Boron and strontium isotope ratio analysis of the Rhine river – tracer for anthropogenic boron emissions?


Environmental monitoring of complex ecosystems requires reliable and sensitive techniques in order to identify the source, fate and sink of anthropogenic contaminants in e.g. aquatic ecosystems. Within this context, the analysis of stable isotope ratios has been proved a valuable tool. Amongst other applications, the analysis of B and Sr isotope ratios has evolved as promising tracer to differentiate water bodies of different origin and their corresponding mixing processes of e.g. freshwater and seawater. Due to its versatile use in industries e.g. as sodium perborate, or more recently as dopant for semiconductors, analysis if boron isotope ratios may be of value as a new tracer for anthropogenic B emissions into the aquatic environment. This contribution aims to investigate, whether it is possible to distinguish between the different discharges of a river on the basis of their B and Sr isotopic signatures, in order to distinguish between natural and human input sources of B into the aquatic environment. Therefore, 76 freshwater samples from one of the most anthropogenically influenced rivers in Germany, the Rhine, were analyzed for their Sr and B isotopic composition. The B isotope composition assessed in the Rhine River shows a large variability of δ11B/10BNIST951a of ca. 30‰, and a B concentration ranging from 11.6 µg L-1 ± 1.3 µg L-1 to 65 µg L-1 ± 6 µg L-1. In contrast to that. the Rhine tributaries are characterized by significantly higher B loads. Additionally, tributaries are characterized by Sr concentrations and Sr isotope ratios significantly different from those of the Rhine. The combination of Sr and B isotopic compositions can be used to distinguish different inputs into a complex river system, and can therefore provide a better insight into possible sources and distribution of anthropogenic B inputs.
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