9 dists overexploit groundwater

CHENNAI: Tamil Nadu extracts close to 74 per cent of its extractable groundwater resources per year and 14 per cent of its taluks come under the ‘over-exploited’ category. Shockingly, nine districts, including Chennai, extract more than 100 per cent of their extractable groundwater, mainly for irrigation purposes.

According to ‘Dynamic Ground Water Resources of India, 2023’, released by the Central Ground Water Board (CGWB), the total annual groundwater recharge of the State has been assessed at 21.59 bcm (billion cubic metres) and annual extractable groundwater resources at 19.51 bcm. The annual groundwater extraction is 14.42 bcm and the stage of groundwater extraction is 73.91 per cent.

Of the total 14.42 bcm of groundwater, as much as 13.48 bcm of water is drawn for irrigation purposes. On the other hand, 0.15 bcm and 0.79 bcm are only extracted for industrial and domestic purposes. Among the districts, Chennai extracts 127.47 per cent of its extractable groundwater, mainly for domestic use. The city has only 0.092 bcm of extractable groundwater, but draws 0.12 bcm, with 0.10 bcm going for domestic usage.

Other districts that extract more than 100 per cent are Dindigul, Madurai, Namakkal, Perambalur, Salem, Thanjavur, Tirupattur and Vellore. Interestingly, except Chennai, all the other districts, use most of it for irrigation purposes.

The study assessed 313 taluks in the State, of which 125 were found ‘safe’. While 100 taluks are ‘over-exploited’, 27 are categorised ‘critical’. As many as 56 taluks are under the ‘semi-critical’ category. Groundwater in five assessed taluks has salinity. Of the 16 taluks assessed in Chennai, 81 per cent (13 taluks) have been identified as ‘over-exploited’.

When asked the reason for the extraction of groundwater in higher quantities for domestic purposes in the city despite Chennai Metropolitan Water Supply and Sewerage Board (Metrowater) supplying more than 1,000 million litres per day (MLD), a Metrowater engineer said the city is also growing parallel to the increasing supply of water.

“Population is increasing in the city as is commercial and industrial activities. Also, peri-urban areas are growing. To meet the increasing demand, residents draw groundwater. Supply of 1,000 MLD is not sufficient as the city requires 1,500 to 1,600 MLD. Source and infrastructure to supply a huge amount of water is limited,” the engineer said.

A senior officer of Metrowater explained that some added areas in the city are yet to get pipe connections, so they extract groundwater. “It will take at least two more years to give pipe connections in those areas. We are developing sources to meet the demand as works of 150 MLD desalination plant is nearing completion and 400 MLD plant is ongoing,” he added.

Marginal increase in groundwater recharge across State

Out of almost 1.09 lakh sq km recharge-worthy area of the State, 31,130 sq km (28.64%) area is under ‘over-exploited’ category, 11,774 sq km (10.83%) under ‘critical’, 19,483 sq km (17.92%) under ‘semi-critical’, 44,393 sq km (40.84%) under ‘safe’ and 1,911 sq km (1.76%) under ‘saline’ categories.

Of the existing 19,505.70 mcm (million cubic metres) annual extractable groundwater resources available, 4,772.59 mcm (24.47%) are under ‘over-exploited’, 2,029.09 mcm (10.4%) under ‘critical’, 3,776.06 mcm (19.36%) under ‘semi-critical’ and 8,927.94 mcm (45.77%) under ‘safe’ categories.

In comparison to the 2022 assessment, the total annual groundwater recharge has marginally increased from 21.11 bcm to 21.59 bcm. The annual extractable resources have also marginally increased from 19.09 bcm to 19.51 bcm and the annual extraction has decreased marginally from 14.43 bcm to 14.42 bcm.

“Consequently, there is a decrease in the stage of extraction from 75.59 % to 73.91 % indicating overall improvement in the scenario. The increase in groundwater recharge is due to changes in rainfall pattern, and decreased extraction is due to decrease in the load on resources for irrigation and domestic sector,” the report pointed out.

It added that the groundwater availability is low in the southern peninsular India, including parts of Karnataka, TN, Telangana and Andhra, due to inherent characteristics of crystalline aquifers. Groundwater level is in the range of 5-10 metres below ground in TN.

Meanwhile, the Chennai Metropolitan Water Supply and Sewerage Board (Metro Water) is trying to bring implement a regulation to mandate industries to use recycled water for industrial purposes and ban the extraction of groundwater. Though the Metro Water has two tertiary sewage treatment plants of 90 MLD capacity, demand for recycled water from industries is low.

Sekar Raghavan of Rain Centre, an expert in groundwater management, explained that citizens were extracting groundwater from deep aquifers, which were difficult to recharge. “Shallow aquifers are good in Chennai, and can be recharged easily. But the wells are being closed in the city. Also, saturation of shallow aquifers is one of the reasons for frequent flooding,” he pointed out.

Raghavan added that after the recent rains, several monitoring wells of Metro Water recorded shallow groundwater levels. “Residents should use groundwater from shallow aquifers during the rainy season and deep aquifers during the summer,” he stated.

He also advised residents to use slotted-casing pipes (those with incisions) instead of PVC pipes for deep bore wells as the former can recharge the deep aquifer by shifting water from the shallow aquifer.