Environmental Engineering Reference
In-Depth Information
Evaluation of retention time in the barrier is by far the most important factor.
This retention time is mainly a function of the hydraulic properties and transport of
materials across the barrier and the necessity to facilitate chemical
reaction/biological time. [SHA 00] details the approach to quantify this retention
time. In short, so that environmental remediation is effective: (i) the time for the
necessary reactions must be lower than the time necessary for polluted subterranean
water to cross the reactive barrier; (ii) the treatment medium must remain effective
for an economical period; and (iii) the reactive medium itself should not introduce
materials that are likely to contaminate subterranean water downstream.
There is a variety of installation techniques for permeable reactive barriers. Their
choice should be based on conditions specific to the sites considered. To choose a
special method of installation, many criteria must initially be considered, including:
the contaminants concerned, depth of the aureole of pollution, extent and importance
of the aureole of pollution, geology, hydrogeology and geochemistry, the type of
barrier to be built, the material to be used for the barrier, configuration of the barrier,
and distribution strategy.
The design considerations for the installation of a permeable reactive barrier
include the excavation or displacement of part of the aquifer material and its
replacement by a porous reactive mixture above the desired treatment depth. The
geotechnics of construction available for the installation of a reactive barrier include:
sheet piled walls; boxes; grouting; deep and shallow mixture of ground; excavation;
continuous trenches; and vertical or directional drillings. The impermeable sections
of the reactive barrier can use standard barrier installation techniques including:
ground-bentonite walls; cement-bentonite walls, geomembranes (polyethylene with
high density) and sheeting piles.
A detailed discussion of the treatments used for permeable reactive barriers is
beyond the range of this section. However, some technologies that are at various
stages of development are listed in Table 15.3. We can note that technology based
on the use of iron filings (with null valence, Fe
0
) is already commercially available.
The work recently undertaken on reactive barriers [EYK 95, LO 97, MAR 97]
strongly suggest that the combined use of modified clays mixed with inert materials
(sand/gravel) offers a means of creating a very profitable
in situ
treatment and
provides a practical solution to the rehabilitation of sites with complex
contamination problems.
The addition of clay will allow the sorption and immobilization of heavy metals
and some organic compounds. In any event, the general mechanisms of behavior
seem to be sufficiently understood to allow us to control the important parameters so
that their potentially beneficial effects can be exploited. Lastly, on the basis of data
