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This study examines seawater intrusion in the southern part of Kebumen District, focusing on the impact of various geological formations on groundwater salinity. Groundwater samples were systematically collected from 11 locations representing alluvial, volcanic, coastal sediment, and karst geological conditions. The samples were analyzed for major ions, including calcium (Ca2+), magnesium (Mg2+), chloride (Cl-), and bicarbonate (HCO3-), as well as Total Dissolved Solids (TDS). Ion ratios such as Mg2+/Ca2+, Cl-/HCO3-, and Mg2+/(Mg2++Ca2+), along with the Fraction of Seawater (fsea), were calculated to assess the extent of seawater intrusion. The study employed the Gibbs diagram method to identify the primary processes controlling groundwater chemistry, revealing that water-rock interactions are dominant in most samples, particularly in those affected by seawater intrusion. The results indicate significant seawater intrusion in alluvial and karst regions, especially at sampling points TP 2, TP 4, and TP 10, while volcanic and coastal sediment areas show minimal intrusion. These findings underscore the critical influence of geological conditions on seawater intrusion and highlight the need for targeted groundwater management strategies. Further research focusing on long-term monitoring is recommended to better understand and mitigate the impacts of seawater intrusion in this region. [ABSTRACT FROM AUTHOR]

Seawater intrusion often occurs in shoreline areas closest to alluvium rock lithology and is caused by excessive groundwater exploration. In Pesanggaran Banyuwangi, saline groundwater was found at approximately 8000 m from the shoreline. Therefore, this study aimed to analyse the spatial distribution of seawater intrusion based on rock lithology, Electrical Conductivity (EC), and Total Dissolved Solids (TDS) in Pesanggaran, Banyuwangi. A descriptive-quantitative survey method was used to assess groundwater quality based on EC, TDS, and spatial distribution parameters. Interpolation with Inverse Distance Weighting (IDW) method was used to map seawater intrusion and its relationship to each parameter. The results showed that moderate-saline groundwater was found in several areas far from the coast with volcanic rock lithology, namely Kalibaru formations (Qpvk) and limestone lithology at Batuampar formation. Groundwater in shoreline areas (Alluvium) was dominated by freshwater due to the deposition process found in alluvium lithology, which could form an aquifer with high quantity. Saline groundwater in Batuampar and Kalibaru formations showed salt minerals flushing related to shallow marine deposition since the tertiary period. In conclusion, Pesanggaran Subdistrict had uneven spatial distribution, and no significant relationship with distance from the shoreline, but it was related to the deformation process for each lithology. [ABSTRACT FROM AUTHOR]