A University of Waterloo researcher is the first to develop a new way to remediate contaminated groundwater by capitalizing on the power of mother nature.
With the help of a specially-designed field testing site built in part by the Southern Ontario Water Consortium (SOWC), Mahsa Shayan was able to test the effectiveness of combining a chemical remediation process with a biological one, using a real-world setting.
In her study, Shayan used a reduced amount of persulfate along with bacteria found naturally in the soil to remediate groundwater contaminated with benzene, toluene and xylene (BTX).
“It was thought that chemical oxidization couldn’t be integrated with the biological process because the oxidants kill the bacteria in the soil,”says Shayan, who recently completed her PhD in civil environmental engineering with supervisors Neil Thomson of Civil and Environmental Engineering and Jim Barker of Earth and Environmental Sciences. “But my study shows that an appropriate persulfate oxidization step that permits some contamination to remain for use by the bacteria allows for these two processes to be combined and work together.”
By reducing the amount of persulfate injected into the contamination site, the bacteria in the soil are able to thrive on the lower concentration of contaminants and essentially help in the tail-end of the remediation process, she says.
“This is important because it means we can use less chemicals, which lowers the cost and decreases the amount of toxins going into our environment during the remediation process.”
Shayan conducted her study at the University of Waterloo’s contamination and remediation testing site at the Canadian Forces Base Borden. This site is unique in that it has a contained aquifer that researchers can access and use to track the migration of subsurface contamination.
The site contains cells bounded by impermeable sheet piling, which are located in the shallow aquifer. This allows for the cell to be deliberately contaminated, a remediation technology applied and the treatment of contaminants tracked as they move through the aquifer without residual contamination or harm to the groundwater system. Having this setup allows researchers to know exactly where, what and how much contamination is introduced so they can get a more precise evaluation of their treatment. This also allows for new technologies for remediation to be tested and demonstrated in a real-world setting without worry of residual contamination or harm to the aquifer.
The University of Waterloo has seven cells at the research site, including a new cell constructed in partnership with SOWC. For Shayan’s experiment, additional equipment funded by SOWC was integral to the field testing, including the installation of Waterloo Emitters (a technology for oxygen diffusion manufactured by Georgetown-based Solinst Inc) and wells at the experimental gate.
“It was a unique opportunity to have access to this type of field testing site,” says Shayan. “It’s rare to have the ability to have so much control over the design of an experiment.”
The first step of the year-long study involved injecting the BTX solution into the cell, which runs 24 metres long, two metres wide and three metres deep.
“I chose a mixture of these chemicals because they are all found in gasoline and are common in contaminated sites,” says Shayan. “These chemicals are toxic and can remain in the subsurface for a long time so there is concern about having these contaminants in the aquifer and possibly getting into the drinking water.”
After leaving the contaminant in the groundwater for 170 days, Shayan injected the persulfate treatment into the cell on two separate occasions, 10 days apart and used only half the amount typically required to remediate a contamination of this size. Shayan’s goal was to remediate 50 per cent of the contamination with persulfate oxidization and 50 per cent with the bacteria. Essentially the injection of the lower dose of persfulate into the contaminated site was designed to not only partially remediate the site, but also spur the bioremediation process.
When persulfate interacts with the contaminants it decomposes into sulphate, she says. The sulphate then migrates down and acts as an energy source for the sulphate reducing bacteria causing a boost in the bacteria’s population and activity. The persulfate also breaks down the contaminant into a simpler compound that the bacteria can feed off of, she adds.
“These sulfate reducing bacteria are abundant in the soil at any petroleum contaminated site, but they need an energy source to survive. In the past, people have used so much persulfate that it kills off these bacteria. What I found is if you use less, you can create an environment that allows these bacteria to thrive and remediate the site in a more natural way.”
Shayan took water samples throughout the experiment to determine how effective the treatment was. After the initial injection of the persulfate, she found there was a temporary decrease in the population and activity of the bacteria. However, further sampling 40 days later revealed the population and activity of the bacteria had increased to levels higher than what they were before the experiment began. Shayan says the samplings also provided evidence that the bacteria were doing the remediation work.
“In the samples, we detected genes of the bacteria and metabolite byproduct of the remediation process. This was enough to show that the sulfate reducing activity was going on. Persulfate may inhibit the effect of the bacteria in the short term, but it will be followed by an increase in population and activity in the long term.”
Shayan found that 150 days after the first injection of the persulfate more than 80 per cent of the BTX was gone from the aquifer thanks to the sulfate reducing bacteria. As for the last 20 per cent, Shayan says it was remediated by methanogenisis bacteria, which are effective at degrading small amounts of BTX and don’t require sulfate to thrive.
“This study shows that mother nature has the capability to help in remediating contamination sites. It’s all about finding the right design and balance.”
Shayan says that the success of her project is further evidence that state-of-the-art, real-world testing sites and facilities, such as those offered by SOWC, are a critical component to the advancement of water research and technology development.