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relative contributions of material from a suspected contaminant source and the dis-
persal rates and transport pathways from the source(s). Moreover, Pb isotopes have
been effectively applied to systematically accumulated and dated deposits to assess
changes in contaminant flux to a river through time. Pb isotopes, then, represent
a powerful tool for determining the source of Pb contaminated sediment when the
source materials can be distinguished on the basis of Pb isotopic ratios. The use of Pb
isotopes will be more effective within smaller catchments with a limited number of
potential Pb sources than within larger basins characterized by a wider range of rock
types with varying Pb isotopic ratios. Limitations of the use of Pb isotopes are related
in part to the inability to effectively distinguish between the Pb of differing industrial
contaminants, and the lack of well-defined protocols for quantitatively estimating the
uncertainties associated with the estimation of Pb contributions from the delineated
sources. The quantification of uncertainties is particularly needed for environmental
forensic investigations. Unfortunately, the signature of end-member Pb sources has
been based in most studies on relatively simple statistics of the source data, such as
the mean. As a result, variations in isotopic ratios inherent in the source materials
(or the sampled river sediments) are not quantitatively considered in the estimation
of Pb contributions, making it difficult to assess the uncertainties involved in the
analysis. More attention will need to be given to the use of rigorous statistical meth-
ods to (1) characterize the variation in Pb isotopic ratios within the source and river
sediments, and (2) utilize an understanding of the variations in the source ascription
process before Pb isotopic methods can be extensively used in forensic analysis.
Method development may benefit from recent advances that have been made in the
geochemical fingerprinting of non-point source pollutants described in Chap. 2 .
Future analyses using Pb isotopes are likely to focus more on the cycling of Pb
within the riverine environment, including its dispersal froma point source in both the
particulate and dissolved forms, its sorption and desorption from sediments, and its
accumulation in biota. Particular attention is likely to be paid to the use of Pb isotopes
to quantitatively determine the provenance of Pb in aquatic biota. While existing
studies suggest that Pb isotopes hold considerable promise in determining the source
of Pb in biota, the analysis is unlikely to be as straightforward as might be expected.
Miller et al. ( 2005 ), for example, used Pb isotopes to examine the accumulation
of Pb associated with orchard soils contaminated by lead arsenate in Rainbow Trout
( Oncorhynchus mykiss ) in the Richland Creek basin of Western North Carolina. They
found that Pb concentrations and Pb isotopic ratios varied between muscle, liver, and
bone (Fig. 4.11 ). Although differences in Pb concentrationwere expected, differences
in Pb isotopic abundances were not because previous studies had suggested that
biological processes do not significantly fractionate Pb. Miller et al. ( 2005 ) attributed
the observed difference in isotopic ratios to the temporary exposure of the trout to
contaminated waters, and to subsequent differences in the ability of the tissues to
excrete Pb. In fact, laboratory studies demonstrated that when rainbow trout were
exposed to high levels of dissolved Pb with a distinct isotopic signature, both the
bone and liver rapidly acquired Pb from the water and took on its isotopic ratios.
Following exposure the bone retained the Pb and the acquired Pb isotopic ratios
for a period of months. In contrast, the liver excreted approximately 50% of the
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