QUANTIFYING THE IMPACT OF ION EXCHANGE ON LONG-TERM SOLUTE TRANSPORT IN CLAY AQUITARDS USING CENTRIFUGATION AND GEOCHEMICAL MODELING

Characterizing and predicting reactive solute transport in low hydraulic conductivity (K) clay-rich aquitards is challenging because the very long transport time for solutes (about 0.5 to 0.8 m per 10,000 years) renders conventional column tests impractical. Furthermore, standard cation exchange batch techniques may not provide realistic data due to higher liquid:solid ratio (200:1) than in situ conditions (1:5) and the impact of carbonate mineral dissolution (~8 wt. % calcite). In this study, a centrifugation technique was developed to accelerate solute flux through low K (1.1x10^-11 m/s) clay-rich core samples to quantify ion exchange processes in a scale model for comparison with observations at the field scale. Duplicate cores (52 mm length x 33 mm dia.) were centrifuged at 330 x g for 3 months to simulate the migration of saline pore water (0.5 M NaCl) through ~17 m thick clay aquitard over ~27,000 years under in situ conditions. Importantly, a liquid:solid ratio of 1:5 was maintained by centrifugation at stresses equivalent to pre-consolidation stress in the aquitard. Effluent breakthrough over the experimental period was simulated with a PHREEQC 1D reactive solute transport model using measured cation exchange capacity (CEC) of ~20 meq/100g. Best fit simulations for breakthrough and exchangeable cations indicated that exchange coefficients K_Na/Ca and K_Na/Mg were lower than determined by static cation exchange batch tests. PHREEQC simulations of the field solute profile obtained from 13 piezometers installed between 3 and 20 m below ground, indicated that despite high CEC, chromatographic separation of Na⁺ and Mg²⁺ was limited to ~1 m by diffusive dominated solute transport. Migration of K⁺ and Sr²⁺ was also delayed. High apparent Ca²⁺ exchange affinity was attributed in part to high Na/Ca molar ratios in porewater. Similar K_Na/Mg and K_Na/Ca for centrifugation and field results indicated that centrifugation may be used to determine long-term reactive solute transport controlled by ion exchange in clay-rich aquitards.