BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 157

the injection wellheads had to be disassembled to perform the inoculation to ensure that
exposure to air was minimized. The addition of bioaugmentation culture directly to the
reinjection line during recirculation operations is not advised as this approach can risk exposing
the culture to oxygen if the pipes/hoses are not flowing full or if the extracted groundwater is
not sufficiently reduced (Trotsky et al., 2010 ).

5.4.2 Semi-Passive Approach

The semi-passive approach refers to pulsed injection of amendment solution to achieve a
large radius of influence around each injection point, and then allow amendments to drift along
with the groundwater. Semi-passive also can refer to systems employing lateral mixing of
electron donor perpendicular to groundwater flow to minimize the number of required injection
wells. Inoculation is generally performed as a single injection event after establishing reducing
conditions that are suitable for bioaugmentation. Recent studies have shown that reducing
conditions adequate for bioaugmentation can be achieved 50 ft (~15 m) or more downgradient
from an injection point, depending on the site's hydrogeologic conditions (Mora et al., 2008 ).
To perform the injections, electron donor amendment is blended above ground to achieve
the desired injection concentration. The amendment used in this approach can be either a
soluble or a slow-release electron donor, although generally semi-passive systems use soluble
donors such as sodium lactate or ethanol, which are immediately bioavailable upon injection
into the subsurface. The frequency of injections is dependent on many factors, including
the electron donor type used, hydrogeologic conditions, competing electron acceptors, and
the concentrations of the target contaminants.

5.4.2.1 Advantages/Disadvantages for Biostimulation
Semi-passive systems are generally favorable for biostimulation at sites that have moderate
to high hydraulic conductivity and moderate hydraulic gradient, allowing ambient groundwater
flow and/or lateral mixing to distribute electron donor. The semi-passive technique has been
successfully applied at many sites (Mora et al., 2008 ) and during the recent ESTCP project
ER-200513 (Trotsky et al., 2010 ).
The primary advantage to the semi-passive technique is that it is a flexible approach that
allows for frequent applications of electron donor, while keeping the operational requirements
and costs low.
Compared to passive and active recirculation approaches, other advantages include:

Ability to distribute and maintain high concentrations of electron donor in a large
radius of influence from individual injection points;

Ability to perform frequent (i.e., monthly to quarterly) amendment injections cost
effectively (on smaller scales);

Large areas can be treated effectively with fewer injection points compared to passive
systems; and

Less O&M and capital requirements compared to active recirculation.
The semi-passive technique does have disadvantages compared to the passive and active
recirculation approaches, including:

Individual injections can take multiple days depending on subsurface conditions;

Vertical mixing may be inconsistent and more dependent on ambient flow conditions
compared to active recirculation.