Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard

SummaryThe impact of cation exchange reactions on the long-term (2 ka) migration of dissolved Ca2+, Na+, Mg2+, Sr2+, and K+ was determined in the upper 20 m of an 80 m thick clay-rich till aquitard in which solute transport is dominated by diffusion. A concentration-depth profile for dissolved solutes was obtained from 13 piezometers installed in the unoxidized and nonfractured zone (the aquitard) between 3 and 36 m below ground (BG). Concentrations of Na+, Mg2+, K+ and Sr2+ decrease with depth through the aquitard to 15–21 m BG, below which the concentrations remain constant. By contrast, the concentrations of Ca2+ showed no clear trend with depth. The effects of cation exchange on the solute concentrations of Ca2+, Na+, Mg2+, and K+ were determined on core samples using cation exchange analysis (n = 11) at in situ pH and solid:liquid (S:L) ratio of 1:200. Exchange coefficients were also determined by batch equilibrations (n = 10) at S:L of 1:1 and Na+ analysis by neutron activation analysis (n = 4). PHREEQC 1D reactive transport simulations of the field solute profiles indicated chromatographic separation was consistent with measured exchange coefficient values and the typical lyotropic series, with the probable exception of exchangeable Ca2+. Our findings suggest that, over depths of 20 m, Mg2+ and Na+ profiles can be simulated using non-reactive diffusive transport because chromatographic separation is limited to about two meters. The limited impact of exchange reactions on transport of these major cations was attributed to high KNa/Ca values and diffusive transport, which smoothed concentration profiles over the long transport times. A chromatographic separation of about 10 m between Sr2+ and Na+ was attributed to high CEC and a thick aquitard. This study presents, for the first time, field observations of chromatographic separation due to salinization in an aquitard.