Assessing long-term impacts of chlorinated hydrocarbons to freshwater aquifers due to diffusive transport and reactions within clayey aquitards

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Due to their widespread industrial usage, chlorinated hydrocarbons are among the most abundant groundwater contaminants across Canada and worldwide. After their release at the surface as dense non-aqueous phase liquids (DNAPLs), rapid vertical migration into the subsurface occurs due to their high density and low viscosity. During transport in the saturated zone, DNAPLs form pools on top of low permeability layers. With time, accumulated DNAPLs are transported by advection in the aquifer and by diffusion into the underlying low permeable units. After reduction of plume strength in the aquifer, back-diffusion occurs from the low permeability sediments that form a long-term contamination source for adjacent freshwater aquifers. Physical and reactive processes affecting chlorinated hydrocarbons in low permeability sediments are not yet fully understood. However, the identification and distinction of relevant processes affecting chlorinated hydrocarbons in low permeability zones is of major importance for estimating plume persistence in adjacent aquifers due to back-diffusion. This research program aims to gain a clear understanding of all physical and reactive processes affecting chlorinated hydrocarbons in saturated low permeability sediments and to assess their impact on long-term plume persistence in adjacent freshwater aquifers due to back-diffusion. The research will be conducted at three sites using advanced high-resolution field methods combined with new analytical methods as well as analytical and numerical modelling. Controlled release experiments are in progress at two sites in ongoing studies, which I initiated more than 15 years ago (near Sarnia and Alliston, ON). These sites are unique because physical and reactive processes can be investigated under controlled conditions, as the initial conditions of the contamination (spill time, volume, composition) are well known. Furthermore, the datasets obtained can be compared with previous investigations, offering the incomparable opportunity to gain understanding of the temporal resolution of physical and reactive processes in low permeability sediments under controlled conditions. The third site (near the city of Florence, SC, USA) offers a real-world scenario with mixed organic contaminants present over a longer time period (50 years). This research will improve prospects for predicting the time scales over which plume persistence due to back-diffusion occurs, and thereby provide an improved basis for remedial related site management decisions.