In different projects, the CRP Henri Tudor in collaboration with the Water Management Agencyhas shown that Luxembourg’s groundwater resources used for drinking water production arecontaminated with trace levels of a series of pesticides from different sources. The LuxembourgSandstone is the aquifer providing the majority of drinking water supplies in Luxembourg. Thisfractured rock aquifer can be regarded as a double porosity system with fractures as transportchannels and sandstone porosity as reservoir. Depending on the local distribution of fractureand pore volumes mean water residence times in the aquifer can vary substantially. The FNRSPRINGproject investigates groundwater residence times with conceptual models andgroundwater dating methods. First estimations based on porosity distributions have given meanresidence times of up to 25 years. Typically, the determination of the environmental fate ofphytosanitary compounds considers mainly degradation rates that apply for the topsoil. Thiscan be explained by the fact that the few reported degradation rates in deeper aquifers aregenerally up to 2 orders of magnitude slower than in the upper first centimetres of agriculturalsoils. Considering the previously mentioned high residence times, a significant attenuation ofpesticide concentrations in the saturated zone and consequently in the spring outcrops cannotnecessarily be excluded.The regulatory framework for the registration of phytosanitary products requires specificationson their environmental fate. Different approaches including laboratory batch experiments andfield-scale tests as well as modelling of pesticide leaching and degradation are applied toassess the environmental risks associated with pesticide applications. Since the degradationhalf-life of any given pesticide is a very essential and sensitive parameter in pesticide fatemodels, a better understanding of the degradation potential in deep aquifers is of stronginterest. Determining the actual existence and the rate of pesticide degradation in the aquifer istherefore of pivotal importance for sound pesticide modelling and water resourcesmanagement.Compound-specific stable isotope analysis (CSIA) is a new promising tool to allow such insightinto naturally occurring degradation processes in deep regions of the aquifer. This approachrelies solely on information contained in the investigated molecules and does not dependent ontracer tests, transport modelling or even detection of metabolites. By analyzing the isotopevalues of organic trace compounds, it records the fingerprint of different sources and can pickup the footprint of chemical and/or biological transformations that the compounds haveundergone. A major challenge of the project will consist in adapting existing analytical preconcentrationmethods in order to achieve sufficient sensitivity to measure stable isotopecompositions in the target analytes at concentrations in the low to medium ng/L range.