Evaluating groundwater vulnerability to seawater intrusion in the coastal region of Bangladesh

Abstract

In Bangladesh’s southwest coastal belt, including Khulna District, seawater intrusion poses a serious threat to groundwater quality. This study evaluates groundwater vulnerability using hydrochemical analysis of 30 (thirty) samples to detect salinity levels and potential intrusion. Results show significant salinization in parts of the study area, with electrical conductivity (EC) up to 3016 µS/cm, total dissolved solids (TDS) up to 1930 mg/L, and salinity up to 0.57 ppt. Most samples are mixed NaCl and NaHCO₃ water types, with Na⁺ as the dominant cation and Cl⁻ and HCO₃⁻ as main anions. Correlation analysis reveals strong relationships between EC and Cl⁻ (R2 = 0.975) and Na⁺ (R2 = 0.835), identifying them as key indicators of seawater intrusion. According to WHO drinking water guidelines, sodium concentrations (65.20–2406.92 mg/L) indicate 17% of the area is highly vulnerable. Chloride levels (10.30–987.99 mg/L) classify 13% as moderately vulnerable and 10% as highly vulnerable, while bicarbonate levels (245.30–668.85 mg/L) mostly meet DOE standards. Trace elements in most of the samples such as As (0.2–0.38 mg/L), Fe (0.00–3.39 mg/L), and Mn (0.00–0.65 mg/L) exceed permissible limits. Salinity levels have risen sharply, linked to reduced surface water flow and rising sea levels during dry season. GALDIT model (groundwater level, aquifer type, hydraulic conductivity, distance from shore, intrusion impact, and aquifer thickness), indicate that vulnerability increases where the water table lies near or below sea level, especially near tidal rivers. Based on the [Cl⁻]/[HCO₃⁻ + CO₃²⁻] ratio, Chandkhali Union (3.56) showed the highest vulnerability, while Dumuria Union (0.02) had the lowest. The vulnerability map highlights the southern region—particularly Laskar, Chandkhali, and Garuikhali Unions in Paikgachha—as most at risk, while northeastern areas like Khulna Sadar and Dumuria Upazilas show low to moderate susceptibility. These findings provide crucial guidance for managing freshwater aquifers and preventing salinization in coastal Bangladesh.