Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 387

generally consists of permeable soil material containing various elements designed to
react with the kinds of contaminants entering the barrier. The whole intent of PRBs is to
provide a chemical-physical sieve or ilter that would capture the contaminants as they
pass through the barrier. In a sense therefore, PRBs are barriers with highly engineered
and eficient attenuation properties and characteristics. They are sometimes also known
as treatment walls.
The major contaminant capture and immobilization processes needed for the engineered
materials in the PRBs to function effectively include (a) sorption, precipitation, substitu-
tion, transformation, complexation, oxidation, and reduction for inorganic contaminants
and (b) sorption, biotic and abiotic transformations, and degradation for organic chemi-
cal contaminants. The types of reagents, compounds, and microenvironment in the PRBs
include a range of oxidants and reductants, chelating agents, catalysts, microorganisms,
zero-valent metals, zeolite, reactive clays, ferrous hydroxides, carbonates and sulfates, fer-
ric oxides and oxyhydroxides, activated carbon and alumina, nutrients, phosphates, soil
organic materials. The selection of engineered materials in the PRBs, such as reagents
and compounds, and the manipulation of the pH- pE microenvironment in the treatment
walls will need to be made on the basis of site-speciic knowledge of the nature of the
contaminants.
The success of PRBs in mitigating contaminant impacts depends on
1. Effectiveness of types of engineered material in the PRB: This depends on a proper
knowledge of the contaminants, barrier material, and the kinds of processes (inter-
actions and bonding mechanisms) resulting from contaminant-barrier material
interactions.
2. Suficient residence time of the contaminant plume in the PRB: There must be suf-
icient residence time in the PRB for contaminant-material interactions and reac-
tions to be fully realized. This is a function of both the permeability of the barrier
itself and the kinds of reaction times needed between contaminants and the bar-
rier material. It would be useless if the contaminant passed through the barrier at
high rates—rates that would not permit reactions to be completed. Conversely, it
would be useless if the contaminant would not penetrate the barrier, hence deny-
ing any opportunity for reactions to occur.
3. Proper intercept of contaminant plume advance: A thorough knowledge of site
hydrogeology is required to allow one to place the barrier for optimum intercept
of the contaminant plume. A knowledge also of the advective velocity is also
required.
For more effective use of PRBs, an enhanced NA treatment zone can be used and placed
ahead of the PRB. Such a case has been shown in Figure 3.11 in Chapter 3. The treatment
zone in the diagram is shown as an optional tool. Contaminant plumes can be channeled
to low through reactive walls by shepherding the plume with a “funnel-gate” technique.
In this technique, the plume is essentially guided to the intercepting reactive wall by a
funnel constructed of impermeable material such as sheet-pile walls, and placed in the
contaminated ground to channel the plume to the PRB (Figure 10.16). Other variations
of the funnel-gate technique exist—obviously in accordance with site geometry and site
speciicities.