Information on groundwater contribution to surface waters is an important but often neglected aspect of water quality, especially in the case of rivers and streams. The contribution of underground water sources to rivers is often significant, both in terms of the quantity of water discharged as well as the flux of nutrients and/or pollutants delivered. In recent years, many catchment hydrology studies have shown, through the use of environmental tracers, that the contribution of groundwater discharge and associated fluxes to river flows is often more important than anticipated. The lack of information on these aspects has important implications in adopting water management policies, since information is needed about the dependency of surface water flows or wetlands on underground sources. The proper hydrological characterization of surface/ground water interactions requires the identification of areas of groundwater discharge, estimation of the magnitude of discharge, its origin and age, as well as the impact of groundwater contribution to the water quality of a river. Methods based in the use of environmental isotopes and other geochemical tracers offer great potential to obtain valuable information on these aspects. This CRP aims to review and test existing and new approaches, coupling mass balance calculations of both water and dissolved substances with environmental tracers to gain further information on the transport processes of water and pollutants.