Assessment of aquifer karstification degree in some karst sites on Java Island, Indonesia

This research was conducted on several springs and underground rivers in two different karst areas: the Gunung Sewu karst in the southern part of Java Island and the Rengel karst in northern Java. The objective was to determine the spatial degree of karstification in springs and underground rivers in these areas using available data on hydrograph recession. Hydrograph recession curves from eight water-level gauges in springs and underground rivers were analyzed and classified for each location based on quantitative parameters to express individual groundwater flow sub-regimes. Discharge was used to generate recessional equations, and these were related to karstification degree on a qualitative scale varying from 1 to 10. The karstification degree calculated using selected data on flood recession ranged from a low value of 3.7 in the Petoyan spring to a high value of 7.7 in the Bribin river. The value of 3.7 (low) indicates that the aquifer is dominated by a network of uniform small-sized voids (diffuse and fissure), the majority of which are open with minimal macro-fissure karst channels. Meanwhile, the value of 7.7 (high) indicates highly developed karstification of the aquifer, which is formed by large open conduits (karst channels). Furthermore, the general degree of karstification in Gunung Sewu is more developed than in the karst region of Rengel, except in the Petoyan spring, located in the western part of the Gunung Sewu karst region. Interestingly, the degree of karstification in Gunung Sewu varied. This may be due to differences in variable surface geomorphology, which is controlled by the differing solubility and thickness of the limestone beds. This study also indicates that there is a fairly strong positive relationship between the degree of karstification and the extent of the catchment area of springs