Table Mountain Group Aquifers Explained

🌍 Understanding the Table Mountain Group Aquifer

The Table Mountain Group Aquifer (TMG) is a massive underground water system found deep within the sandstone mountains of the Western Cape — stretching beneath areas like Table Mountain, Stellenbosch, Hermanus, Worcester, and the Cederberg.

It’s made up of hard quartzitic sandstone and other layers of ancient rock that have cracks and fractures. Rainwater seeps into these cracks high up in the mountains, slowly filtering down until it gathers deep underground. This naturally filtered water becomes a clean and sustainable source of groundwater that can be accessed through a borehole.

In simple terms:
🌧️ Rainwater falls on the mountain → 💧 filters through the rock → 🪨 collects in cracks → 🚰 is pumped out via boreholes for use.

The Table Mountain Group Aquifers are deep, naturally formed groundwater systems that occur within the ancient sandstone layers of the Table Mountain Group. These layers form part of the Cape Supergroup – a geological formation that stretches across the Western Cape and includes the rock foundations beneath Table Mountain, the Hottentots Holland Mountains, and the Cederberg range.

These aquifers are recognised as some of South Africa’s most reliable and high-quality groundwater sources, supplying water to farms, homes, and municipalities.

The Table Mountain Group consists mainly of extremely hard quartzitic sandstones, interbedded with thin shale and siltstone layers. Over millions of years, these rocks were compressed, folded, and uplifted to form the Cape Fold Mountains. Cracks and fractures developed during this process – and these now act as natural conduits and storage zones for groundwater.

Because the rock itself is not porous, most of the water movement happens through these fractures. This makes the aquifers “secondary aquifers,” meaning they store and transmit water through cracks rather than through open pore spaces.

Rainfall on the mountain slopes infiltrates slowly into the ground. Water seeps into the cracks and joints within the sandstone and eventually collects in the deeper fractured zones. These layers can extend hundreds of metres underground and form interconnected groundwater reservoirs.

When a borehole is drilled into one of these water-bearing fractures, the result is a sustainable and often high-yield water source. The water is typically clear, fresh, and low in dissolved minerals, making it suitable for domestic, agricultural, and industrial use.

The Table Mountain Group Aquifers extend across a large area of the Western Cape. Major zones include Table Mountain and the Cape Peninsula, the Hottentots Holland Mountains near Somerset West, the Franschhoek and Stellenbosch regions, and the Cederberg Mountains and Overberg area.

They are generally divided into two broad types:

1. Unconfined or primary aquifers – closer to the surface and directly replenished by rainfall.
2. Confined or deep aquifers – located deeper underground, often under natural pressure, and tapped by deep boreholes.

Drilling into Table Mountain Group formations requires specialised equipment because the rock is extremely hard. Boreholes are typically drilled using down-the-hole (DTH) hammer rigs designed to penetrate dense sandstone. Once drilled, steel casing is installed to stabilise the borehole walls and prevent collapse.

Locating productive fracture zones is essential. Hydrogeological assessments and geological mapping help identify the most likely areas to yield high volumes of groundwater. In the Western Cape, boreholes tapping into TMG aquifers often provide excellent yields – anywhere from 0.5 to over 20 litres per second – depending on the number and size of the fractures encountered.

The TMG aquifers play a crucial role in supporting both ecosystems and people. They feed springs, wetlands, and rivers across the Cape region. The water that emerges from these systems is often of exceptional quality and has been a lifeline during droughts and municipal shortages.

Because these aquifers recharge slowly, responsible water extraction and monitoring are vital to ensure long-term sustainability.

Quick Reference Summary:

Rock Type: Hard quartzitic sandstone
Depth Range: 50 m – 1,000 m+
Typical Yield: 0.5 – 20 litres per second
Water Quality: Excellent (low mineral content)
Recharge Source: Rainfall and mountain runoff
Drilling Method: Down-the-hole hammer (DTH)
Common Locations: Somerset West, Stellenbosch, Cederberg, Table Mountain

The Table Mountain Group Aquifers are among the most valuable natural water systems in South Africa. Their combination of depth, geological stability, and recharge reliability makes them ideal for sustainable borehole water use. With the right geological knowledge and drilling technique, these aquifers provide a dependable source of clean water for homes, farms, and businesses across the Western Cape. Find out more about our FAQ on boreholes here.

Sources of reference
Researchgate
Researchgate map image “Extent of TMG formations and sub-groups in the Western Cape and the study area.”
Table Mountain Ground Water
Table Mountain Water Source Area maps

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