Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 86

Chapter 4
Radiogenic Isotopes

Abstract Radiogenic isotopes have been widely used to assess an extensive range
of geological processes. In this chapter, we focus on the use of three radiogenic
isotope systems (Sr, Nd, and Pb) to determine the source of sediment and sediment-
associated contaminants in riverine environments. We begin by examining the past
and continuing use of Sr and Nd isotopes to determine the provenance of sediment
at large spatial scales before exploring their potential use to track anthropogenically
contaminated sediment at much smaller (local to regional) spatial scales. We then
turn our attention to the use of Pb isotopes as a tracer of Pb contaminated sediments
in riverine environments. Given the many and increasing ways in which radiogenic
isotopes can be applied to environmental, geomorphic, and hydrologic issues, the dis-
cussion is not meant to be exhaustive. Rather, it is intended to provide an overview
of the sorts of methodological approaches that have been used to address the sed-
iment/contaminant source problem in riverine environments, and the strengths and
weaknesses inherent in the approach.

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Keywords Sr Nd Pb isotopes Sediment provenance Pb contamination

4.1 Introduction

In the previous chapter we briefly described the use of fallout radionuclides as envi-
ronmental tracers to gain insights into the source and transport processes of sediment
and sediment-associated contaminants in river systems. Other isotopes have also been
extensively used to trace sediment and any associated contaminants in the near sur-
face environment, including fluvial systems (see Hoefs 2009 and Allègre 2008 for
an overview of isotope geology and analysis). Of particular importance have been
various radiogenic isotopes. Radiogenic isotopes are the daughter products generated
from the decay of a radioactive parent. Here we focus on three isotope systems, Sr,
Nd, and Pb. Their use as effective tracers hinges on several inherent characteristics,
including the fact that they (1) can be precisely and accurately measured, (2) tend
to exhibit isotopic abundances that vary widely within geological materials, and
(3) are not significantly fractionated by physical, chemical or biological processes
that alter the isotopic abundance observed within a given source material as it is
dispersed through the river system. Thus, the isotopic ratios (fingerprints) of the