Subsurface Investigation of Freshwater-Seawater Interface on Gowa-Takalar Coastal Aquifer, Indonesia

Gowa-Takalar Coastal Aquifer is an unconfined aquifer located in the southward of Makassar City. The Gowa-Takalar area has a heavy groundwater exploitation particularly on dry season. In this area, the groundwater is utilized to support water need for agriculture activities, marine tourism facilities, and new settlement development. The highly economic activities in this area, it causes an intensive groundwater exploitation. Since this area is located around coastal area, this area has highly susceptibility to suffer from seawater intrusion. In order to maintain the continuity of groundwater supply, a special study is needed on hydrodynamics of fresh-sea water interface as the basis for the formulation of a sustainable management model. This study as an early step of groundwater management research emphasizing on basic of aquifer and groundwater data. The research method consisting of reconnaissance study for geological & hydrogeological mapping, groundwater chemistry laboratory analysis, and geo-electrical investigation. Geological condition of this area is composed of alluvial and coastal deposits, volcanic deposits, and marine sedimentary rocks interbedded with volcanics. These materials form three hydrogeological characteristics: extensive and moderately productive aquifer, locally productive aquifers, and poorly productive aquifers of local importance. The chemical composition of groundwater from dug wells show the chloride concentration vary from 3.55 ppm to 81.65 ppm, with chloride-bicarbonate ratio is 0.008 – 0.035 indicating aquifer without sea water invasion. However, based on vertical electrical sounding, the resistivity layers vary from 0.02 to 62.50 ohm-m indicating the present of seawater intrusion into coastal aquifer.