Vulnerability assessment of nitrate leaching on the regional scale using isotope techniques Zusammenfassung: Groundwater is an important source of drinking water but its quality is threatened by contamination with nitrate. In order to prevent groundwater contamination knowledge about potential nitrate sources their pathways and time lag between sources and drinking water wells is required. Still i) simple methods to quantify nitrate fluxes are missing ii) the applicability of stable isotopes of nitrate in nitrate source identification in grassland systems remains unknown and iii) there is a lack of physically based modeling approaches focusing on the spatially resolved estimation of groundwater vulnerability to nitrate contamination. Thus the objectives of the thesis are to i) quantify nitrate leaching in soils ii) identify major sources of nitrate in aquifers and iii) determine the intrinsic and nitrate specific aquifer vulnerability. The objectives were realized in a pre alpine area with oligotrophic aquifers used for drinking water supply. Different isotopic and environmental tracers were used to understand the local hydrogeology and the fate of nitrate and related dynamic transformation processes. At farmed grassland sites analysis of nitrate concentration in soil water below the root zone using suction cups was combined with groundwater recharge rates estimated from vertical deuterium profiles in soil to calculate nitrate fluxes. Stable isotopes of nitrate were analyzed in surface water soil water and groundwater and compared with the distinct isotope composition of nitrate in potential sources. Local tritium concentrations were used in combination with a lumped parameter model to calculate the mean residence times (MRTt) and MRTt distributions of local groundwater wells. The intrinsic and nitrate specific vulnerability of groundwater was spatially assessed (GIS) based on the approximation of the total mean residence time (MRT) and the residence time distribution as well as consulting data on nitrate leaching.Measured nitrate concentration below the root zone varies between 0 and 90 mg NO3 L 1. Estimated groundwater recharge rates cover a range of 430 to 926 mm yr 1. Nitrate leaching (0.2 to 41 kg NO3 N ha 1 yr 1) at grassland sites is highest in spring and fall. At farmed sites nitrate leaching is positively related to fertilizer input rather than land use category (organic conventional). Nitrate isotope values indicate that nitrate derive from ammonium nitrification of multiple sources such as atmospheric deposition mineral ...