fernkloof Faults are situated either side of Lemoenkop. The Hermanus Fault extends from Droëkloof through the town towards the sewage treatment works.
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GRAPHIC: www.nationalgeographic.org/media/aquifer-illo/ The Mount Pleasant/ Gateway Faults are splays off the Hermanus Fault where the Gateway Wellfield is located. Our aquifer water is contained in a layer of rock that can store water due to the fractured sandstone that allows surface water to flow into it. There it is confined by a layer of impermeable rock above it. By drilling through this confining layer we can access the water. Our major concern is, of course, that we should not extract more water from this aquifer than is replaced by precipitation. Drawing too much water could deplete groundwater levels and cause reductions in spring/seep discharge. Less water could have a potential impact on groundwater-dependent fynbos ecosystems in major aquifer recharge areas such as the Fernkloof Mountains. Drawdown of groundwater levels below sea level may allow the intrusion of sea water into the aquifer, with serious consequences. For this reason, groundwater levels are kept at a minimum of two metres above sea level at the Gateway Wellfield, ensuring no sea water intrusion can occur. In addition to the wellfield abstraction boreholes, there are several boreholes that monitor water levels and water quality, and will shut off all pumping if any problems are detected by the sophisticated telemetry monitoring system. The major aquifer recharge area in the Fernkloof Mountains is also regularly monitored, in terms of groundwater, hydrology and ecology. What about unregulated private boreholes? Private boreholes mostly tap into a shallower aquifer called the Nardouw Aquifer of the Table Mountain Group. It overlies the Peninsula Aquifer and pumping from it should not affect our municipal groundwater supply, as the two aquifers are separated by a thick, relatively impermeable aquitard unit. However, there is minimal control over how deep private boreholes
ABOVE: Fractured and brecciated quartzitic sandstone, typical of the Peninsula Aquifer. PHOTO: Dylan Blake RIGHT: Over 100 people attended a recent public talk at Whale Coast Conservation's Green House to get the low-down on the water situation in Hermanus. PHOTO: WCC
are sunk or how much water is extracted, so we don’t really know (although Nardouw Aquifer monitoring boreholes in the town suggest groundwater levels are currently stable). What everyone at the talk was most anxious to know was if we will have enough water to cope with the population growth in Hermanus. It is projected that our yield from De Bos Dam and Table Mountain Group (TMG) aquifers (maximum combined total of 6 million cubic metres per annum) will only be enough for about another 3 – 7 years (depending on whether low or high growth scenarios occur). Then additional water will need to be sourced from elsewhere. Theewaterskloof is a potential option. However, the dam itself is not reliable and has been described as ‘nothing more than a big evaporation pan’. Also, a large pipeline would have to be constructed. There is strong groundwater from the primary Stanford Aquifer; however a pipeline would also have to be built to pump water to Hermanus. Other regional TMG aquifer or surface water schemes within the Overberg region may potentially provide additional water. By the 2030s we will have to start reusing grey and brown water by purifying it to a potable standard and by the 2040s we will have to desalinate sea water, which will make water extremely costly and energy-dependent – and who knows where our energy will come from 20 years from now? This indicates the importance of using water sparingly, even during periods of elevated rainfall and no restrictions. Dylan Blake is an Associate and Principal Geologist at Umvoto Africa, who has worked on the TMG aquifers in the Hermanus region for almost 14 years. He is working towards an MSc on the TMG aquifers in the Steenbras and Nuweberg areas, as part of the City of Cape Town’s wellfield developments.
