Model Answer
0 min readIntroduction
Water is a crucial resource, and its availability is significantly influenced by the geological characteristics of the subsurface. The ability of rocks to store and transmit water, known as their water-bearing properties, is determined by their porosity and permeability. Porosity refers to the void space within a rock, while permeability describes the interconnectedness of these voids, allowing water to flow through them. Understanding these properties is fundamental to groundwater exploration and management, impacting areas from agriculture to urban water supply.
Water-Bearing Properties of Rocks
The water-bearing capacity of rocks varies significantly based on their origin, composition, and structure. Rocks are broadly categorized into igneous, sedimentary, and metamorphic types, each exhibiting distinct characteristics.
Igneous Rocks
Generally, igneous rocks have low porosity and permeability due to their crystalline structure and lack of interconnected voids. However, this can vary:
- Intrusive Igneous Rocks (Granite, Diorite): These rocks cool slowly, forming large crystals with minimal pore space. They are typically aquicludes, meaning they impede water flow.
- Extrusive Igneous Rocks (Basalt, Rhyolite): Rapid cooling results in smaller crystals and sometimes vesicular textures (containing vesicles – gas bubbles). Vesicular basalts can have moderate porosity but often limited permeability unless fractured. Lava flows can create permeable layers if fractured.
Sedimentary Rocks
Sedimentary rocks are generally the best aquifers due to their inherent porosity and permeability. They are formed from the accumulation and cementation of sediments.
- Sandstone: Highly porous and permeable due to the rounded shape and size of sand grains, and the presence of interconnected pore spaces. It forms excellent aquifers.
- Limestone: Can be highly porous and permeable, especially if karstified (dissolved by groundwater, creating caves and fissures). Karst topography is a significant feature associated with limestone aquifers.
- Shale: Composed of fine-grained sediments, resulting in low porosity and permeability. It acts as an aquiclude, preventing water flow.
- Conglomerate: Variable porosity and permeability depending on the size and sorting of the clasts. Poorly sorted conglomerates have lower permeability.
Metamorphic Rocks
Metamorphic rocks exhibit variable water-bearing properties depending on the parent rock and the degree of metamorphism.
- Slate: Formed from shale, it retains low porosity and permeability, acting as an aquiclude.
- Schist & Gneiss: These rocks can develop secondary permeability due to fracturing and foliation (layered structure). However, their overall permeability is generally lower than sedimentary rocks.
- Marble: Formed from limestone, it can be porous and permeable, especially if it retains some karst features from its parent rock.
- Quartzite: Formed from sandstone, it generally has low porosity and permeability due to the interlocking quartz grains.
| Rock Type | Porosity | Permeability | Water-Bearing Capacity |
|---|---|---|---|
| Granite | Low | Very Low | Poor |
| Basalt | Low to Moderate | Low to Moderate | Moderate (if fractured) |
| Sandstone | High | High | Excellent |
| Shale | Low | Very Low | Poor |
| Limestone | Moderate to High | Moderate to High | Good to Excellent (especially karstified) |
Conclusion
In conclusion, the water-bearing properties of rocks are intrinsically linked to their geological characteristics. Sedimentary rocks, particularly sandstone and karstified limestone, generally serve as the most productive aquifers. Igneous and metamorphic rocks typically exhibit lower permeability, often acting as confining layers. Understanding these variations is crucial for effective groundwater resource management and sustainable water supply, especially in the context of increasing water demand and climate change.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.