Seawater-groundwater interactions in coastal multi-aquifer systems of Puducherry, India, using combined hydrogeochemical, radon isotope, and unsupervised machine learning approach: Implications for sustainable groundwater management.

Seawater intrusion into freshwater coastal aquifers alters the chemical composition of groundwater due to the dissolution of excess salts and other organic and inorganic constituents into the freshwater environment. The current research aims to identify the dominant hydrogeochemical and salinity dynamics along the five dominant aquifers in the study region. A total of 376 groundwater samples were collected: Alluvium (n = 108), Cuddalore (n = 163), Kadaperikuppam (n = 26), Thuruvai (n = 30), and Vanur-Ramanathapuram formation (n = 49). Results suggest significant seawater intrusion along the eastern coastal tracts of the Alluvium (seawater mixing index: 37.0% of samples exceeding the limit) and Cuddalore formations (seawater mixing index: 12.5% of samples exceeding the limit), with aquifer freshening prominent along the western sectors. Hydrochemical processes such as water-rock interaction, reverse ion exchange and microbial sulphate reduction were prominent in the Alluvium, Cuddalore, and Vanur-Ramanathapuram formations. In the aquifers of Kadaperikuppam and Thuruvai, anthropogenic sources and carbonate weathering were found to be dominant. Results from the self-organising map suggest that saline groundwater is prominent in about 28.7% of Alluvium, 12.8% of Cuddalore, 15.3% of Kadaperikuppam, 36.6% of Thuruvai, and 10.2% of Vanur-Ramanathapuram formations. Integrated ionic ratios, seawater intrusion indices, and Radon isotopes confirm aquifer-specific salinisation sources. To address the challenges, targeted aquifer management practices such as the use of treated surface water and rainfall, controlled well withdrawal, relocation of wells away from saline intrusion zones, rejuvenation of tanks and ponds, and establishment of salinity/Radon monitoring wells were suggested to enable early detection of seawater intrusion. The findings link hydrochemical evidence with appropriate management strategies to mitigate seawater intrusion and to protect the pristine groundwater resources for local communities and future generations.