(BAT) Reference Document for the Production of Chlor-alkali ...

4.5.3 Containment
Chapter 4
Description
Containment techniques consist in cutting-off the exposure pathway from a contaminated site
towards receptors. This is achieved by confinement of the contaminated material either at the
original location or after removal.
Technical description
For the containment of contaminated soil at the original location, barriers such as cappings or
cut-off walls are frequently used, either during emergency preliminary activities or as
permanent techniques. Otherwise the polluted soil or sediment is removed and confined in a
permanent storage facility. If necessary, the incurring groundwater is treated. The excavated soil
is then replaced with uncontaminated soil.
Each cut-off wall system has limitations with respect to emplacement depth and to uncertainty
concerning permeability. Barriers may intend to surround the contaminated zone entirely or
remove the potential for groundwater flow through the source. Examples include slurry walls,
grout walls/curtains and sheet pile walls. For cappings, low permeability materials, such as
compacted clays, natural soils mixed with stabilisers or bentonite, or geosynthetic membranes
are frequently used to inhibit infiltration [ 244, Hinton and Veiga 2001 ].
In one case, a decision has been taken At a mercury cell chlor-alkali site in Skutskär (Sweden),
a decision was made to build a barrier and dredge the sediment from the bottom of the harbour
area and place it behind the barrier. The volume of sediment was is 500 000 m 3 and the amount
of mercury is 4000 kg. The content of mercury varied varies between 1 and 110 mg/kg dry
substance, with an average of 24 mg/kg [ 75, COM 2001 ], [ 278, Verberne and Maxson 2000 ].
{Please TWG provide information on the actual site and if the decision was implemented.}
At a mercury cell chlor-alkali site in Pavlodar (Kazakhstzan) the following containment
techniques were implemented from 2002 – 2005 [ 262, Ilyushchenko et al. 2008 ]:
construction of bentonite cut-off walls (15 – 20 m deep, 0.5 m wide) underneath the cell
room building by backfilling a trench with a mixture of bentonite clay and water which
was deep enough to reach water-resistant basalt clay below the aquifer;
removal of the upper soil layer outside the cut-off wall (50 cm depth, mercury content
> 10 mg /kg) and replacement with clean soil;
monitoring network with more than 100 observation boreholes.
At a mercury and diaphragm cell chlor-alkali site in Rheinfelden (Germany), the
PCDD/PCDF-contaminated soil with > 1 Zg TEQ/kg was removed and substituted or, in some
cases was contained by removing the top soil, fitting a geo textile and covering the bottom soil
again with non-contaminated soil. The largest share of contaminated soil with PCDD/PCDF
concentrations < 10 Zg TEQ/kg was deposited on a landfill while most of the soil with
PCDD/PCDF concentrations > 10 Zg TEQ/kg was incinerated as hazardous waste. Soil with
PCDD/PCDF concentrations < 1 Zg TEQ/kg remained without action taken but restrictions to
agricultural use applied [ 240, Otto et al. 2006 ].
WORKING DRAFT IN PROGRESS
Achieved environmental benefits
The achieved environmental benefit of this technique is a prevention of pollution dispersion and
transfer to biota.
Environmental performance and operational data
At a mercury cell chlor-alkali site in Pavlodar (Kazakhstzan) the mercury concentrations in
groundwater inside the site perimeter remained high after the implementation of containment
techniques while significant local decreases were observed outside [ 262, Ilyushchenko et al.
2008 ].
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