Seawater Intrusion into Shallow Aquifers in the Coastal Area of Belawan, North Sumatra, Indonesia
Abstract
Seawater intrusion represents a major threat to groundwater sustainability in rapidly urbanizing coastal areas, particularly where groundwater abstraction and land-use changes are intensifying. This study aimed to evaluate the spatial distribution, intensity, and controlling factors of seawater intrusion into shallow aquifers in the coastal area of Belawan, North Sumatra, Indonesia. Thirty shallow groundwater samples were collected from wells located at approximately 2-km intervals and analyzed for chloride (Cl⁻), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and electrical conductivity (EC). Seawater intrusion was evaluated using the chloride-to-bicarbonate ratio (CBR), while groundwater was classified according to chloride concentration and electrical conductivity. Chloride concentrations ranged from 2 to 31,160 mg/L, whereas EC values ranged from 186 to 26,400 µS/cm. Among the 30 sampled wells, 20 were classified as freshwater, three as brackish water, and seven as saline water. Based on the CBR classification, three wells exhibited low seawater intrusion and five showed high seawater intrusion. Spatial analysis revealed that seawater intrusion was predominantly concentrated in densely populated coastal zones characterized by permeable sandy-gravel aquifers and intensive land development. The results provide updated hydrochemical evidence on the extent of seawater intrusion in the Belawan coastal area and may support groundwater monitoring, coastal land-use planning, and sustainable groundwater management.
Keywords: seawater intrusion; coastal aquifer; groundwater salinization; chloride-to-bicarbonate ratio; electrical conductivity; hydrochemistry.
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