HYDROCHEMICAL ASSESSMENT OF UNCONFINED AQUIFER SYSTEM IN BAYAT MELANGE COMPLEX, KLATEN, INDONESIA

Abstract

Thegeological setting of Bayat that has a complexity in structural geology as well as a variety of lithology because this region is interesting to be investigated from thehydrogeological point of view. Hydrogeological assessments were conducted based on thehydrogeological and geological mapping, as well as hydrochemical analysis. Groundwater level measurements were carried out at 51 measurement points. There were 25 groundwater samples to be analyzed the ion content by using Ion Chromatography and ICP-OES. There are seven (7) lithology units i.e. Schist-phyllite-marble, Nummulitieslimestone, Diorite intrusion, Pebbly sandstone, Layered limestone, Colluvial and Alluvial deposit based on thesurface geological mapping. The geological structure investigation in Temas Hill conduct the trends of structural orientation that are northeast to southwest. They become the point of infiltration and circulation pathways in the crystalline bedrock.Hydrochemical analysis obtainfive (5) groundwater facies. There are Ca-Mg-HCO₃, amix of Ca-Na-HCO₃-Cl, Na-HCO₃, Ca-SO₄ and Na-Cl. The concentration abundances of cation are Na⁺>Ca²⁺>Mg²⁺>K⁺while HCO₃^->Cl^->SO₄^2->NO₃^- represent anion content. A Gibbs diagram shows that the dissolved ions are dominated by rock weathering process. Na⁺ ion is derived from the weathering of feldspar-rich mineral, Ca²⁺ and Mg²⁺ indicate through limestone dissolving. The high Cl^- ion content in some samples is predicted from the mixing of upwelling fossil water through Trembono strike-slip fault.