Fingerprinting TCE in a bedrock aquifer using compound-specific isotope analysis

Area of Science:

• Environmental Geochemistry
• Hydrogeology
• Isotope Geochemistry

Background:

• Persistent trichloroethylene (TCE) contamination led to the 1994 shutdown of a municipal well in a fractured dolostone aquifer in Guelph, Ontario.
• Several industrial sites with known subsurface TCE contamination are located near the affected municipal well.
• One industrial property implemented a comprehensive bedrock monitoring network, while others lacked detailed subsurface investigation.

Purpose of the Study:

• To identify the sources of persistent trichloroethylene (TCE) contamination impacting the municipal well.
• To evaluate the utility of a dual isotope approach using carbon (C) and chlorine (Cl) compound-specific isotope analysis (CSIA) for source identification.
• To assess the role of biodegradation in the fractured dolostone aquifer.

Main Methods:

• Compound-specific isotope analysis (CSIA) of carbon (δ(13)C) and chlorine (δ(37)Cl) was performed on TCE and cis-1,2-dichloroethene.
• Samples were collected from the impacted municipal well and numerous monitoring wells within the bedrock.
• Analysis included volatile organic compounds (VOCs), inorganic parameters, and dual isotope signatures (δ(13)C vs. δ(37)Cl).

Main Results:

• A wide range of isotope values for TCE and cis-1,2-dichloroethene were observed, indicating diverse sources or processes.
• The dual isotope plot clearly separated municipal well TCE samples from those originating from the monitored industrial property.
• CSIA results indicated that the intensively studied industrial property was not the primary source of TCE in the municipal well under non-pumping conditions.
• Evidence of microbial reductive dechlorination of TCE was found in specific zones of the bedrock aquifer.

Conclusions:

• The dual CSIA approach effectively distinguished TCE sources in the complex fractured dolostone aquifer.
• Early implementation of CSIA with strategically placed multilevel monitoring wells could have expedited source identification.
• CSIA, combined with VOC and redox parameter analysis, is crucial for understanding contaminant fate in complex fractured rock systems, especially with biodegradation present.
• Further investigation is recommended when the municipal well resumes full operation.