Groundwater is the water present beneath Earth’s surface in soil pore spaces and in the fractures of rock formations. It is a vital component of the hydrosphere and serves as the primary source of freshwater for nearly half of the global population. In India, groundwater is the backbone of agriculture, supporting over 60% of irrigated land and 85% of rural drinking water needs.
Geological Occurrence of Groundwater
Groundwater does not exist in vast underground “lakes” but rather in the tiny spaces within geological materials. Its occurrence is defined by two vertical zones:
Zone of Aeration (Vadose Zone)
This is the layer nearest to the surface where the gaps between soil and rock contain both air and water. Water in this zone is held by surface tension and is known as soil moisture.
Zone of Saturation (Phreatic Zone)
Located below the zone of aeration, this is where all pore spaces are completely filled with water. The upper limit of this zone is known as the Water Table.
Key Hydrological Properties of Rocks
The ability of a geological formation to hold and transmit water depends on two properties:
- Porosity: The percentage of the total volume of a rock or sediment that consists of open spaces (pores). It determines how much water the rock can hold.
- Permeability: The ability of a rock or sediment to transmit fluids through its interconnected pores. It determines how easily water can flow through the material.
Types of Groundwater Reservoirs (Aquifers)
An Aquifer is a saturated geological unit that can yield a significant quantity of water under ordinary hydraulic gradients.
| Aquifer Type | Characteristics |
|---|---|
| Unconfined Aquifer | The water table is exposed to the atmosphere through the soil above. It is also called a “phreatic” or “water-table” aquifer. |
| Confined Aquifer | Trapped between two impermeable layers (Aquitards). The water is under pressure, and when tapped by a well, the water level rises above the aquifer’s top. |
| Artesian Aquifer | A type of confined aquifer where the hydraulic pressure is high enough to push water to the surface without a pump. |
| Perched Aquifer | Occurs when an impermeable layer (like clay) is situated above the main water table, “trapping” a small lens of water in the aeration zone. |
Global Distribution of Groundwater
Groundwater distribution is highly variable, dictated by climate (recharge) and geology (storage).
- Large Sedimentary Basins: The most productive aquifers are found in deep sedimentary basins such as the Indo-Gangetic Plain (India), the Great Artesian Basin (Australia), and the High Plains (Ogallala) Aquifer (USA).
- Hard Rock Areas: Regions with igneous or metamorphic rocks (like the Deccan Plateau) have low primary porosity. Groundwater here is stored only in secondary features like fractures, fissures, and weathered zones.
- Karst Regions: In limestone areas, groundwater forms large underground channels and caves due to the chemical dissolution of rock.
Groundwater Dynamics and Movement
- Recharge: The process by which water from the surface (rain, rivers) moves down into the aquifer.
- Discharge: The exit of groundwater to the surface, appearing as springs, base flow in rivers, or through human-made wells.
- Aquiclude: A geological formation (like clay) that may contain water but is incapable of transmitting it fast enough to furnish a supply.
- Aquitard: A layer that retards the flow of water (low permeability) but does not stop it completely.
Status of Groundwater in India
India is the largest user of groundwater in the world, extracting more than the USA and China combined.
- Over-Exploited Blocks: Found primarily in Punjab, Haryana, Rajasthan, and Tamil Nadu.
- Dynamic Groundwater Resources: The Central Ground Water Board (CGWB) monitors “Stage of Extraction” (Ratio of extraction to recharge). A stage above 100% is categorized as “Over-exploited.”
- Salinity Issues: In coastal regions (e.g., Gujarat, Tamil Nadu), over-pumping causes “Saltwater Intrusion,” where seawater enters freshwater aquifers.
Important Trivia for Prelims
- Specific Yield: The volume of water that an aquifer will release under gravity.
- Piezometric Level: The level to which water will rise in a well-penetrating confined aquifer.
- Connate Water: Water trapped in the pores of sedimentary rocks at the time of their deposition (often saline).
- Juvenile Water: New water of magmatic origin that has never been part of the hydrological cycle before.
- Atal Bhujal Yojana: A central sector scheme aimed at sustainable groundwater management through community participation in water-stressed blocks.