Use of Natural Tracers for Understanding Porewater Residence Time and Solute Transport: Challenges Encountered in Paleozoic Rocks of the Michigan Basin

Abstract:

Geoscience programs that focus on long-term waste management in the geosphere commonly aim to develop a conceptual model that explains porewater residence time and the mechanisms and time scales for solute transport. Diffusion is the dominant transport process in low-permeability rocks so measurement of rock diffusion properties is important, but measurements at the laboratory scale can be highly variable due to changes in lithology, porosity, confining pressure and pore-fluid composition at scales of cm to 10’s of m. The natural analogue approach, using tracers such as Cl, Br, ¹⁸O and ²H that commonly display conservative behaviour, provides a means to scale up the results of laboratory measurements to understand the larger-scale controls on porewater residence time and solute transport. The principal challenge in applying this approach is the difficulty of obtaining high-quality tracer concentration data from porewater in low-permeability rocks. Additional insight into solute transport and reaction processes could be gained by collecting data for the concentrations of major cations but these data are even more difficult to obtain. Using data from the geoscience site characterization for the Ontario Power Generation proposed Deep Geologic Repository at the Bruce site in southwest Ontario, this presentation will illustrate some of the problems with standard methods for obtaining porewater compositional data . Methods such as core squeezing and advective displacement simply do not work because the permeability and the water content are too low. The crush-leach method has been used but is susceptible to artifacts from dissolution of soluble minerals (halite, anhydrite, gypsum and celestite), ion exchange and difficulty in assigning a porosity value to specific solutes. In order to overcome these limitations and improve the outcome from the natural analogue approach, a new method has been developed that extracts porewater directly into hydrophyllic cellulosic paper by placing it in direct contact with the rock. Results will be presented to demonstrate that this method has potential to provide high-quality data for major cations, anions and porewater isotope systems (¹⁸O, ²H, ⁸⁷Sr/⁸⁶Sr) of interest, thereby extending the use of the natural analogue technique to a greater range of solute tracers.