Determining recharge area in ULUBELU geothermal field, LAMPUNG, Indonesia using stable isotope data

The Ulubelu geothermal field is located in Lampung Province, Indonesia, about 125 km west of Bandar Lampung City. This field is utilized as a power plant with an installed capacity of 220 MW operated since 2012. The Ulubelu geothermal system is a high enthalpy water-dominated reservoir and has temperature around 265 °C. This study aims to determine the elevation of the recharge area in the Ulubelu geothermal system based on the values of δ²H and δ¹⁸O isotopes to support the geothermal use sustainability. Major ions, δ²H, δ¹⁸O, and δ¹³C_(DIC) isotopes analysis were carried out on hot springs, lake water, cold springs, and rainwater. The content of the major anions and cations indicates that the hot springs from the Way Panas Group are chloride-type water originating from the reservoir meanwhile the Danau Hijau and Pagar Alam groups are condensates acid-sulphate fluids. Moreover, the calculation using a Na/K and silica geothermometer shows that the reservoir has a temperature around 229 ± 4 °C. Analysis of the δ²H and δ¹⁸O isotopes content informs that the origin meteoric water seeped into the geothermal reservoir was δ¹⁸O −6.65‰ and δ²H −39.5‰ ranging from 1108 to 1570 masl elevation. Overlaying with the surface terrain map depicts that the recharge area originates from the peak of Mt. Rendingan in the north of Ulubelu and Mt. Sula in the west with 42.9 km² total area. In conclusion, this study elucidates the recharge area elevation in the Ulubelu geothermal system, offering crucial insights for sustainable geothermal resource management and reservoir origin understanding, thus highlighting its potential for sustainable energy production.