Elsevier

Journal of Hydrology

Volume 332, Issues 1–2, 1 January 2007, Pages 110-122
Journal of Hydrology

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

https://doi.org/10.1016/j.jhydrol.2006.06.025Get rights and content

Summary

The 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.

Introduction

Understanding the mode of transport and geochemical reactions on the long-term development of solute profiles in low hydraulic conductivity clay-rich aquitards is important to protect underlying groundwater resources from anthropogenic contaminants. The transport of conservative tracers in aquitards (e.g., δ18O, δD, and Cl) by molecular diffusion is well established (Desaulniers et al., 1981, Hendry and Wassenaar, 1999, Hendry et al., 2000). To date, transport of typically non-conservative (Na+, K+, Ca2+, Mg2+) cations has been satisfactorily described without considering mineral-water reactions, despite long pore-water residence times. Hendry and Wassenaar, 2000, Hendry and Wassenaar, 2004 demonstrate that depth trends in Na+ and Mg2+ observed between the shallow saline oxidized zone and background concentrations at 20 m depth in two clay aquitards can be approximated to diffusive mixing. The apparent lack of cation exchange within these clay matrices was surprising because reactive smectitic clay minerals constitute about 50–60% of the clay minerals at their study sites and cation exchange processes are identified as a major control on the distribution of pore-water chemistry within clay aquitards (Hendry et al., 1986), delta aquitards (Manzano et al., 1993), and in a compacted bentonite barrier (Wersin, 2003). Exchange processes can result in chromatographic separation of dissolved cations, as reported in freshening groundwater in sandy aquifers with relatively low cation exchange capacities (CEC) (e.g., Appelo and Postma, 1996, Valocchi et al., 1981).

The objectives of this study were (1) to determine the cation exchange capacity of a clay aquitard, and the likely composition of exchangeable cations, and (2) to quantify the role of ion exchange in controlling solute migration through the aquitard over a long time period (>2000 years). These objectives were met by (1) combining CEC analysis, batch techniques and PHREEQC reaction path modeling to obtain realistic cation exchange properties for the clay aquitard, (2) characterizing changes in cation exchange properties in the aquitard with depth, pore-water solute composition and ionic strength, and (3) quantifying the impacts of cation exchange processes on the long-term migration of solutes in the aquitard, by applying the PHREEQC 1D reactive solute transport code.

Section snippets

Field site

This study was conducted on core and pore-water samples collected from an 80 m thick, laterally extensive, plastic, clay-rich (39% sand, 26% silt, and 35% clay wt) till research site (hereafter called the King site). The King site is located 140 km south of Saskatoon, Saskatchewan, Canada (51.05°N latitude, 106.5°W longitude). This site has been the focus of many studies which have described the physical characteristics of aquitard material (Shaw, 1998, Shaw and Hendry, 1998), solute transport

Collection of core and pore-water samples

Core samples (76 mm dia. × 1.52 m long) were collected from 0.91–2.21 m, 6.08–7.60 m, 9.22–10.74 m and 15.15–16.34 m BG in August 2001 using Shelby tubes. After extraction from the Shelby tubes, cores were double coated in paraffin wax and stored at 4 °C in a humidity-controlled environment until testing.

Ten piezometers (termed the BJ series piezometers) were installed in August 2001. Piezometers were constructed of 50 mm ID schedule 40 PVC pipe attached to a 0.2 m long × 0.05 m ID plastic wound well screen

Pore-water chemistry

Pore-water chemistry from individual BJ series piezometers in the aquitard remained stable over the two year sampling period (n = 4; see Table 1 and Fig. 1 for representative values for June 2002). Depth profiles for Na+ and Mg2+ were similar; concentrations were greatest at the top of the aquitard and gradually decreased with depth to uniform background values at >15 m. Depth profiles for K+ and Sr2+ were similar with variable concentrations near the top and a gradual decrease to uniform

Summary and conclusions

Field and laboratory measurements used in conjunction with PHREEQC modeling were used to describe, for the first time, the effects of chromatographic separation due to salinization within a thick, clay-rich aquitard. Although cation exchange processes are active in the aquitard, the extent of chromatographic separation of aqueous Mg2+ and Na+ profiles developed over a period of a few thousand years is subtle; cation exchange retards the migration of Mg2+ about two meters (at 10 m BG) relative to

Acknowledgements

Funding for this work was provided by the Saskatchewan Potash Producers Association and the National Science and Engineering Research Council of Canada. Technical assistance of B. Boldt-Leppin, E. Defiendorf, R. Kirkland, A. Lieu, J. Muise, and M. Pitz is gratefully acknowledged. CEC analysis was facilitated by B. Goetz. M. Andersen critically reviewed a provisional draft.

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