Karst groundwater conservation based on hydrochemistry and isotope stable analysis in Obi Island, South Halmahera

Abstract This paper presents an analysis of the hydrogeological characteristics of the Kelo karst area in Obi Island, South Halmahera, focusing on hydrochemistry and isotopic methods to delineate the groundwater recharge zones and propose conservation measures. Despite its natural state, the Kelo karst area has not been previously studied for hydrogeology nor exploited for resources. Unregulated exploitation of karst areas poses significant environmental risks, leading to the degradation of their strategic and hydrological functions. This study investigates spring characteristics through hydrochemistry analysis, stable isotopes (δ 18 O and δ 2 H), and groundwater mixing processes. Hydrochemistry and isotopic analyses elucidate the hydrogeological features, including groundwater origin and recharge mechanisms. The predominant groundwater facies in the study area exhibit a Ca-Mg-HCO 3 − composition, reflecting the dissolution processes of both limestone and igneous rock formations. Isotopic analysis and local meteoric water line examination indicate a recharge zone elevation ranging from 612 to 945 meters. Geological features such as fracture zones, lineaments, faults, and sinkholes significantly influence the determination of recharge areas. Protection of these recharge zones is imperative for groundwater conservation, ensuring the sustained availability and quality of groundwater amidst the development pressures on karst landscapes.