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