2.7.1.1 Controlling the Contamination Source in Unsaturated Zones
The fastest and most effective treatment option in this case is excavation. The
advantages of this technique are:
• Short operating time therefore limiting dissemination of the contaminant plume
• Reduced residual contamination
Excavation is usually implemented in zones with easy access (with the exception
of contaminated zones located under building basements and pipe networks).
The major drawback of this technique is the need to treat the excavated contaminated soils. Biological treatments of soil are technically and economically better
suited for this type of contamination than thermal, physical, and chemical treatments.
The entire contaminated source in the unsaturated zone can be treated ex situ, but this
option is not only expensive (estimated in the order of 120,000 €), it is not
eco-efficient (the off-site treatment facilities are located far away, implying long
transport distances). Onsite treatment was therefore preferred as the most costefficient option because, in addition to not requiring transport, excavated soils
could be reused on site. The selected strategy involved three treatment options:
landfarming, windrows, and biopiles. Treatment by landfarming or windrows calls
for relatively long treatment durations, and the waste streams are difficult to control,
particularly gases. They may also have a negative visual and olfactory impact. For
these reasons, a biopile system was selected. The treatment cost was 45,000 €
(including leachate and atmospheric effluent treatment).
The biopile was designed as follows: a geomembrane was installed under the pile
to recover the leachates, while another geomembrane was placed on top of the pile to
limit rainwater infiltration and gas release. These barriers facilitate the collection of
aqueous and gaseous effluents in preparation for their treatment.
The limits of the excavation were dictated by: (1) geotechnical considerations at
the top of the water table; and (2) soil concentrations that do not generate contamination in the groundwater (leaching tests have been carried out to determine the
residual concentrations in soils that do not release contaminants in the leachates
above permissible values).
2.7.1.2 Controlling the Contamination Source in Saturated Zones (Free
Product)
Four existing free product recovery techniques have been assessed: dual-phase
extraction, vapor extraction/groundwater extraction, free product recovery with
groundwater extraction (pumping/skimming), and drainage trenches
(skimming only).
Dual-phase extraction was not selected because it required too many suction
wells in the building area, and was not compatible with the problems inherent to
104
S. Colombano et al.
The fastest and most effective treatment option in this case is excavation. The
advantages of this technique are:
• Short operating time therefore limiting dissemination of the contaminant plume
• Reduced residual contamination
Excavation is usually implemented in zones with easy access (with the exception
of contaminated zones located under building basements and pipe networks).
The major drawback of this technique is the need to treat the excavated contaminated soils. Biological treatments of soil are technically and economically better
suited for this type of contamination than thermal, physical, and chemical treatments.
The entire contaminated source in the unsaturated zone can be treated ex situ, but this
option is not only expensive (estimated in the order of 120,000 €), it is not
eco-efficient (the off-site treatment facilities are located far away, implying long
transport distances). Onsite treatment was therefore preferred as the most costefficient option because, in addition to not requiring transport, excavated soils
could be reused on site. The selected strategy involved three treatment options:
landfarming, windrows, and biopiles. Treatment by landfarming or windrows calls
for relatively long treatment durations, and the waste streams are difficult to control,
particularly gases. They may also have a negative visual and olfactory impact. For
these reasons, a biopile system was selected. The treatment cost was 45,000 €
(including leachate and atmospheric effluent treatment).
The biopile was designed as follows: a geomembrane was installed under the pile
to recover the leachates, while another geomembrane was placed on top of the pile to
limit rainwater infiltration and gas release. These barriers facilitate the collection of
aqueous and gaseous effluents in preparation for their treatment.
The limits of the excavation were dictated by: (1) geotechnical considerations at
the top of the water table; and (2) soil concentrations that do not generate contamination in the groundwater (leaching tests have been carried out to determine the
residual concentrations in soils that do not release contaminants in the leachates
above permissible values).
2.7.1.2 Controlling the Contamination Source in Saturated Zones (Free
Product)
Four existing free product recovery techniques have been assessed: dual-phase
extraction, vapor extraction/groundwater extraction, free product recovery with
groundwater extraction (pumping/skimming), and drainage trenches
(skimming only).
Dual-phase extraction was not selected because it required too many suction
wells in the building area, and was not compatible with the problems inherent to
104
S. Colombano et al.