Contaminants:
Benzene-toluene-ethylbenzene-xylene (BTEX)

Reactive Media:
O₂

Construction:
Trench and Gate

Point of Contact:
Marc Bowles
Komex International, Ltd.
Tel: 403-247-0200
Fax: 403-247-4811
Email: mbowles@
calgary.komex.com
16th Avenue, NW
Suite 100
Calgary , Alberta T3B 0M6 Canada

A pilot-scale permeable reactive barrier (PRB) was installed at the East Garrington gas plant in Alberta, Canada in September 1995. Initial concentrations of up to 12 mg/L of BTEX (benzene, toluene, ethylbenzene, and xylene) were detected. The gas processing plant was contaminated by condensate, lube oil, flare pit wastes, and other materials. The goal of the pilot-scale demonstration was to contain the BTEX onsite and ensure that only treated ground water migrated offsite.

The site is underlain by 10-16 ft of low-conductivity glacial till composed of silty clay and cobble-rich deposits that grade into a clay-rich sandy to silty basal unit. This is underlain by a silty shale with occasional interbedded sandstone units. The contaminated aquifer extends from the near the surface to 10 ft below ground surface (bgs). No information is given on porosity, transmissivity, hydraulic conductivity, etc.

Two 145-ft-long cut-off trenches were excavated at right angles to each other through the fine-grained glacial sediments down to the relatively impermeable bedrock. The bottom and the downgradient sides of the trenches were then sealed with an impermeable, synthetic liner before being filled with highly permeable aggregate. The PRB systems consists of three 6-foot-wide modular treatment gates in series. They were constructed of vertical culverts that inject air into the contaminated ground water, which promotes hydrocarbon degradation. The residence time inside the treatment gate is approximately 24 hours. The treated ground water then passes through an infiltration gallery composed of thin vertical trenches filled with highly permeable gravel.

The cost of the system was approximately $67,200. This includes design, construction, and materials.


Conceptual schematic of trench and gate system.

A passive permeable reactive barrier was chosen as a remedy for the site because of its low maintenance costs, despite the longer timeframe required for remediation. More specifically, a trench-and-gate system was selected over a funnel-and-gate system because of its advantages in low permeability sediments such as glacial tills. Compared with traditional stand-alone barriers, the combination of a cut-off wall and adjacent drainage trench (1) improves drainage of the contaminated zone; (2) increases the size of the capture zone both horizontally and vertically; and (3) prevents damming effects such as mounding which force contaminants around or under funnel walls.

Results of the pilot-scale project show that the contaminant plume has been captured and treated by the trench-and-gate system. Recent sampling yielded BTEX concentrations below 10 µg/L at the treatment gate, and no contaminants have been detected offsite. Monitoring equipment includes soil moisture sensors, tensiometers, and pressure transducers installed upgradient, downgradient, and along the trench. Experiments conducted in the system using artificially contaminated water suggest that total BTEX concentrations up to 2.5 mg/L can be effectively treated. Sampling will continue on a biannual basis.

Lessons Learned

During installation, an unusually high water table led to trenching problems. The high water table increased installation costs, but had no effect on maintenance costs.

Air sparging was found to be an effective method for enhancing biodegradation through the addition of oxygen to the treatment cell. Experiments using Oxygen Releasing Compound were not effective. However, the addition of phosphorus increased degradation rates.

SITE-SPECIFIC REFERENCES

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