Groundwater Vulnerability to Seawater Intrusion in Parangtritis Beach, Yogyakarta Special Province, Indonesia

Abstract The consequences of increased tourism, population growth, and agricultural activities on groundwater resources are becoming increasingly concerning. The tourism industry in the Parangtritis Tourism Area has been expanding, leading to a rise in the number of hotels and visitors. This growth, along with the expanding population and the demand for groundwater in agriculture, has contributed to a reduction in groundwater reserves. Overexploitation of groundwater can result in several adverse effects, such as declining water tables and deteriorating water quality due to seawater intrusion. Seawater intrusion in coastal areas poses a serious threat, potentially hindering economic development, endangering local cultures, and contaminating groundwater sources. This study is critical as it aims to identify the geological and hydrogeological conditions and assess areas at risk of seawater intrusion in the study area. The research involved observing surface geological conditions, mapping hydrogeology, and applying the GALDIT method to assess the vulnerability to seawater intrusion. The findings reveal diverse lithology in the study area, including fine sand, coarse sand, sandy clay, floatstone, wackestone, andesite, and andesite breccia. Observations from 80 dug wells or boreholes indicated that groundwater use in the study area is limited to an unconfined aquifer with a thickness of up to 101 meters. The GALDIT index results show that the Parangtritis tourism area has a moderate to very high vulnerability to seawater intrusion. Areas with very high vulnerability are primarily concentrated in coastal zones with extensive groundwater usage, covering 4.04% of the total study area. Additionally, high vulnerability areas cover 11.23% of the total research area, characterized by high hydraulic conductivity values in aquifers and dominated by settlements and agricultural land use.