Determination of Groundwater Recharge Area Based on Isotope Data in Makassar Coastal Area, South Sulawesi, Indonesia

Abstract The development efforts of the Indonesian government towards the Makassar metropolitan area in South Sulawesi are intended to support its position as a key food supplier for the new capital city of Nusantara (IKN) in East Kalimantan. These developments are expected to lead to population growth and an increase in the demand for freshwater resources. Groundwater remains the primary source of water for residents in this area, specifically Makassar, the main and core city. The availability of groundwater is closely related to water infiltration and the level of extraction. Therefore, this study aimed to determine the groundwater recharge area of the Makassar coastal area using isotope methods, to ensure sustainability of the resources. A total of 19 water samples, including rainwater, groundwater, river, and seawater samples, were collected and analyzed. TDS and Cl- concentrations were also measured alongside groundwater isotope analysis to identify groundwater potential contamination in the area. The results showed that groundwater in the Makassar coastal area had a common origin, primarily derived from rainwater, based on an isotope comparison diagram between δ 18 O and δD. However, some samples were affected by saltwater intrusion, as indicated by groundwater isotope results approaching the isotopic composition of seawater, confirm by high TDS and Cl- values. The groundwater recharge area, specifically for unconfined aquifer was identified at an altitude ranging from 4.88 to 49.92 masl. In comparison, the recharge area for semi-unconfined aquifer was located at an altitude between 20.75 to 37.92 meters above sea level (masl).