Hydrogeochemical assessment of groundwater and saline water intrusion along the coastal aquifer in Udupi region of Karnataka (southern India)

Groundwater is extensively utilized for drinking, irrigation, and industrial activities globally. However, its quality is deteriorating significantly, particularly in coastal regions where saline water infiltration is a major issue. This study investigates the coastal aquifer of Udupi district in Karnataka, India, covering approximately 80 km², through hydro-geochemical analysis of groundwater samples from 57 spatially distributed locations, pre- and post-monsoon. The major anion and cation parameters analyzed include Cl⁻, HCO₃⁻, SO₄²⁻, NO₃⁻, Na⁺, Ca²⁺, K⁺, and Mg²⁺. The analysis revealed a pH range of 2.9 to 8.3, indicating acidic to slightly alkaline conditions, and total dissolved solids (TDS) concentrations ranging from 110 to 6683 mg/L, predominantly showing saline characteristics toward the coastline. Hydrogeochemical assessment, including Chadha’s plot, identified Na-Cl type water in most samples, indicating the influence of salinity. Approximately 21% of pre-monsoon and 67% of post-monsoon samples were deemed unsuitable for irrigation based on permeability index values, with 2 to 3% exhibiting high salinity hazards. The variation in correlation coefficients between pre-monsoon and post-monsoon periods suggests that saline water intrusion, freshwater recharge, and geochemical interactions influence the coastal groundwater system. The molar ratio of Na⁺ to Cl⁻ suggested a reverse cation exchange process, highlighting the complex interactions between saline and freshwater. Overall, 79% of pre-monsoon and 80% of post-monsoon samples were deemed suitable for irrigation, while 21% and 20% were classified as completely unsuitable, respectively.