Table 9. Concentration and δ15N values for TNT
understood and the cause for these trends remains
unclear. Further investigation through additional
extracted from LAAP groundwater collected in
sampling and analysis may aide in clarifying these
September 1998.
The wells at LAAP were sampled a second time
Plume
Leading edge
in September 1998. Again, the TNT was extracted
Transect 1
from the groundwater and the concentration and
Lower terrace
MW 141
MW 112
δ15N values determined (Table 9). These data
TNT conc. (g/L)
2870
75.1
showed the same concentration gradients along
δ15N (‰)
13.3
17.3
each transect. The statistical analysis of the data
(Table 10) showed a clear increasing trend in the
Transect 2
δ15N values for TNT, indicating that the TNT is
Lower terrace
MW 141
MW 110
being fractionated and that it has become isotopi-
TNT conc. (g/L)
2870
1060
δ15N (‰)
cally heavier. These results strongly suggest that,
13.3
10.0
as the TNT migrates through the soil, there is a
Transect 3
process taking place that removes the lighter iso-
Upper terrace
MW 140
MW 108
topic fraction of TNT from the groundwater. It was
TNT conc. (g/L)
unclear why a decrease in the δ15N values for TNT
1920
567
δ15N (‰)
9.7
13.8
was seen along transect 2. Close examination of
the site map (Fig. 1) showed that well 110, in the
Transect 4
lower terrace, is directly beneath transect 4 in the
Upper terrace
MW 085
MW 108
upper terrace. Also, the δ15N value for TNT from
TNT conc. (g/L)
5950
567
this sample is the same as that of the TNT found
δ15N (‰)
10.1
13.8
in the high concentration well along transect 4
(well 085). These data strongly suggest the possi-
Table 10. Statistical comparison of δ15N values for TNT extracted from LAAP
groundwater collected in September 1998.
Mean
SE
F
Pooled SE
t
Transect 1
Sample 141
13.26
0.956
6.32
1.83
3.16
Sample 112
17.35
2.402
Transect 2
Sample 141
13.26
0.956
1.42
1.05
4.42
Sample 110
9.98
1.137
Transect 3
Sample 140
9.69
0.592
3.10
0.48
12.22
Sample 108
13.85
0.336
Transect 4
Sample 085
10.10
0.689
4.20
0.54
9.77
Sample 108
13.85
0.336
Critical t values
95%
99%
99.9%
t-values
2.78
4.60
8.60
df = 3
Critical F values
95%
99%
F values
9.28
29.0
df 3,3
15
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