BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 151

Two other techniques have been used to improve the distribution of Dhc after injection.
One method is to use a forced gradient or recirculation system (discussed in Section 5.4 ).
The other technique is to directly inject the culture in a large number of locations to achieve a
better initial distribution of Dhc . This technique permits the Dhc to be injected away from
oxidized areas (e.g., near an injection well or plume fringes) or downgradient in a number of
locations to minimize the time required for the Dhc to be transported downgradient naturally
through growth and detachment. The downside of this approach may be the additional cost of a
direct-push rig to accomplish the injections.

5.4 BIOREMEDIATION CONFIGURATIONS EMPLOYING
BIOAUGMENTATION
Because almost all bioaugmentation applications require the addition of an electron donor,
it is important to consider the electron donor delivery method and bioremediation system
configuration.
The typical electron donor emplacement methodologies used for bioremediation include
(adapted from ITRC [ 2005 ]):

Conventional injection wells: Electron donors are injected into an existing or newly-
installed well. Typically a network of wells is used to inject relatively large volumes of
liquids containing a soluble electron donor. Conventional wells often are used for
moderate to high permeability aquifers or treatment zones.

Direct-push injection points: A network of relatively closely spaced points is usually
used, with injections of a relatively small volume of a soluble electron donor at each
injection point. Direct-push injection is most applicable for shallow sites with relatively
homogeneous conditions with a moderate to high permeability, although it also can be
used in sandy clays or silty sands.

Trenching: Trenches are usually backfilled with a large mass of a solid electron donor
(e.g., mulch or chitin) and/or a long-lived liquid electron donor such as vegetable oil,
often mixed with sand. Trenches can be used in aquifers with any degree of permeabil-
ity, as long as the permeability of the trench is at least as high as the formation.

Hydraulic or pneumatic fracturing: Fracturing of low permeability regions creates
zones in which electron donor (or culture) may be injected over greater distances.
Either solid or liquid electron donors may be emplaced during or immediately after
fracturing. Fracturing is generally used in low permeability conditions or at highly
heterogeneous sites where the low permeability zones require treatment.
Various bioremediation approaches (classified as active, semi-passive or passive), can be used
in conjunctionwith bioaugmentation. In the following subsections, these approaches are described.
The advantages and disadvantages of each approach for achieving and maintaining optimal
conditions for bacterial growth (biostimulation) are considered, as these also impact bioaugmenta-
tion. Afterwards, the practical implications of each approach for successful bioaugmentation are
discussed.

5.4.1 Active Recirculation Approach

Active recirculation uses pumping (extraction) and reinjection of groundwater and electron
donor across a treatment area. Often, the goal of a recirculation system is to control ground-
water flow and donor distribution across the target treatment area in a more precise manner