Making the invisible visible — tapping into groundwater must form part of Cape Town’s future water supply
Data from Cape Town’s ‘Day Zero’ crisis shows that during and after the drought, groundwater levels stayed within natural ranges and essentially, the aquifers remained full. Sustainable groundwater use must form part of the future water supply in Cape Town.
In August 2017 I wrote in Daily Maverick that “the Western Cape province in general and the City of Cape Town, in particular, are facing an unprecedented water crisis.” Dam levels had dropped to below 20% of capacity by the end of summer 2017 and the winter rains had failed to arrive.
The threat of Day Zero — the day when municipal supply was to be switched off and citizens of Cape Town would have to fetch and carry their own water from 200 points of distribution spread across the city — was narrowly averted by a dramatic reduction in municipal water use and the winter rains of 2018 pushing dam levels up to 76% in October of that year.
It was a close call, far too close for comfort.
Looking back, this all seems so long ago. Good winter rains allowed the dams to fully recover by October 2020, by which time the Covid-19 pandemic was in full swing. State Capture, load shedding and the riots and looting in July 2021 added to our woes before Russia decided to invade Ukraine, creating another global crisis. These and many other stories have pushed water out of our minds and onto the backburner.
But now is exactly the time that we should be focusing on both water use and supplies if we want to avoid or reduce shortages in the future. Any geography or hydrology student will tell you that South Africa is a water-scarce country, and an ever-growing population results in our water supplies coming under more and more pressure.
What we did learn during the drought is that it is not easy to produce new water. Desalination was touted as the saviour, but the plants developed during the drought were very small in relation to demand (1.5% of target demand of 450ML/d), expensive and reliant on a failing Eskom electricity supply. The Atlantis Water Resource Management Scheme was resuscitated (but also small at 3% of the target demand) and development of the Table Mountain Group Aquifer and the Cape Flats Aquifer by the City of Cape Town did not contribute to the city’s water supply during the crisis.
What did contribute to averting Day Zero — but difficult to quantify — was development of local groundwater supplies. The drilling of boreholes at private homes, hotels, businesses, schools, shopping malls and critical service delivery facilities contributed to reducing the use of potable municipal water from around 1,200ML/d down to 450ML/d — a stout effort by any measure.
While there was much talk about the PoDs (points of distribution) and residents collecting water from springs, the politicians and city officials were silent on how to keep critical service delivery facilities supplied with water and functional during the crisis.
The Western Cape Government does not have the mandate to manage and supply water. This is the responsibility of national government. However, they do have the mandate to run hospitals, schools and youth care centres. In response to the crisis, the provincial government investigated developing local groundwater supplies at 95 key hospitals, clinics, social development facilities and offices — a story that has never really been publicly told. This led to the implementation of 61 groundwater supply systems, 38 of which were in Cape Town.
Because groundwater cannot be seen, knowledge about it remains subject to mystical claims and much speculation. One only has to think back to the concerns expressed about the impact of the drought on aquifers, saline intrusion and overexploitation; or worse yet — the so-called powers of the water diviner. It is the responsibility of the professional hydrogeologist to make the invisible visible.
Groundwater is governed by the laws of science and there is a large body of well-qualified South African specialists who understand the occurrence and behaviour of the resource. At last count, this included 137 PhD graduates and more than 400 students with MSc degrees in hydrogeology. Sadly, it is also true that there is a body of chancers and charlatans who misrepresent the resource.
In my work with the Western Cape Government, we deployed about 150 data loggers across the province. Data loggers were deployed at each of the 38 facilities at which groundwater supply systems were implemented. These devices are hung in boreholes and allow us to measure depth to groundwater as often as we like, but typically between every 30 minutes and three hours. Together with measurements made manually, we can assess the response of the groundwater system to both natural influences and pumping.
A review of this data allows us to observe groundwater levels at the height of the drought and during the post-drought period of average to above-average rainfall. This is a good indicator of the state of aquifers.
It is well established that groundwater levels in and around Cape Town are deepest at the end of summer (April) and shallowest at the end of winter following recharge (October). The graphs presented below show that the drought had limited, if any, effect on groundwater levels and hence the volume of groundwater held in storage.
Monitoring at a hospital on the Cape Flats showed that in April 2018 the water table was 3.3m below surface. Subsequent April measurements were 3.8m, 3.4m and 3.1m, with the latter measurement being the result of the early rains that fell in the summer of 2021. Nonetheless, these are all in the same range and are not significantly different. The winter highs of 2018, 2020 and 2021 were also similar at a depth of about 2.0m. Our measurement of October 2019 was affected by pumping and not reflective of natural conditions. This range of 2m between winter and summer is within that observed previously in the Cape Flats Aquifer.
Integration of various databases showed most private domestic boreholes were drilled in the leafy suburbs stretching from Rondebosch southwards to Lakeside. Drillers were kept busy and unverified claims suggested more than 100 drilling rigs set up shop in Cape Town to meet the drought-driven demand. Naturally, this raised concerns about the impact on the underlying aquifers. Groundwater level data from hospitals in Newlands and Wynberg demonstrate that these concerns were unfounded.
The pattern and range of groundwater fluctuations observed at Newlands and Wynberg were similar to those observed on the Cape Flats. Importantly, there was no declining trend. We also did not observe declining trends in other boreholes we monitor. This indicates that neither the drought nor the localised groundwater abstraction negatively impacted the underlying aquifers in any significant way.
Groundwater has been used for decades in the Philippi Horticultural Area for irrigation, supplying Cape Town with some 70% of its fresh produce. This large-scale and widespread abstraction may have a different effect to that of the smaller, localised groundwater use. The data showed that during and after the drought, groundwater levels stayed within natural ranges and no negative impact on the aquifers could be detected. Essentially, the aquifers remained full.
The use of groundwater within the metropole is small in relation to the recharge the aquifers receive — even when the rainfall received was about 60% of the average and the large downpours critical for recharge were absent.
With this knowledge, sustainable groundwater use must form part of the future water supply in Cape Town. In addition to municipal abstraction from the Atlantis Aquifer, the Cape Flats Aquifer and the Table Mountain Group Aquifer, this can include use of groundwater for garden and sports field irrigation through to the supply of shopping malls, hotels and critical service delivery facilities such as hospital, clinics, prisons and youth care centres.
However, this has to be underpinned by appropriate and coordinated monitoring, considered review and informed management by the mandated authority. This is the only way we can make this invisible resource visible and achieve sustainable groundwater use in Cape Town.
Through engaging with students and an intern programme for early career hydrogeologists, the monitoring could provide a much-needed boost to capacity building in this field. DM
Dr Roger Parsons is a hydrogeologist with almost 40 years’ of experience in the field. He is the Director of Parsons & Associates Specialist Groundwater Consultants and was recently awarded the Ground Water Medal for his outstanding service and dedication to furthering the science and technology of groundwater. He is past chair of the provincial branch of the Ground Water Division. He writes in his private capacity.