This paper demonstrates, using selected case studies in the Upper Danube basin, how isotopes may serve as a way of better implementing the groundwater related part of the EU Water Framework Directive requirements in the Danube basin. The following examples of conceptual models elaborated in the 1990s with use of isotopes in the Upper Danube basin show how isotopes can be used to help understand: a) potential pathways and travel time of contamination from groundwater to the river from Southern Vienna basin; b) vulnerability of river/wetland ecosystems in the Regelsbrunn wetland; and c) delineation of Danube-originated and local groundwaters in the Pannonian basin. Isotope methods could be also used in other parts of the Danube basin. For example, because the groundwater inflow via tributaries can be important, we also propose longitudinal chemical and isotopical monitoring of the main Danube tributaries, e.g., the Tisza river, which drains the major part of the Great Pannonian basin. Finally, we argue that open questions highlighted in the previous Joint Danube Survey, especially those on the identification of hotspots of nutrient inputs could be addressed using isotope techniques, and therefore we recommend addition of isotopes to the sampling programme of the coming Joint Danube Survey in 2007.

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