As the National Department of Water and Sanitation announced on Monday that Western Cape dam storage levels are at 35.7%, compared to 62% last year, a geologist told Constantia residents that the Table Mountain aquifer was highly productive and recharged by rainfall every nine months.
However, he debunked perceptions of an underground lake or dam beneath Cape Town that we can tap into at any time.
He cautioned borehole users that if water was sucked out faster than it is was recharged, it would cause subsidence if not properly monitored.
Professor Chris Harris, of the University of Cape Town (UCT) Department of Geological Sciences, said there was no water shortage but there was not enough for drinking.
He was the guest speaker at a water workshop organised by Constantia businessman Andrew Pollock (“A way to beat drought”, Bulletin, May 4).
The workshop was held before the Constantia Residents’ and Ratepayers’ Association’s AGM at the Alphen Centre on Wednesday October 11, the day the City announced that Cape Town was experiencing its worst drought in recorded history.
Consumption remains too high, at 607 million litres of collective usage a day against the crucial consumption target of 500 million litres. Meanwhile, Dr Kevin Winter of Cape Town’s Future Water Institute at UCT, was the guest speaker at the Bergvliet Meadowridge Ratepayers’ Association AGM.
He said we could not assume the availability of water in the dry years when the water table was lowered through excessive pumping.
He added that the annual recharge was some years better than others.
Xanthea Limberg, Mayoral committee member for informal settlements, water and waste services; and energy, said borehole registration was compulsory in terms of the water restrictions.
She emphasised that those using borehole water, treated effluent water, spring water or well points are not exempt from the current water restrictions. “We urge them to use all water resources sparingly during this drought crisis. Groundwater is also a finite resource and can be completely depleted,” she says.
The National Department of Water and Sanitation is responsible for monitoring underground water but did not respond to questions about this by the time this issue went to print.
Terry Winstanley, head of environmental law at firm Cliffe Dekker Hofmeyr said the amount of water being extracted from boreholes could be policed.
The Department of Water and Sanitation is the custodian of water resources and the National Water Act stipulates that the state holds all water, regardless of its origin, in trust for the nation and allocates use rights.
Gabby and Sam de Wet of De Wet’s Boreholes recently installed a borehole for Soil for Life in Constantia.
Over the past year they have been inundated with requests to install boreholes. Their work has doubled from three boreholes a week to six. They say to allow for a successful installation they have to drill into the aquifer and that there is virtually no risk of any ground caving in. They have been installing boreholes for the past 30 years and experienced numerous droughts, the first in November 2001. They said none of their boreholes have caused subsidence.
“The aquifer is a substantial source which has to be managed well and we suggest any groundwater user to use this wisely and considerately,” says Mr De Wet.
Dr Tony Rebelo of the South African National Biodiversity Institute (SANBI), who lives in Bergvliet, says the Bulletin’s region covers two different geologies: granite and sand. It is not surprising that the most affluent suburbs are on the granites. Historically, the Cape Flats were impassable with vleis and wetlands in winter and dry, loose sand in summer.
“The suburbs expanded into the Cape Flats as technology allowed these areas to be drained with canals, and flattened. Still, the Cape Flats has a huge underground aquifer that forms seasonal wetlands in winter, and draws down deeply in summer.
“As water demand for gardening has increased over the years, homeowners with well-points have seen their summer water supply run dry as others have put in much deeper boreholes.
“The current drought has exacerbated the situation. The city should monitor aquifer use, but if the situation does become dire and seawater invades the aquifer, residents nearer the sea will undoubtedly alert officials and action can easily be taken,” said Mr Rebelo.
“It’s about day zero and how to ready ourselves for when taps run dry,” Mr Pollock told the assembled 220 people at the AGM. “The greenness of Constantia is because of its boreholes. We are not villains. We are not stealing from the aquifer, we are responsible about water usage,” said Mr Pollock.
After the meeting he said residents’ minds were put to rest about the historic irrigation of Constantia gardens using underground water. Almost everyone who attended the meeting owns a borehole.
Professor Harris presented research data collected over 20 years from many springs on the lower slopes of Table Mountain. Concentrating on the main ones – Albion, Newlands and Main Spring near St Cyprians, water comes from various sources collectively yielding about 10 million litres each day, one percent of Cape Town’s drinking water needs.
He debunked perceptions of an underground lake or dam beneath Cape Town and that we can tap into it at any time.
Instead, he said groundwater exists in the pore space in unconsolidated sand A long-term aim of his research is to determine the average recharge rate of the springs.
Comparison between monthly oxygen and hydrogen isotope data of the springs and rainfall is consistent with about 50% recharge in nine months.
Comparison between data for the top of Table Mountain and UCT is consistent, with significant recharge on the lower slopes of the mountain.
Residents were asked to bring water samples from their boreholes to understand the nature of the aquifer.
Testing will be for electrical conductivity and the oxygen and hydrogen isotope ratio to explore if the Constantia aquifers are connected, and to estimate the average recharge rate.
Some borehole water may contain a component of mains water from leaking pipes, which can be tested using this approach.
Professor Harris said the outcome will be to produce a map of groundwater composition, and this will take about one month to complete.
Results and feedback will be provided to residents.
After the talk, Mr Pollock described easy, inexpensive ways he has gone off the water grid. Water must be analysed as most borehole water is too acidic for copper pipes and geysers. The PH level in Constantia is about 5.5 but needs to be between 7.8 and 8.2. Use a control mechanism for purifying borehole water for drinking, such as limestone chips or white line.
Retired wastewater treatment official Dave Crombie said it is illegal to use grey or borehole water in municipal pipes.
This can be avoided by fitting a backflow valve.
Residents can report contraventions of the water restrictions (evidence should be provided to assist the City’s enforcement efforts) to firstname.lastname@example.org or send an SMS to 31373. To register boreholes, visit www.capetown.gov.za/thinkwater, and once registered you will receive necessary signage free of charge.