Turner Valley Gas Plant
Mercury at the Plant – a Thing of the Past
The Turner Valley Gas Plant was among the earliest gas processing facilities in Alberta. When Stantec went to the Plant in 2002 to help remediate the historic site, they knew mercury was present because of the mercury control switches in the dozens of gauges used throughout the complex.
Little was known of the health hazards around mercury until the 1970s. It was spilled, workers breathed its vapours, and broken gauges were generally disposed of without containment.
Higher levels of mercury have been found near the buildings on the gas plant site, rather than spread across the whole property.
"The reason for that, we think, is that most of the mercury was used in pressure gauges inside the buildings," Stantec project manager Bruce Dewar says. "The thought is that when they’d break, workers would either chuck the mercury out the door or it would cling to their boots and they would track it out."
After the gas plant closed in 1985, the Alberta government resolved to clean up the Turner Valley Gas Plant and open it as an historic site. Mercury remediation was high on the to do list.
Mercury is considered a toxic substance, and studies done in the 1990s indicated there were certain "hot spots" at the plant that had to be cleaned up to reduce the risk to visitors and those who would staff the site.
Intact mercury gauges were located, drained and disposed of in the Gasoline Plant, the Scrubbing Plant, and the Compressor Plant as part of initial plans to clean up the location in 1999.
Stantec came in to refine the remediation process, focusing on contamination outside on the site, and inside the plant’s buildings.
Using criteria established by Health Canada, Canadian Council of Ministers for the Environment, and the United States Environmental Protection Agency, the Stantec team took samples from inside buildings over the summer of 2002, and soil samples from along the tour pathway in the fall of 2003.
Inside, the concern was around breathing in mercury vapours from residue left in the floor, walls and ceiling. Air sampling found elevated levels in the fractionating building, scrubbing plant, the compressor plant, the sulphur plant, and the gasoline plant.
The buildings were mapped into grids of five-metre square sections, then vapour tests were taken at three different heights in every five-metre quadrant of each building.
"What we were looking for was the potential for inhalation from different groups," Dewar says. "The waist level tests mimicked the average height of a child; samples taken around five feet off the ground were considered the average adult breathing space, and floor level was worst-case scenario, where mercury would be trapped in cracks in the floor."
Dewar notes a vast majority of the tests showed no detectable mercury vapours. And the standard applied to the site was high: twice as stringent as the occupational health and safety standard created for commercial areas estimated for people working on site eight hours a day, five days a week.
Coincidentally, while the tests were being completed, Alberta reduced its commercial exposure guidelines from 0.05 to 0.025 milligrams per cubic metre – the level that was followed in the gas plant remediation program from the beginning.
The crews scrubbed the floor, and sealed all cracks in the concrete with silicone sealant to ensure any mercury residue was sealed in. The mercury vapour testing was completed again and it was found that all the areas were well below the eight hour exposure limit.
Outside, soils showing mercury contamination were excavated to a depth of one to two feet, and the contaminated soil trucked to an Alberta Environment-approved landfill. The holes were backfilled with clean soil obtained from a local source.
For the majority of the plant, site specific soil mercury guidelines (21 mg/kg) were followed. This guideline is similar to the industrial exposure guideline provided by the government. Surface soils showing mercury concentrations greater than this were excavated and removed from the site.
The tour path was sampled every 25 meters for mercury contamination. On this area of the plant specifically the more stringent residential soil guideline of 6.6 mg/kg was applied to the soils. Surface soils along the path found to exceed 6.6 mg/kg were excavated, removed from site for disposal, and the holes backfilled with clean material.
With mercury remediation contained to the most stringent guidelines, including the meeting of residential standards, proper disposal procedures and an ongoing management plan in place, mercury no longer poses a hazard at the site.
