Geological Factor Causing Water Decrease of the Baturagung Reservoir, Based on ERT Geoelectric Application

Abstract

The Baturagung reservoir, a small artificial lake is located in the Gedangrejo village, Karangmojo district, Gunungkidul regency, Yogyakarta Special Region, Indonesia. Initially, the reservoir was built on hilly geomorphology with volcanic sandstone lithology intended to meet the agricultural water needs of the people in the surrounding area, and as a tourist spot as well. Since 2019, there has been a decrease in the volume of reservoir water so that the reservoir is never fully filled with water. Due to the decrease in reservoir water volume, residents' water needs are not being met, and visitors to this place are reduced. Therefore, it is important to accomplish a study with the purpose to identify the factors that cause reservoir water depletion, such as the conditions of geomorphology, lithological variations, geological structures, and hydrogeology of the area. The methodologies used in this study are field surveys, surface mapping, and the application of the electric resistivity tomography method. The results show that there is a weak zone with a resistivity ranges 4.24– 45.9 Ωm, northeast-southwest trend, interpreted as a fault crossing the study area. It is deduced that the fault is active, causing the tearing of the geomembrane underlying the reservoir, initiating water leakage from the reservoir to the southwest direction.