Environmental Engineering Reference
In-Depth Information
(month 39) showed signs of EDC dechlorination (only the 6 m bgs transect
was sampled); sampling in November 2006 (month 91) confirmed this.
It was postulated that the cause of the EDC mass removal was microbial
colonization of the barrier by iron-reducing and/or hydrogen-utilizing bac-
teria (i.e., the hydrogen formed by the corrosion of the ZVI was acting as an
electron donor for reductive dechlorination of the EDC and other CHCs).
To evaluate this, a single core was retrieved in late February 2008 from the
front face of the reactive iron barrier, collecting upgradient indigenous aqui-
fer material and ZVI from the barrier. The core was taken at 20° from vertical
using a Geoprobe push tube, and split into two samples for analysis. One
sample (Sample A, representing indigenous aquifer material) was taken at a
depth of 3 m bgs approximately 1 m upgradient of the reactive iron barrier.
Another sample (Sample B) was taken approximately 0.15 m into the reactive
iron barrier at a depth of approximately 6 m bgs.
The samples were analyzed by the Centre for Marine Bio Innovation at the
University of New South Wales. A clone library of small subunit ribosomal
ribonucleic acid (RNA) genes for each of the samples was constructed. The
deoxyribonucleic acid (DNA) was extracted from both samples using the Fast
DNA Spin Kit for Soil (Qbiogene). The small subunit ribosomal RNA genes
present in the DNA extracts were amplified using the polymerase chain
reaction (PCR) with universal bacterial primers and cloned for sequencing.
The libraries consisted of 56 and 44 clones for Samples A and B , returning 49
and 44 sequences, respectively. Table 12.3 (Stening et al., 2008) summarizes
the most abundant sequences in the clone libraries.
Despite the fact that the clone library generated from Sample B had
slightly fewer clones than the library generated from Sample A, the former
contained two sequences belonging to known dehalorespiring microbes
(
Dehalococcoides
and
Dehalobacter
) while the latter contained none. This
indicates that the reactive iron barrier harbors higher concentrations of
dehalorespiring microbes than the aquifer upgradient. The fact that deha-
lorespiring microbes were detected in a clone library constructed using uni-
versal bacterial primers suggests that they constitute a larger percentage of
TABLE 12.3
Composition of Clone Libraries from Samples A and B
Closest Relative
% of Library
Comments
Clone Library A
Dechlorosoma suillum
48
Chlorate respirer; Fe(II) oxidizer
(sand)
Azospira
25
Nitrate respirer
Beta Proteobacteria
9
Anaerobically cycles iron redox
Clone Library B
D. suillum
16
Chlorate respirer; Fe(II) oxidizer
(iron)
Desulfovibrio
10
Sulfate-reducing bacterium
Desulfosporosinus
5
Sulfate-reducing bacterium
Dehalococcoides
2
Dehalorespirer
Dehalobacter
2
Dehalorespirer
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