Groote Schuur groundwater probe
Springs, boreholes have enough good-quality water to irrigate historical estate, study shows
GROOTE Schuur Estate in Cape Town has been home to the VOC granary, Cecil John Rhodes and South Africa’s presidents. The Groote Schuur Minute between the ANC and FW de Klerk, then president of South Africa, was also signed here.
In a new development, to ensure that these historical grounds are well maintained in an eco-friendly way, the estate has been the site of a study to determine its groundwater potential for irrigation.
Groundwater is water found underground in porous sandy strata, and in the fractures and fissures of rock formations. Where groundwater occurs in reasonable quantities, it is referred to as an aquifer.
This study is part of a larger irrigation storage and supply system
‘Two possible boreholes drilled in 1961 have been discovered. These had been forgotten over the past decade due to work done’
upgrade to be undertaken by the Department of Public Works, the client. The lead consultant (civil engineering) is Aurecon and the groundwater assessment and hydrocensus have been undertaken by Muizenberg-based water and earth s ci ences consultancy Umvoto Africa.
The hydrocensus has involved investigating all current water points such as boreholes and springs within and surrounding the project area.
As much information as possible has already been gathered on factors such as construction of existing boreholes – depth, diameter, casing – yield of the borehole, how much water is used, what it’s used for and samples for hydrochemical analysis. This gives a good idea of the aquifer characteristics in the vicinity.
This on-site research is being supplemented with a desktop-based groundwater assessment – using data sources such as geological and hydrogeological maps, Google Earth mapping, old reports and Department of Water Affairs databases.
Using the Department of Water Affairs’ old National Groundwater Database, Umvoto has already discovered two possible boreholes drilled in 1961. These had been forgotten over the past decade due to the numerous contractors that have been doing work on the site over the years, says Umvoto managing director Rowena Hay.
“The project has been interesting because of its historical context, and also unravelling the complex surface water-groundwater interactions that occur in the area. These interactions have led to the development of numerous springs, which have been used for many years, in the Wynberg-Newlands-Rondebosch and city bowl areas. Two springs – Nursery Spring and Glenn Spring – are visible in the estate.”
Springs occur when groundwater emerges naturally on the earth’s surface. In this case, the springs are fed by groundwater leaking out of the Peninsula Formation sandstone that forms Table Mountain, which lies above the Cape Granite Suite and Malmesbury Group shale, which are relatively impermeable.
Rainwater flows into open surface fractures and faults within the Peninsula Formation aquifer, and then into larger fracture systems that extend from the Cape Peninsula into the underlying Malmesbury Group shale. Groundwater emerges at specific points or diffuse areas as springs or seeps, where these fracture and fault systems have closed sufficiently in the Malmesbury Group (either due to mechanical weakness or weathered clay infill) to cause the pressure head within the water to force it to the surface.
The perennial, strong spring flow and high water quality suggests the spring water is from the Peninsula Formation aquifer, despite exiting from the Malmesbury Group shales, which are generally low-yielding with poor water quality, says Hay.
Umvoto completed the report last month. Early indications are there is sufficient groundwater from both estate springs as well as current boreholes to supply a large portion, if not all, of the estate’s irrigation requirements, keeping this historical part of Cape Town in good condition for future generations.
Future work will include measuring seasonal spring flow, as well as developing infrastructure to incorporate the springs into the current irrigation system. The sustainable yields of the current boreholes, as well as determining whether new pump infrastructure will be required, will also be investigated.
FOCUSED: Umvoto trainee hydrogeologist Sinawo Jack samples water from the borehole next to Genadendal in the Groote Schuur Estate.