Periodic Table--Boron

Boron has two naturally-occurring stable isotopes, ¹¹B (80.1%) and ¹⁰B (19.9%). The mass difference results in a wide range of d¹¹B values in natural waters, ranging from -16 to +59 ‰ (data from references within Vengosh et al., 1994). Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)₃ and B(OH)₄ (Schwarcz et al., 1969). Boron isotopes are also fractionated during mineral crystalization (Oi et al., 1989), during H₂O phase changes in hydrothermal systems (Spivack et al., 1990; Leeman et al., 1992), and during hydrothermal alteration of rock (Spivak, 1985). The latter effect (species preferential removal of the ¹⁰B(OH)₄ ion onto clays results in solutions enriched in ¹¹B(OH)₃; Schwarcz et al., 1969) may be responsible for the large ¹¹B enrichment in seawater relative to both oceanic crust (Spivack and Edmond, 1987) and continental crust (Spivack et al., 1987). All of these effects combine to produce B isotopic variations in hydrologic systems that can be very useful. Boron isotopic ratios have been used to trace the origin of water masses (Palmer and Sturchio, 1990), to track the evolution of brines (Vengosh et al., 1991 a, b; Moldovanyi et al., 1993), to determine the origin of evaporites (Swihart et al., 1986; Vengosh et al., 1992), and to examine hydrothermal flow systems (Leeman et al., 1992).

Boron in groundwater might derive from leaching of country rocks, infiltration of meteoric salts, mixing with adjacent groundwaters, and contamination by anthropogenic sources. While boric acid and borate minerals are widely used in industrial applications, the main use of boron compounds (especially sodium perborate) is as a bleaching agent in detergents. This usage causes high concentrations of boron in wastewaters worldwide. Vengosh et al. (1994) note that each of these sources has a distinctive boron isotopic signature (eg., the d¹¹B of seawater is 39‰ and that of average continental crust is 0 +/- 5 ‰).

In one study in Israel (Vengosh et al., 1994), raw and untreated sewage were found to have d¹¹B values ranging from 5.3 to 12.9 ‰, overlapping the compositions of natural non-marine sodium borate minerals (-0.9 to 10.2 ‰). However, these values were significantly different from regional uncontaminated groundwater (~30 ‰) and seawater. Furthermore, groundwater contaminated by recharge of treated sewage had a high B/Cl ratio and a distinctive d¹¹B signature of 7 to 25 ‰. Elemental B and d¹¹B variations reflect both mixing with regional groundwater and the boron isotope fractionation caused by boron removal by adsorption onto clays. Therefore, boron isotopes have a high likelihood of being a very useful tracer in groundwater systems in which the role of clay and minerals can be clearly identified, as a tracer for anthropogenic boron and as a tracer for seawater contamination.

Source of text: This review was assembled by Carol Kendall, Eric Caldwell and Dan Snyder, primarily drawing from Vengosh et al. (1994) and Nimz (1998).