ECONOMICS AND VALUATION OF BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 311

Typically, the culture then is disbursed further from the well by injecting additional “chase”
water. The water injected immediately following addition of the culture normally is made
anaerobic to avoid exposing the culture to oxygen that could reduce the number of viable cells
in the injected culture.
Assuming that a bioaugmentation culture contains 10 11 cells/L, then 1 L could theoretically
inoculate an aquifer volume of approximately 33 cubic meters (m 3 ) of the subsurface, equiva-
lent to 1,177 cubic feet (ft 3 ), assuming a porosity of 30% and a target concentration of 10 7 cells/
L of groundwater. If 1 L of culture is injected over a 3 m (10 ft) screened well located within a
homogeneous, isotropic aquifer such that it is uniformly distributed in the water, then the
calculated average cell density could be achieved out to a radial distance around the well of
1.9 m (6.2 ft).
However, to achieve the theoretical distribution of the inoculum requires that: (1) sufficient
water be injected with the culture to push or carry it out to this radius; and (2) the microorgan-
isms will be transported like a conservative tracer. In reality, water volumes are often insuffi-
cient to push the culture to this radius and microorganisms will “stick” to the aquifer solids and
be retarded in their migration from the injection point. The degree of stickiness is a function of
various physical and chemical properties of the cells and the aquifer. Generally, injected cell
density will fall off logarithmically with distance from the injection point and cells will be
transported further in higher permeability zones than in low permeability zones. The initial
distribution of the culture is important, but as discussed later in this section, active growth of
the microorganisms in situ generally will be needed to achieve the distribution required to meet
the remedial objectives (Hood et al., 2006 ).
Given this fact, achieving observable degradation rates within short time frames often
requires that the culture be injected at a significant number of discrete locations or at closely
spaced locations. Dechlorination will be achieved more quickly because a sufficient population
of microorganisms will be established at more locations and the time required for the popula-
tion to spread to areas between injection locations will be shorter. However, injection at more
discrete locations will increase the implementation costs for bioaugmentation.
The water used to push the microorganisms out into the formation should be anaerobic to
preclude exposing dechlorinating bacteria to oxygen. However, the time to collect and modify
(if required) the groundwater (or municipal tap water) used as chase water will increase costs.
Bioaugmentation culture often is added with the electron donor solution to simplify the
injection process. Experience with numerous injections has demonstrated that Dhc cultures
are not negatively impacted by high concentration electron donor solutions in the subsurface
near the injection point. This approach has been conducted successfully at many sites (Jeff
Roberts, SiREM Laboratories, personal communication, 2010).
11.2.2.3 Growth Following Injection
The growth rate of the culture directly affects the time needed to achieve meaningful
results following bioaugmentation, since actively growing cultures distribute throughout the
subsurface much more quickly than slow growing cultures. After injection, bioaugmented
microorganisms may migrate only a short distance from the injection point before they are
filtered out by the aquifer matrix. Consequently, microbial growth in the subsurface is
generally necessary for effective bioaugmentation, since the progeny of introduced microbes
must move with groundwater and colonize downgradient regions to bioaugment a reasonable
volume of the aquifer.
Dhc have been shown to grow and spread downgradient relatively quickly under favorable
conditions (Schaefer et al., 2009 ). Growth of Dhc in situ requires several key conditions,