STUDY HYDROGEOLOGY, HYDROGEOCHEMISTRY, AND ISOTOPE TO IDENTIFY THE FLUCTUATION MECHANISM OF LAKE WATER IN BATUR, INDONESIA

• Main Author: Aprillia, Putri
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/50173
• Institution: Institut Teknologi Bandung

Lake Batur is located on the Mount Batur caldera which hydrologically is a closed basin system, which does not have an inlet or outlet. Based on measurements and previous studies, it was stated that there was a significant rise in the water level of Lake Batur, which caused the sinking of housing, agriculture, temples and several other public facilities. This research was conducted to determine the mechanism of lake water level changes, where there were no significant changes in hydro-meteorological conditions in Batur and its surroundings. This research covers the Lake Batur caldera area as the main location and other volcanic lake caldera on the island of Bali which includes Lake Beratan, Buyan and Tamblingan as a comparison. The study area is located at an altitude of about 1000 to 1050 meters above sea level. The main lithology composing the research area is volcanic rock which is dominated by agglomerates, lava, tuff, and a little lava and ignimbrite. The method used in the study includes the measurement of the physic-chemical parameters of water, including lake water, warm and cold springs, shallow wells, rivers and rainwater. Measurement of the concentration of soil gas and dissolved isotope 222Rn (Radon-222) was carried out to determine the condition of permeability and influx below the subsurface to the area around the lake. In addition, mineralogy and stable isotope tests of ?18O and ?2H were also carried out to determine the mechanism of hydrogeology in the lake. Based on the physical and chemical parameters of water, Lake Batur's water characteristics are comparable to warm springs that appear around the lake show the characteristics of magmatic fluid, that is, a high total dissolved ion around 1200 mg / L, the presence of Cl-, SO42- anions and cations are high. Lake Batur water conditions also have characteristics that is different from rainwater and other volcanic lakes, as shown by the low dissolved ions (less than 100 mg / L). The stable isotope measurements show the value of Batur’s lake is heavier than hot springs (-2 to -4 ‰ ?18O) and has a different value with the isotopic value of meteoric water. Radon activity values indicate the existence of permeable fissures to release volcano-magmatic fluid and fill into lakes. Hydro-meteorological analysis around the lake also indicates that there is no significant effect of precipitation-evaporation on lake water level rise. In addition to seasonal fluctuations, the water level shows an increasing trend from year to year which is quite significant in the last 20 years. The results of this study suggested that the presence of magmatic fluid input into the lake is relevant the volcanic activity in the Batur. The discharge of magmatic fluid input through a process of "hydrothermal overpressure", results in the fracturing of lava, of which typically poor quartz. Furthermore, magmatic fluid is penetrated into the surface and rise the water level. The hydrothermal overpressure is supported by tomographic imaging geophysical data around the study area. Further research needs to be done to strengthen the study results including per-depth sampling, adding radon points to the caldera rim boundary, measuring 13Carbon isotopes rare element concentration from lake water and model calculation balance between surface and groundwater input to detect level changes in the Lake