HYDROGEOCHEMICAL CHARACTERISTICS AND SEAWATER INTRUSION OF SHALLOW GROUNDWATER IN MENGKIRAU, KEPULAUAN MERANTI, RIAU, INDONESIA

Abstract

Shallow groundwater is the primary source of freshwater in coastal areas and small islands, but is highly vulnerable to seawater intrusion. This study aimed to determine the hydrogeochemical characteristics of shallow groundwater and the extent of seawater intrusion in the study area. Groundwater samples from 16 dug wells were analyzed for physicochemical properties, while seawater intrusion and hydrochemical facies were evaluated using the Revelle Index (RI) and Piper diagram, respectively. The results revealed groundwater ranging from freshwater in inland areas to brackish and saline water near the coast, with some samples exceeding domestic water quality standards. Elevated total dissolved solids (TDS), electrical conductivity (EC), salinity, iron (Fe), and manganese (Mn) were associated with reducing conditions, organic-rich sediments, and seawater mixing. The Revelle Index (RI) indicated moderate to high intrusion, particularly in coastal areas. Groundwater was dominated by the Na(K)-Cl(SO₄) facies, reflecting the combined influence of seawater intrusion and water-rock interaction.