GROUNDWATER - Water Quality Engineering in Natural Systems - page 194

ensure efficient performance of the system. In enhanced
bioremediation , a system for injection of nutrients and
circulation through the contaminated portion of the
aquifer is used.
When contaminants are organic compounds that can
be used by microorganisms as growth substrates, it may
be possible for the indigenous microbial population to
biodegrade the contaminants at significant rates in the
absence of engineered intervention. This is called intrin-
sic bioremediation , natural attenuation , or bioattenua-
tion . Intrinsic bioremediation is considered a suitable
remediation strategy only when natural contaminant
biodegradation occurs faster than migration, resulting
in a stable or shrinking contaminant plume.
The major advantage associated with in situ bioreme-
diation is that the contaminants are destroyed in place,
with minimal transport to the surface. An important
factor that can limit the feasibility of in situ bioremedia-
tion is the availability of the contaminant for microbial
attack. That is, contaminants that have extremely low
solubilities, sorb strongly to solids, or are otherwise
physically inaccessible.

Solidification and stabilization techniques involve
treating contaminated soil to alter the physical charac-
teristics of the soil and reduce the leachability and
mobility of contaminants within the soil. Soils are
usually treated in place with mixing augers or digging
tools adapted to mix the additives with the contami-
nated soil. Common additives include cement, lime
spikes with fly ash or sodium silicate, asphalt or bitumen,
and various organic polymers. In some applications,
combinations of additives are used together. By adjust-
ing the quantity of additives, permanent solidification of
the soil can be achieved (called monolithic treatment ),
or the soil can be made more amenable for transport
after treatment. Soil solidification and stabilization
techniques appear to be most effective in the stabiliza-
tion of metals and organic compounds like PCBs.
Physical barriers to prevent groundwater flow are
commonly called groundwater cutoff walls and include
slurry walls, grout curtains, sheet piling, and compacted
liners or geomembranes. Construction of a slurry wall
is illustrated in Figure 5.29. Physical barriers are most
effective in shallow aquifers that are bounded below
by a solid confining layer of bedrock or clay. A slurry
wall , the most popular type of cutoff wall, is constructed
by excavating a narrow trench 0.5-2 m (1.5-6 ft) wide
around a contaminated zone and filling the trench with
a slurry material. The slurry acts to maintain the trench
during excavation and usually consists of a mixture of
soil or cement, bentonite clay, and water. Soil-bentonite
cutoff walls typically allow some contaminant transport
across the wall, depending on the variability in hydraulic
conductivity in the bentonite. Trenches are usually
dug using an excavator or backhoe, and sandy aquifers
less than 18 m (60 ft) thick and underlain by an imper-
meable layer or bedrock are most amenable to slurry

5.8.2.8 In Situ Reaction Walls. In situ reaction walls
are excavated trenches containing material that reacts
with the contaminant in the groundwater. Water flows
in one side of the trench and out the other side of the
trench, hence the name reaction wall . In some cases,
low-permeability cutoff walls are used to direct the
groundwater flow through the reaction wall. These
systems have been referred to a funnel-and-gate systems ,
where the impervious barrier is the “funnel” and the
reaction wall is the “gate.” Chemical, physical, and bio-
logical treatment barriers have been used in practice. As
an example, granular elemental iron placed in a reaction
wall induces the dechlorination of some chlorinated
contaminants (e.g., TCE and PCE) and the removal of
dissolved metals, such as chromium, Cr(VI), through
reduction and precipitation. Reaction rates are typically
slow by aboveground treatment standards but are suf-
ficiently fast for groundwater systems.

5.8.2.9 In Situ Containment. The migration of con-
taminants can be restricted by using various contain-
ment options. In cases where residual contaminants are
in the vadose zone, the site can be capped to minimize
rainfall contact and subsequent percolation of the con-
taminant into the groundwater. Surface caps are usually
constructed using either natural soils, commercially
designed materials, or waste materials, and are typically
sloped for rainfall to run off rather than infiltrate.
Examples include clay, concrete, asphalt, lime, fly ash,
and synthetic liners.

Figure 5.29. Construction of a slurry wall. Source : Permeable
Reactive Barrier network (2005).