Applied tracers, artificial and environmental, are widely used to characterize groundwater flow paths and to estimate system parameters. They are increasingly used in research settings to understand solute transport phenomena in strongly homogeneous karst and fractured aquifers, which are motivated e.g. by environmental concerns related to water resources and water protection evaluation or to disposal and release of wastes. However, there are also many practical uses for environmental isotopes in understanding and quantifying of groundwater bioremediation. The practical use of tracers in environmental hydrogeology and groundwater remediation is quickly increasing. The characteristic isotopic signature of many pollutants and the changes in isotopic composition during specific biological, chemical, and physical processes may yield unique information on the origin of pollutants and on their fate in soil and groundwater. Especially for natural attenuation studies, isotope analysis can provide essential information. Conservative and reactive tracers can greatly aid in the design and evaluation of enhanced bioremediation strategies by providing a reliable way to measure in-situ contaminant decay, oxido-reduction processes rates, and zones of influence. This session will be focused on two following topics: - Estimation of water origin, water flow-paths and groundwater dynamics in heterogeneous karst systems by combined use of tracers and mathematical modelling; - Understanding and estimating of pollutant transport and bioremediation processes using isotope methods.