11B/10B variations of dissolved boron in a freshwater–seawater mixing plume (Elbe Estuary, North Sea)

This study is the first to utilize the boron isotopic composition (δ11B), in combination with elemental concentrations (like B, Na, and Cl) and salinity, as a sensitive tracer for discerning distinct solute sources and constraining geochemical processes in a freshwater–seawater mixing plume. Two distinct series of water samples, collected in the Elbe Estuary and North Sea (Germany) in September 1994 and during an extreme winter flood event in February 1995, have been examined. Boron isotopic compositions and B concentrations have been determined by negative thermal ionization mass spectrometry with analytical uncertainties of ±0.3‰ (2σmean) and ±1% (2σ), respectively. The least saline brackish water (salinity of <0.50) shows a δ11B value of +23.9‰ (0.25 mg B/l), at variance with seawater from the open marine North Sea (salinity of 34.70), which is characterized by a δ11B value of +40.3‰ (4.7 mg B/l). The effects of admixture of immense amounts of rainwater during the centennial flood (in February 1995) relative to less extreme meteorologic conditions prevailing earlier (in September 1994) are documented in this study. The flood event led to considerable 11B enrichment in the freshwater–seawater mixing plume (recorded by a shift on the order of δ11B≈14‰ at 0.25 mg B/l in the least saline brackish water), accompanied by comparatively minor changes in elemental abundances. Well-defined correlations among δ11B values, B concentrations, and salinity are interpreted in terms of binary mixing between riverine input waters and North Sea seawater by a process of straightforward dilution. These relationships favor a model of conservative behavior of boron during estuarine mixing, without involving significant interactions between dissolved and adsorbed boron.