New methods to assess groundwater contamination will help keep our drinking water safe.
Humans need water to regulate our body temperature, to protect our joints, and to combat disease. But groundwater, the source of drinking water for over one-third of the world’s population, is being contaminated by products like gasoline, jet fuel, and solvents. Researchers are working to clean-up our groundwater, and one method that they are exploring is monitored natural attenuation (MNA). With MNA, a range of physical, chemical and biological processes can be used to naturally reduce (attenuate) contaminants. However, evaluating the effectiveness of MNA is challenging because of a lack of data from contaminated sites.
University of Guelph engineering professor Beth Parker and her team at the G360 Institute for Groundwater Research are tackling this challenge head-on. The team, which includes U of G Environmental Microbiologist and professor Kari Dunfield, and post-doctoral fellow Philipp Wanner took a multi-disciplinary approach to study a historic manufacturing facility in southwestern Ontario with a groundwater source contaminated by toluene, a harmful environmental pollutant. The site is undergoing cleanup using trees, a process called phytoremediation. Using novel multi-depth samplers, which are sensitive enough to examine data on a fine scale, the team measured contamination levels in plant and root tissues to confirm if and where reactions that break down toluene occurred.
The elements that make-up chemical contaminants such as toluene include hydrogen and carbon and each of these elements can have different masses due to extra neutrons called isotopes. The composition of the heavier and lighter isotopes in the contaminant can change if degradation reactions occur making these isotope measurements diagnostic of subsurface contaminant destruction (i.e. removal). By measuring toluene-specific stable hydrogen and carbon isotope ratios in the groundwater, the team uncovered the groundwater response to phytoremediation treatment over a period of two years. Theirs is one of the first field studies to examine groundwater remediation in the long-term and uses new methods to examine how contaminants respond to treatment—bonds between light isotopes are preferentially severed as contaminants break down, which results in a higher number of heavy versus light isotopes in the parent compounds.
The results show that depending on season, toluene biodegradation and therefore natural toluene attenuation occurs throughout the entire contaminated area and not just at the periphery of the contaminated area, as previously thought. As well, oxygen levels in the groundwater conditions can change rapidly based on season, which affects toluene biodegradation and natural attenuation.
“We learned that relying on a single measurement in time could lead to false estimations of biodegradation rates,” says Parker. “We’ve found that oxidation-reduction conditions in the water can change rapidly throughout the year. Understanding this variance will help us determine the best remediation strategy at a given site.”
This work was supported by BP Canada, the University Consortium for Field-Focused Groundwater Contamination Research, and the Natural Sciences and Engineering Research Council of Canada. Beth Parker holds an NSERC Industrial Research Chair in Hydrogeology. Kari Dunfield holds a Tier 2 Canada Research Chair in Environmental Microbiology of Agro-Ecosystems.
Wanner P, Aravena R, Fernandes J, BenIsrael M, Haack EA, Tsao DT, Dunfield KE, Parker BL. Assessing toluene biodegradation under temporally varying redox conditions in a fractured bedrock aquifer using stable isotope methods. Water Res. 2019 Nov 15. doi: 10.1016/j.watres.2019.114986.