Environmental Engineering Reference
In-Depth Information
basis depending on the depth, width, and saturated thickness of the plume
which controls the overall dimensions of the system. Several other aspects
of the subsurface construction procedure need to be considered such as the
need for dewatering during excavation, the means and costs of GW and soil
disposal, health and safety, and disruption to site activity. The continuous
and funnel-and-gate PRB designs are the two PRB types which are currently
being used for full-scale applications.
3.2.3.1 Continuous PRBs
For any site where GW flow and plume geometry are well understood and
there is no construction constraint, the continuous barrier (Figure 3.3a) is the
best design choice (Gavaskar et al. 2000). Continuous PRBs are the most com-
mon field installations operating today and they have relatively minimal
impact on the natural GW flow conditions at a site. Since the barrier covers
the full width of the plume, the continuous barrier design generally requires
a large amount of reactive medium compared to the funnel-and-gate system.
The prevailing ZVI cost can be a major factor influencing the capital cost and
hence, the choice of design. Continuous barriers do not have to be buried in a
below low permeability zone of the aquifer as long as the hydraulic conduc-
tivity of the saturated aquifer is less than that of the PRB (Figure 3.3h and i).
3.2.3.2 Funnel-and-Gate PRBs
In the funnel-and-gate PRB configuration with low permeability funnels, the
GW is directed toward the reactive medium (the gate) (Figure 3.3b) (Birke
et al. 2003). The funnel can be made of sheet piling, slurry walls, or some
other material penetrating into an impermeable layer (aquitard) to prevent
contaminant underflow (Gavaskar 1999) (see Section 3.3). Impermeable fun-
nels are generally keyed up to 1.5 m into the aquitard. In an extremely large
contaminant plume or highly heterogeneous aquifer the funnel-and-gate
system can be modified to have multiple gates (Birke et al. 2003). Multiple
reactive media in a separate vertical “treatment train” might be considered
at a specific site with a combination of contaminants (Figure 3.3c). Care must
be taken so that the reactions do not interfere or limit one another (Gavaskar
1999). GW velocity within the treatment zone is usually 2-5 times higher than
that resulting from the natural gradient, depending on the funnel:treatment-
zone ratio (Day et al. 1999). To ensure that GW flow does not occur beneath
the system, funnel-and-gate systems must be keyed into an underlying low
permeable zone (Lai et al. 2006).
It is very important to ensure that there is no gap between the permeable
and impermeable wall funnel joints. Since the construction consists of both
permeable and impermeable barriers, the construction cost is high compared
to a continuous barrier design using less reactive materials (Gavaskar 1999).
Typically, the ratio of funnel length:permeable treatment zone is <6. The
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