Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 98

differences among the isotopes (Keinonen 1992 ; Komárek et al. 2008 ; Balcaen et al.
2010 ; Cheng and Hu 2010 ;Bird 2011 ). This is an extremely beneficial trait in that the
Pb isotopic composition of the ore deposits does not change during mining, smelting,
or other industrial or biological processing. Thus, the Pb released from a contaminant
source retains the isotopic signature of the source material as it moves through the
river system. As a result, the observed differences in Pb isotopic ratios between the
sourcematerials and the alluvial sediments at any given site is a function of themixing
with Pb in the sediments from the various sources that exist, allowing for a quantitative
analysis of Pb provenance (Cheng and Hu 2010 ). Third, the most effective tracers
exhibit a wide range of values in geological materials. In the case of Pb, the isotopic
composition of a geological material depends upon the relative proportion of U, Th,
and Pb in the system, various mixing processes associated with metamorphism, and
the age of the rocks and minerals during which U and Th can decay to the radiogenic
isotopes of Pb (Keinonen 1992 ; Cheng and Hu 2010 ). Although the Pb isotopic com-
position of ore deposits on a global scale is highly variable (Bird 2011 ), there is a
strong tendency for ore-derived Pb to be more radiogenic. As a result, Pb ratios within
anthropogenically derived materials created from those ores tend to differ from the
more geogenic Pb typically found within non-mineralized rocks that underlie the
basin (Hopper et al. 1991 ). It is often possible, then, to effectively fingerprint both
natural and anthropogenic sources of Pb contained within alluvial sediments.
4.3.2 Applying Pb Isotopes as a Tracer in Riverine
Environments
Although Pb isotopes have recently been used in riverine environments for a wide
range of purposes (e.g., the elucidation of Pb cycling dynamics involved with sed-
iment/rock/water interactions (Ip et al. 2007 ), they primarily have been used to
determine contaminant source(s) or to construct a chronology of source/pollutant
loading rates to the river. The analyses presume that the analyzed sediment is a
mixture of particles from all of the potential Pb sources in the catchment, and that
the relative abundance of the radiogenic Pb isotopes in the sediment reflects the
contributions from each source. In addition, it is assumed that the isotopes behave
conservatively with respect to physical processes (e.g., hydraulic sorting) that may
lead to the partitioning of particles on the basis of size and/or density into specific
sedimentologic units. While there is some evidence to suggest that physical sorting
processes may lead to minor differences in the isotopic ratios from one location in
the river to another (Bird 2011 ), it currently appears that differences related to such
grain size affects are likely to be negligible along contaminated river systems. It
follows, then, that the observed temporal and spatial variations in Pb isotopic ratios
within alluvial sediments will predominately reflect the input and mixing of Pb from
different natural and anthropogenic sources each of which are characterized by a
different Pb isotopic composition.