A STUDY OF TRICHLOROETHYLENE (TCE) RELEASE RATE FROM BORDEN, ONT. AQUIFER SEDIMENTS THROUGH INTERMITTENT PURGING EXPERIMENTS AND MODELLING

Contaminants stored in aquifer regions accessible only by diffusion produce ‘tailing’ in response to remediation wherein a rapid initial mass and/or concentration reduction is followed by sustained release at a low rate (or concentration). The objective of this research is to quantify the effects of non-linear sorption in high sorbing grains on tailing behavior for grains representative of a surficial glacial aquifer in Borden, Ontario. We combine laboratory experiments and numerical simulations to achieve this objective.

Mass release rate (termed ‘desorption’) studies are conducted using trichloroethylene (TCE), which exhibits non-linear sorption to the aquifer material. The desorption batch experiments are conducted with two sizes of grains (0.8-2mm, 2.8-4mm) which were pre-equilibrated over several months with TCE at high (~1000 mg/L) or low (~2 mg/L) concentrations. Standard tubes installed on a purge and trap system are purged intermittently (~weekly) over several months.

The pattern in the data with the larger grains exposed to the high concentration shows extensive tailing over the experiment duration (~3 months). Preliminary numerical simulations of retarded (due to sorption) intragranular diffusion suggest that a nonlinear sorption model provides a superior fit to the data compared to a linear model. Numerical simulations of the paired experiments (high and low concentrations for each grain size) will be used to evaluate the evidence for a contribution of nonlinear sorption to tailing and to explore simplified models that could be useful to practitioners. The improved understanding of the impacts of nonlinear sorption due to kerogen within these high sorbing grains will contribute to an improved conceptual understanding of the mechanisms that cause chlorinated solvent persistence and tailing in surficial aquifers.