A multi-isotope approach has been used in the Marano lagoon (NE Italy) and parts of its catchment area to identify causes of increased View the MathML source pollution. The hydrogeochemical features of different water types and potential sources of View the MathML source were characterized using the isotopic composition of View the MathML source (d15N, d18O, and ?17O) and other source-related species such as B (d11B), water (d2H and d18O) and View the MathML source (d34S and d18O). Water samples from the lagoon, its tributary rivers, the groundwater up-welling line, groundwater, sewer pipes, and open sea water have been collected at quarterly intervals in the years 2009–2010. The results indicate that the View the MathML source load in the lagoon was not only derived from agricultural activities but also from other sources such as urban waste water, in situ nitrification and atmospheric deposition. The d34S signature in the lagoon clearly denotes the largely prevailing origin of aqueous View the MathML source from seawater, and practically points to the absence of any appreciable redox process involving S species in the lagoon. It also supports the existence of a connection between the lagoon and the nearby Tagliamento river.