Analyses of heavy metals in soils reveal the degree of contamination but the problem of identifying the source of contamination and the distinction between multiple sources in a single sample often remains unsolved. The present
study was initiated to test the assumption that different
contaminating sources can be distinguished on the basis of their characteristic lead isotope ratios and furthermore that by comparing contaminated with uncontaminated samples, it might be possible to estimate the relative amounts of contaminants from different sources.
In Switzerland the main sources of lead contamination are leaded gasoline, sewage sludge used as fertilizer, and exhausts from waste incinerators. Contaminating lead essentially is derived from ores with distinct, but generally unradiogenic isotopic signatures which depend on the age of the ores. In contrast, the Pb isotopic composition of uncontaminated soils depends on the U/Pb and Th/Pb ratios of their parent rocks and generally is more radiogenic than that of industrial lead. Thus the following components can be distinguished (Table 1):
Soil samples from urban and suburban areas were collected from 4 profiles to depths of up to 90 cm. Lead was extracted from the size fraction < 0.2mm by hot, 2n HNO3 during 2 hours.
In diagrams involving only isotopic ratios, the compositions of contaminants as well as of uncontaminated soils deviate only slightly from linear arrays, and thus the source of the contamination cannot be defined unambiguously. However, if the Pb concentration of the contaminants is high compared to the background concentrations it is possible to define the isotopic composition of the combined contaminants and furthermore, if only two contaminating components are present, their relative proportions. The results show that only the top 30-40 cm of soils are contaminated, below this depth the concentrations and isotopic compositions of Pb remain approximately constant. In 3 of the 4 profiles the 208Pb/204Pb and the 206Pb/204Pb ratios of the combined contaminants vary from 37.51 to 37.82, and from 17.60 to 17.89, resp., which suggests a contamination by varying proportions of waste incineration exhausts, sewage sludge and aerosol Pb. In a fourth case the 208Pb/204Pb and 206Pb/204Pb ratios were found to be 38.04 and 18.11 respectively, i.e. higher than the examined components indicating the presence of an unknown additional component with relatively high 208Pb/204Pb and 206Pb/204Pb ratios.
In profiles from uncontaminated soils overlying carbonate rocks, the isotopic ratios remain approximately constant with increasing depth in spite of drastic decreases of the Pb-concentrations from 140 to 50 ppm.
The major advantage of employing lead isotopes to environmental studies is that they offer the possibility to distinguish anthropogenic inputs from natural accumulations and that they allow roughly to estimate the proportions of major contaminants in soils. Furthermore, the presence of additional contaminants with distinct isotopic signatures can be established.
Table 1
206Pb/204Pb 208Pb/204Pb Pb conc [ppm]
Traffic exhausts 17.32 ± .09 37.09 ± .10
Aerosols # 17.53 ± .08 37.37 ± .11 appr. 1300 *
Sewage sludge 17.82 ± .07 37.64 ± .05 appr. 200 **
Waste incineration exhausts 17.95 ± .02 37.83 ± .02 x000
Background Pb ## >18.8 >38.6 10-20
* Values for 1985/86, ** Values for 1993 # Assuming that aerosol Pb mainly represents a mixture of background Pb and traffic derived Pb then according to its isotopic composition they amount to 15 %, resp. to 85 %. Locally other components may contribute to the aerosol Pb. ## Background Pb = leachable lead from uncontaminated soils.