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natural sources. Cu associated with the surface horizons of the contaminated soils,
and which was mostly bound to the organic fraction, exhibited a signature similar to
the Bordeaux mixture used as a fungicide treatment (with or without a minor amount
of fractionation). Interestingly, Cu associated with the water soluble fraction of the
SPM possessed
65 Cu values similar to Cu dissolved in the water. Thus, most of the
Cu associated with the SPM was interpreted to have been derived from the water.
The investigation is important in that it demonstrates that Cu isotopes may not only
be used to gain insights into redox processes within an aquatic environment, but Cu
sources and dispersal pathways.
ʴ
5.4 Mercury Isotopes
Hg, a priority global pollutant, exhibits a highly complex biogeochemical cycle,
a feature attributed to the fact that it possesses a stable gaseous form (Hg 0 ) and
may undergo a range of redox, phase, and biologically mediated transformations. In
fact, methylmercury, the chemical form predominantly found in biota, is primarily
produced by sulfate reducing microbes. Until recently, understanding the sources and
biogeochemical cycling of Hg in the near surface environment relied on an analysis
of total Hg concentrations within the media of interest, its chemical speciation,
and/or the application of mass balance and geochemical models (Sonke et al. 2010 ).
However, with the recent ability to analyzed for Hg isotopes in abiotic and biotic
materials it has become clear that numerous inorganic and organic reactions result in
Hg isotopic fractionation (Table 5.1 ), producing large variations in the Hg isotopic
composition of both natural and anthropogenic materials (Fig. 5.5 ) (Bergquist and
Blum 2009 ). These variations in Hg isotopic values are now being used to determine
Hg source(s), pollution histories, and transformations during biogeochemical cycling
(Bergquist and Blum 2009 ; Yin et al. 2010 ; Sonke et al. 2010 ; Mil-Homens et al.
2013 ; Sherman and Blum 2013 ). In fact, it could be argued on the basis of the
number of publications on the topic that more effort has been devoted to Hg isotope
systematics for environmental studies than for any other of the non-CHONS.
Hg possess seven stable isotopes, and unlike themajority of the elements variations
in Hg isotopic values results from both mass-dependent and mass-independent
processes (Table 5.1 ). Mass-independent fractionation of Hg has only been observed
to affect the odd isotopes ( 199 Hg, 201 Hg). Thus, the ratio of 199 Hg to 201 Hg appears to
be particularly useful for characterizing chemical pathways, such as photochemical
reduction, associated with mass-independent fractionation processes (Bergquist and
Blum 2009 ). Mass-dependent isotopic compositions are reported using
ʴ
notation
relative to the NIST Hg standard 3133 as expressed by:
1
202 Hg
198 Hg Sample
/
202 Hg
ʴ
=
198 Hg SRM 3133
×
1
,
000
(5.3)
202 Hg
/
 
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