Hydrogeological Controls on Freshwater Preservation in a Coastal Sand-Dune Aquifer in part of the Mahanadi delta, Eastern India

Coastal aquifers in tropical regions are highly vulnerable to salinization, yet local hydrogeological settings can create natural barriers that sustain freshwater resources. This work investigates a ~145 km² sand‑dune alluvial aquifer along the eastern coast of India, bounded by the Bay of Bengal to the south and tidal-influenced rivers to the east and as well in the west. It is observed that a long stretch of freshwater zone, present near and parallel to the coast, despite the presence of highly saline groundwater adjacent to the inland north side and the sea on the eastern side. Hydrochemical analyses show total dissolved solids (TDS) consistently low and range from (52-830 mg/l), and Chloride (Cl - ) ranges from (10-237 mg/l). Groundwater levels in the well located in this zone were observed below mean sea level during dry periods, yet no salinity ingression or measurable bromide was detected. It indicates the presence of hydrogeological barriers at a certain depth, mostly clay. As clay restricts the flow of water, it protects the upper shallow aquifer from being contaminated by saline water. Stable isotope (δ 2 H and δ 18 O) signatures further show recharge from local precipitation rather than marine intrusion. However, samples from the marginal area of the zone are observed with high TDS (1078-5390 mg/l), Chloride (291-2424 mg/l) in the northwestern and southern parts of the aquifer. This may be attributed to occasional cyclone‑induced flooding and saline inundation in the adjacent low-lying coastal zone via rivers during high tide, which remain restricted at the periphery of the freshwater zone. The mixing of seawater is confirmed by the presence of high Chloride (Cl - ) & Bromide (Br - ) content and similar stable isotopic characteristics to seawater. Overall, the clay‑bound sand‑dune system effectively restricts the lateral and upward movement of the saline water. Therefore, this freshwater zone should be carefully protected and monitored from anthropogenic activities. These findings provide a valuable analogy for coastal aquifer sustainability and for its better management