CHENNAI: Astudy by Anna University in 2018 has revealed that over a span of 44 years, the groundwater recharge potential of Chennai and its neighbouring districts of Kanchipuram and Tiruvallur has been reduced by 63 per cent. Rapid urbanisation coupled with vanishing water bodies and insufficient rejuvenation of the aquifer has led to this drastic drop, said experts.
To get to the crux of Chennai’s annual struggle for water and plummeting groundwater levels, it is pertinent to understand the relationship between the city’s geological structure and distribution of water resources. Experts say that factors like geology, geomorphology, soil nature, drainage potential, rainfall and land use are key to indicate occurrence of groundwater in any location.
Chennai’s geology
Tamil Nadu, like any other southern state, is mostly made up of Charnockite rocks, which has good water absorption qualities. In the Chennai aquifer system, which consists of three neighbouring districts, the Western and Southeastern parts are hard rock regions — consisting of Gneissic and Charnockite rocks, which store a moderate amount of groundwater, potable in nature.
The Northwestern parts majorly consist of Gondwana rocks that yield groundwater of poor quality. But the aquifer in this region is of two types — one with a thick top layer, where water is not absorbed much, and one that is porous in nature, retaining maximum amount of water. The rest of the central area, which all 20 taluks of Chennai district are part of, has Alluvium deposits. With maximum water retention capacity of groundwater, these areas are highly exploited due to extraction.
“The whole of Chennai is made up of Alluvium soil and lined with coastal aquifers which is very favourable for groundwater retention. But Kanchipuram and Tiruvallur districts are made up of hard rocks and have relatively low potential to store groundwater. To compensate this, nature has given 2,000 plus water bodies in these two districts which are slowly dying,” said Sekhar Raghavan, director of Rain Centre.
In addition to this, 218 Million Cubic Metres (MCM) of run-off water can be harnessed every year from the Chennai aquifer region, pointed out a study by the Central Groundwater Board. As the city is formed on a slight slope, this run-off rainwater can be tapped through recharge wells or temple ponds instead of draining into the sea through stormwater drains, said Sekhar Raghavan.
What went wrong
Research studies show that unregulated extraction of water especially in the North, Northwestern, and Northeastern parts of Chennai and along the coastal aquifer has made the groundwater table saline in some places and dry in others. Other reasons for water shortage are shrinking of floodplains and vanishing water bodies due to encroachments.
According to the 2017 Central Ground Water Board’s report on Chennai Aquifer System, all 20 taluks of Chennai district fell under the over-exploited category. Similarly, four of 33 taluks in Kanchipuram district, 12 out 46 taluks in Tiruvallur district and two out of 10 taluks in Vellore district were highly critical.
Water for domestic use has been pumped from the Arani and Koratalai River Basin by the Metro Water Board to cater mainly to North Chennai. Per the report, this region falls under semi-critical and critical categories, as groundwater table is close to going dry due to over-extraction.
An official from the Public Works Department said that taluks earlier marked as safe zones, like Ponneri, Gummudipoondi, Kolur, Poondi, Tiruvallur and Thandalam, are now closely moving to the critical category. “Borewells which Metro Water Board draws water from are running dry. Private tankers are driving deep into suburbs and nearby districts for water. Most of Vellore is at a critical category due to this,” said the offiical.
Falling groundwater levels directly impact agriculture, forest and wetland cover, established by a study presented at the Indian Science Conference in 2018 by two geology professors from Anna University.
It also stated that waterbody cover has decreased from 15.5 per cent in 1973 to 11.16 per cent in 2013 to a low 8.2 per cent in 2017. “The results were recorded using satellite images and remote sensing. Built-up areas are increasing since 1973 and is now at 82 per cent. But agriculture, forest and waste land has been decreasing,” said Elango L, professor, Geology department of Anna University, who was part of this study.