CHARACTERIZATION OF UNDERGROUND RIVER PASSAGES CONTROLLED BY ALLOGENIC RECHARGE: INSIGHTS FROM AN ARTIFICIAL TRACER TEST IN THE GUNUNGSEWU KARST AREA, INDONESIA

The Ponjong Hydrogeological Subsystem is a part of the Gunungsewu Karst Area on Java Island, Indonesia, and is uniquely influenced by allogenic recharge. This allogenic river system exists because the karst in this subsystem develops over a Tertiary volcanic formation. The contact between the Wonosari Formation limestone and the volcanic rocks, primarily composed of explosive volcanic materials from the Semillir Formation, allows this system forming. In this area, allogenic river catchments are dominated by relatively impermeable volcanic rocks and steep gradients, which often lead to flash floods. This study aimed to determine the characteristics of the cave and conduit networks within the underground river system influenced by allogenic recharge. An artificial tracer test using fluorescent dyes with breakthrough curves recorded by a fluorometer was conducted. Data analysis revealed that large, single conduits predominantly shape the cave networks in the study area. Due to the large discharge and steep gradient of the underground river system, a single conduit is formed instead of a maze-like cave system. The identification of connectivity between the allogenic rivers and the Gremeng Resurgence highlights that the karst drainage basin of the Gremeng Resurgence is not part of the Beton Karst Drainage Basin, as previously suggested by researchers who used methods other than artificial tracer tests. The development of large underground river systems indicates a high vulnerability to groundwater pollution and the ability to release groundwater reserves quickly. This poses a challenge for future environmental management, particularly in relation to water quality and water resource quantity management. © 2025 Earth Science Division, Azerbaijan National Academy of Sciences. All rights reserved