Master Recession Curve (MRC) analysis to characterize karst aquifers of several springs in the north side of the Karangbolong (Gombong) karst area

Karst aquifers have triple porosity (diffuse, fissure, and conduit) which makes their characterization difficult, and often requires a combination of particular methods and investigation over a long period. The purpose of this study is to analyse the components of the flood hydrograph and create a master recession curve (MRC) in karst aquifers that recharge several springs on the north side of the Karangbolong Karst Area (Gombong). The springs studied include Kalisirah, Jumbleng, and Kalikarak springs. The data used are time-series discharges recorded every 15 minutes from November 2018 to March 2020. Furthermore, the reconstruction of the flow regime for MRC is carried out with the help of RC 4.0 software, which is at the same time able to define the level of karst aquifer development. The results showed that Kalisirah and Kalikarak Springs have a complex discharge regime with a degree of karstification in class 8, while Jumbleng Springs in class 5. Analysis of the components of the flood hydrograph reinforces the results of the calculation of the karstification degree. The time to the peak (Tlag) of the Kalisirah and Kalikarak Springs is relatively fast (1.94 and 1.44 hours), which indicates that conduit flow has developed, while Jumbleng spring has a longer Tlag of 2.69 hours. Calculation of time to base flow (Tb) both manually (by flood events analysis) and automatically (by MRC) shows that Kalikarak Springs has the longest time with an average of about 31 hours which reflects that karst aquifers which contribute to it are still quite good in storing groundwater, while Jumbleng spring has the fastest Tb value with an average of 17.25 hours which reflects the shortest release of water storage compared to the other two springs

Identification of Karst Underground River Catchment Areas with Artificial Tracer Tests and Water Balance in Banteng Cave Springs (Karst Gombong Selatan, Central Java)

The karst hills of Gombong Selatan have abundant potential water resources, especially in locations that have underground springs and rivers. The connectivity between the subsurface passageways that is difficult to know can threaten the potential of water resources, one of which is due to pollution caused by uncontrolled human activities. Therefore, identification of catchment systems and boundaries of water catchment areas in karst aquifers is needed that can contribute to sustainable water resources management policies. This catchment identification needs to be conducted because previous studies have never explored the eastern side of this karst area. This study aims to (1) define the underground river flow connectivity (upstream-downstream) of Banteng Cave; and (2) limiting the water catchment area of Banteng Cave. The method used to determine the subsurface connectivity system was carried out through an artificial tracer test, while the catchment area was delineated using a water balance approach. The results showed that the underground river of Banteng Cave has connectivity with Lake Blembeng, as evidenced by a change in watercolour after the tracer test and breakthrough curve (BTC) analysis of the tracing test results. BTC analysis shows that the Banteng Cave passageway has one main passage and does not have a tunnel branch. The estimated area of the Banteng Cave catchment used a water balance approach, which is 141.73 hectares. The Banteng Cave karst catchment conditions are dominated by the formation of valleys and karst cones accompanied by the appearance of valleys and karst hills that are quite evenly distributed, indicating that the Banteng Cave karst catchment is included in the advanced karst development phase. Furthermore, this research contributes significantly to increase knowledge regarding the characteristics of void karst development in aquifers which in the future are very important for determining water resources management policies

Analysis of underground river network connectivity in Barat Cave, Karst Karangbolong, Central Java, using the Artificial Tracer Test Method

The connectivity of the Barat Cave underground river system needs to be known to support the implementation of proper environmental management so that water resources can be maintained sustainably. However, the mapping of underground river paths is often hindered by conditions of narrow cave passages, a barrier blocking the flow of water (siphon), deep underground lakes, underground waterfalls, as well as paths filled with water. This research was conducted in Barat Cave, Karangbolong Karst Area. The purpose of this study is to determine the upstream-downstream connectivity system in this underground river and define the characteristics of the passageway based on quantitative analysis of the transport parameters from the tracer test results. This underground river network analysis needs to be done because previous research has never analyzed this underground river network. The research method used in this study is divided into three stages, namely the pre-field stage, the field stage, and the post-field stage. The pre-field step includes determining the location of the study, collecting secondary data, and studying the literature. The field stage consists of a hydrogeological survey to find information on the presence of caves, springs, sinking stream ponors, or luweng in the study area, instantaneous discharge measurements, and tracer tests. The post-field stage includes data processing and analysis. The results showed that the Barat underground river system originated from the Kalimas sinking stream, Mblabak Cave, Pendok Cave, and Pagilangan sinking streams, then merged into a single tunnel without a flow breaker to the Barat Cave, Pengantin Cave, and appeared in the Kalikarak springs to become a surface river, with a tunnel pattern in the form of curvilinear branchwork. The transport parameters for the underground system tracing of the Barat cave have an advection value of 86.528 m/hour, a dispersion of 0.092 m2/second, a dispersivity of 3.38 meters, and a recovery of 63%. The transport value of the tracing test parameter is influenced by the characteristics of the passageway and underground river flow conditions