Fractured rock aquifers: on the importance of springs

Recently some of Umvoto’s geologists and hydrogeologists went on a hunt for discharge points (groundwater-fed springs) along a large-scale fault network that cuts through the Table Mountain Group (TMG) fractured aquifers. This was in connection with a project that Umvoto is involved in with the City of Cape Town that was launched towards the end of 2020. This project, TMG aquifer exploration and wellfield development as part of the City of Cape Town’s New Water Programme, is an exciting project Umvoto has been involved in various forms since 2002 and aims to help diversify City of Cape Town’s water supply in response to the recent “Day Zero” drought.

Densely fractured Table Mountain Group Rock with a groundwater-fed spring
Densely-fractured Table Mountain Group Rock: these fractures allow water to flow through, otherwise impermeable/solid rock and result in invisible groundwater being discharged as springs.

Fractured rock aquifers

To our delight, we were able to physically see the nature of the fault zone (titled the “Steenbras-Brandvlei Megafault Zone”) and its intensely broken network. These fractures, planes of weakness in the earth’s crust that form a break in a rock unit, are of interest to hydrogeologists as they allow water to flow through, otherwise impermeable/solid rock. Physically seeing closely-spaced fracture sets allows you to visually understand how water could possibly move through rock. Spring water discharging from these fractures are representations of generally “invisible” groundwater from (sometimes very deep) underground daylighting at surface, and they give us an important insight into various physical and chemical processes that occur within aquifers at depth. Fractured aquifers are an important groundwater resource in the Western and Eastern Cape, as represented by the extensively outcropping and fractured rocks of the TMG aquifers.

Isotope sampling

During this exercise, we also took stable isotope samples from the springs, that we hope will help us better delineate groundwater flow paths from source (i.e. where the water seeps into the rock) to discharge (i.e. where the water exits the aquifer) point. Stable isotopes, specifically oxygen-18 (18O) and deuterium/hydrogen-2(2H), are widely used in many hydrogeological investigations – this is because they are useful environmental tracers for tracking the movement of water, and understanding  processes that occur within the hydrological cycle. Isotopes are atoms of the same element that have differing atomic masses due to a differing number of neutrons, and they occur in fixed relative  abundances to one another. This isotope analysis is an important step in the process to understanding these large fractured TMG aquifer systems as a whole.