Groundwater Vulnerability of Ngemplak and Kalasan in North-East of Sleman District, Yogyakarta, Indonesia Ratha Phok* 1,2 , Doni Prakasa Eka Putra 3 , and Heru Hendrayana 3 1 Department of Environmental Geology Engineering, Gadjah Mada University, Indonesia 2 Department of Water Resources & Rural Infrastructure Engineering, Institute of Technology, Cambodia 3 Department of Environmental Geology Engineering, Gadjah Mada University, Indonesia rathaphok0621@yahoo.com Abstract: Intrinsic vulnerability of groundwater was studied to generate in the purpose of delineating groundwater protection zone in Ngemplak and Kalasan, Indonesia, whose mainly water supply is from groundwater. Another study of specific groundwater vulnerability was conducted in order to assure the quality of groundwater for the supply and to know the actual contaminant based on land-use. DRASTIC method and SVV method are two methods of vulnerability mapping and are used for the evaluation. The resulted maps conducted from these methods are validated using the actual contaminant concentration through the impact of on-site sanitation of the settlements, for example nitrate as it is evidenced to be very stable contaminants and conservative ion in groundwater. Considered in different hydrogeological setting, these two methods have produced various results at the certain site. However, its reliability has been drawn upon the nitrate concentration at the study areas, and it is concluded that SVV method gives more realistic and applicable result than DRASTIC method. Keywords: Intrinsic & Specific groundwater vulnerability; DRASTIC; SVV methods; nitrate contamination. 1. Introduction Groundwater is used to serve a lot of services in many countries in the world, especially, in developing countries. It thus plays a fundamental (but often little appreciated) role in human well-being, as well as that of some aquatic and terrestrial ecosystems (Foster et al., 2002). The concept of groundwater vulnerability is derived from the assumption that the physical environment may provide some degree of protection of groundwater against natural and human impacts (Vrba & Zaporozec, 1994). Technically, there are two types of vulnerability; intrinsic vulnerability and specific vulnerability. According to Vrba & Zaporozec (1994), Gogu & Dassargues (1999); modified from Putra (2007), the term intrinsic vulnerability is used to define the vulnerability of the groundwater to contaminants generated by human activity as a function of natural factors such as hydrogeological factors, hydrology, the overlying soil and geological material. While the term specific vulnerability is used to define the vulnerability of groundwater to a particular contaminant or group of contaminants by taking account the contaminant properties and their relationship with the various components of intrinsic vulnerability. Many intrinsic vulnerabilities methods have been developed in the past two decades but basically there are only three mainly groups of these techniques referred as hydrogeological complex and setting methods (HCS), parametric methods (included matrix systems (MS), rating systems (RS), and point count system models (PCSM)), and analogical relation and numerical model (AR). The intrinsic methods applied in the case study are both from the point count system model. DRASTIC method is a method developed for the U.S. Environmental Protection Agency (EPA) by Aller et al. (1985, vide Aller et al., 1987). In the other hand, Hoelting and SVV method, which take the unsaturated zone into consideration, was developed by the Geological Survey of the individual states of the Federal Republic of Germany in 1995 by Voigt et al., (2004); Putra (2007) respectively. Since vulnerability is not an absolute characteristic but subjective, relative, non-measurable, dimensionless property (Vrba & Zaporozec, 1994), the reliability of the each vulnerability maps is still be a topic to be discussed among hydrogeologist. From many research works, Gugo & Dassargues (1999) has suggested that the only way of gaining confidence in vulnerability is by comparing the results of various techniques and analyzing their consistency with the actual contaminant at the study area. There are three main objective of this study such as, (1) To determine the hydrogeological condition of the study area, (2) to determine the intrinsic and specific vulnerability of groundwater in two study areas using two different methods; (3) To compare the suitable groundwater vulnerability mapping method between DRASTIC and SVV for the study area. 2. Study Area 2.1. Location of Study Area