Characterization of shallow groundwater in coastal aquifer of urbanized area using stable isotope and hydrochemical approaches

BACKGROUND AND OBJECTIVES: Cirebon City, located in West Java, Indonesia, is one of the coastal cities formed from alluvial deposits with aquifer rocks in form of gravels and sands in the plains, as well as sands in the hills. Due to its urbanized nature, the city experiences a substantial need for access to clean water. A significant number of residents remain reliant on drinking water providers for safe water, given the contamination of seawater in specific regions, while some continue to draw from shallow groundwater. As groundwater abstraction can intensify seawater intrusion, especially in urbanized area, it is important to investigate this matter. This study sought to qualitatively and quantitatively analyze the degree of seawater pollution affecting the shallow groundwater in Cirebon City, Indonesia. METHODS: Water samples were collected from 44 shallow groundwater wells around Cirebon City. Following collection, the samples were examined for their significant water chemistry content using ion chromatography and for their isotopic composition through a liquid water isotope analyzer. Interpretation was done using graphical tools such as Piper and scatter diagrams, as well as mathematical equation to approximate mixing proportion between fresh groundwater and seawater and its proportion. FINDINGS: Stable isotope analysis revealed that eight shallow groundwater samples were salinized due to the incorporation of seawater, with the proportion of seawater ranging from approximately 17.3 to 54.9 percent. These samples have very enriched oxygen-18 isotope composition values up to -3.9 permil. Another 14 samples were mixed with surface water, and the remaining 22 samples were fresh groundwater. Analysis of the hydrochemical properties indicates that fresh shallow groundwater primarily exhibits a bicarbonate composition. In contrast, the salinized shallow groundwater is identified as sodium chloride type, with chloride concentrations reaching a maximum of 6980 milligrams per liter and sodium concentrations peaking at 4963 miligrams per liter. Salinized shallow groundwater samples were up to 1.5 kilometer inland from coastline, which can be considered as an environmental concern. CONCLUSION: The seawater contamination into shallow aquifer have reached up to 1.5 kilometer inland and with seawater proportion is up to 55 percent. Throughout the contamination process, the chemical composition of groundwater is modified not solely due to mixing, but also as a result of ion-exchange mechanisms. The significant rate of seawater contamination necessitates urgent action from all stakeholders, with local government leadership being essential. To effectively address this problem, it is vital to limit the extraction of shallow groundwater by enhancing the capabilities of the government's water piping services and to either slow down or completely halt the advancement of seawater intrusion through the adoption of artificial groundwater recharge strategies.