els of specific compounds is a key for determin-
Theory
ing the fate of contaminants of interest (COI) in
Carbon is primarily composed of two isotopes,
12C, which accounts for 98.89% of all carbon, and
an environment. The conditions of the system,
13C, which accounts for approximately 1.11%
such as temperature, moisture, COI concentration,
available carbon, chemistry, pH, and Eh, that
(Craig 1957, Faure 1986, Hoefs 1987). The relative
abundance of 13C is expressed as δ13C values,
exist during the process can affect the dominant
which represent the 13C to 12C ratio relative to a
remediation pathway by governing the rates of
standard ratio from the Pee Dee Belemnite (PDB)
individual biological, geochemical, and physical
Formation, Upper Cretaceous, South Carolina.
processes. Not all transformation processes create
more benign compounds. Measuring the direction
of which is 14N and 0.4% is 15N (Watson 1985).
and rates of competing processes under different
The relative abundance of 15N in a sample is
conditions is necessary for predicting the fate of
determined vs. that in atmospheric nitrogen.
compounds undergoing natural attenuation, and
Isotope ratios are reported as a difference, rather
allows one to set a time-scale for remediation of
than absolute concentrations, because this pro-
an ecosystem and to prioritize remedial efforts.
vides a more useful measure in the description of
Rates for attenuation of a COI can be estimated
isotope behavior, as well as being more precisely
in laboratory studies conducted under represen-
measured (Friedman and O'Neil 1977, Lajtha and
tative field conditions, but there remains a need
Michener 1994). These difference values--δ or
to confirm the rates with field data. One approach
"del"--are calculated using eq 1 and are expressed
is through modeling and subsequent model vali-
in parts per thousand, often referred to as "per mil"
dation. This approach typically uses changes in
and generally denoted as 0/00 . Compounds that
concentrations of readily measured reactants, fi-
are depleted of the heavy isotope relative to the
nal products, or both, to evaluate the success of
standard have negative δ values, while com-
describing intermediate processes. While labora-
pounds that are enriched in the heavy isotope have
tory analysis indicates the potential pathways for
positive δ values.
attenuation of a COI, in-situ analysis is required
to provide confirmation of pathways and rates of
δ = ([R/Rs) 1]) 1000
(1)
disappearance. For assurance that these processes
are in fact occurring, it is desirable to use field
where R is the absolute isotope ratio in the sample,
monitoring to confirm both laboratory tests and
and Rs is the absolute isotope ratio in the standard.
model predictions.
Degradation pathways for 2,4,6-trinitrotoluene
(TNT) have been documented previously
Previous uses
Stable isotope analysis has been used to address
(McCormick et al. 1976). In soil systems, TNT is
a broad range of topics in C, N, O, H, S, and Cl
believed to be reduced to several isomeric amino-
cycling. The natural differentiation in stable iso-
dinitrotoluenes, then sequentially to diamino-
tope values, such as δ13C and δ15N levels, can be
nitrotoluenes. Recent evidence indicates that these
used as tracers for following the long-term dynam-
mono and diamino compounds can form covalent
ics of natural systems (Balesdent et al. 1987, 1988).
bonds to natural organic matter (Thorn 1995,
In the oil industry, petroleum compounds have
Thorne and Leggett 1998) and thus be humified.
been linked to the source rock formation from
The influences of moisture, temperature, and soil
which they have migrated by relating the δ13C val-
organic carbon on the fate of processes affecting
TNT can be evaluated in the laboratory using 14C
ues of specific petroleum fractions with those in
TNT to measure respiration or humification or
the kerogen (organic) fraction of the source rock
assimilation, and the resulting rates can be used
(Stahl 1980). In soil science, soil organic matter
for making field predictions. Two potential limi-
turnover rates have been estimated by comparing
changes in δ13C values for different soil organic
tations arise from this approach: 1) 14C labeled
compounds generally can not be used for field con-
fractions (O'Brien and Stout 1978, Boutton et al.
1980), and δ13C has been used for determining
firmation, and 2) there may be some isotopic frac-
tionation during the process (Galimov 1981). An
sources of CO2 evolved from soil (Reardon et al.
innovative alternative that has received limited
1979). Stable isotope ratios have also been used to
estimate long term changes in plants and soils
as naturally occurring tracers (Van de Velde et al.
(Freeman and Hayes 1992).
1995).
Stable isotope analysis has also been employed
2
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