The Origin of Toba Caldera Aquifers Based on Hydrogeology and Hydrogeochemistry, Northern Sumatra, Indonesia

The Toba Caldera exhibits complex geological processes with unique lithological characteristics serving as aquifers. Groundwater in the Toba Caldera is the primary water source for the surrounding communities. However, detailed research on hydrogeology, hydrogeochemistry, and groundwater recharge areas has yet to be conducted in the Toba Caldera. Therefore, the objective of this study is to determine the characteristics of groundwater in the Toba Caldera to ensure its sustainable supply to Lake Toba and meet the needs of the local population. The methods used in this study consist of groundwater level measurements, groundwater chemistry measurements, rock observations, geochemical analysis, and isotope analysis. The lithology of the study area comprised alluvial, lava, rhyolite, microdiorite, sandstone, tuff, limestone, granite, conglomerate, metasediment, and siltstone. The overall groundwater flow pattern moves toward Lake Toba in the Toba Caldera. Geochemical measurements displayed that groundwater in the Toba Caldera has a pH of 6 to 8.5, TDS between 50 and 1,000 ppm, EC from 100 to 2,250 µS/cm, and temperatures of 15 to 50°C. The hydrostratigraphy includes aquifers, aquitards, and aquifuges. The Piper Diagram results indicated a dominance of calcium, magnesium, alkali, and bicarbonate types, while the Kurlov Diagram confirmed a similar composition with the dominance of calcium, alkali, and bicarbonate. Isotope analysis revealed that groundwater in the Toba Caldera primarily originated from meteoric water with minor contributions from magmatic water, paleowater, and H2S. Isotope analysis also estimated that the groundwater recharge in the Toba Caldera comes from 1,200 to 1,665 masl.