G360 PROJECT TEAM: Dr. Beth Parker, Dr. Ramon Aravena & Joanna Olesiuk

A detailed study of a trichloroethene (TCE) source zone and dissolved phase plume in a fractured dolostone aquifer was undertaken to assess the spatial distribution, temporal evolution and contribution of natural processes on contaminant attenuation. The research site is in Guelph, Ontario and is the location of an automotive manufacturing facility where TCE, as a DNAPL, entered the subsurface nearly three decades ago. Previous depth discrete rock core analysis indicated that the bulk of the contaminant mass, primarily TCE, resides in the rock matrix in the upper 20 m of the bedrock (Kennel, 2008). Long term TCE concentration data, from a dense monitoring network representing groundwater in the fractures, indicate decreasing trends at over 80% of monitoring wells in the top 10 meters of the bedrock. Over a ten year monitoring period, degradation products cis-1,2 dichloroethene (cis-DCE), vinyl chloride (VC), ethene and ethane are increasingly detected, with the latter two providing evidence of complete dechlorination, likely due to mainly microbial but also abiotic processes.

Using data from 18 monitoring wells in the shallow bedrock (0 to 4 metres below top of bedrock), 46%, 74% and 86% reduction in TCE plume area, average concentration and mass, respectively, was calculated between 2003 and 2013. The position of the plume centre of mass shows a general receding trend, migrating less than 10 meters in an upgradient direction. In the intermediate bedrock zone (4 to 10 meters below top of rock), data from six monitoring wells, indicate a 44%, 87% and 93% reduction in TCE plume area, average concentration and mass, respectively, over the same period. Shallow bedrock zone cis-DCE trends indicate 67% and 29% reduction in plume area and mass, respectively, with an increase in average concentration of 115% between 2003 and 2013. Average concentration and mass of VC are also increasing within the plume, as are detections of ethene and ethane. Decreasing trends are also observed when comparing equivalent TCE concentrations, indicating some degree of TCE mass loss due to complete destruction rather than transformation into degradation products. Results of TCE Compound Specific Isotope Analysis (CSIA), show 13C enrichment along the groundwater flow path, providing evidence that biodegradation, specifically in areas where favourable redox conditions exist, is occurring and is increasing over time. Although the centre of mass is near stationary, the TCE plume front is retracting, and concentrations are decreasing primarily due to combination of matrix diffusion, sorption and complete degradation.

2013 TCE distribution in shallow bedrock. Plume boundary at 5 µg/L.