Hydrogeochemical Conditions in Groundwater Systems with Various Geomorphological Units in Kulonprogo Regency, Java Island, Indonesia

Geomorphological (landform) aspects have long been known to control groundwater conditions in an area. Thus, combining the hydrogeological and geomorphological aspects (lithology, genesis, and morphology) becomes a prospective approach for understanding and delineating the hydrogeochemical processes in an area. The idea is then applied in Kulonprogo, Java, Indonesia, that consists of several landforms with minimum anthropogenic influence, in order to identify and quantify the hydrogeochemical processes that are responsible for hydrogeochemical facies changes in each landform. The groundwater facies based on Kurlov classification in each landform are strongly influenced by the water–rock interaction process as it presented in the Gibbs curve. The magnitude of saturation indices and mass transfer is also diverse that caused a distinction of hydrogeochemical facies and processes in each landform. For instance, the evolution of groundwater in the denudational hill to the fluviomarine plain occurs from Ca–HCO₃ to Na?+?K–Ca–HCO₃. The analysis of Durov diagram and inverse modeling—using PHREEQ—reveals that the hydrogeochemical processes that occur in most of the landform are ion exchange, weathering or dissolution, and precipitation. Further, oxidation–reduction and mixing only occur in few landforms. The further investigation from mass balance calculation that constructs from inverse modeling reveals some interesting findings and hypotheses, such as the construction of gypsum probably found in the deeper layer on swale as a result of pyrite dissolution of 1.074?×?10^?3 mmol, and it is responsible in escalating Ca²⁺ and SO₄^2?. Another finding is that although the calcite mineral mostly related to the past-marine environment, such as in the east denudational hill, the calcite in the west part is formed as a breakdown of 3.225?×?10^?3 mmol anorthite.