16 Natural Radioactive Isotopes in Water in Relation with Geology and Hydrological Investigations in the Territory of Luxembourg Zornitza Tosheva, Harald Hofmann and Antoine Kies 1 University of Luxembourg, 2 Monash University, 1 Luxembourg 2 Australia 1. Introduction Luxembourg is situated in the middle of Europe and is surrounded by Belgium in the north- west, France in the south-west and south, and Germany in the east (Figure 1). It is one of the smaller countries in Europe with a surface area of 2.600 km 2 and a population of approximately 500.000 inhabitants. The northern third, known as 'Eislek', has a plateau character with an average elevation of 450 m, whereas the southern two-third are know as the 'Gutland', good land in English. This region has a more varied topography and an average elevation of 240 m. Groundwater is the main water source for drinking water supply in the Grand Duchy of Luxembourg. About 60% of the country’s drinking water comes from so called “Luxemburg Sandstone”, which forms the most important aquifer. Most of the groundwater captures are natural springs or shallow wells and represent 80 % of the volume of this aquifer. The majority of the springs are distributed along the main river valleys. The unconfined part of the sandstone is located in the centre of the country and covers about 300 km 2 with a surface to water table depths ranging from 0 to 100 meters (Colbach, 2006). Except from some bores in the north of Luxembourg and some in the south, natural springs are mainly used for public drinking purpose. The radionuclide content of more than 300 springs and bores was determined as part of 2 monitoring campaigns, in a first step to relate the physico-chemical parameters with local geological conditions and in a second to obtain data on groundwater pollution regarding agricultural contaminants, such as pesticides, nitrate etc. The second campaign allows a general quality assessment of the groundwater resources in the country, which is also required by the European Water Framework Directive (EU, 2000). Radio-nuclides are of interest for two reasons. Firstly, a high activity poses a health risk. An example is dissolved radon (222Rn) in groundwater that degasses when the water leaves the aquifer. Radon concentrations increase in the air in buildings where the water is trapped, such as spring trappings, or when used like in households. The inhalation of radon and the deposition of its daughter products in the lungs can cause lung cancer. Secondly, radio- nuclides can be useful tools as tracers in the hydrological / hydro-geological cycle.