CHENNAI : Indian Institute of Technology Madras Researchers have proposed implementing a rapid groundwater recharge technology for combined flood and drought mitigation near Ayankulam village in Thisayanvilai taluk of Tirunelveli District, Tamil Nadu.
A proposal for this project has been submitted to the Tirunelveli District Administration, which had requested IIT Madras to study this phenomenon.
Video Bytes of by Dr. Venkatraman Srinivasan, Assistant Professor, Department of Civil Engineering, IIT Madras, can be downloaded from the following links:
ENGLISH VIDEO BYTE – https://drive.google.com/file/d/1f425o8yE-aKRUYxMD0spye9wMKk0gy8K/view?usp=sharing
TAMIL VIDEO BYTE – https://drive.google.com/file/d/1k7kfwv3T1BhBpDfH2BWSnzjgY5Px5p6X/view?usp=sharing
An open agricultural well in Ayankulam was widely reported to recharge an estimated 1,500-2,500 litres of water every second for several weeks without overflowing. The recharge water for this well was from the excess overflow of an adjacent minor irrigation tank due to the record monsoon rains in Nov-Dec 2021.
The well became a local attraction and was called a “miracle well” since typical wells would fill and overflow in a matter of hours at such recharge rates. Local villagers have been practicing this ad-hoc method of well recharge during episodic intense monsoon years for several decades. They claim that this practice increased the local water table in a 10-15 [km] radius from the well.
The region surrounding this village is considered a dry belt with hot summers. Many small land-holding farmers have abandoned their farming practices and work as labourers in larger farms or other labour-based occupations. Being close to the coastal zone, several agricultural and domestic wells in the region suffer from saltwater intrusion due to excessive pumping and lowering of groundwater levels. Even extending the agricultural water availability by a few months each year can positively impact the livelihoods of these farmers.
A team led by Dr. Venkatraman Srinivasan, Assistant Professor, Department of Civil Engineering, IIT Madras, visited the site between 2nd and 3rd December 2021 to investigate this phenomenon and explore the potential of this well and possibly other wells in the region for use as rapid aquifer recharge during monsoon floods for storage and withdrawal during the dry summer months.
The IIT Madras Team suggested implementing a ‘Rapid Recharge Technology,’ which, when developed, can bring several potential benefits to the region, including:
Ø mitigate floods and droughts,
Ø create a subsurface dam for water storage without evaporation loss,
Ø distribute water automatically and equitably throughout the region,
Ø filter and clean water when managed appropriately, and
Ø prevent and reverse saltwater intrusion in coastal aquifers.
Explaining the potential of this pilot project to benefit the region, Dr.Venkatraman Srinivasan, Assistant professor, Department of Civil Engineering, IIT Madras, said, “This is a win-win situation for all stakeholders. The excess water which causes devastating floods and would otherwise empty into the ocean is being channelled to recharge the groundwater for storage and retrieval in the dry summer months.”
Further, Dr.Venkatraman Srinivasan said, “The unique hydro-geology of the region allows the implementation of this rapid aquifer recharge. In most other places, wells do not sustain such high injection rates and would easily overflow.”
The rapid groundwater recharge technology proposed for this study area is different from conventional rainwater harvesting or well recharge. For example, in Chennai, groundwater recharge is practiced in every household by collecting rooftop rainwater. This comprises of 1000s of recharge structures, each capable of recharging about 150,000-200,000 litres of rainwater over a good monsoon season.
In contrast, the proposed recharge technology at Ayankulam village will consist of a few dozen wells, with each well capable of recharging 1,50,000 to 2,00,000 litres of water every minute during floods events.
While the unique karst aquifer hydro-geology allows such high recharge rates, these come with challenges. While rooftop rainwater is relatively clean, floodwater from rivers carries sediments that can clog the well in the long term. Hence sediment traps need to be designed to remove suspended sediments before recharging the wells. Additionally, water quality parameters need to be continuously monitored to ensure that it is safe for recharge.
The IIT Madras team performed an on-field site survey of the injection well and 20 additional wells in a one-km radius. The wells were geo-tagged using GPS equipment. Water level measurements taken from these wells were combined with digital elevation map data to obtain groundwater hydraulic gradients. These are used to predict the direction and flow of groundwater in the region. Water samples were also collected from 13 wells for fingerprinting analysis of water quality parameters.
A preliminary assessment by IIT Madras indicated that subsurface geology of the region was likely a ‘Fractured/Karst aquifer,’ which has hydraulic conductivities that are orders of magnitude larger than conventional aquifers. This unique hydro-geology of Thisayavilai taluk and surrounding areas enables rapid recharge during floods. The recharged water will be stored below ground and retrieved during dryer months.
Unlike surface water storage, groundwater storage losses due to evaporation are negligible. Additionally, the recharge of groundwater has the potential to mitigate saltwater intrusion in coastal groundwater aquifers.
Currently, while groundwater recharge is practiced only during episodic(about once every 4-5 years) extreme flood events in one well, it is possible to design a system that can recharge the aquifer annually (every monsoon season) through multiple injection well systems. Such a system of annual aquifer recharge will facilitate sustainable water use in the dry summer months. Further detailed studies are required to develop and optimize this flood and drought mitigation technology.
