50
10
40
8
Concentration
30
6
20
10
4
δ15N
0
2
10
0
20
10
20
30
40
50
0
60
Time (days)
Figure 9. Concentration and δ15N of extractable TNT over time for incubation study 2.
however, the precision of the measurements indi-
cates that there is uncertainty whether or not this
Table 8. Concentration and stable isotope data
is real. Further investigation of this possible trend
for TNT extracted from LAAP groundwater.
may be appropriate, based on results of the field
studies conducted at LAAP discussed below.
Plume
Leading edge
Transect 1
Field studies
Lower terrace
MW 141
MW 112
Isotope ratios across the contaminated plume at LAAP
TNT conc. (g/L)
2660
116
The objective of this experiment was to deter-
δ13C (‰)
20.3
21.9
mine if isotopic fractionation of the TNT in the
δ15N (‰)
13.21
6.14
groundwater was occurring as the TNT migrated
through the soil. Groundwater samples were col-
Transect 2
lected from six monitoring wells along four
Lower terrace
MW 141
MW 110
transects at LAAP in April 1998. The concentra-
TNT conc. (g/L)
2660
772
δ13C (‰)
tion of TNT in the groundwater near the source
20.3
18.5
δ15N (‰)
region of the plume ranged from 1870 up to 5110
13.21
9.30
g/L. Concentrations along the leading edge
Transect 3
ranged from 116 to 772 g/L. Table 8 shows the
Upper terrace
MW 140
MW 108
data for the pairs of wells along the four transects.
TNT conc. (g/L)
1870
542
A significant difference between the concentra-
δ13C (‰)
20.9
17.8
tion of TNT in the center of the plume and that at
δ15N (‰)
9.44
14.74
the leading edge was evident for both the upper
and lower terraces. Although no significant dif-
Transect 4
ferences were found in the δ13C values for TNT,
Upper terrace
MW 085
MW 108
the δ15N values for TNT suggest possible trends.
TNT conc. (g/L)
5110
542
As predicted, the δ15N values for TNT increased
δ13C (‰)
*
17.8
δ15N (‰)
11.36
14.74
as the TNT moves away from the source area in
the upper aquifer, but in the lower aquifer the
*Data for this point were not available owing to a chromato-
opposite occurs. These results are not completely
graphic interference.
14
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