Hydrological and Biogeochemical Characterization of the Danube River System Using Isotopes

Meeting Danube Basin monitoring and management objectives such as those implied by the EU Water Framework Directive requires a comprehensive understanding about the hydrological and biogeochemical functioning of not only the river system but also the connections between groundwater and surface water across the basin. While hydraulic and geochemical measurements can provide some of this understanding, it is often difficult to obtain knowledge about some of the more critical aspects of basin functioning or it can take decades of intensive monitoring before adequate interpretations can be made. Isotope hydrology approaches can often provide critical insights on surface water/groundwater interactions and biogeochemical cycling with only moderate effort and cost. Such information can help clarify local behaviors as well as overall basin responses. Approaches using “environmental” stable and radioactive isotopes (i.e., isotopes that are already in the environment and not intentionally applied) have been used to understand sources and losses of water in the Danube, the importance of groundwater discharge, basin residence times, tributary mixing, and nitrate cycling using isotope methods. We review existing studies as well as present new isotope data that reveal important spatial and temporal dynamics occurring in the Danube River, tributaries, and across the basin.