Environmental Engineering Reference
In-Depth Information
Water
Lake Ontario
202
Hg
∂
Sediment (1)
Concentrations of Hg in (uncontaminated) aqueous
sources are too low for directly measuring isotope ratios
by MC-ICP/MS. However, Foucher and Hintelmann (2004)
were able to determine fractionation of Hg isotopes in
mine leachate, contaminated groundwater and rivers. Not
surprisingly, the initial
Zooplankton (1)
201
Hg
Δ
Diporeia hoyi (1)
199
Hg
Mysis relicta (1)
Δ
Alewife (2)
Slimy sculpin (2)
Rainbow smelt (2)
202
Hg in the receiving water body
was identical to the source material at the mine. However,
over a length of 3000 m, signifi cant amounts of Hg evapo-
rated from the river, resulting in a progressive enrichment
of heavy isotopes in the water.
As a proof-of-concept study, my colleagues and I collected
large quantities of polar snow. After melting it down under
clean room conditions and preconcentrating the Hg in 4 L of
sample onto gold traps in the fi eld, the Hg isotope ratio was
determined for the original snow sample. At total Hg con-
centrations of 5.2 to 9.9 ng/L (n
δ
Lake trout (2)
Shipiskan Lake
Sediment (1)
Zooplankton (1)
Longnose sucker (4)
Northern pike (4)
Lake trout (5)
Lake whitefish (9)
Cli Lake
3), initial
δ
202
Hg values
Sediment (1)
Zooplankton (2)
3.10‰ were obtained for snow collected
in early May 2007 at Ny Ålesund, Svalbard. Considering that
background sediments typically have negative
between
2.32 and
Pond smelt (2)
Stickleback (4)
202
Hg values,
the observation of such large positive deviations is remark-
able. More recently, a new method using ion exchange resins
has been developed to concentrate Hg from pristine water
samples, facilitating the Hg isotope ratio measurement at
nanograms per liter levels (Chen et al., 2010).
δ
Lake trout (5)
Lake whitefish (5)
Round whitefish (5)
-3
-2
-1
0
1
2
3
4
5
Deviation (‰)
202
Hg in three freshwater lake food webs. Data were
converted from Jackson et al. (2007), and deviations are expressed
relative to a standard solution of Hg (but not relative to SRM NIST
3133, which was not measured in this study).
FIGURE 4.2
Air
Owing to the extremely low concentration of typically
1.5-2 ng/m
3
Hg in air, this matrix is as challenging for
MC-ICP/MS measurements as water samples. The only
study reporting Hg(0) in atmospheric samples (Sonke et al.,
2008) collected Hg(0) emissions from the passively degas-
sing volcano—Vulcano, Italy—and determined
0.16‰ and was always the most negative biologic sample.
The top predator lake trout ranged from
1.43‰
and always displayed the most positive values. The wide
variation in
0.57 to
1.74
202
Hg among samples of the same fi sh spe-
cies collected at different locations is intriguing. However,
the database and number of analyzed samples is still very
small. It is too early to decide whether the variations refl ect
differences in Hg sources, differences in habitat and MMHg
accumulation, or differences in Hg cycling (e.g., photore-
duction, methylation, and demethylation). Nevertheless, a
clear trend was visible in all three ecosystems, with
δ
0.36‰ for this source.
Aquatic Food Web Samples
15
N, which is used in food web studies to identify
trophic levels of specimen and
Similar to
δ
13
C, which is used to
identify food sources, one of the great potentials of applying
Hg isotope ratios is to better understand Hg bioaccumula-
tion. However, except for fi sh, which typically have high
enough Hg concentrations and provide suffi cient sample
mass for a precise isotope ratio determination, most other
aquatic biota are either low in Hg or are available only in
small quantities (e.g., zooplankton, algae, and benthos) or
both, which makes isotope ratio measurements challenging.
A comprehensive investigation measured Hg isotope
ratios in food chains of Lake Ontario and two boreal lakes
(Jackson et al., 2007). Hg isotope variations from the origi-
nal literature have been converted to
δ
202
Hg
increasing with increasing MMHg. However, it must be
noted that all data so far report
δ
δ
202
Hg for total Hg—that
is, the composite value for
δ
Me
202
Hg and
δ
202
Hgi (Hgi
inorganic Hg). This means that in fi sh (
95% MMHg)
the composite
Me
202
Hg. In lower-food-
chain organisms such as zooplankton (up to 40% MMHg),
δ
δ
202
Hg refl ects
δ
202
Hg is a concentration-weighted average of
δ
Me
202
Hg
and
δ
202
Hgi. At the other end of the spectrum, the mea-
sured
202
Hgi.
Bearing in mind the vast differences in the geochemistry
and bioaccumulation behavior of MMHg and inorganic
Hg, it is likely that individual samples may exhibit very
different
δ
202
Hg in sediments (
1% MMHg) represents
δ
δ
202
Hg values and are
202
Hg increased with
increasing Hg concentration of the specimen. Depending
on the lake, zooplankton showed between
summarized in Fig ure 4.2. In general,
δ
δ
Me
202
Hg and
δ
202
Hgi values. Therefore, compari-
2.60‰ and
sons based on
δ
202
Hg must consider potential bias when
