Modeling of Seawater Intrusion in Karst Area of Tuban Region, East Java Province, Indonesia

Carbonate aquifer in coastal areas is very susceptible to seawater intrusion, especially if groundwater exploitation is high. To identify the distribution of seawater intrusion both horizontally and vertically, hydrophysics and geophysical-resistivity surveys have been carried out. Investigations were carried out at 143 observation points in the form of dug wells, boreholes, and springs to obtain seawater intrusion’s horizontal and vertical distribution. In situ measurement of groundwater’s electric conductivity and pH is carried out using portable instruments. The geoelectric resistivity investigation using the vertical electrical sounding with Schlumberger array was carried out at 25 points. The results of the hydrophysics survey are presented in the maps of groundwater flow direction, the distribution of electrical conductivity, and pH. These three maps show that the horizontal distribution of seawater intrusion occurs along the tributary pattern. Based on the geoelectric profile, the groundwater conditions in the study area can be classified into three zones; freshwater, saline water in the karst plane, and saline water in the hilly area. The saline water in the hilly region is interpreted as connate water. After this study, it can be concluded that seawater intrusion in the karst plain topography is classified as a natural process, and developing mangrove forests along tidal rivers may be the best option to prevent widespread seawater intrusion.