Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 104

determine that the Pb was derived from the mines, but the ore body from which the
Pb was derived.
Downstream trends in Pb isotopic ratios also reflected temporal changes in the
amount of Pb derived from the mills within the channel bed sediment. In 2000,
206 Pb/ 207 Pb values within the lower channel bed sediments tended to decline down-
stream toward values observed within the Ordovician rocks, indicating a decrease in
the contributions of Pb from the mills. In contrast, 206 Pb/ 207 Pb ratios in channel bed
sediments in 2002 exhibited no downstream trend; rather, the 206 Pb/ 207 Pb ratios were
either indistinguishable from the Cerro Rico type ore deposits, or fell between the
Cerro Rico and Porco types of ore deposits (Fig. 4.9 b). Changes in the isotopic ratios
were coincident with a decrease in: (1) Pb concentrations at any given site along the
channel; and (2) the downstream extent to which elevated Pb concentrations could
be observed (Fig. 4.9 a). Miller et al. ( 2007 ) argued that the changes in Pb concen-
tration and Pb isotopic ratios reflected annual differences in system hydrology and
upland/tributary sediment inputs to the river. During wet years, such as 2000, exten-
sive runoff from the mines and mills allowed large quantities of Pb-contaminated
sediment to be transported downstream over longer distances, resulting in relatively
high Pb concentrations. However, the proportion of the Pb derived from the mines
decreased downstream, as indicated by the isotopic ratios, because relatively large
quantities of uncontaminated, but Pb-enriched fine-grained sediments eroded from
upland areas were delivered to the channel via tributaries. During relatively dry years,
such as 2002, the transport of Pb-contaminated sediment from the mines and mills
was reduced, resulting in a decrease in Pb concentrations within the channel bed sed-
iments. However, most of the Pb found in the channel was derived from the upstream
mining operations because less sediment was eroded from the surrounding uplands
and delivered to the channel. The study by Miller et al. ( 2007 ) demonstrated that the
source of Pb to the channel can vary over relatively short time frames as a function
of the annual variations in catchment hydrology and sediment transport processes
within a catchment. This is especially true in higher energy rivers with large sediment
fluxes, such as the Rio Pilcomayo.
Within larger river systems contaminated by multiple anthropogenic Pb sources,
or which may be underlain by a wide range of bedrock types, the Pb isotopic sig-
nature of the alluvial sediments is likely to vary such that well-defined mixing lines
do not exist on binary scatter plots (Ip et al. 2007 ). As a result, determining the
relative contribution of Pb from the identified sources becomes much more difficult.
Ayraultetal.( 2012 ), for example, found that there were systematic trends in Pb con-
centration and 206 Pb/ 207 Pb ratios within dated cores obtained from various locations
along the Seine River (Fig. 4.10 a, b). However, when combined on a bivariate plot
of 206 Pb/ 207 Pb ratios versus Pb concentration, the data formed a complex pattern.
Although complex, they were able to show that the data reflected systematic changes
in Pb input to the system such that two mixing lines could be defined, each represent-
ing a different time period. Between 1927 and 1968, sediments in cores B2 and M1
fell within a narrow range of values that were consistent with Pb from the Rio Tinto
ore body. Pb from the Rio Tinto was thought to be used extensively for centuries
in the Seine River basin, particularly after 1850, and was therefore referred to as