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Can Water Flow Through Granite?

Published in Hydrogeology 4 mins read

No, water generally cannot flow easily through the solid, intact rock matrix of granite. While water can certainly be found within granite formations, its movement is typically restricted to fractures, fissures, and weathered zones, rather than permeating through the dense rock itself.

Understanding Granite's Properties

Granite is an igneous rock formed from the slow crystallization of magma below the Earth's surface. Its characteristic properties, such as its high density and tightly interlocked mineral grains (like quartz, feldspar, and mica), make its inherent porosity and permeability very low.

The Role of Porosity and Permeability

To understand water flow through rocks, it's essential to grasp two key concepts:

  • Porosity: The amount of empty space (pores) within a rock.
  • Permeability: The ability of fluids (like water) to pass through these interconnected pores or fractures.

Granite typically has very low primary porosity (pores within the rock fabric) and even lower primary permeability. This means there are few interconnected spaces for water to travel through its solid mass.

Granite's Water-Bearing Capacity

The provided reference clearly states the practical implications of granite's structure:
granite, due to its high density will yield almost no water to a well, even if it is relatively close to the surface. This highlights its poor capacity for water storage and transmission through its solid rock body.

In contrast, other rock types behave very differently. For instance, the reference notes: "However, sandstone in contrast, which is a porous rock, may lie extremely deep in the ground and can contain water at the rate of hundreds of litres per minute." This stark difference underscores granite's low capability for water flow through its matrix.

How Water Exists in Granite Formations

While water doesn't easily flow through granite's solid matrix, it can definitely be found within granite geological formations. This occurs primarily through:

  • Fractures and Joints: Over geological time, tectonic forces, weathering, and stress relief can create cracks, fissures, and joints in granite bedrock. These secondary features provide pathways for water to infiltrate and accumulate. Water in wells drilled into granite often comes from intercepting these water-filled fractures.
  • Weathered Zones: Near the surface, granite can undergo weathering, transforming its minerals into clay and creating more porous, permeable material. Water can flow more readily through this weathered granite (regolith) than through fresh bedrock.
  • Fault Zones: Major geological faults within granite can create extensive zones of fractured and crushed rock, acting as conduits for groundwater flow.

Comparison: Granite vs. Porous Rocks

The table below summarizes the difference in water-bearing capacity between granite and a porous rock like sandstone, based on the provided reference:

Feature Granite Sandstone
Porosity Very Low (primary) High (primary)
Permeability Very Low (primary); relies on fractures/joints for water movement High
Water Yield Yields almost no water to a well, even if close to the surface Can contain water at hundreds of litres per minute, even at great depths
Water Storage Primarily in secondary structures (fractures, faults, weathered zones) Within the rock matrix (interconnected pores) and potentially fractures too
Density High Variable, often lower than granite

Practical Implications for Water Wells

For practical applications, such as drilling a water well, understanding granite's properties is crucial:

  • Wells drilled into granite rely entirely on intercepting water-bearing fractures. Locating these can be challenging and often requires hydrogeological surveys.
  • The water yield from a granite well is highly variable, depending on the number, size, and interconnection of the fractures encountered.
  • In regions where granite is the dominant bedrock, groundwater resources can be limited and localized compared to areas with highly porous aquifers like sandstone or gravel.

In conclusion, while granite itself is not a porous medium through which water flows readily, water can still be a vital resource within granite landscapes by occupying and moving through its network of fractures and weathered zones.