3-D CONTAMINANT TRANSPORT IN ARGILLACEOUS SEDIMENTS: A CASE STUDY USING NUMERICAL MODELING OF STABLE ISOTOPES AND CHLORIDE TO UNDERSTAND LONG TERM BRINE MIGRATION

Many studies that have applied stable isotopes as tracers within the argillaceous sediments of the Canadian Prairie have assumed that solute transport can be described by a diffusion-dominated 1-D model. Here we present a case study of contaminant transport in a shallow groundwater system (≤10 m depth), where advection and diffusion in both horizontal and vertical directions must be considered. The current extent of a brine plume emanating from a potash mine has been mapped by measuring 18 vertical profiles of stable isotopes and chloride. These data demonstrate that the peak chloride concentration associated with the brine plume is restricted to the upper 5 m of the subsurface. This is in contrast to horizontal migration of the plume which extends up to 500 m laterally and a distance of 2.5 km longitudinally from the brine source. The horizontal extent of the plume aligns with a series of small wetlands, which would suggest that the distribution of solute in the subsurface is controlled by the surface water distribution and subsequent vertical diffusion. However, water level, chloride and stable isotope data suggest that advective transport in the subsurface may be a more important control on plume migration. Remediation efforts have focussed on pumping saline water from the wetland immediately down-gradient of the brine source. However, chloride data demonstrate that the brine is accumulating in a small wetland formed within the local topographic low, 2.5 km down-gradient from the source of the plume. Therefore, prediction of the ultimate fate of the brine plume, and the design of an effective remediation program, requires an understanding of the subsurface transport processes. We use 3-D numerical modeling of stable isotopes and chloride to constrain the duration of plume migration and to assess the relative contributions of diffusion and advection on plume migration. The results of this study provide a platform for the forward prediction of plume migration and the design of monitoring and remediation programs in shallow clay-dominated groundwater systems.