to trace short term cycling of organic matter
may be possible in the laboratory. If so, the reac-
through food chains in ecosystems (Peterson et al.
tion and transport components could be more
1985, Peterson and Fry 1987). It has been used to
mechanistically included in models.
study microbial roles in elemental cycling (Blair
et al. 1985; Coffin 1989; Coffin et al. 1990, 1994;
tracers to track the processes that take place dur-
Coffin and Cifuentes 1993), to identify carbon
ing TNT degradation. Dual tracers allow measure-
sources that support food chains in complex
ment of attenuation whether the process is gov-
erned by nitro/amino functionality or aromatic
aquatic ecosystems (Coffin 1989; Coffin et al. 1990,
ring/toluene functionality, or both. The following
1994; Coffin and Cifuentes 1993; Cifuentes et al.
two strategies were used: measuring carbon-based
1996a,b), and to analyze nitrogen cycling (Peterson
changes in the soil, and nitrogen-based changes
and Fry 1987; Hoch et al. 1992, 1996). As a result
in solution.
of technological limits, preliminary studies have
focused on the major component of elemental
cycles in complex pools with numerous sources.
Soil carbon
Incorporation of TNT into soil humic material
Recently, with the attachment of gas chromato-
or covalent bonding of amino-daughter products
graphs (GC) inlet to the isotope ratio mass spec-
of TNT with the same may result in a change in
trometers (IRMS), research has moved to study-
the δ13C of the soil during or after binding. Mea-
ing the cycling of specific compounds in
suring differences between δ13C values in TNT and
ecosystems (O'Malley et al. 1994; Trust et al., in
in soil is complicated by the following three fac-
press) and to identifying individual compounds
tors:
as biomarkers to determine microbial roles in bio-
geochemical cycles (Pelz et al. 1998).
Scant data are available on δ13C values for
Recent developments allow stable isotope analy-
TNT from different sources and soil δ13C val-
sis to be used for evaluating remedial efforts in
ues vary with parent material and biological
environments that are contaminated with petro-
chemical processes.
leum hydrocarbons. This technology provides us
The δ13C values may change very slowly
with the capability to identify sources of contami-
under field conditions.
nants in ecosystems (O'Malley et al. 1994; Kelley
The amount of TNT-derived carbon binding
et al., in press). In-situ degradation of organic con-
to soil, and hence the change in δ13C of the
taminants is studied using 13C analysis of CO2
soil carbon after attenuation, may be too small
evolving from soil (Cifuentes et al. 1996a,b; Van
to measure relative to the native carbon lev-
de Velde et al. 1995). Finally, specific biomarkers
els in the soil. Low-carbon soils would have
have been used to evaluate the bacterial assimila-
less "masking" ability than high-carbon soils
tion of contaminants (Pelz et al. 1998).
and would, in theory, require less difference
It should be feasible to use a similar technique
in soil and TNT δ13C values to detect changes.
for estimating TNT attenuation rates in subsur-
Deep aquifer soils, however, may contain
face soils. Since TNT was manufactured from
very low masses of organic carbon, resulting
petroleum-based compounds, the carbon ring in
in detection limit difficulties.
TNT should retain depleted δ13C values similar
to those that are measured in petroleum com-
Solution nitrogen
pounds. A primary fate of TNT in many environ-
The amino reduction products of TNT are
ments appears to be bonding to or incorporation
believed to bond directly through the amino func-
with the soil organic fraction rather than mineral-
tional group to the soil humic material (Thorn
ization (Pennington et al. 1995, Pennington 1996);
1995). Therefore, one or two of the nitro groups
therefore, significant evolution of 13CO2 derived
on the TNT must first be reduced to amino groups.
from TNT is not expected.
The reduction of nitro groups, whether chemically
or biologically, may result in isotopic fractionation
because of reaction-rate differences between 15N
Rationale
and 14N. If so, a relatively greater percentage of
TNT in a contaminated area undergoes two
14N should be reduced and involved in reactions
major processes: reaction with the soil through
which it moves, and transport with the ground-
with either humic material or clay. This process
would leave relatively more 15N in the TNT
water. These processes simultaneously influence
the fate of TNT in soil and groundwater systems,
but separation of reaction and transport processes
values of TNT in groundwater would evolve
3
Previous Page
