Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 100

data it could reasonably be argued that the decreasing Pb concentrations observed
downstream of Site 11 was the result of dilution of sediment-born Pb by relatively
'clean' sediments from the underlying bedrock. However, the increasing 208 Pb/ 206 Pb
ratio indicates that Pb characterized by a higher 208 Pb/ 206 Pb ratio is entering the
channel from other source(s). Shepherd et al. ( 2009 ) argued that this additional
Pb was associated with diffuse anthropogenic sources from tributaries draining an
abandoned coalfield. The abrupt and sustained increase in 208 Pb/ 206 Pb between sites
24 and 27 was interpreted to result from either: (1) an increased influx of diffuse Pb
from Durham; or (2) the influx of tetra-ethyl lead associated with leaded gasoline.
The point to be made here is that the analysis by Shepherd et al. ( 2009 ) provides
a much more detailed understanding of Pb sources and their relative contributions
to the contamination of the river than would be possible by only examining the Pb
concentration data.
The quantification of Pb contributions from a particular source is often conducted
by combining observed spatial variations in isotopic ratios along a channel with
the analysis of three-component (bivariate) scatter diagrams depicting differences
between two isotopic ratios (Elbaz-Poulichet et al. 1986 ; Miller et al. 2002 ; McGill
et al. 2003 ; Kurkjian et al. 2004 ; Bird et al. 2010a , b ). When isotopic ratios of the
analyzed samples form a linear trend, Pb within the samples is typically interpreted
to be derived from two primary end-member sources (as was the case of Sr and Nd
plots). In the case of Pb contaminated alluvial sediments, one end-member typically
represents Pb from the underlying bedrock or the soils developed from it, whereas
the other represents Pb from a significant anthropogenic source (Erel et al. 1997 ;Bird
2011 ). The signature of the 'geogenic' Pb source may be determined by analyzing a
number of different materials including channel bed sediments from uncontaminated
tributaries or areas upstream of the contaminant influx (Keinonen 1992 ), uncontam-
inated alluvial terraces (usually comprised of pre-historic sediments) (Miller et al.
2002 , 2007 ; Church et al. 2004 ), alluvial sediment found at depth within a sediment
core that pre-dates anthropogenic contamination (Church et al. 2004 ), or the direct
analysis of the underlying bedrock (Miller et al. 2007 ) (Fig. 4.7 ).
Miller et al. ( 2007 ) provided an example of the use of bivariate scatter diagrams to
assess the source and dispersal patterns of Pb along the Rio Pilcomayo downstreamof
the Cerro Rico de Potosi precious metal-polymetallic tin deposits of Bolivia. Mining
and milling of the deposits has continuously occurred since 1545 and resulted in
the severe contamination of the river by a wide-range of trace metals and metalloids
(Hudson-Edwards et al. 2001 ; Miller et al. 2007 ). Miller et al. ( 2007 ) found that (1)
bedrock units within the catchment exhibited different 206 Pb/ 207 Pb and 206 Pb/ 208 Pb
ratios, and (2) alluvial sediments containedwithin pre-mining terrace deposits formed
a linearmixing line inwhich theOrdovician andMesozoic rocks served as the isotopic
end-members (Fig. 4.8 a). In marked contrast, samples collected in 2000 from the
highly contaminated channel bed fall along a separate mixing line in which the
isotopic end members were formed by Ordovician rocks and mine/mill processing
waste from the operations at Cerro Rico (Fig. 4.8 b). Samples collected in 2000,
then, showed that the channel bed sediment was dominated by natural Pb from the
underlying Ordovician rocks and Pb from the Cerro Rico ore deposits that were