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).
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||Moldovanyi, E. P., Walter, L. M. and Land, L. S. (1993). "Strontium,
boron, oxygen, and hydrogen isotope geochemistry of brines from basal strata
of the Gulf Coast sedimentary basin, USA." Geochim. et Cosmochim. Acta,
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||Palmer, M. R. and Sturchio, N. C. (1990). "The boron isotope systematics
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||Schwarcz, H. P., Agyei, E. K., McCullen, C. C. (1969). "Boron isotopic
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