Hydrochemistry temporal variation of Anjani Underground River, Jonggrangan Karst, Java Island, Indonesia

Abstract

Anjani underground river was one of the biggest water resource in Jonggrangan Karst. The objective of this research was to describe hydrochemical temporal variation which was expected to assist in defining the aquifer conditions that recharge Anjani underground river in more detail. Sampling was carried out for one year covering dry and wet seasons, as well as the flood events. The hydrochemical analysis was conducted based on dissolved major ion concentration and physicochemical properties. In addition, SIcalcite and log PCO2 were calculated to understand the temporal variation of aggressiveness on calcite mineral. The results showed that all water samples performed HCO3-Ca type. The chemical constituent in water varied temporally which showed high during dry season and decreased during wet season. The high chemical constituent during dry season was caused by the domination of slow-flow (diffuse) so the water had a longer residence time in aquifer, allowing water-rock interaction to be more intensive. Meanwhile, the low dissolved constituent during wet season was caused by input from quick-flow that changed water-rock interaction into dilution by precipitation process. Generally, all water samples was saturated with respect to calcite, except during flood events. One of the factors that affected this condition was CO2 supply from the allogenic river that allowed the dissolution process to be more intensive.