Hydrochemical and environmental isotopes analysis for characterizing a complex karst hydrogeological system of Watuputih area, Rembang, Central Java, Indonesia

The karst hydrogeology systems of the Watuputih Hills region of Central Java, Indonesia, have many springs with varying discharge and are composed of formations with complex geological structures. This work characterized the karst hydrogeology by studying 50 hydrogeological features (caves, springs and wells) and by analyzing the chemical-physical properties of groundwater in the field (pH, temperature, EC, HCO₃⁻, ²²²Rn) and the major ions and stable isotopes of the groundwater samples in the laboratory, along with the stable isotope content of rainwater sampled over 1 year. Hierarchical cluster analysis of the water samples identified three hydrochemical groups: groundwater flowing through carbonate rocks (groups C2 and C3), through quartz sandstones and volcanic rocks (group C4), and through carbonate rocks and the siliciclastic rocks (quartz sandstones) underneath them (group C1). Springs with large discharge, typified as artesian fault-guided springs, were categorized into group C1. These springs are Sumbersemen, Brubulan Tahunan, and Brubulan Pesucen, with mean discharges of 1,516, 165, and 95 L/s, respectively. Based on the results of the stable isotope analysis, the d-excess calculation and the ²²²Rn concentrations, groups C2, C3, and C4 associate with shallow groundwater systems that dominantly flow through pores, whereas group C1 associates with a deep groundwater system controlled by geological structure. The geological structure also determines the groundwater flow in the cave streams. The shallow groundwater system is sourced by local rainwater, while the deep groundwater system displays a relationship with the groundwater in the northern hills at an elevation >375 m above sea level.