Groundwater Mixing Processes in Karst and Volcanic Systems: A Hydrogeochemical Study in North Java, Indonesia

Groundwater resources in the northern part of eastern Java are influenced by contrasting volcanic and karst lithologies, particularly within the Lasem Volcano Complex and Tuban Karst Hills. This study characterizes groundwater hydrogeochemistry and identifies groundwater mixing processes across these two aquifer systems. Field surveys during the dry season included measurements from 112 dug wells, 39 boreholes, and 12 springs, covering groundwater levels, physicochemical parameters (pH, electrical conductivity, total dissolved solids, and temperature), major ions, and stable isotopes (δ¹⁸O and δ²H). Spatial analyses show that groundwater moves from upland volcanic and karst hills toward the northern coastal plain. Physicochemical analyses indicate that deep groundwater in volcanic aquifers is more stable and less mineralized than shallow groundwater, whereas karst aquifers display the opposite pattern due to rapid flow through fractures and conduits. Hydrogeochemical diagrams (Piper, Kurlov, and Schoeller) reveal dominant calcium–bicarbonate in karst aquifers and calcium–magnesium–bicarbonate in volcanic aquifers, with sodium–chloride types in coastal zones indicating seawater intrusion. Groundwater mixing zones were detected at the boundary between the Tuban and Lasem groundwater basins. Stable isotope data confirmed a meteoric origin for most groundwater, with some samples showing evaporation effects and seawater interactions. The conceptual hydrogeological model highlights lithological contacts and converging groundwater flow paths as the main controls on groundwater mixing in this study area.