In developing solutions for many of the world’s environmental problems, there’s an increasing number of potential tools in the remediation engineer’s toolbox. One of the keys to success is selecting the right tools to get the best possible combination of environmental, social and economic outcomes.
This was the case at a former fuel storage facility in Québec, Canada that had been used to store gasoline and diesel for 40 years. Over the years, hydrocarbons leaking from the tanks had leached into the soil. Although the storage tanks and most other structures on the site had been removed, contaminants were still in the soil. These contaminants had the potential to enter a sewer line near the site and then be carried off the property, with the possibility of migrating into groundwater.
Technology helps choose the best-fit remediation method
To determine the best way to remediate the site and prevent the impact on groundwater, the client engaged Golder to analyze alternative technologies using GoldSET®, Golder’s proprietary multi-criteria analysis web-based application. This tool helps determine the most sustainable alternative by balancing the economic, social, environmental and technical impacts of the various choices.
One possible remediation technique that the Golder team considered was dig-and-dump – digging up the impacted soil and then dumping it in a landfill designed to keep the contaminated soil isolated. This alternative would have been relatively low cost, so it “passed” on economic grounds. However, this alternative would also have been noisy and disruptive to the community, particularly a nearby school, so it failed the “social” aspect of sustainability.
Another alternative was to use in-situ chemical oxidation remediation methods, but this also had potential for negative impacts on the community as this technology involves transportation and injection of a large quantity of chemicals and it was expensive.
While many remediation projects must be performed quickly (so the property can be moved on to its next use), this project did not have the same time constraints. This luxury of time opened possibilities for using another technique, one that is low in social and environmental impacts as well as relatively low in cost, but one that takes time to work.
This technique, called bioremediation, uses natural biological processes to degrade hydrocarbons, producing carbon dioxide as a by-product. The work is done by providing native bacteria ideal growth conditions by injecting oxygen and nutrients via vertical injection wells. The enhanced bacteria have an appetite for hydrocarbons, which they ingest and convert to carbon dioxide.
Boosting remediation processes with super-oxygenated water
While there were fewer time pressures on this project than with many Golder has worked on, there was still a need to fix the problem before the contaminants had a chance to spread.
One of the disadvantages of bioremediation is that it tends to gradually slow down over time. This is partly because as the bacteria underground do their work, they absorb much of the oxygen naturally available between the grains of soil. Without oxygen, the bacteria can continue their work in an “anaerobic” state, but the cleanup moves more slowly.
There are several methods for speeding productivity of the underground microbes. In this case, the team implemented “super-oxygenated water” (SOW)*, a technique pioneered by Golder, to improve the speed of remediation. This technique involves dissolving high purity oxygen into water, using either a pressurized aeration tower or a pressurized chamber. Both methods are very effective, as over 90% of the oxygen injected (as SOW) is transferred into the groundwater. SOW technology can produce water dissolved oxygen concentrations of up to 40-50 mg/litre, which is five to six times the natural dissolved oxygen concentration of groundwater.
For some SOW projects, Golder has used municipal water supplies, but in this case the team was able to develop a sustainable loop in which groundwater was pumped to surface downstream of the groundwater plume, boosted with oxygen, and then reinserted by a series of injection wells on the impacted soil zone.
To keep the extra oxygen in position where it was needed, Golder added an oleophilic nutrient to the injected water. Those nutrients bonds to the hydrocarbon to eliminate or decrease the need to reapply on regular basis.
The compressor and other equipment fit inside a building on site, and power was provided by the electrical grid. The underground pipes supplying the injection wells were installed below the frost line, so that the remediation work could continue through Quebec’s cold winters.
After only two months of operation, the oxygen concentration in the impacted plume was increased from 1 ppm to 45-55 parts per million (ppm), the hydrocarbon degrading bacteria counts increased from 200 to over 5,000,000 CFUs or colony forming units (a measure of bacterial viability) per cc, and the hydrocarbon concentration in the groundwater were decreased by over 90%s.
Golder won a 2009 Brownie Award from the Canadian Urban Institute in the category “Sustainable Remediation Technologies and Technological Innovation” for this important project.
To learn more about the advantages of bioremediation, see Super-oxygenated Water: Your Sustainable ‘Drink’ of Choice for Groundwater Remediation.
*Editor’s note: The SOW process was developed in 2004 by Golder’s remediation team in Montreal, Canada (Eric Bergeron, Sylvain Hains, Christian Gosselin and J-P Davit).