The application of B isotope geochemistry as a tracer for anthropogenic contamination of natural aquatic systems represents a novel approach in the field of environmental sciences. Anthropogenic B mainly derives from sodium perborate typically added as a bleaching agent to textile and laundry detergents. As natural non-marine sodium borate minerals typically used for the production of sodium perborate are characterized by specific B isotope variations, the end use of detergents is expected to result in discharge of B with well-constrained isotopic signatures to the aquatic environment. In order to investigate the potential of B isotope systematics for discerning natural versus anthropogenic sources and modeling geochemical cycles we have analyzed a suite of groundwater and contaminated recharge derived from several geographically and temporally (1918 to recent) separated compartments located within a hydrochemically well-studied solid waste disposal site. The first B isotope data for groundwater and contaminated recharge presented in this study will show that the natural and anthropogenic B sources are characterized by substantial differences in their B isotope signatures and that the pattern reflects a mixing relationship characterized by systematically increasing B concentrations and 11B/10B isotope ratios with an increasing contribution of anthropogenic B.
11B/10B isotope ratios were determined by Negative Thermal Ionization Mass Spectrometry (NTIMS) on an NBS design 6 inch radius of curvature 60° sector solid source mass spectrometer at the State University of New York at Stony Brook. By adopting the refined NTIMS technique (Hemming and Hanson, 1994) B samples in the order of 1-10 ng were analyzed (3-4 replicates per sample) by a direct loading procedure with an analytical uncertainty of ± 0.4 ” (2smean) determined on sample replicates. B isotope values are given in conventional d notation relative to NIST SRM-951 standard (11B/10B ratio of 4.0014 ”) (Hemming and Hanson, 1994). B concentrations were determined by ICP-OES and NTIMS isotope dilution techniques with analytical uncertainties (2s) of ± 5 % and ± 1.4 %, respectively.
B concentrations and B isotopic compositions in groundwater and contaminated recharge from the solid waste disposal area under study show considerable variations from 20 ppb to 3.0 ppm and from -7.1 to +16.5 ”, respectively. Based on B isotope systematics, the existence of two distinct groups is suggested: (1) A suite of samples derived from compartments located within the solid waste disposal site (1918-1924, 1925-1935, 1950-1960, 1961-1989, and 1990 to recent) is characterized by variations in B concentration of 70 ppb to 3.0 ppm and by a wide range in d11B of -7.1 to +9.0 ”. The samples define a two-component mixing curve and, moreover, align along the mixing curve in a way which systematically correlates increasing B concentrations and d11B values with increasing age of the respective solid waste compartments. Consequently, the low-B / low-d11B and high-B / high-d11B endmembers may be interpreted to approach the natural (uncontaminated) and anthropogenically contaminated B sources, respectively. The range in d11B obtained for groundwater and contaminated recharge samples which are to a variable degree influenced by the anthropogenic B source overlaps with the B isotope signatures reported for non-marine sodium borate minerals (-1 to +10 ”) (Oi et al., 1989) and sodium perborate and detergent products (-6 to +8 ”) (Barth et al., 1995). (2) In contrast, another suite of samples derived from an area adjacent to the solid waste disposal site is characterized by a narrow range in B concentration of 20 to 120 ppb and by a wide range in d11B of -0.2 to +16.5 ”. The observation that these samples are set apart from the above mixing curve may be taken to suggest that groundwater from this area is influenced by an additional (natural or anthropogenic) B source, thus demonstrating the potential of B isotope systematics for discerning different flow regimes of groundwater systems. In conclusion, the data reported in the present study suggest that anthropogenic pollution of groundwater dated back to 1950-1960 can still be recorded by B isotope systematics and that the application of this innovative stable isotope tracer to environmental sciences may contribute to monitoring the anthropogenic impact on natural aquatic systems.
Barth, S., et al., GSA Annual Meeting A-421/422 (1995).
Hemming, N.G. & Hanson, G.N. Chem. Geology (Isot. Geosci. Sect.) 114, 147-156 (1994).
Oi, T., et al., Geochim. Cosmochim. Acta 53, 3189-3195 (1989).