Application of Centrifuge Modelling to Assess Long-Term Brine Migration in Thick Clay Tills, Saskatchewan, Canada

摘要

Thick clay tills below potash mine tailings and brine ponds are important natural barriers protecting fresh water aquifers in Saskatchewan. Centrifuge modelling was applied to assess geochemical processes and impacts of dilute brine (0.1 M NaCl) migration through clay till core samples obtained from non saline, unoxidised clay till at a mine site. At accelerated gravity of 876 x g, during each day of centrifuge modelling, flow driven through a 52 mm length core simulated transport through a 45 m thick clay over ～2100 years. Hydraulic conductivity, monitored continuously before and during brine permeation, was low and relatively constant (8.5 x 10~(-12) m/s), confirming that confining stresses prevent permeability increase. Core density and porosity measurements confirmed that reactive transport occurred under realistic in situ conditions. Breakthrough (defined by C/C_0 = 0.5) of non-reactive Cl~- occurred at 25 days. The slight delay for Na~+ breakthrough, associated with small increases in Ca~(2+) and Mg~(2+) concentrations in the effluent were attributed to ion exchange, but carbonate reactions were relatively unimportant. Cation exchange analysis of core samples was inconclusive without corrections for porewater Na~+ and carbonate dissolution. Geochemical processes that were observed during centrifuge modelling were successfully simulated using the 1D geochemical transport code PHREEQC. Solute transport was dominated by diffusion with limited ion exchange to delay Na~+ migration. Brine would not break through a 45 m thick clay for tens of thousands of years. Based on these findings, the thick clay till at this site, if laterally extensive and undisturbed by drilling activity, is likely to act as an effective natural barrier to vertical brine migration over the long term.