Turner Valley Gas Plant

Containment System

Not to diminish the engineering elegance of the groundwater gathering and flood protection system newly installed at the historic Turner Valley Gas Plant . . . but it really is just a big bathtub, a $7-million, 15.4-hectare bathtub with a fancy drain.

The project manager who built it says so.

The price tag includes design, construction and any operating costs the structure incurs in the next two years. Its purpose is to channel all of the water that flows through and over the site of the decommissioned gas plant plus remove the hydrocarbons contaminating the soils and groundwater before the water spills into the Sheep River.

At the same time this highly-engineered bathtub controls the flow of the water within the site, it protects against the river raging past each spring. The need for groundwater containment and cleaning became urgent when the Sheep River flooded three times in 2005. Torrents of water severely eroded the river bank and leached out hydrocarbons that had leaked from the plant long ago.

The base of the bathtub is bedrock, which is a relatively impermeable shale in this area. It is walled on its downhill sides with bentonite, to separate the site from the river. Bentonite is a highly-absorbent Wyoming clay. When water is stirred into it, it swells dramatically and becomes impermeable. Excavation to build the bentonite wall so it could form a seal with the uneven bedrock reached as deeply as five metres below the soil surface.

Just as a bathtub slants toward its drain, the sloping bedrock and the bentonite wall funnel all of the surface and groundwater into a state-of-the-art "treatment corridor" built at the lowest point adjoining the river. That "treatment corridor," a row of cleaning systems, removes the hydrocarbons so the water draining into the Sheep River is clean.

It may actually be cleaner than water in the area has ever been. Historians say it was naturally occurring gas and oil seeps that attracted investors and led to the drilling of the Dingman 1 natural gas well, which blew in dramatically on May 14, 1914.

The success of that and other wells gave birth to the energy industry in Alberta. The federal government has declared it a national historic site, and the provincial government has been working to reclaim the site and open it as a tourist attraction.

The principles incorporated to channel and clean the water have been used in other places, says Bruce Dewar, Project Manager, Environment, Stantec Ltd., but this installation is innovative in the way it applies those principles to the complexities of the site, and in the combination of passive and active water treatment technologies.

The complexities:

  • Separated from the gas plant property by only a chain link fence, there stands a Talisman Energy Inc. sour gas compressor station which is serviced by active high-pressure underground pipelines.
  • Utilities lines belonging to the Town of Turner Valley underlie Sunset Boulevard along the northern boundary of the gas plant site.
  • The bedrock, largely covered by a layer of gravel, is a series of three shale terraces rising unevenly 10 metres from the meander of the river, which forms a J as you face north. Shale is a common, fine-grained, sedimentary rock whose original constituents were clays or muds. Any fissuring tends to be parallel to the base layer. Shale is readily seen across the river on the south escarpment.

"The bedrock here is discharging water, contributing water into the groundwater rather than having water on top of it discharging water into the bedrock," Dewar says. "Different hydrostatic pressures make it discharge at different places. This is also contiguous bedrock. There are not a whole lot of cracks or fissures. It’s hard shale, relatively impermeable, very uniform."

The total length of all the legs of bentonite dyking is approximately 750 metres.
Because the Talisman battery station has live underground high-pressure gas production lines, Stantec could not excavate to encircle the whole gas plant property with bentonite. The engineering team chose to use weeping tile -- actually a perforated pipeline that water would drain into -- to demarcate the northern boundary.

"We wanted to put it along the northern edge of the middle terrace, but we couldn’t excavate to the depth we wanted because of all the live lines there. If we moved it south would it really serve its purpose? So we talked to the Town, which owns utilities under Sunset Boulevard (to the north of the Talisman plant). The Town wanted to upgrade its utilities. We said if we go in and put the line under the road, we’ll help you and upgrade the utilities at the same time. We put it as deep as possible on the clay layer under the gravel, directly underneath their new sanitary line."

The thinking is that it may well take a year for water to cross the site. Rate of flow depends on the amount of water in the ground, the amount of precipitation, and the kinds of soils. The general slope on the property is from south to north, in conjunction with the river.

Using only gravity, the weeping tile and bentonite walls direct the water towards a three-stage "treatment corridor" which has been built with extra space available for any advances in technology that may yet be developed.

Stage 1 is a skimmer that separates out the heavier hydrocarbons, the oils visible floating on the water -- F1s and F2s, in the jargon.

Stage 2 is an air sparger. It is a grid of 5-cm pvc piping that pumps air into the water, like a fish tank aerator. Its purpose is to speed up the natural evaporation of any light hydrocarbons and add oxygen to support the growth of microbes which eat hydrocarbons. Among the light hydrocarbons may be low concentrations of benzene, toluene, ethylbenzene and xylene, which are known collectively as BTEX.

The air sparger compartment contains "geotextile" curtains, made of a material which allows the water to pass through but catches the gas-gobbling microbes and any metals that precipitate out of the water, to stop them from flowing into the following compartment.

Stage 3 contains two activated carbon filters, like an industrial version of a kitchen water filter. The carbon captures the polycyclic aromatic hydrocarbons, chemical compounds primarily formed by incomplete combustion of carbon-containing fuels, but also found within crude oil.

"Water passes over the carbon matrix and any hydrocarbons not volatilized (evaporated) will preferentially stick to the carbon, versus staying in the water," Dewar says. "Carbon has binding sites, a million little cracks and crevices, and an ionic charge difference between particle and carbon" makes the particles adhere to the carbon.

Tubs filled with brine sit on top of the carbon cells to force the water down through them and provide insulation against freezing. The corridor has been designed to work year-round; time will tell whether water flow is little enough that it can be shut down during the winter to save energy. It will be carefully monitored through the first two years to develop benchmarks.

There’s also an insulated lid over the entire corridor, which will let in precipitation but serve to keep out wayward tourists, animals and autumn leaves.

To accommodate varying conditions, valves at each end either passively or actively control the rate water moves through the corridor. Should spring floodwaters rise to the level of the outlet, a one-way valve will prevent river water from flowing backwards into the corridor.

Remote instrumentation is in place to alert Stantec of any problems, and technicians visit periodically to collect water samples from the monitors stationed in each of the treatment corridor compartments.

Sampling and analysis has shifted from every week to every four because initial results were excellent. Analysis, including for the various hydrocarbons and mercury, covers almost 80 characteristics (parameters is the jargon), from ionic balance to pH and turbidity. Results are measured against both the Canadian Drinking Water Guidelines and the Canadian Council of Ministers of the Environment Guidelines for the Protection of Aquatic Life. Monitoring will continue far into the future.

Should reclamation of the plant and property be deemed adequate for the plant to be safe as a public venue, visitors will stroll the grounds on a blend of native grasses.

The security fence is on the inside of the bentonite wall, so area residents will have access to a five- to six-metre-wide pathway along the top of the wall.

Rafters floating down the river will see only the intimidating armour of riprap protecting the retaining wall, standing half a metre taller than the highest floodwaters reached in 2005.