Boron isotopic composition of freshwater lakes from central Europe and possible contamination sources

The boron isotopic compositions of common synthetic boron products, municipal wastewaters from Switzerland, and three Swiss freshwater lakes were investigated. The δ¹¹B values (δ¹¹B values are normalized to the standard NIST SRM-951) of synthetic Na-borates (-0.4 to 7.6‰) and Ca-Na-borates (-13.4 to -4.9‰) overlap with those of natural borate minerals and hence suggest that the isotopic signal of anthropogenic boron is not modified during the manufacturing process. As a result it is possible to predict the isotopic composition of synthetic boron products and their potential impact upon contamination of water resources. The δ¹¹B values of municipal wastewaters from two locations in northern Switzerland (-7.7 to -4.5‰) reflect utilization of Na/Ca- and/or Ca-borates depleted in ¹¹B. Freshwater lakes from Switzerland (Lake Zurich, Greifensee, Lake Lugano) yielded a δ¹¹B range of -1.7 to 7.1‰ and boron concentrations of 17 to 102 mg L^-1. The boron isotopic ratios decrease with increasing boron concentrations, indicating mixing between anthropogenic boron with a low δ¹¹B signature and meteoric boron with a heavier isotopic signal. We suggest that the isotopic composition of meteoric boron over central Europe has δ¹¹B values in the range of ca. 10 to 20‰, whereas in coastal areas the marine component is larger with a higher ¹¹B/¹⁰B ratio (δ¹¹B ~ 30‰).