BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 159

installed at the time of injection, and therefore it can be difficult to know if reducing conditions
are present before (and after) the inoculation.
Inoculation in passive systems is generally performed in the same injection locations used
for electron donor addition. Because survival and effectiveness of the inocula requires highly
reducing conditions, bioaugmentation may have to occur after electron donor addition has
resulted in appropriate conditions (Ellis et al., 2000 ; Major et al., 2002 ). However, if the aquifer
is sufficiently reducing prior to initiation of biostimulation, inoculation can be performed
simultaneously with the initial electron donor addition. In most cases, only one inoculation is
required.
A passive approach can be a successful method for bioaugmentation (Dybas et al., 1998 ;
Lendvay et al., 2003 ). Kovacich et al. ( 2007 ) showed that Dhc moved significant distances
downgradient under ambient groundwater flow, reaching levels of 10 6 -10 7 cells/L at wells 45 ft
(~15 m) downgradient within only 8 months. Similarly, a recent field-scale comparison of
“active” and “passive” bioaugmentation (ESTCP Project ER-200513; Trotsky et al., 2010 )
showed that Dhc could spread over considerable distances following passive bioaugmentation
(i.e., a one-time injection, relying on migration with groundwater for further distribution).
The passively-injected Dhc cells migrated at least 30 ft (10 m) downgradient, causing complete
dechlorination to ethene over that distance. The measured travel time for the Dhc was only
1.5-3 times longer than the conservative tracer (bromide).

5.5 CONCLUSIONS
Bioaugmentation will be most successful if exposure to oxygen is minimized, the aquifer
pH is near neutral, and sufficient electron donor is provided to meet the demand of the
chlorinated solvents and competing alternate electron acceptors. Many different types of
fermentable carbon substrates can be employed to provide a source of hydrogen to the Dhc
bacteria to drive reductive dechlorination. The choice of substrate will largely depend on the
system configuration and cost. If aquifer conditions are not immediately suitable, precondi-
tioning through the addition of electron donor and/or buffer is often required to lower the ORP
to less than 75 mV and change the pH to near neutral. Electron donor and buffer selection will

be influenced by the system configuration and cost considerations.
Currently there are no firm guidelines with respect to the amount of culture to inject.
Typically, practitioners aim to achieve a minimum of 10 7 Dhc /L in situ for a target treatment
volume, as empirically this density has generally corresponded with the presence of ethene.
A more complex model for estimating dosage effects has been developed (Schaefer et al.,
2009 ) and verified in the field (Schaefer et al., 2010a ), and it is expected to be available soon as
a module for a widely-used groundwater transport model (Torlapati et al., 2012 ).
There are various techniques for distributing the culture, including using the electron donor
solution to push the culture following injection, recirculation, or multiple direct push injections
of culture. The relative effectiveness of these various approaches is still under study, but all
of the techniques have been widely used. Laboratory column studies have shown that most
Dhc remain near the point of injection, and the more mobile cells have been shown to have the
same or better activity that the original inoculum. However, several field-scale studies have
demonstrated that Dhc can travel hundreds of feet from the injection well over time, presum-
ably through growth and detachment processes.
There are a variety of bioremediation system configurations (i.e., active, semi-passive and
passive) where bioaugmentation can be employed. All three general approaches have been
successful for bioaugmentation applications. Time is required for Dhc to migrate throughout