Oleh: Mochammad Prahastomi M.S, S.T., M.Sc. (Departemen Geologi, FMIPA UI)
The emergence of a new lake after being hit by a tropical cyclone is a natural phenomenon that is quite rare to witness, especially in Indonesia. This is very interesting, especially when viewed in the context of Hydrogeology. Hydrogeology is a branch of applied geology where the focus of study is on the movement of groundwater and the interaction of groundwater with the rocks it passes through.
The emergence of the lake in Kupang, East Nusa Tenggara (NTT), is inextricably linked to the complex hydrogeological system within the limestone region. Kupang and its surrounding areas are regionally composed of Pleistocene and Neogene rocks. Pleistocene rocks dominate the Kupang area and are composed of coralline limestone. This coralline limestone typically forms the karst landscape in NTT.
Karst areas are found in abundance in Indonesia, for example in Padalarang and Sawarna, West Java. These karst areas are unique, characterized by the formation of caves and underground rivers. These caves are the result of the dissolution of meteoric water entering through cracks in the limestone. This meteoric water dissolves limestone due to its low lime saturation (CaCO3) content. These cracks in the limestone can enlarge over time. This long-term dissolution process is what forms the caves and underground rivers in karst environments.
Underground river in the Sawarna karst area (Personal documentation)
How are karst areas related to lake formation?
Cyclone Seroja appears to have played a significant role in the formation of the lake. Geologically, this lake is often referred to as a karst lake. Recent cyclones have been accompanied by increased rainfall intensity, which infiltrates the ground. This high volume of rainwater increases erosion and limestone dissolution. This groundwater can emerge to the surface as springs.
One reason why these springs are only now emerging is likely related to the formation of new underground river channels that eventually connected to the surface, triggered by heavy storms and heavy rainfall in the area. This is supported by observations of relatively high water discharge, which could indicate the presence of subsurface river flow emerging to the surface as springs.
The lakes that form are usually temporary. They are controlled by the flow of groundwater that rises to the surface. Therefore, if the subsurface water supply recedes, the lake water level will also decrease. This certainly requires further study regarding the management and evaluation of groundwater potential in highly complex karst areas. This is an opportune time to conduct comprehensive research on the emergence of these springs to accurately map the groundwater potential in this area.
Collaboration between universities, ministries, and local governments is key to successful groundwater management in areas with severe water shortages. If properly managed and developed, this karst lake’s potential could be developed into a tourist attraction offering exotic natural beauty.
The process of limestone dissolution in karst areas. The source of the karst lake water is indicated to come from a new connecting path between the underground river and the topography (orange box).
Karst lake tourism potential, Oman (Personal documentation)
