Abstract
This study was conducted on the Petoyan Spring, one of the most important springs in the western part of Gunung Sewu Karst, Java, Indonesia. The main purpose of the study is to define the diffuse flow characteristics of the spring. It also focuses on identifying the relationship between the diffuse flow proportion and several hydrochemical parameters to provide a better aquifer characterization. Water-level monitoring and discharge measurements were conducted over an eight months period in order to define the discharge hydrograph. The calculation of recession constants, including the diffuse flow (Kb), fissure flow (Ki), and conduit flow (Kc), was done by selecting several flood hydrograph events. Furthermore, the base flow-separation analysis was conducted to determine the diffuse flow proportion throughout the research period. Water sampling for hydrogeochemical analysis was taken every week and every hour for two selected flood events. A bivariate plot analysis between the diffuse flow proportion and certain hydrochemical parameters was made in order to describe the hydrogeochemical processes. The results showed that the diffuse- flow significantly dominates the spring discharge during non-flooding and flooding recession periods. Conversely, during the rising limb period, the diffuse proportion decreased drastically due to the conduit flow supply. This fact was also supported by the high Kb and short Tp (time to peak) and Tb (time to base flow) values. Hydrogeochemically the correlation between the diffuse flow proportion and major dissolved elements during the flood recession periods was strong because the conduit flow domination was replaced by the diffuse flow, causing the domination of water-rock interaction. According to the diffuse flow and its relation to its hydrochemistry, it is apparent that small fractures in the Petoyan Spring aquifer still provide good storage for the diffuse flow, even though the conduit fracture has already been developed.
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