Biomedical Engineering Reference
In-Depth Information
With respect to the need for bioaugmentation, Lu et al. (
2009
) showed that high levels of
Dhc
(10
7
cells/L) were found only at sites where complete dechlorination to ethene was
detected. Some sites with detectable but lower numbers of
Dhc
did not exhibit complete
dechlorination. Lu et al. (
2009
) concluded that detection of
Dhc
DNA in a groundwater sample
was strong evidence that dechlorination to ethene or VC would occur, but “a failure to detect
Dhc
DNA in a sample of groundwater should not be taken to mean that dechlorination will stop
at the level of dichloroethenes.”
MBTs also can provide insight into the ecology of
Dhc
spp., which may prove useful in
assessing bioaugmentation. For example, Behrens et al. (
2008
) showed that
Dhc
strains with VC
reductases (
bvcA
and
vcrA
) were spatially separated from the early stage PCE- and
TCE-dechlorinating bacteria, with the VC dechlorinators further downgradient. Similar separa-
tion in space or time seems likely in many situations, because
Dhc
do not compete well for
hydrogen against other PCE and TCE dehalogenating bacteria (Flynn et al.,
2000
; Becker,
2006
). It is therefore reasonable to expect that
Dhc
populations will be most numerous at
locations where late stage dechlorination (
cis
-DCE and VC reduction) is occurring and
Dhc
spp.
have little or no competition for that niche.
Monitoring after biostimulation or bioaugmentation confirms that
Dhc
numbers (and VC
reductase genes) increase over time, and levels
>
10
7
cells/L appear characteristic of optimized
systems (Lee et al.,
2008
; Scheutz et al.,
2008
; Amos et al.,
2009
; van der Zaan et al.,
2010
).
At such population densities,
Dhc
must constitute a dominant member of the community,
considering that groundwaters typically have total bacterial numbers of 10
7
-10
9
cells/L, based
on recoverable bacteria (Hirsch and Rades-Rohkohl,
1988
). Consequently it is not surprising
that such numbers would be found only at sites where complete dechlorination is occurring.
Establishing a specific trigger for whether bioaugmentation is needed involves some
judgment, but it is reasonable that if the
Dhc
numbers exceed 10
6
cells/L across most of the
target treatment zone, then complete dechlorination is either already measurable or is likely to
be evident soon. In interpreting
Dhc
numbers, generally it will be important to analyze several
samples from across the site to understand the spatial distribution and ensure that adequate
numbers of
Dhc
are present throughout the majority of the site.
4.7.1.2 Compound Specific Isotope Analysis (CSIA)
Compound specific isotope analysis is a diagnostic chemical analysis with many potential
environmental uses (Hunkeler et al.,
2008
; Schmidt et al.,
2004
). CSIA measures the naturally
occurring isotopic composition of the chemicals of concern. Because the relative abundance of
different stable isotopes can reflect the influence of different processes acting on the con-
taminants, these results can be useful in many assessments, including:
1. Elucidating key biotic and abiotic reaction processes and separating these degradation
losses from those due to physical attenuation processes, such as dilution, sorption and
volatilization.
2. Providing a powerful line of unequivocal evidence for
in situ
degradation, and estimating
degradation rates.
3. Predicting the progress and extent of plume migration, in conjunction with groundwater
models.
For chlorinated solvents in groundwater, CSIA has proven to be a sensitive tool
for determining that natural biodegradation is occurring (Sherwood Lollar et al.,
2001
;
Nijenhuis et al.,
2007
) or for assessing and monitoring enhanced reductive dechlorination
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