Integrated isotope hydrology and hydrogeochemical approach for delineating groundwater flow systems in Denpasar-Tabanan Groundwater Basin, Bali, Indonesia

Study region Denpasar-Tabanan Groundwater Basin, Indonesia. Study focus Bali Island faces intensifying groundwater exploitation due to rapid urbanization and tourism growth. This generates raising concerns regarding groundwater sustainability of the basin. Previous study in the basin has focused on groundwater head changes, however comprehensive flow system characterization using hydrogeochemical and isotope tracers remains limited. This study aims (1) to elucidate a major hydrogeochemical process controlling groundwater evolution, (2) to determine recharge elevation zones and mechanisms, and (3) to delineate flow systems within the aquifers. In this study, major ion elements (F, Cl, NO 3 , PO 4 , SO 4 , Na, K, Ca, Mg, HCO 3 , Si) of water samples were determined using ion chromatography, titration, and spectrophotometry methods, in conjunction with environmental isotopes (δ 2 H, δ 18 O) were analyzed utilizing isotope ratio mass spectrometer. The major ions and groundwater origins were combined to delineate flow systems within the basin. New hydrological insights for the region Water isotope signatures reveal groundwater recharge originating from effective elevations between 100 and 700 m above sea level. Combination of groundwater origins and hydrogeochemical signatures reveal common recharge signature and progressive groundwater evolution along hierarchical groundwater flow systems. Shallow and deep groundwater indicates the development of intermediate flow systems with human pressure contributes to groundwater evolution. The conceptual model demonstrates that groundwater in the lowland discharge area is sustained by recharge originating from the middle basin.