Application of boron isotopes for tracing sources of anthropogenic contamination in groundwater

Natural sodium borate minerals from non-marine evaporite sequences are used for world production of sodium perborate, an industrially manufactured bleaching agent added to a variety of detergent formulations and cleaning products. During end use, water-soluble boron compounds are discharged with domestic aqueous effluents into sewage treatment plants, where little or no boron is removed and, hence, the anthropogenic boron load is almost entirely released into the aquatic environment. Natural sodium borate minerals are characterized by a rather narrow range in boron isotopic composition within the large natural variations, such that an isotopic approach may be used to decipher an anthropogenic boron source (mainly from industrial perborate, the dominant use of mined boron) in a given natural aquatic system that is characterized by a distinctive local background signature. This paper presents new boron isotope data for a series of industrial sodium perborate monohydrate and tetrahydrate products. 11B/10B isotopic ratios were measured by NTIMS (Negative Thermal Ionization Mass Spectrometry) with an analytical uncertainty of ±0.5‰ (2σmean). Boron isotope values of sodium perborate monohydrate and tetrahydrate products, reported as δ11B values (in per mille) relative to a mean 11B/10B ratio of 4.00125 measured for the NIST SRM-951 boric acid standard (δ11B={[(11B/10B)Sample/(11B/10B)Standard ]−1}×103), show variations from −3.9 to +0.9‰ and −4.8 to +0.5‰, respectively, which are within the δ11B range of natural sodium borate minerals. The application of this innovative stable isotope tracer technique for monitoring pollution of natural aquatic systems is discussed in the framework of two case studies which demonstrate the usefulness of boron isotopes for constraining distinct solute sources (anthropogenic vs natural) in groundwater.