Groundwater

1. Basic concepts and groundwater movement

Groundwater forms mainly when precipitation and surface water infiltrate the soil and percolate downward into saturated geological formations. The unsaturated or vadose zone above the water table contains both air and water. Below it, interconnected pores or fractures are saturated. Recharge can occur through rainfall, riverbeds, tanks, canal seepage and irrigation return flows; discharge occurs through springs, streams, pumping and evapotranspiration where groundwater is shallow.

Porosity describes the proportion of void space in a material, whereas permeability describes its capacity to transmit water. Clay can have high porosity but low permeability because its pores are small and poorly connected for flow. Coarse sand and gravel commonly form productive aquifers. An aquitard transmits water slowly and restricts movement between aquifers. Therefore, a porous rock is not necessarily a good source for a high-yielding well.

An unconfined aquifer has the water table as its upper boundary. A confined aquifer lies beneath a relatively impermeable layer and contains water under pressure. Water in a well tapping it rises to the potentiometric level; it flows naturally at the surface only when this level is above ground elevation. Pumping produces drawdown and a cone of depression. Groundwater generally moves from higher to lower hydraulic head, not necessarily in the same direction as surface slope.

• A perched water table is a local saturated zone above the main water table, supported by a discontinuous low-permeability layer.
• Groundwater sustains river baseflow during dry periods, while some river reaches lose water to underlying aquifers.
• Deep groundwater is not automatically renewable on human timescales or free from contamination.

2. Regional distribution of groundwater in India

The Indo-Gangetic-Brahmaputra plains contain extensive alluvial aquifer systems comprising layers of sand, silt, clay and gravel. Their considerable thickness and storage generally support high-yielding wells. However, aquifer productivity varies locally, and clay layers can separate shallow and deeper groundwater. Intensive irrigation has caused major depletion in parts of Punjab, Haryana and western Uttar Pradesh, while shallow groundwater and waterlogging occur in other alluvial areas.

Much of peninsular India consists of hard rocks such as granite, gneiss and basalt. Fresh crystalline rocks have little primary porosity; groundwater occurs mainly in weathered mantles, joints and fractures. Storage is often limited and discontinuous, making borewell yields highly variable even over short distances. In the Deccan basalt region, weathered zones, fractures and some vesicular flow horizons can store water, whereas massive basalt layers may restrict movement.

Arid western India receives limited and irregular recharge, while high evaporation and geological conditions contribute to salinity in many locations. Coastal and deltaic aquifers may be productive but are vulnerable to seawater intrusion when excessive pumping lowers freshwater heads. Himalayan groundwater commonly emerges as springs from fractured rocks and slope deposits. Springshed recharge areas do not always coincide with visible surface-water catchments, so geological mapping is important for spring restoration.

• Groundwater potential depends on rainfall, lithology, weathering, fractures, slope and recharge opportunities, not rainfall alone.
• Alluvial plains generally provide greater regional storage than hard-rock terrains, but neither setting guarantees sustainable extraction.
• In islands, excessive pumping can disturb thin freshwater lenses overlying saline groundwater.

3. Assessment, extraction and environmental impacts

The Central Ground Water Board and state agencies periodically assess dynamic groundwater resources. Annual extractable groundwater resource is estimated after allowing for natural discharge from annual recharge. The stage of groundwater extraction is annual extraction divided by annual extractable resource, multiplied by 100. Assessment units are generally administrative units such as blocks, mandals or talukas. A national or state average therefore cannot establish whether an individual village has a sustainable water balance.

Under the stage-based classification, an assessment unit is safe at extraction of 70% or less, semi-critical above 70% and up to 90%, critical above 90% and up to 100%, and over-exploited above 100%. Over-exploitation means annual abstraction exceeds the assessed annually extractable resource; it does not mean that the aquifer is already empty. Similarly, a safe category is not a certificate of drinking-water quality or an assurance against seasonal shortages.

Expansion of private wells, assured access to irrigation, subsidised electricity and procurement incentives for water-intensive crops have increased extraction. Groundwater allows farmers to irrigate on demand, but uncoordinated pumping creates a common-pool resource problem: one user’s abstraction can lower neighbouring wells. Depletion raises pumping costs, disadvantages households unable to deepen wells, reduces dry-season streamflow and can damage groundwater-dependent ecosystems.

• Land subsidence can occur where declining groundwater pressure causes compressible aquifer sediments to compact.
• Canal seepage and excessive irrigation can create shallow water tables, waterlogging and secondary salinisation where drainage is inadequate.
• Greater irrigation efficiency does not necessarily reduce basin-wide extraction if farmers expand irrigated area or shift to thirstier crops.

4. Groundwater quality and contamination

Groundwater contamination may be geogenic, originating in rocks and sediments, or anthropogenic, caused by human activities. Arsenic contamination is associated with parts of the Ganga-Brahmaputra alluvial region, notably areas of West Bengal, Bihar, Uttar Pradesh and Assam. In many affected aquifers, reducing conditions mobilise arsenic associated with iron minerals. Long-term exposure can cause skin lesions and cancers; arsenic cannot reliably be detected by taste, smell or appearance.

Excess fluoride occurs in parts of Rajasthan, Telangana, Andhra Pradesh, Karnataka and other states, commonly where fluoride-bearing minerals interact with groundwater under favourable geochemical conditions. Prolonged consumption can cause dental and skeletal fluorosis. Nitrate pollution frequently reflects fertilisers, manure, leaking sewers and poorly located sanitation systems. Elevated nitrate poses particular risks to infants, including methaemoglobinaemia.

Salinity may arise from geological sources, evaporation, irrigation return flows or seawater intrusion. Industrial effluents and waste-disposal sites can introduce heavy metals and organic pollutants. Contamination may persist because groundwater moves slowly and aquifer cleanup is difficult. Prevention through source protection, safe sanitation, effluent control and regular testing is usually more effective than remediation. Boiling does not remove dissolved arsenic, fluoride or salts and can concentrate them.

• A deeper borewell is not a universally safe substitute: water quality and hydraulic connections must be tested.
• Artificial recharge requires source-water quality checks; contaminated runoff can transfer pollutants into an aquifer.

5. Governance and sustainable management

Water is primarily a State List subject under Entry 17, subject to Union powers concerning inter-state rivers and river valleys under Entry 56. Groundwater governance therefore involves both state institutions and central agencies. The Central Ground Water Authority was constituted in 1997 under the Environment (Protection) Act, 1986. It regulates specified groundwater withdrawals through mechanisms including no-objection certificates, while state laws and authorities also operate. Groundwater access has historically been strongly associated with landholding, complicating management of a shared aquifer.

The National Aquifer Mapping and Management Programme develops aquifer maps and management plans. Atal Bhujal Yojana, launched in 2019, is a Central Sector Scheme supported by the World Bank and designed around community-led groundwater management in selected areas of Gujarat, Haryana, Karnataka, Madhya Pradesh, Maharashtra, Rajasthan and Uttar Pradesh. Its emphasis includes water budgeting, public disclosure and demand-side interventions, rather than simply constructing more recharge structures.

Sustainable management must combine recharge protection with extraction control. Suitable measures include restoring tanks and wetlands, protecting floodplains, harvesting roof runoff and building scientifically sited recharge structures. Crop diversification, carefully designed power incentives, irrigation scheduling and community water budgets can reduce demand. Recharge structures require permeable receiving formations, available aquifer storage and clean source water. Aquifer-scale monitoring and equitable access are essential because conservation by one farmer can otherwise be offset by increased pumping by another.

• Conjunctive use means coordinated use of surface water and groundwater to improve reliability and manage waterlogging or depletion.
• Springshed management combines geological investigation, recharge-area treatment and community protection of mountain water sources.
• Track water levels, extraction, water quality and ecological outcomes rather than judging success only by structures constructed.

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