Water-Bearing Characteristics of Rocks | Geology Optional Notes for UPSC PDF Download

Factors Affecting Water-bearing Capacity of Rocks

• Process of Sedimentation

  Sedimentation occurs through various natural processes like weathering. Different agents such as water, gravity, and air play crucial roles in the formation of sediments. For instance, sediments deposited by air are typically well-sorted and possess high porosity, while carbonated rocks may exhibit variable pore space and limited water-bearing capacity due to poor porosity. Additionally, chemically precipitated sedimentary rocks tend to have low water-bearing capacity due to water being stored in dissolution cavities, which are common in carbonates.

• Expansion of Gases

  Lava and mud flows containing gases can lead to the creation of cavities as these gases escape, forming bubbles. Despite having interconnected voids, rocks like pumice may exhibit low water-bearing capacity due to this gas-expansion process.

• Crystallization of Igneous Rock

  The water-bearing capacity of igneous rocks is influenced by the inter-crystalline spaces within them. This aspect must be taken into account when determining the water-bearing status of igneous rocks.

• Mechanical Disruption

  Mechanical forces acting on rocks can create secondary structures that enhance their water-bearing characteristics. It's important to note that rock classification based solely on lithological features may require adjustments based on regional tectonic conditions.

• Chemical Action

  Dissolution caused by carbonic and organic acids can increase the water storage space in impermeable mediums. The presence of springs in carbonate rocks is often attributed to the chemical action of water on these rocks.

• Cementation

  Increased cementation in sedimentary rocks can reduce their water-bearing capacity.

• Sediment Compaction

  Rock compaction due to burial leads to a reduction in pore space, subsequently decreasing water-bearing capacity.

• Metamorphism

  The process of metamorphism can decrease both porosity and permeability in rocks. High temperatures and pressures resulting from metamorphic processes can lower the water-bearing capacity of rocks.

Weathering and Biological Processes

Weathering

• Weathering leads to an increase in porosity of rocks. Physical weathering plays a role in widening cracks and joints within rocks, thereby enhancing secondary permeability and increasing water yield in impermeable media.
• Chemical weathering also contributes to the water-bearing capacity of rocks. For instance, the dissolution of carbonate rocks by acidic water creates dissolution pits and cavities, elevating the water-bearing capacity of impermeable sedimentary rocks.

Biological Processes

• Biological processes in the shallow regions impact the water-bearing capacity of rocks significantly. Plant roots, for example, create fractures in rocks, while soil water is enriched by humus, further enhancing water retention.
• Additionally, animal burrows play a role in increasing the water-bearing capacity of rock formations.

Water-bearing Capacity of Sedimentary, Metamorphic, and Igneous Rocks

• Sedimentary Rocks
  • Sedimentary deposits can be porous when loose, but compacted sedimentary rocks may retain some porosity. Cemented rocks are mostly impervious.
  • The water-bearing capacity of sedimentary rocks depends on factors like grain size, porosity, and permeability.
  • Permeability is crucial for water yield, with grain size influencing storage capacity. For example, coarse sandstone has good storage but poor yield.
  • Claystone has high porosity but poor water yield.
• Igneous Rocks
  • Igneous rocks like granitic and basaltic rocks are generally poor sources of water due to their compact nature.
  • Weathering and joint development can increase their water-bearing capacity over time.
• Metamorphic Rocks
  • Metamorphic rocks, originating from sedimentary rocks, vary in water-bearing capacity based on their parent material and metamorphism.
  • Rocks like slate may be porous but have limited water-bearing capacity, while schist and gneiss are more complex and have poorer capacity.