In September 1999, during routine daily inspections, water and methane gas were noted bubbling to the surface of the excavated clay base in three locations of sub-cell 3. Field investigations by Clean Harbors and independent subsurface and groundwater specialists, in cooperation with the Ministry of the Environment and experts from the University of Western Ontario and the University of Waterloo, concluded that the water and gas originated in the 'interface aquifer' - the zone located about 40 metres (130 feet) below the surface, where the clay till meets the bedrock.
Researchers identified several factors unique to sub-cell 3 that contributed to this anomaly. First, the upward pressure of water and gas trapped in the aquifer exceeded the weight and self-bonding strength of the remaining clay below the sub-cell 3 excavation.
Second, an elevation of the bedrock below the affected area contributed to the pressure differential, which in turn thinned the clay between the base of the sub-cell and the underlying bedrock aquifer. The elevated bedrock also contributed to a localized build-up of water and methane pressure directly below sub-cell 3.
Since the anomaly was discovered, no waste has been placed in the area where the water and gas seepage occurred. Groundwater testing confirms that the area has remained isolated from any waste management activities.
The Ontario Ministry of the Environment approved Clean Harbors' plan to secure sub-cell 3 and restore the environmental integrity of the landfill. This remediation project was undertaken in May, 2000 and completed in November 2001.
The Plan
The plan for restoring sub-cell 3 was designed to maintain an upward flow in the affected area to protect the bedrock aquifer. The design reflected sound science and input from the Ministry of the Environment as well as two third-party technical reviewers.
The remediation project involved construction of a granular stone drainage layer, called a hydraulic control layer, over the affected area. The entire hydraulic control layer was then encased in about 15 metres (50 feet) of layered and compacted clay. A thick fabric filter blanket separates the two layers to prevent clay particles from clogging the drainage areas.
Wells equipped with pumps were installed from the ground surface down into the granular layer. By pumping out any water that might collect in the granular layer, an upward movement into the landfill was created and maintained. The movement of water toward the surface ensures that contaminants are held in the landfill and away from groundwater.