Permeable Reactive Barriers in Europe - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 300

• A perched water table also existed in the fill covering the surface
of the site. In wet seasons, if allowed to enter the reactive treatment
zone, this water could dominate flow through it and unacceptably
reduce the residence time. This perched water therefore had to be
prevented from entering the reactive zone.
• Proximity to buildings and cost prevented the use of sheetpiles to
form the reaction chamber and the funnel of the F&G system. At the
time, all previous reactors had been formed within sheetpile boxes.
• If a slurry trench cutoff was used to form the funnel, then it was
imperative that the iron filings should not be inundated and blocked
by slurry. The iron would have to be contained or the slurry wall
would have to be constructed first.
• Excavation next to a slurry wall, to install a reactive treatment zone,
could cause local collapse of the cement-bentonite and/or a poor
seal between the wall and the iron filings. It would be undesirable to
have the possibility of a preferential flow path at this interface.
• The cleanup was being undertaken voluntarily and was not driven
by regulatory requirements. It was therefore particularly impor-
tant that those working on or adjacent to the project should not be
exposed to contamination. Early in the design study, it was decided
that there should be no hand excavation of contaminated soil or
work near it, for example to form or fill the reactor. Personal protec-
tive equipment could have enabled hand excavation but the risks
were deemed inappropriate for a voluntary remediation.
After consideration and rejection of many reactive treatment zone designs,
the in situ reactor configuration was developed as best fitting the site con-
straints. In place of the previously used horizontal flow reactive treatment
zones, the flow was arranged as vertical, in a 12 m tall by 1.2-m-diameter
steel reactor shell, which was filled with iron filings as shown in the figure.
This design enabled the reactor to be placed between the contamination and
the site boundary. This could not have been achieved with a horizontal flow
regime as the design calculations had shown that the flow path length needed
to be at least 5 m plus entry and exit zones to collect and disperse the flow.
The reactor was placed in an enlargement in a cement-bentonite cutoff
wall that was used to funnel the flow to the reactor. This wall was toed
into the deep aquiclude layer and the enlargement was taken to a depth of
slightly over 12 m to accommodate the reactor shell. The cutoff and enlarge-
ment penetrated through the clay layer on which the chlorinated solvents
were retained. However, as the cutoff material was designed to have a per-
meability of <10 -9 m/s, minimal downward migration of the solvents would
occur. The vertical flow direction within the reactor ensured that the full
depth of the iron filings was saturated whatever the seasonal variation in
groundwater level. The piping of the flow into the reactor and the change