BIOAUGMENTATION WITH DEHALOCOCCOIDES: A DECISION GUIDE - Bioaugmentation for Groundwater Remediation - page 128

genes are present and represent a large fraction of the total bacterial numbers, bioaugmentation
is almost certainly not necessary. Conversely, the presence of such biomarkers only in bioaug-
mented samplers is powerful evidence that they are needed.
There are some caveats to consider when using this method and interpreting the data.
Notably, the environment inside a well can differ in important ways from the surrounding
aquifer. Oxygen and ORP levels may differ, pH and geochemistry may be modified within the
well environment and the biological community also may be different. The time of incubation
also may be an important consideration. Incubation for roughly 30 days is typical, and should
be sufficient for many sites. However, lag times can extend to several months in some cases,
possibly yielding false negative results.
4.7.2.2 Push-Pull Tests
One relatively rapid and inexpensive field method to assess microbial activities is the
so-called “push-pull” test procedure. Push-pull tests involve injection through a well into the
surrounding aquifer, and later extraction of water from the same well (Istok et al., 1997 ).
The injected solution contains one or more tracers, as well as reactive solutes such as electron
donors. After some incubation time - referred to as drift time, as the injected solution moves
downgradient and unamended groundwater moves into the capture zone from upgradient - the
groundwater is extracted and analyzed. The method can be used for in situ testing of several
biological and chemical processes (e.g., Istok et al., 2004 ). Of course, the drift time must be
sufficient to allow in situ growth and measurable dechlorination without allowing too much of
the injected solution to move beyond the well's capture zone.
The push-pull test has been proposed as a method to directly assess the need for bioaug-
mentation by adding augmentation cultures to some wells or portions of an aquifer with
comparisons to biostimulated controls (Lee, 2006 ). Inclusion of a reactive tracer such as
trichlorofluoroethene that is not generally found in groundwater can increase the power of
the technique and allow accurate in situ dechlorination rate measurements (Field et al., 2005 ).
This technique is still considered innovative and can require considerable planning and data
analysis, but it may have specialized applications. In addition to measuring dechlorination rates,
this method also may allow measurements of the in situ rates of consumption of other
amendments (e.g., electron donor).
4.7.2.3 Field Plots
More robust data can be obtained from field test plots. However, field testing is expensive
and the heterogeneity at the field scale can lead to relatively high variability and extensive
sampling networks. Although the results can be the most conclusive evidence that bioaugmen-
tation is or is not beneficial, the cost and effort may be difficult to justify given the develop-
ment and demonstrated value of less costly diagnostic tests. Field tests can make sense,
however, if project managers are committed to using in situ bioremediation, if the site is large
or has a complex hydrogeology or contaminant mixture and if the managers have the time to
optimize the design and operations by using field testing as a first phase of full-scale treatment.
For example, in one case study (ESTCP, 2003 ), significant ethene concentrations were
detected in the groundwater from a TCE- and VC-contaminated area at Cape Canaveral Air
Station, Florida, but not until approximately 3 months after continuous lactate injection was
initiated (Figure 4.5 ). In this case, bioaugmentation was not tested, but the results clearly
showed that it would not be essential for treatment, and that the biostimulation lag time
would be roughly 3 months.