Application of boron isotopes for tracing sources of anthropogenic contamination in groundwater
Introduction
Anthropogenic contamination of natural water resources deserves serious consideration as a major problem in environmental protection. The information carried by chemical and isotopic signatures of water samples is, however, often difficult to decipher because of the number of natural and anthropogenic solute sources that may contribute to aquatic systems. Boron isotopic compositions and concentrations provide new insight into hydrogeologic processes (e.g. Davidson and Bassett, 1993; Vengosh et al., 1994; Bassett et al., 1995). In fact, boron isotopes can be used as tracers for discerning distinct solute sources in natural waters since (a) boron is highly soluble in aqueous solutions and therefore an ubiquitous minor or trace constituent in nearly all water types, (b) the boron isotopic composition is controlled by several known parameters among which the solute source compositions and isotope fractionation processes related to adsorption/desorption, mineral precipitation and dissolution, and volatilization are the most relevant, and (c) the relatively large mass difference between the two stable isotopes of boron, 10B and 11B, leads to a wide range of boron isotope variations in nature (δ11B≈90‰, Barth (1993); for δ11B notation see Eq. (1)below). Natural waters (such as seawater, river water, rainwater, groundwater, brines, geothermal fluids, and fumarole condensates) encompass a large range in δ11B of≈76‰. The lowest δ11B values at −16% are reported for groundwater from the Great Artesian Basin in Australia (Vengosh et al., 1991a). The most 11B-enriched reservoirs known to date are saline groundwaters in Israel and brines from the Dead Sea and Australian salt lakes with δ11B values up to +60‰ (Vengosh et al., 1991a, Vengosh et al., 1991b, Vengosh et al., 1994).
A variety of applications are known where anthropogenic water-soluble boron compounds are discharged to the aqueous environment, including the use of boronated fertilisers, insecticides, corrosion inhibitors in anti-freeze formulations for motor vehicle and other cooling systems, buffers in pharmaceutical and dyestuff production, and the use of boric acid solutions for the control of nuclear reactions (the absorption of neutrons by the 10B isotope). By far the most significant application of anthropogenic boron compounds is, however, the use of industrially manufactured sodium perborate (either as monohydrate or tetrahydrate; see structural formula in Fig. 1), added as a bleaching agent to a variety of detergent formulations and cleaning products. The raw materials are mainly natural Na-borate minerals (borax (tincal) and kernite; for structural formulae see Table 1) from large non-marine evaporite deposits in the U.S.A. (e.g. Boron, Searles Lake) and western Turkey (e.g. Kirka) which account for almost 90% of world production of sodium perborate (Harben and Bates, 1984). These deposits were formed in lacustrine (salt lake) environments during periods of Neogene volcanic activity (Harben and Bates, 1984; Palmer and Helvaci, 1995). During end use of perborate-enriched detergents and cleaning products, the anthropogenic water-soluble boron compounds are discharged with domestic aqueous effluents into sewage treatment plants, where little or no boron is removed during conventional processing of the wastewaters (Waggott, 1969). Hence, the anthropogenic boron load is almost entirely released into the aqueous environment by entering a receiving surface water system, usually a river or lake, where further dilution occurs. Covariations observed between B concentrations of lacustrine and riverine freshwaters and P concentrations or anionic detergent signatures (Tartari and Camusso, 1988) support that sodium perborate is to be considered as the major anthropogenic boron source.
Non-marine Na-borate minerals (see Bassett (1990) for a review of borate minerals) are characterized by a rather narrow range in boron isotopic composition within the large natural variations (δ11B values from −0.9 to +10.2‰ and −5.4 to −1.7‰ for Na-borates from the U.S.A. and Turkey, respectively; McMullen et al., 1961; Swihart et al., 1986; Xiao et al., 1988; Oi et al., 1989; Palmer and Helvaci, 1995). Therefore, 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. In view of analytical difficulties related to the detection of organic and inorganic chemicals (and intermediate products produced by bio-degradation) derived from the discharge of detergents and cleaning products to surface water and groundwater systems, the approach to use boron isotopes for tracing such type of pollution, as exemplified by some few recent studies (e.g. Vengosh et al., 1994; Bassett et al., 1995), offers a new perspective. The boron isotopic signature of commercial borax products, however, is still largely unknown and indicated by only two values reported for products from the U.S.A. (δ11B=−1.3 to +3‰; Xiao et al., 1988; Vengosh et al., 1994) and one value reported for a product from Turkey (−4.9‰; Xiao et al., 1988). In this paper, new and precise boron isotope data, measured by NTIMS (Negative Thermal Ionization Mass Spectrometry), are presented for a series of industrial sodium perborate monohydrate and tetrahydrate products that have been manufactured in Europe (Germany, Austria, and Italy). 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.
Section snippets
Experimental
The NIST SRM-951 boric acid standard (used as reference material) and sodium perborate sample solutions containing around 10 ng boron were loaded directly (without prior separation of boron by ion-exchange column chemistry) by addition of 1 μl 1 M HCl and 1 μl B-free seawater onto rhenium filaments (for analytical details see Hemming and Hanson (1994)). The 11B/10B isotopic ratios were measured by NTIMS (Negative Thermal Ionization Mass Spectrometry) using an NBS design 15 cm radius of curvature
Results
The results of replicate boron isotope analyses performed on the NIST SRM-951 boric acid standard and industrial sodium perborate products are listed in Table 2, Table 3, respectively. The boron isotopic compositions of the sodium perborate samples are reported as δ11B values (in per mille) relative to a mean 11B/10B ratio of 4.00125 measured for the NIST SRM-951 standard whereSodium perborate monohydrate and tetrahydrate samples are
The manufacture of sodium perborate
Sodium perborate is formulated either as sodium perborate monohydrate, NaBO3·H2O, or as sodium perborate tetrahydrate, NaBO3·4H2O (see Fig. 1), with the latter being the form that is predominantly used at present (Raymond and Butterwick, 1992). Sodium perborate tetrahydrate is usually produced by treatment of borax (almost in the pentahydrate form, Na2B4O7·5H2O) with sodium hydroxide and hydrogen peroxide according to the reaction (Raymond and Butterwick, 1992)
Conclusions
- 1.
The narrow and overlapping ranges in the boron isotopic compositions of non-marine Na-borate minerals (δ11B values from −5 to −2‰ and −1 to +10‰ for Na-borates from Turkey and the U.S.A., respectively; McMullen et al., 1961; Swihart et al., 1986; Xiao et al., 1988; Oi et al., 1989; Palmer and Helvaci, 1995) and anthropogenic boron compounds such as sodium perborate (−5 to +1‰), representing the dominant use of mined borates, indicates that the industrial manufacture process leaves the original
Acknowledgements
I wish to express my special thanks to G. N. Hanson for the access to the NBS mass spectrometer at the State University of New York at Stony Brook, U.S.A., and to N. G. Hemming for his advice during mass spectrometric boron isotope analysis. Two anonymous reviewers are sincerely thanked for constructive and helpful comments. The help by B. Vitale, T. Rasbury, and D. McDaniel is appreciated. This work was supported by a grant provided by the Deutsche Forschungsgemeinschaft (DFG) to S. B. which
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