G360 PROJECT TEAM: Dr. Beth Parker, Steven Chapman, Ryan Kroeker, Maria Gorecka, Rashmi Jadeja, Marina Nunes, Tim Speirs & James Hommersen (MSc Candidate)
In 1982, a train derailment near Sarnia, Ontario resulted in the release of ~50,000 litres of parachlorobenzotrifluoride (4-CBTF), a dense non-aqueous phase liquid (DNAPL). Emergency response measures recovered about half of the volume released, leaving about 25,000 litres in the subsurface. The release occurred into a surficial clayey aquitard in a physiographic region of southwestern Ontario, referred to as the St. Clair Clay Plain, which has been well-characterized via many prior research studies, including diffusion studies (Parker, 1996), large scale laboratory column experiments with DNAPL releases (O’Hara, 1997; O’Hara et al., 2000), controlled field DNAPL releases (Kirkpatrick, 1998), detailed assessment of contamination from industrial site DNAPL releases (Lane, 2001), as well as many other studies on depths of active fractures (Burke, 1997) and aquitard characteristics. Thus, this release site offered a unique opportunity to characterize and evaluate source zone evolution and plume formation from an aged 4-CBTF DNAPL source, a contaminant for which little is known about its environmental fate, and where the release conditions were well documented (i.e. known timing and volume, estimates of mass removal during preliminary remediation, etc.). G360 initiated this project with funding from CN Rail and NSERC and collaborated with Golder Associates to conduct a detailed study of this release.
Golder employed several high resolution investigation techniques including soil vapour sampling, cone-penetration testing (CPT) and ultraviolet induced fluorescence profiling (UVIF), as well as conventional soil boring and installation of monitoring wells. G360 expanded on this through collection of continuous cores with detailed subsampling for 4-CBTF and potential degradation products, and installation of multilevel monitoring wells (MLS) comprised of 7-channel CMT systems (Einarson and Cherry, 2002), at key locations informed by prior investigations. Together these high resolution datasets combined with numerical modeling provided a refined Conceptual Site Model (CSM) for evolution of the DNAPL source and downgradient plume transport and fate. Given the low solubility and high sorption potential of 4-CBTF, continued DNAPL persistence occurs more than three decades after the release, but also with significant mass transfer into the matrix off fractures and sand lenses. Plume mobility is strongly attenuated by diffusion from fractures into the matrix in the shallow active flow zone and underlying unfractured zone. Potential for contamination of deeper bedrock water supply aquifers below the 40 meter thick aquitard is also low given diffusion controlled transport below the depth of active fractures. The refined and robust CSM is key to evaluation of the long term effectiveness of the current site management approach of natural attenuation, leading to better site management decisions.