IMPLICATIONS OF CATION EXCHANGE ON THE LONG-TERM MIGRATION OF NATURALLY OCCURRING SOLUTES IN UNOXIDISED CLAY TILL

A study was undertaken to examine the impact of cation exchange on the migration and porewater chemistry of solutes through a thick unoxidized, non-fractured, clay-rich till (35 wt. % clay) using centrifugation. Despite the very low hydraulic conductivity of this material (1.2 x 10^-11 m/s), flow driven by accelerated gravity (234-423 x g) allowed us to model 20,000 years of advective-dispersive transport in 40 days.

Duplicate subsamples of unoxidized clay till core (50 mm length) were collected from 13 m depth, sterilized by gamma radiation, saturated under vacuum, and installed in a UFA modeling centrifuge. Porewater, collected from a 13 m deep piezometer located on site, flowed from a micro-burette reservoir through the core samples to effluent collection caps within the centrifuge. At centrifuge startup, the centrifuge speed was gradually increased so that stresses at the base of the samples were equivalent to in situ preconsolidation pressure (550 kPa). After establishing steady flow and measuring the hydraulic conductivity using a flow rate method, the reservoir water was replaced (t₀) with NaCl at a concentration equivalent to that at the top of the overlying unoxidised zone (I=0.5).

Breakthrough (C/C₀=0.5) of the conservative solute Cl occurred t=20 days, yielding an effective porosity, n_e of 0.19 (n_T=0.29). During this time, decreased Na (relative to Cl) was partly balanced by increased Ca, indicating ion exchange and possibly, calcite dissolution. However, retardation of Na appeared to be small (R=1.12). The cation exchange capacity (CEC) of 5 sub-samples of each core, dissected afterwards, was similar to that of original till samples CEC. The insignificance of retardation by exchange at this site, despite mineralogy dominated by reactive smectite, was attributed to low clay fraction in the till and low CEC (36-42 meq/L, 59% Ca, 24% Mg, 17% Na). The apparent lack of cation exchange supports earlier geochemical studies of long-term solute transport at this research site and contrast with other studies which demonstrate significant reactivity within clay aquitard systems. This study showed that centrifuge modeling is rapid and reproducible.