CHENNAI: Large volumes of freshwater from underground aquifers are silently draining into the Arabian Sea along the Kanniyakumari coast, carrying nutrients that could alter coastal ecosystems, according to a new scientific study published in the international journal, Scientific Reports.
The research, conducted by scientists from the University of Madras and VO Chidambaram College in Thoothukudi, quantified what is known as submarine groundwater discharge (SGD), the natural movement of groundwater from coastal aquifers into the ocean through sediments below the shoreline.
Published in the peer-reviewed journal, Scientific Reports, part of the journal Nature, the study used radon isotope tracing and nutrient analysis to detect and measure the flow of groundwater into coastal waters along Tamil Nadu's southernmost coastline.
"Groundwater contains much higher concentrations of radon compared to seawater. This contrast allows scientists to identify and estimate groundwater discharge even when it occurs beneath the seabed," the researchers noted.
Water samples were collected from coastal wells and sediments in the intertidal zone at several locations along the Kanniyakumari coast during pre-monsoon and post-monsoon periods.
The analysis estimated groundwater discharge rates ranging from 0.01 to 0.54 cubic metres per square metre per day during the pre-monsoon season, increasing to 0.04 to 0.98 cubic metres per square metre per day after the monsoon.
According to the researchers, groundwater flow into the sea intensifies following the monsoon as rainfall replenishes aquifers and increases subsurface water movement toward the coast.
"Seasonal rainfall plays a crucial role in groundwater dynamics. Post-monsoon recharge enhances the hydraulic gradient and leads to greater discharge of groundwater into coastal waters," the study observed.
The researchers identified Enayam, Mandaikadu, and Muttam beaches as prominent zones where groundwater discharge into the sea is particularly high.
FROM FARMS AND TOILETS TO THE SEA
Besides freshwater loss, the study found that groundwater acts as a carrier of nutrients, such as dissolved inorganic nitrogen, phosphorus, and silicate, which are transported from land into the marine environment.
These nutrients may originate from natural geological processes as well as human activities, including fertiliser use, agricultural runoff and wastewater discharge, the researchers said.
Scientists cautioned that excessive nutrient input into coastal waters could contribute to ecological problems such as algal blooms, oxygen depletion, and shifts in marine biodiversity.
At the same time, the findings underscored the need to better understand the groundwater dynamics in coastal regions where freshwater resources are under increasing pressure.
"Quantifying submarine groundwater discharge is essential for sustainable coastal management and for assessing hidden losses of groundwater resources," it noted.
The researchers said radon-based tracing techniques provide a reliable scientific tool to identify groundwater discharge zones and assess their environmental impact along coastal regions.