Mitigating Risk
02/05/2006
by -- By: Prem Manicks
There are many advantages to cleaning up and reinvesting in brownfield properties. The redevelopment can take pressure off of undeveloped and open lands. Brownfields are also often fully serviced so there is less need for investment in new infrastructure. But brownfields also come with risks, including the presence of contaminants.
Risk assessment uses the tools of science, statistics and numerical modeling to analyze risk-related information and evaluates the likelihood of adverse human health or ecological effects that may occur or are occurring because of exposure to environmental contaminants. Risk is not anticipated at a site unless all of the following are present: a hazard, a receptor and an active exposure pathway route.
The following case studies illustrate how a risk-based approach was used in combination with a risk mitigation approach. In both cases, a detailed risk assessment was completed and determined that the contaminants present in the soil and groundwater would pose a potential human health exposure to site users. The mitigation techniques described below were used to eliminate the potential exposure routes. This allowed the sites to be redeveloped in a safe and cost-effective manner.
1) VOLATILE ORGANIC CONTAMINANTS IN SOIL AND GROUNDWATER
A site located in Toronto is situated within a historically contaminated former industrial zone. A detailed site characterization was completed and revealed that significantly elevated levels of perchloroethylene, trichloroethylene, dichloroethylene and vinyl chloride were found in both soil and groundwater. These contaminants, known as "chlorinated solvents," were used as a degreasing agent during the former industrial operation.
The project consisted of constructing a 60-metre long, two-lane roadway over the impacted lands to provide access to neighbouring lands. The project began with conducting a detailed human health and ecological risk assessment with no mitigation measures at the site. It was determined that chlorinated solvents in soil and groundwater would volatilize and may pose a human health risk to both construction workers and pedestrians. Therefore, a risk mitigation measure was required to proceed with the construction of the roadway.
Risk mitigation technique
The objective of the risk mitigation was to manage potential human health risk at the site by eliminating the volatilization exposure pathway. The selected risk mitigation measures included excavation of impacted soils at the site and installation of a vapour barrier between the impacted groundwater and the surface, at a depth below the sub-surface utilities.
The critical steps involved the following:
* excavation of the existing soils to a depth below the utility lines;
* installation of a spray-on vapour membrane and backfill of the trench with soils that meet the provincial regulation;
* installation of a vapour monitoring system at the base of the excavation for long-term monitoring and to evaluate the effectiveness of the vapour barrier; and,
* installation of utility lines within the clean backfilled material.
Design
A liquid-based membrane was selected for use as the vapour barrier at the site. A spray-applied membrane technology that is applied at a thickness of approximately 0.2 centimetres to the surface of a thin geotextile sheet was used.
The advantages of the technology was that it is a liquid that is applied cold by spraying that provides excellent sealing of penetrations such as utility lines. It is impermeable to chlorinated solvent vapours up to approximately 20,000 ppm and it can be applied directly to the soil surface.
Vapour monitoring
For future vapour monitoring, a 150-millimetres diameter perforated PVC pipe with a 15-mm perforated tube inside was installed along the length of the membrane. The perforated pipe was fitted with a 90-degree elbow fitting and a 150-mm PVC pipe extended to the proposed surface grade at the south end of the membrane. The pipe was fitted with 200-mm diameter, steel-cased flush-mounted cover that was cemented in place at the road surface.
To confirm the seal provided by the membrane, vapour samples will be collected on a regular basis to ensure the membrane's effectiveness.
Drainage system
The membrane is impermeable to water and, therefore, a potential exists for water accumulation within the membrane area. As a precautionary measure, a drainage system was installed to protect against possible water accumulation in the trench. The drainage system consists of a 150-mm rigid perforated drain surrounded by 19 mm of clear stone located along the length of the membrane and connected to the city sanitary sewer at the proposed sanitary manhole. The vapour membrane and drain pipe slope toward the street to allow drainage to occur.
The construction of mitigation barrier and roadway was completed in eight weeks, which provided the essential access to neighbouring lands.
2) PETROLEUM HYDROCARBON CONTAMINATION
A site in southwestern Ontario was discovered to have benzene, toluene, ethylbenzene and xylenes contamination in groundwater. These contaminants are generally associated with petroleum hydrocarbons releases. A series of subsurface investigation programs completed at the site indicated that these contaminants were likely migrating from off site. A physical cleanup of the site was estimated to cost a few million dollars.
A multi-story residential apartment building, including a basement, was proposed for the site. Detailed risk modeling indicated that benzene, toluene, ethylbenzene and xylenes in groundwater would volatize through the soil media and reach the floor slab areas. It would then enter the building indoor spaces through foundation cracks and residents may come into contact with inhalation of vapours. To make this project viable, a sound risk mitigation measure would be necessary.
The selected mitigation measure consisted of constructing a passive vapour collection system beneath the building floor slab.
Design of passive vapour collection system
Two sets of horizontal vapour collection pipes were constructed beneath the proposed building footprint. One set for the basement area and another set for the ground floor of the proposed building. The horizontal pipes were connected to two risers and the vapour mitigation system would operate on a passive mode. The passive system was designed with adequate capacity to capture any vapours that may migrate close to the concrete slab. Vapours will be captured by the vapour collection system before they enter the area beneath the concrete slab.
Ambient air movement over top of the building roof where the effluent pipe is located creates low negative pressure, which is then conveyed within the entire piping system located underground. This negative pressure then slowly draws any vapours located both within the trenches and within the granular material the slab. The vapour collection system is designed to prevent vertical migration of vapours through cracks, joints and conduits in the concrete flooring.
Contingency system
An active vapour extraction system was put in place as a contingency system. Air monitoring was conducted to ensure that no contaminant vapour release occurs inside the building. In the event of a contaminant release from the subsurface into the building, the system would be converted to an active system using an exhaust fan.
The vapour collection system was designed to collect all vapours, and to prevent any vapours from entering the building. This resulted in a 100 per cent risk reduction.
Contaminated property that poses environmental risk can be managed and put to productive use. There are proven techniques available to mitigate environmental risk. However, each site must be carefully considered prior to selecting an appropriate method. Provincial governments in Canada, through their respective environmental ministries, have developed guidelines and regulations to allow risk assessment to be used as an effective tool for developing contaminated properties. These risk assessment techniques have significantly evolved over the last decade and are gaining wider acceptance among various stakeholders.
Prem Manicks, P.Geo, is the head of risk assessment and hydrogeology services at Trow Associates Inc. in Brampton, Ontario. He can be reached by e-mail at prem.manicks@trow.com.
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