Analysis of Stable Isotopes 18O and 2H and Sodium Adsorption Ratio (SAR) for Groundwater Studies in Coastal Area in Semarang City, Central Java Province Indonesia

Water is the most important basic need for sustaining life, and groundwater remains the primary source of this need in the coastal area of Semarang City. This indicates that the recharge of the source must be monitored as an important part of water resource management. Therefore, this study aimed to determine the origin of groundwater through the analysis of stable isotopes 18 O and 2 H, as well as the calculation of the sodium adsorption ratio (SAR) in Semarang City. The purpose is to determine the origin of deep wells and the quality of water for irrigation. The methods used in this study included hydrogeological mapping, testing groundwater samples, and the analysis of stable isotopes 18 O and 2 H. Semarang City was located in an area with geological components, including the Alluvium and Damar Formation, as well as geological structures such as rectification. Chemical parameter testing was carried out by comparing the SAR aspect of 30 samples with their electrical conductivity (EC). From the analysis of stable isotopes 18 O and 2 H, most of the samples were obtained from deep groundwater, while 1 was found in shallow aquifers. Furthermore, the stable isotope composition of 29 samples still followed the Global Meteoric Water Line (GMWL), indicating that they were obtained from global rainwater and had experienced mixing.