Correlation of the hydrogeological systems and landslide potential in the Northern Menoreh Hills, Purworejo Regency, Central Java, Indonesia

The hydrogeological system of Menoreh Hills, Purworejo Regency area is generally built by fissure aquifers consisting of volcanic pyroclastic breccia, lava, intrusion, and laharic breccia intensively jointed. On the other hand, there are also semikarstic and intergranular aquifer systems compiled by carbonate clastic sedimentary rocks of the Sentolo Formation. Hydrogeological, the presence of groundwater is inserted in rock crevices (fissure aquifers), and occupying either cavities or inter-granular pores (semi karstic - intergranular aquifer). In general, the value of groundwater pressure is relatively high, proven from the fact that the hydraulic head in volcanic fissure aquifer ranges 150m – 400m, while in a carbonate aquifer is about 110m – 150m above sea level. Groundwater pressure is one of the triggering factors for landslides. Influenced by hilly topography with moderate to steep slope inclination of 4° – 35° (13% – 70%), thick soil (2m – 5m), high rainfall (>2000mm/year), and saturated soil (water content =13% – 39%), the Menoreh Hills represents a high potential for landslides. The potential grades can be grouped into low, medium, and high categories. Based on hydrogeological studies, there is a correlation between the hydrogeological system represented by the types of an aquifer with the potential of landslides. Volcanic fissure aquifer type indicates higher landslide potential than the carbonate intergranular aquifer type.