TRACING THE ORIGIN OF SPRING WATER BY USING ENVIRONMENTAL ISOTOPES IN THE SOUTHERN SLOPE OF MOUNT MERAPI, INDONESIA

Abstract

The first and most fundamental question to conserve a springs is where the boundaries of springs’ recharge zone. Research in determining recharge zone of Wadon springs (631 m.asl) which located at the southern slope of mount Merapi has been conducted. Topography and hydro-geological data used to find out the locations of groundwater sampling or recharge zone hypothetic, meanwhile the isotopic compositions (H, O, and C) utilized to determine the origin of groundwater. The isotopic compositions in groundwater samples were collected from wells, springs, and boreholes at southern slope of mount Merapi. Merapi meteoric water line (MMWL) describing the relationship between isotopic compositions and elevation [6] was used as a reference of isotopic compositions of water samples to find out the origin of groundwater. Based on the δ²H and δ¹⁸O values and MMWL, it can be concluded that the recharge zone of Wadon springs is ranging from the elevation of 650 m.asl (Pagerjurang) to 1260 m.asl (Plawangan). The δ¹³C value as well as tritium supports the conclusion; mostly δ¹³C in recharge zone and Wadon springs comes from the same source of carbon; that is -17.74±0.27‰ to -10.96±0.66‰ Pee Dee Belemnite (PDB); this holds for δ¹³C in the range of groundwater Dissolved Inorganic Carbon (DIC). Based on the tritium concentration decreasing from ≥ 4.20 TU to <4.20 TU exhibits that groundwater also comes from recharge zone to Wadon springs. By referring to the recharge elevation and topography maps, it can be estimated that the area of recharge zone is 8.033 km².