Environmental Engineering Reference
In-Depth Information
6.0e + 8
5.0e + 8
4.0e + 8
3.0e + 8
2.0e + 8
LTU 1-natural attenuation
LTU 2-traditional landfarming
1.0e + 8
0.0
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (study months)
Figure 7.25
Changes in biomass in the LTUs.
machine detection limits and too low for estimation. The risk associated with
the soil toxicity is based on the toxic equivalency factors (TEFs) for the seven
most carcinogenic PAH homologues (Table 7.2). These compounds have been
indicated by a superscript “a” in Table 7.15. Molecular weight appears to
determine the sequence of degradation (low molecular weight to high). LTU
1, which showed the greater biomass and the higher proportion of
Pseudomo-
nas
bacteria, also showed a slight, but insignificantly greater, PAH reduction.
LTU 2 appears to have achieved the concentration plateau sooner than LTU
1, suggesting a higher rate of degradation. In the final month of analysis,
both LTUs had detectable concentrations of the toxic five-ring PAH,
dibenzo(a,h)anthracene. We hypothesize that the natural biosurfactant pro-
duction by indigenous pseudomonads in the LTUs became great enough to
desorb this PAH from the soil particles.
7.4.2.3 Microbial characterization
The calculations for microbial biomass are based on the assumption that 1
pmole PLFA = 2.54 × 10
4
cells (White and Ringelberg, 1998). Although total
microbial biomass increased in both LTUs during the initial phase (Figure
7.26), LTU 2 showed the greatest increase. During the second phase, LTU 2
showed a decline in cell numbers, whereas LTU 1 remained constant. At 22
months, LTU 1 had greater biomass than LTU 2 (
p
= 0.008). Tilling did have a
significant impact on biomass, rapidly increasing the cell numbers. However,
