Groundwater depletion and contamination

Detail

1. What groundwater depletion is

• Groundwater: water stored underground in the gaps of soil, sand and rock.
• Aquifer: a layer of rock or sediment that holds this water and lets it flow. Wells and tubewells draw water from aquifers.
• Water table: the top level of this underground water.
• Recharge: rain, canal water and river water soaking down into the aquifer and refilling it.
• Groundwater depletion: when more water is pumped out than is recharged, the aquifer slowly empties.
• Extraction is greater than recharge → the water table falls.
• The pump has to lift water from deeper → pumping costs rise (more electricity or diesel).

• Shallow wells and hand pumps go dry → wells fail. Farmers then drill deeper and pay more.

• Stage of groundwater extraction: the yearly groundwater taken out, shown as a percentage of the yearly groundwater that can be safely taken out. Above 100% means the aquifer is being "mined", because more leaves it each year than enters it.

2. How assessment units are classified (CGWB)

• The CGWB (Central Ground Water Board) and the states assess groundwater in assessment units: blocks, mandals or talukas.
• 2024 assessment, all-India [2][3]:

Category	Units (2024)	Share
Safe	4,951	73.4%
Semi-critical	711	10.5%
Critical	206	3.05%
Over-exploited	751	11.1%
Total	6,746	—

• National average stage of extraction: 60.47% (2024) [2].
• Trend: the share of over-exploited units fell from 17.24% (2017) to 11.13% (2024) [2][3].
• Total yearly recharge rose by about 15 BCM (billion cubic metres) between the 2017 and 2024 assessments [2].

• Extraction fell by about 3 BCM over the same period [2].

• Where over-exploitation is concentrated: north-west India, that is, parts of Punjab, Haryana, Delhi and western Uttar Pradesh [4].

• States above 100% extraction: Punjab, Haryana and Rajasthan. In the 2022 assessment Punjab stood at 165.99% and Rajasthan at 151.07% [4]. These three states had more over-exploited units in 2022 than in 2017 [4].

3. Causes of depletion across India

• Tubewells in the Green Revolution belt: heavy tubewell irrigation in Punjab, Haryana and western Uttar Pradesh has lowered the water table.
• Soil salinity (salt build-up in soil): intensive irrigation is also making the soil saltier in these areas.
• Too much water is applied and drainage is poor → the water table rises in places, or water evaporates from the surface.

• The water leaves its dissolved salts behind → a white salt crust forms → crop yields fall.

• Private pumping in villages and cities:

• Farmers run their own private tubewells.
• Urban housing societies and colonies pump water from their own borewells.

• Together they drain aquifers faster than rain can refill them. This threatens food security (enough food for everyone, every year).

• The Ganga basin under stress:

• The basin feeds over 500 million people.
• It is stressed by changing glacier melt, water diverted away from the river, heavy groundwater pumping and dams.
• Less water stays in the river, so less soaks into the aquifers below.

4. Case study: the Punjab groundwater crisis

Why it happened (cause → process → result):

• HYV crops:
• From the 1960s, the Green Revolution brought HYV (high-yielding variety) seeds of wheat and paddy.
• Paddy (rice) needs standing water for months. It is not a natural crop for semi-arid Punjab.

• So demand for irrigation water rose sharply.

• Free electricity:

• Farms got free power → there was no cost to running pumps longer → over-pumping.

• Agro-chemicals:

• Pesticides and fertilisers seeped down through the soil into the aquifers → depletion and contamination happened together.

How bad it is:

• About 80% of Punjab's area is classed as overexploited (NCERT).
• Across a large part of the state, water lies deeper than about 30 m (NCERT: Fig. 1.17, June 2022 map).
• Stage of extraction: 165.99% (2022 assessment) [4]. The 2024 assessment put it at about 156%, with about three-quarters of its blocks over-exploited.
• The lesson: short-term food security, with Punjab as India's "granary", came at a long-term cost to its water.

Responses:

• Punjab Preservation of Subsoil Water Act, 2009: this law stops paddy from being transplanted before a notified date in June. Planting then comes closer to the monsoon, so less groundwater is pumped in the hottest weeks.
• DSR (direct-seeded rice): rice seed is sown straight into the field instead of being transplanted into flooded nurseries and fields. It uses less water.
• Crop diversification: moving from paddy to maize, pulses, cotton or fruits that need less water.
• Power-subsidy reform: debates on metering farm power or giving cash to farmers who use less.
• Better use of canal water: using canal water efficiently so that less groundwater is pumped.

5. Groundwater contamination: the big picture

• Groundwater contamination: harmful substances in groundwater go above safe drinking-water limits. These limits are set by the BIS (Bureau of Indian Standards).
• Two kinds of source:
• Geogenic: it comes naturally from the rocks and sediments (arsenic, fluoride, uranium).

• Anthropogenic: it comes from human activity (nitrates from fertiliser and sewage, heavy metals from industry).

• Over-withdrawal makes both worse: it raised fluoride in Rajasthan and Maharashtra and arsenic in parts of West Bengal and Bihar. Heavy metals and nitrates are found elsewhere.

• Annual Ground Water Quality Report 2024 (CGWB, released December 2024) [5]:
• It is based on samples from 15,259 monitoring locations [5].
• Nitrate was above the limit in 19.8% of samples. This is the most widespread problem [5].
• Fluoride was high in 9.04% of samples [5].
• Arsenic was high in 3.1% of samples [5].
• 100% of samples met BIS standards in Arunachal Pradesh, Mizoram, Meghalaya and Jammu and Kashmir [5].
• Contamination was found in isolated pockets of Rajasthan, Haryana and Andhra Pradesh [5].

6. Arsenic contamination

• Nature: geogenic. The arsenic is naturally present in the sediments.
• Where it comes from: young (Holocene, meaning laid down in roughly the last 10,000 years) alluvial sediments. Alluvium is the silt and sand that rivers deposit.
• Process:
• Rivers deposit sediments rich in iron oxide, with arsenic stuck to its surface.
• In the oxygen-poor water underground, the iron oxide dissolves → arsenic is released into the water.

• Heavy pumping pulls this water quickly through the aquifer and makes the problem worse.

• Where: the Ganga-Brahmaputra plains of West Bengal, Bihar, Uttar Pradesh and Assam, and across the border in Bangladesh. This is the world's worst-known arsenic belt.

• Health effects: long exposure causes skin lesions (dark patches and hard spots on the palms and soles) and cancers of the skin and internal organs.
• Response: the Jal Kranti Abhiyan (2015) proposed arsenic-free wells.

7. Fluoride contamination

• Nature: geogenic. It comes from fluoride-bearing hard rocks (such as granite and gneiss) in dry areas.
• Process:
• Water stays in contact with these rocks for a long time → fluoride dissolves into it.
• Low rainfall means little fresh water to dilute it → the concentration builds up.

• Over-withdrawal pulls up older water from deeper down, which has more fluoride.

• Where: Rajasthan, Maharashtra, Telangana (Nalgonda district), Andhra Pradesh, Gujarat and Karnataka. This is mostly the dry Peninsular hard-rock belt and the Thar margin.

• Health effects: fluorosis:
• Dental fluorosis: teeth become stained and mottled.

• Skeletal fluorosis: bones become stiff and bent, and joints become painful.

• Remedy: rainwater recharge (sending rainwater into the aquifer) dilutes the fluoride.

8. Uranium, nitrate and salinity

• Uranium in Punjab's Malwa:
• Malwa is the region of Punjab south of the Sutlej.
• Uranium has been found in 9 districts: Bathinda, Moga, Faridkot, Fatehgarh Sahib, Ferozepur, Ludhiana, Muktsar, Patiala and Sangrur [6].
• Arsenic above the limit has been found in Mansa, Faridkot and Sangrur [6].

• CGWB 2024: in Punjab, 32.6% of samples were above the uranium limit, 4.85% were above the arsenic limit and 0.44% were above the selenium limit. Trace elements were tested at 908 locations [7].

• Nitrate: it comes from heavy use of nitrogen fertiliser and from leaking sewage and septic tanks. It is the most common contaminant in India [5].

• Salinity intrusion (seawater entering coastal aquifers):
• Too much pumping near the coast → fresh groundwater pressure falls → seawater moves inland into the aquifer.
• Wells turn salty and cannot be used for drinking or irrigation.
• Seen in Saurashtra (Gujarat) and coastal Tamil Nadu.

9. Contamination belts of India

Contaminant	Source	Main belt	Rock/sediment setting	Health effect
Arsenic	Geogenic	Ganga-Brahmaputra plains (West Bengal, Bihar, UP, Assam)	Young Holocene alluvium	Skin lesions, cancer
Fluoride	Geogenic	Dry hard-rock belt (Rajasthan, Telangana, Andhra Pradesh, Gujarat, Karnataka, Maharashtra)	Granite and gneiss	Dental and skeletal fluorosis
Uranium	Geogenic (worsened by pumping)	Malwa, Punjab [6]	Alluvium of the semi-arid south-west	Kidney damage
Nitrate	Anthropogenic	Widespread (19.8% of samples, 2024) [5]	Farmland, settlements	Blue-baby syndrome in infants
Salinity	Seawater intrusion	Saurashtra, coastal Tamil Nadu	Coastal aquifers	Water unfit for drinking and irrigation

Prelims Hooks

• Arsenic in India's groundwater is geogenic. It comes from Holocene alluvium in the Ganga-Brahmaputra plains, not mainly from industry.
• Fluoride and the Nalgonda district (Telangana) are a classic pair. Fluoride is tied to hard rocks in dry areas and causes dental and skeletal fluorosis.
• Nitrate is India's most common groundwater contaminant: 19.8% of samples above the limit, ahead of fluoride (9.04%) and arsenic (3.1%) (CGWB Annual Ground Water Quality Report 2024) [5].
• CGWB 2024: 751 of 6,746 assessment units (11.1%) are over-exploited. The national stage of extraction is 60.47% [2][3].
• The share of over-exploited units fell from 17.24% (2017) to 11.13% (2024). Trap: India's groundwater did not simply get worse everywhere [2].
• States with stage of extraction above 100%: Punjab, Haryana, Rajasthan [4].
• Uranium in groundwater belongs to the Malwa belt of Punjab (Bathinda, Moga, Faridkot and others) [6].
• Salinity intrusion into coastal aquifers: Saurashtra and coastal Tamil Nadu.
• Punjab Preservation of Subsoil Water Act, 2009: it delays paddy transplanting to save groundwater.

Mains Points

• Causes of depletion (GS-I/III): the HYV wheat-paddy cycle since the 1960s, free farm power, tubewells growing without control, and urban borewells. Rain cannot refill aquifers as fast as they are emptied. North-west India (Punjab, Haryana, Delhi, western UP) is the over-exploited core, with Punjab at 165.99% (2022) [4].
• Effects: falling water tables, rising pumping costs and failed wells, soil salinity, and a threat to long-term food security. Pumping also brings up arsenic, fluoride and uranium. Depletion is therefore a public-health crisis too.
• Pattern of contamination: geology decides the contaminant. Young alluvium gives arsenic in the Ganga-Brahmaputra plains. Old hard rock in dry areas gives fluoride in the Peninsula and Rajasthan. Farming and towns give nitrate everywhere. Coastal pumping draws in seawater in Saurashtra and Tamil Nadu.
• Policy responses: CGWB assessments and quality reports, Punjab's 2009 subsoil water law, DSR and crop diversification, power-subsidy reform, arsenic-free wells (Jal Kranti Abhiyan), rainwater recharge to dilute fluoride, and safe piped water under the Jal Jeevan Mission. The recovery from 17.24% to 11.13% over-exploited units between 2017 and 2024 shows that recharge can work [2].