IIT-M team finds wells interconnected through underground limestone tunnels in Thisaiyanvilai region  

Tamil Nadu Assembly Speaker M. Appavu inspecting the well at Aayankulam in Tirunelveli district.

Tamil Nadu Assembly Speaker M. Appavu inspecting the well at Aayankulam in Tirunelveli district.

A team of researchers from IIT-Madras has recommended to Tirunelveli district administration massive recharge of wells in Thisaiyanvilai region, which have been interconnected by underground limestone caves, with rainwater to check seawater intrusion and rejuvenate farming operations.

When floods ravaged the otherwise dry region in December 2021, a well in Ayankulam in Thisayanvilai taluk devoured thousands of litres of water every second for many weeks. Local farmers call it a ‘miracle well’ as they believe that recharging this well improves groundwater table to a radius of up to 10 km.

Tamil Nadu Assembly Speaker M. Appavu, who is representing Radhapuram Assembly Constituency, and Collector V. Vishnu, who visited the site, roped in the IIT–M team to conduct a study. Led by Venkatraman Srinivasan of Civil Engineering Department, the IIT-M team visited the site for a preliminary investigation.

It performed four field campaigns and surveyed more than 260 wells in the area, and selected 163 of them for monitoring and data collection measurements. These measurements include basic water quality parameters and water-level measurements were taken to compute groundwater hydraulic gradient to assess the direction and the flow rate of groundwater. Additionally, aquifer pumping drawdown and recovery tests were performed on several wells to estimate hydrogeological properties and aquifer connectivity.

Based on these studies, the team confirmed that Thisayanvilai and the surrounding areas have widespread subsurface limestone and calcareous deposits, which dominate the local hydrogeology.

“These limestone formations are susceptible to chemical dissolution when they come in contact with groundwater or experience repeated wetting and drying cycles. These reactions initiate the formation of small pore cavities, which form preferential flow paths... These preferential flow paths create micro-fractures that accelerate chemical reactions and cause further dissolution of limestone rock. Over thousands of years, this process can form a subterranean stream network of interconnected caves and caverns through which water can flow, which is termed ‘karst aquifer systems’,” said Mr. Srinivasan.

To confirm the presence of karst features in Thisaiyanvilai region, the researchers employed GoPro cameras and underwater drones in a dozen wells and obtained video images of well walls and caverns. Fire and Rescue Services personnel entered some of the wells and inspected the walls and caves, and recorded videos and photos that confirmed karst aquifers with a network of interconnected underground streams.

“Wells that intersect these subterranean streams typically have two or more openings of varying sizes. In these special wells we can recharge water at high rates without overflowing. Ayankulam is one such well,” said Mr. Srinivasan.

The researchers have identified these karst features in about 14 wells in Ayankulam, Keeraikaranthattu, Sattankulam, Suviseshapuram, Idaichivilai, Ramadapuram, etc. Now, they have concluded that Ayakulam well, besides other waterbodies, is partially responsible for groundwater table increase in the region.

Increasing the groundwater table also prevents intrusion of seawater into coastal aquifers. This is especially helpful in bringing water security to vulnerable coastal communities.

“It is estimated that Ayankulam well has recharged 500-600 crore litres of water in this one season. It is proposed that by implementing the rapid recharge technology in a surrounding well-cluster, the recharge of Ayankulam well can be more than doubled. Additionally, five other potential well clusters can be used to implement this technology. When implemented in tandem, this technology has the potential to transform farming in this region by bringing more area under cultivation,” said Mr. Srinivasan.

The pilot project will implement groundwater recharge over an area of 30-40 square km. However, with additional funding and resources, this project can potentially expand its reach to cover an area of 200 square km.

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Printable version | Aug 10, 2022 10:51:45 am |