Identification of seawater intrusion signatures through geochemical evolution of groundwater: a case study based on coastal region of the Mahanadi delta, Bay of Bengal, India

The study dealt with seawater intrusion process in a coastal aquifer system in the Mahanadi river delta region in the east coast of India along the Bay of Bengal. The aquifers of Mahanadi delta are characterized as shallow aquifers ( 50 m). Electrical conductivity (EC) of groundwater varied from a fresh of 146 μS/cm (NW of the Mahanadi delta) to a saline of 33,900 μS/cm (close to sea coast) with cation dominance in the order Na + > Ca 2+ > Mg 2+ > K + and anion dominance of Cl − > HCO 3 - > SO 4 2 - . The hydrochemical facies changed from Ca–Mg–Na–HCO 3 type to Na–Cl type along the groundwater flow direction due to ion exchange processes. A strong positive correlation ( r > 0.9) between Cl − with EC, Na + , Mg 2+ , Ca 2+ , SO 4 2 - and K + was observed, which indicated the influence of seawater on coastal aquifer. The ionic ratios (Na + /Cl − , HCO 3 - /Cl − , Mg 2+ /Ca 2+ , SO 4 2 - /Cl − , Ca 2+ /( HCO 3 - / SO 4 2 - )) also suggested that the groundwater is affected by seawater intrusion. Stable isotope compositions (δ 18 O and δ 2 H) varied from − 1.86 to − 6.87 ‰ for δ 18 O and from − 10.79 to − 45.42 ‰ for δ 2 H, implying the mixing of saline water and fresh groundwater in the coastal region of the Mahanadi delta. The proportion of seawater in groundwater was estimated to vary from 0% in the upper-delta formation to 72% in the lower-delta formation of the Mahanadi delta (close to seacoast), which was due to inland intrusion of seawater. In a first ever study on this coastal aquifer along the Bay of Bengal, where a large population is dependent on agriculture, seawater intrusion into the fresh groundwater has been quantified. The issue of seawater intrusion into the coastal aquifer in this region may become a serious disaster, if appropriate management strategies are not implemented in time.