A UK-based geotechnical contractor is developing
a way of cleaning potentially hazardous ground
contamination. The process involves placing a permeable
reactive barrier in the flow path of contaminated
groundwater.
A team of engineers and academics
is piloting an innovative
but simple process that
exploits the natural flow of
groundwater to clean up contaminated
land.
The most common approach to contaminated
land remediation in the
United Kingdom is “dig and dump” -
removing potentially hazardous material
to a controlled landfill site.
Equally well-established is the practice
of isolating a pollutant source
by enclosing it in impermeable
slurry walls, made from a mix
of clay and grout, with a compacted
clay capping layer.
Although the containment
option is arguably more sustainable
than removing contaminated
material, it can be
perceived as a “toxic timebomb”.
UK-based geotechnical
contractor Keller Ground
Engineering is one of a handful
of European firms developing
a technique that promises
to provide an inexpensive, safe
and highly effective way of
cleaning potentially hazardous
ground contamination.
The process involves placing
a permeable reactive barrier
(PRB) in the flow path of
contaminated groundwater.
The PRB is so called because
it contains reactive materials
that immobilize and/or neutralize
the pollutants as they pass
through it. The type of materials
used depends on the nature
of the contaminant.
“A key difference between
PRBs and other in situ groundwater
remediation techniques,”
according to Keller’s business
development director Martyn
Singleton, “is that the contamination
moves through the treatment zone.”
With most other techniques it is the
other way round, which makes it difficult
to ensure that all the contamination
has come into contact with the
treatment reagent.
The process can be achieved by one
of two approaches. With the funnel and
gate technique, impermeable walls
direct contaminated groundwater to
gates containing a reactive material.
Continuous PRBs consist of trenches,
filled usually with gravel and a
reagent, positioned within the flow of
groundwater.
Keller favours the former for its
greater versatility. “The gates are usually
prefabricated steel boxes encapsulated
within an impermeable trench
constructed in the ground,” says Keller
engineer Dr. Denis Greene. “This
allows for removal or exchange of the
reactive material within the gate.”
Keller’s first PRB contract at
Monkstown in Northern Ireland cost
the client £750,000, about £250,000
less than conventional dig and dump.
A more recent project at Portadown
cost £500,000, saving the client an
estimated £1.6 million.
Costs are also favourable when
compared with a perimeter slurry wall
and capping system, Keller says,
because the length of barrier required
to funnel or intersect the contamination
plume is far less than that required
to enclose a pollutant.
The company readily admits that
the use of PRBs is technically challenging,
from the conceptual design of
treatment through to site installation.
With this in mind, it is working closely
with scientists at Queen’s University
Belfast to identify pollutants that can
be treated using PRB technology, and
to match them to the most appropriate
neutralizing reagents.
“High-quality site characterization
is especially important
for implementing a PRB
correctly,” says Dr. Greene. “It
is essential to determine the
spread and concentrations of
contaminants in the groundwater
and to establish the local
geology and hydrogeology
(pattern of groundwater flow).”
It is normally necessary to
carry out a trial to confirm the
degradation of the contamination
and the calculated half-life
- the time needed to half the
contamination concentration -
and the volume of reactive
material required.
Typical design may be
based on 10 to 30 years of
treatment before the source has
attenuated. Keller’s experience
so far suggests that because
this is a passive system, operating
costs and maintenance are
relatively low.
The UK’s Environment
Agency (EA) recently produced
new guidelines on the
use of PRBs, and the technique
now falls within its remit to
“promote and encourage the
effective use of sustainable
remediation technologies”.
Such is the EA’s enthusiasm for
PRBs that it is even relaxing the licensing
requirements for operators wanting
to implement a PRB scheme.
For these reasons, the approach
looks set to become widely adopted in
the UK and beyond.
Contact, Dr. Alan Bell, e-mail: alan.bell@keller-ge.co.uk.
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