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

Journal of Hydrology (2007) 332, 110– 122

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Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard W.A. Timms *, M.J. Hendry Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada, S7N 5E2 Received 27 July 2005; received in revised form 11 June 2006; accepted 27 June 2006

KEYWORDS Ion exchange; Aquitard; Diffusion; Reactive solute transport; Groundwater

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. ª 2006 Elsevier B.V. All rights reserved.

* Corresponding author. Present address: Water Research Laboratory, School of Civil and Environmental Engineering, University of New South Wales, Manly Vale, NSW 2093, Australia. Tel.: +61 2 9949 4488; fax: +61 2 9949 4188. E-mail address: [email protected] (W.A. Timms).

Introduction Understanding the mode of transport and geochemical reactions on the long-term development of solute profiles in low

0022-1694/$ - see front matter ª 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.06.025

Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard hydraulic conductivity clay-rich aquitards is important to protect underlying groundwater resources from anthropogenic contaminants. The transport of conservative tracers in aquitards (e.g., d18O, dD, 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, 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 porewater 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.

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 (51.05N latitude, 106.5W 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 processes with the aquitard (Hendry and Wassenaar, 1999; Hendry and Wassenaar, 2000; Hendry et al., 2000; Hendry et al., 2003; Hendry and Wassenaar, 2004). Texture and index properties of the till are uniform with depth; the natural water content is 15.7%, dry and bulk densities are 1870 and 2170 kg/m3, specific gravity is 2700 kg/m3, and porosity is 0.31 (Shaw and Hendry, 1998). The mineralogy of the clay-size fraction of the till (n = 5) consists of 50–60% smectite, 5–15% illite and trace quantities of kaolinite, feldspar, quartz, vermiculite and chlorite (Hendry and Wassenaar, 2000). The till contains 0.56–

111

1.2 wt% total inorganic carbon (TIC; n = 9, 0.8–32.8 m depth; Shaw, 1998) and an average of 0.6 wt% organic carbon (n = 31; Hendry et al., 2003). TIC is a minimum of 0.56 wt% at 2–3 m and otherwise 1% throughout the thick clay till. Microprobe analysis shows carbonate to be dominated by calcite and dolomite (slightly non-stoichiometric with respect to Ca2+ and Mg2+; Hendry and Wassenaar, 2000). If calcite and dolomite are assumed to each account for half the measured TIC, then the calcite content of the clay till is 2.3–5 wt%. The hydrogeology and pore-water chemistry of the King site were characterized by Shaw and Hendry (1998), Hendry and Wassenaar (2000) and Hendry et al. (2000). The till is divided into two hydrogeologic zones: the upper fractured and oxidized zone (0 to 3–4 m BG) and the underlying nonfractured and nonoxidized till (3–4 to 77–76 m BG), hereafter termed the aquitard. Within the nonoixidized till, Eh measurements by a HYDROLAB instrument at screen depth were 29 ± 29 mV (SHE) between 8.5 and 40 m depth (June 2002, unpublished data) indicating stable redox conditions due to buffering. The laterally extensive nature of the till suggests flow can be represented by a 1D vertical flux. The depth to the water table ranges from near ground surface in spring to near the top of the nonfractured zone during winter. Analysis of slug tests in the nonfractured till and falling and constant head laboratory tests and oedometer tests yields geometric mean hydraulic conductivity values between 5.4 · 1011 and 4.3 · 1012 m/s. A downward pore-water velocity of 0.5–0.8 m per 10,000 years was calculated using measured hydraulic conductivity values, gradients, and porosities (Shaw and Hendry, 1998) and verified using stable isotopes of water (Hendry and Wassenaar, 1999). Naturally occurring solutes, dominated by Na+ and SO2 4 , are present at concentrations up to 70,000 mg/L total dissolved solids in the oxidized zone, and gradually decrease to background concentrations of 3200 mg/L at >15 m BG (Hendry and Wassenaar, 2000). The origin of Na–SO4 dominated saline groundwater within the oxidized zone of the clay till is attributed to oxidation of reduced sulphur, causing dissolution of calcite and subsequent release of Na+ from ion exchange processes due to increased Ca2+ (Hendry et al., 1986). Diffusive transport modeling by Hendry and Wassenaar (2000) suggests diffusion from the oxidized to the unoxidized zone occurred over the past 2–3 ka for  Na+, Mg2+, K+, SO2 4 and alkalinity. Concentrations of Cl remained low throughout the profile.

Materials and methods 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

112 0.2 m long · 0.05 m ID plastic wound well screen and installed in individual boreholes. One meter of silica sand was placed in the annular space around the screen intake zone using the method outlined by Wassenaar and Hendry (1999). The remainder of the annular space was filled with bentonite pellets and chips. Depths to the mid-point of sand packs of each piezometer was 2.63 (usually above the water table), 3.51, 4.53, 5.83, 6.64, 7.50, 10.64, 13.68, 16.58 and 19.70 m BG. BJ series piezometers were sampled for chemical analyses on four occasions between September 2001 and July 2003 (14 Sept., 2001, 16 Nov., 2001, 5 June, 2002 and 23 July, 2003). Water chemistry data sets (total of 13 sets of data) were also obtained from an additional four piezometers (termed the B-series piezometers) installed in 1995 by Shaw and Hendry (1998). These piezometers were completed at 14.9, 21.0, 28.9 and 36.1 m BG. All samples were collected with pre-cleaned and sterilized PVC bailers, using the procedure outlined by Hendry and Wassenaar (2000). A HYDROLAB was used to measure pH and temperature in situ, at screen depth. The remaining samples were field-filtered using 0.45 lm cellulose acetate membrane filters. After filtering, alkalinity was immediately determined by end point titration using a HACH field alkalinity kit. Subsamples of the filtered waters were acidified to pH 2 with ultrapure nitric acid for analysis of cations and metals by ion chromatography (Na+, K+, Mg2+, Ca2+). Non-acidified splits were retained for Cl and SO2 analysis by ion 4 chromatography. Sr2+ analyses were measured using ICP-MS techniques on acidified pore-water samples collected from 14 B-series piezometers completed between 2.3 and 61.0 m BG on eight occasions. As was the case with all other pore-water samples, these samples were collected with pre-cleaned and sterilized PVC bailers, using the procedure outlined by Hendry and Wassenaar (2000).

Cation exchange capacity Exchangeable cations were measured on core samples in the laboratory using a modified CEC method at a solid to liquid (S:L) ratio of 1:200 (Carter, 1993). An apparent CEC (CECa) was calculated from exchangeable cation concentrations. In addition, a batch technique at a S:L ratio of 1 was applied to determine the distribution of exchangeable cations at varying solute concentrations. Exchange properties of clay till were determined at depths similar to piezometer data used to correct for porewater composition (e.g. 3.5 m piezometer, 3.8 m depth till sample). The CECa and exchangeable cation composition of 1 g clay samples were determined at in situ pH for 11 samples from between 3.8 and 21.8 m BG using the method of Carter (1993) modified by using Buchner funnels and unbuffered 0.5 M BaCl2 extracting solution. In brief, this method involved adding approximately 50 mL of unbuffered 0.5 M BaCl2 to 1 g of clay and allowing it to stand overnight. The slurry was then transferred to a Buchner funnel connected to vacuum, and washed with additional 0.5 M BaCl2 to collect a total leachate volume of 220 mL. Leachates were analyzed using standard atomic absorption methods. Exchangeable cations were determined from Na+, Ca2+,

W.A. Timms, M.J. Hendry Mg2+ and K+ in the BaCl2 solution. CECa was calculated as the sum of exchangeable cations minus pore-water cations. In this calculation, pore-water concentrations for the BJ series piezometers from the June 2002 sampling period were converted to meq/100 g pore-water using (Appelo and Postma, 1996): Naþ ðmeq=100 gÞ ¼ Naþ ðmeq=LÞ=10=qb  h

ð1Þ

where qb is bulk density and h is porosity. For example, for our clay till (qb of 2.19 g/cm3 and porosity h, of 0.29; Shaw and Hendry, 1998), 75.5 meq/L Na+ (1736.5 mg/L) in solution was equivalent to 1 meq/100 g of exchangeable Na+ due to the high S:L ratio (5.033:1). Batch tests determined the distribution of exchange cations (Kd, used to derive KNa/Ca) at varying solute concentrations in the clay till. The core obtained from 12.95–13.13 m BG was maintained at in situ moisture content during batch test preparation. A series of duplicate 20 g samples from the core, dissected to <2 mm particle diameter, was reacted with 20 mL of NaCl for 60 minutes at a groundwater temperature of 5 C. Tests were conducted using a range in NaCl concentrations of 0.1, 0.2, 0.5, 2.0 and 5.0 M. All tests were conducted in duplicate (n = 10). At the end of the reaction period, the aqueous phases were separated by centrifugation at 3000 revolutions per minute and analyzed for Na+, K+, Mg2+, Ca2+, Cl, and SO2 using ion chromatography 4 and alkalinity by titration. Four samples were analyzed by neutron activation analysis (NAA) at the University of Alberta. Because the ion chromatograph methods proved inaccurate at very high concentrations, our discussion of the batch test data is limited to the NAA data. Cation exchange composition and capacity were determined on solids after batch testing, using the modified method of Carter (1993) described above.

Geochemical modeling of cation exchange analysis and pore-water chemistry data PHREEQC code Version 2.7 (Parkhurst and Appelo, 1999) was used for geochemical modeling of cation exchange analysis, and subsequently for 1D reactive transport modeling. Ion exchange was described using the Gaines Thomas expression, which uses equivalent fractions for solid-phase concentrations and applies to homovalent and hetereovalent exchange (Bjerg and Christensen, 1993). The exchange coefficient KNa/Ca for an exchange site with a unit charge of X and equivalent fraction b was defined as K Na=Ca ¼ ½NaX½Ca2þ 0:5 =½CaX0:5 ½Naþ  þ ¼ bNa ½Ca2þ 0:5 =b0:5 Ca ½Na 

ð2Þ

PHREEQC simulates exchange reactions using solute activity rather than solute concentration, thus accounting for apparent variability of KNa/Ca due to high ionic strength and solute complexation. The exchange coefficients were recalculated in the form of exchange half equations as given in the PHREEQC database. PHREEQC reaction path models were developed to simulate cation exchange analysis of samples from 3.8 m, 13.5 m and 21.8 m depth and to quantify the impact of calcite dissolution and barium salt precipitation on measured exchange properties. In the first step of the PHREEQC model, 1 g of clay

Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard exchanger (moles of exchanger X calculated as a fraction of CEC in meq/100 g) was equilibrated with corresponding pore-water chemistry (Solution 1) to determine moles of CaX2, MgX2, NaX and KX. Secondly, a 220 mL solution of 0.5 M BaCl2 (Solution 2) was mixed with 0.13 mL (density 2.1 g/cm3, porosity 0.29) of pore-water (Solution 1). This mixture was reacted with moles of exchangers determined in step one and between zero and 4.2 wt% (0.42 mM per g clay assuming TIC was 50% calcite) calcite at atmospheric pCO2 of 3.5. This PHREEQC model of the cation exchange analysis technique was carried out on the bench, in equilibrium with atmospheric CO2, and so differed from the PHREEQC models for in situ solute transport at the King site. Exchange coefficients were varied to obtain a bestfit with observed Ca2+, Na+, Mg2+, and K+ leachate concentrations. The PHREEQC model developed for cation exchange analysis also enabled determination of the efficiency of the displacement of cations by 0.5 M BaCl2, along with the possible interference of barium salt precipitation (BaCO3 and BaSO4) on CECa. 1D reactive geochemical transport models were developed to simulate the cation profiles at the study site. The PHREEQC code explicitly calculates mineral precipitation, dissolution and ion exchange, assuming chemical equilibrium between each advection and dispersion time step. The structure of the PHREEQC v2.7 code necessitated constant CEC and exchange coefficients during transport for all model cells. A total of 260 cells of 0.1 m length were modeled to represent the zone from 4 to 30 m depth at the study site. Other model parameters were dispersivity of 0.005 m, and 2 cell shifts with a time step of 3.15 · 1010 s, to achieve a total model time of 2000 years and a flow velocity of 1 m per 10,000 years. PHREEQC models were insensitive to dispersivity due to diffusion dominated transport behavior.

Results and discussion 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 background values. By contrast, Ca2+ concentrations at the top of the unoxidized zone were approximately equivalent to background concentrations (12.7 mM), but with a zone of lower concentrations (9.0 mM) at 7.5 m BG. Assuming that the migration of solutes with higher exchange affinity would be relatively restricted, we estimated the relative exchange affinity as the depth at which pore-water concentrations were within 25% of background concentrations. For example, background Na+ concentrations occurred between piezometers at 19.7 and 21.0 m BG, whereas background Mg2+ concentration occurred between piezometers at 14.9 and 16.6 m BG (Table

113

1). These subtle differences suggest a chromatographic separation of Na+ and Mg2+ of 3–6 m. In a similar manner, we determined the Sr2+ solute front was delayed relative to Na+ by 8–9.8 m (Fig. 1); background Sr2+ concentration occurred between piezometers at 11.2 and 11.7 m BG. We were unable to identify chromatographic effects from the solute profiles of K+ and Ca2+. The observed chromatographic separations occurred over a transit time of 2– 3 ka (Hendry and Wassenaar, 2000) and transit distance of up to 16.5 m, assuming a starting depth of 4.5 m at the top of the unoxidized zone. Ionic strength, calculated using the PHREEQC code, ranged from 0.06 molality at 20 m BG to 1.32 molality at 3.5 m BG (Table 1). The dominance of Na+ over other cations increased towards the ground surface (decreasing Ca/ Na; Table 1).

Cation exchange capacity Cation exchange properties of the clay till were determined using CEC and batch analysis techniques and geochemical modeling. Apparent cation exchange values (CECa) (n = 7), varied from 26.9 to 56.0 meq/100 g (Table 2). CECa was dominated throughout the profile by exchangeable calcium and increased towards the ground surface, with the exception of a sample from 3.8 m BG. Duplicate analyses at 13.8 m BG indicated good repeatability for the CEC analysis. PHREEQC models of CECa data indicated calcite dissolution could cause CECa to be overestimated, and enabled a range of modeled values (CECm) to be determined. A series of non-unique PHREEQC models were found to fit leachate cation concentrations from CEC analysis (Table 3). In general, a lower CECm was associated with higher exchange coefficients and increased calcite dissolution (Fig. 2). PHREEQC models for leachate cation concentrations indicated partial calcite dissolution (0.02–0.11 mM of calcite per gram of sample, representing 8–37% of available calcite) in both fresh (21.0 m) and saline (3.5 m BG) solutions. Similar to other studies, it was found that the relative values of exchange coefficients, rather than absolute values of exchange coefficients were of critical importance. For example at 3.8 m depth, there were several possible models that were consistent with experimental data and CECm of 10–25 meq/100 g. PHREEQC modeling further showed precipitation of BaCO3 and BaSO4 (SI > 0) did not appear to impact CECa results (data not presented). Furthermore, PHREEQC modeling demonstrated the efficiency of this CEC technique because 99.9% of exchange sites were occupied by Ba2+ during equilibration, effectively displacing all other cations. A CECm of 20 meq/100 g was considered representative for this till unit. Physical index properties detailed in ‘‘Field site’’, within the clay-rich till unit are known to be uniform indicating that surface area and charge density were likely to be relatively constant. Independent batch techniques support relatively low exchange coefficients and high CEC values. Also, the derived CECm of 20 meq/100 g was consistent with an estimate of 26 meq/100 g determined from clay and organic carbon content using (Appelo and Postma, 1996): CEC ðmeq=100 gÞ ¼ 0:7  ð%clayÞ þ 3:5  ð%carbonÞ

ð3Þ

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W.A. Timms, M.J. Hendry

Table 1

Representative solute chemistry of pore-water sampled from BJ (n = 9) and B (n = 5) series piezometers (June 2002)

Depth (m)

pH

EC (ls/cm)

Ca (mg/l)

Mg (mg/l)

K (mg/l)

Na (mg/l)

Cl (mg/l)

SO4 (mg/l)

HCO3 (mg/l)

Ca/Na (mg/l)

Mg/Na (mg/l)

Ia

CBEa

3.51 4.53 5.83 6.64 7.5 10.6 13.7 14.9b 16.6 19.7 21.0b 28.9b 36.1b 42.9b

7.5 7.5 7.4 7.2 7.2 6.9 6.9 7.0 7.1 6.9 7.1 7.1 7.0 7.1

48,701 45,960 39,509 34,743 27,547 15,803 7726 5558 5190 3943 3644 3415 3378 3535

515.4 557.8 602.0 441.4 360.1 372.4 445.6 529.5 705.0 555.4 510.6 509.4 510.1 506.1

9375.4 7370.0 6182.7 4498.6 3347.0 1118.3 324.6 232.4 158.8 188.2 193.0 177.6 173.8 175.5

0.8 37.6 21.9 39.6 33.3 23.7 9.4 13.9 18.8 14.4 12.4 10.8 11.3 12.5

11,878.0 10,042.0 9373.5 7238.7 5145 2956.4 1693.6 897.6 1376.9 344.9 276.4 207.4 200.8 239.5

54.4 59.8 70.0 78.0 92.2 135.6 141.7 114.7 128.3 92.9 120.5 47.9 18.7 60.9

67,287.5 56,812.3 43,596.1 36,260.3 24,610.4 11,020.7 4183.6 3044.2 2581.5 1938.7 1853.3 1869.9 2137.0 1997.2

897.7 – – – 1178.4 – 877.5 697.3 – 546.0 – 543.1 549.0 494.7

0.02 0.03 0.04 0.03 0.04 0.07 0.15 0.34 0.29 0.92 1.06 1.41 1.46 1.21

0.75 0.69 0.62 0.59 0.62 0.36 0.18 0.24 0.11 0.52 0.66 0.81 0.82 0.69

1.23 1.05 0.88 0.72 0.52 0.26 0.13 0.10 0.12 0.07 0.07 0.06 0.06 0.06

7.5 9.4 0.1 5.8 2.6 0.2 9.8 5.6 0.4 8.3 0.2 0.2 7.3 1.3

Molar ratios Ca/Na and Mg/Na are provided for calculation of exchange coefficient values. Piezometer depth is mid-screen. a Ionic strength I, and charge balance error CBE was calculated by PHREEQC. b B-series piezometers.

0

K

Na

Mg

Sr

Ca

D

E

Depth (m)

5

10

15

20

25

A

30

0

100 200 300

mM

B 0

200

C 400

0.2

mM

0.6

1

mM

1.4

0

0.05

0.1

0.15

-10

mM

0

10

20

mM

Figure 1 Measured solute, pH and exchangeable cations profiles at the field site compared with best-fit PHREEQC simulation (transport time = 2000 years, velocity = 1 m/10,000 years, CEC = 1.5 eq/L or 20 meq/100 g, De = 2 · 1010 m2/s, KNa/Ca 0.01, KNa/Mg 0.1, KNa/K 0.6, KNa/Sr 0.08) for (a) Mg2+, (b) Na+, (c) K+, (d) Sr2+ and (e) Ca2+. Solid lines represent PHREEQC simulations (diffusion and exchange) and dashed lines mixing with no ion exchange (diffusion only). Vertical error bars for analyses (two year sampling period, n = 4) represent the length of sand pack of each piezometer (1.0 m length).

Furthermore, the derived CECm was typical of other aquitards (Manzano et al., 1993; Yang and Barbour, 1992). The CECm values for the aquitard cores were at least one order of magnitude greater than those of groundwater systems in which the controls of ion exchange processes on pore-water chemistry are significant (Valocchi et al., 1981; Dance and Reardon, 1983; Bjerg and Christensen, 1993; DeSimone et al., 1997).

Cation exchange coefficients Exchange coefficients corrected for calcite dissolution were determined by PHREEQC modeling (‘‘Cation exchange capacity’’ under ‘‘Results and discussion’’). Modeled exchange coefficients based on CECm were lower than exchange coefficients based on CECa (Table 3). This finding

Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard 0.5

NaX

115

CaX2

Equivalent fraction

0.4 0.3 0.2 0.1

Calcite dissolved (mM)

0.12

0.1

0.08

0.5

KNa/Ca

0.4

0.3

0.2

0.1

0 5

10

15

20

CEC (meq/100g)

Figure 2 Non-unique cation exchange parameters that fit observed leachate concentrations (3.8 m depth). The relationship between CECm, KNa/Ca exchange coefficients, calcite dissolution and equivalent fractions of exchangeable Na+ and Ca2+ is shown.

Table 2 Depth (m)

2.2 3.8 4.6 5.8 6.1 7.6 9.8 13.5 13.5 15.8 21.8

Apparent cation exchange capacity (CECa) determined by standard CEC analysis of core samples from the study site Pore water (meq/100 g)

Exchangeable = extract  pore water (meq/100 g)

Extract (meq/100 g)

Ca

Mg

K

Na

Ca

Mg

K

Na

Ca

Mg

K

Na

CECa

– 0.36 0.37 – 0.30 – 0.24 0.30 0.30 – 0.37

– 10.72 8.14 – 4.97 – 1.24 0.36 0.36 – 0.21

– 0.001 0.013 – 0.014 – 0.008 0.003 0.003 – 0.005

– 6.9 5.9 – 4.2 – 1.7 1.0 1.0 – 0.2

36.9 26.1 38.4 26.8 35.5 19.9 26.0 18.7 19.2 19.3 22.5

13.3 17.3 17.3 11.0 13.7 16.6 7.6 5.2 5.1 5.1 5.1

1.1 1.7 2.7 1.5 1.1 1.3 1.6 1.6 1.4 1.6 1.6

9.0 11.3 12.6 6.7 10.5 11.5 4.5 3.1 3.1 2.1 1.3

– 25.8 38.0 – 35.2 – 25.7 18.4 18.9 – 22.1

– 6.5 8.9 – 8.6 – 6.4 4.8 4.8 – 4.9

– 1.7 2.7 – 1.2 – 1.6 1.6 1.4 – 1.6

– 4.1 6.5 – 6.2 – 2.7 2.1 2.1 – 1.1

– 38.1 56.0 – 51.1 – 36.4 26.9 27.1 29.7

All data are in meq/100 g. Conversion of pore-water concentrations from mg/L to meq/100 g assumes porosity of 0.29 and bulk density of 2.16 g/cm3.

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W.A. Timms, M.J. Hendry

Table 3 Summary of apparent and modeled cation exchange capacities (CECa, CECm), cation exchange coefficients and solid:liquid (S:L) ratios, for saline (3.8 m piezometer), transition (13.5 m piezometer) and background (21.8 m piezometer) solute concentrations at the study site Depth (m) 3.8

13.5

21.8

Method

S:L ratio

CECa Batchc

1:200 1:1

CECm

1:200

CECa Batchc

1:200 1:1

CECm

1:200

CECm

1:200

CEC (meq/100 g)

KNa/Ca

38.10 25 20 17 25 20 17 15 12 10

0.025 0.007 0.009 0.011 0.03 0.05 0.06 0.08 0.25 0.4

26.9 25 20 17 25 20 17 15

0.1a 0.06 0.08 0.09 0.09 0.15 0.18 0.21

29.5 25 20

0.35 0.35 0.55

KNa/Mg

a

0.27 – – – 0.25 0.3 0.35 0.4 0.4 0.45

a

Calcite dissolvedb (mM)

KNa/K a

0.005 – – – 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

– – – – 0 0.08 0.085 0.10 0.11 0.11

0.22a – – – 0.2 0.27 0.3 0.31

0.08a – – – 0.08 0.08 0.08 0.08

– – – – 0 0.025 0.029 0.045

0.6 0.6 0.7

0.02 0.02 0.02

0 0.02 0.05

Non-unique PHREEQC models which match observed CEC results are presented over a range of CEC values with variable exchange coefficients and calcite dissolution. a Exchange coefficients calculated using solute concentrations rather than solute activities. b Calculated by PHREEQC models matching observed CEC leachate concentrations. c Calculated from Kd value (Table 4, Fig. 3), assuming CEC of 38.1 meq/100 g, Ca2+ 12.8 mM, bCa 0.68 (3.8 m depth); and Kd value for 3720–9100 mg/L assuming a CEC of 27 meq/100 g, Ca2+ 11.1 mM and bCa 0.68 (13.5 m depth).

suggests cation exchange measurement of small laboratory samples under static flow conditions and at differing S:L ratios may not reflect in situ conditions. In our study, calcite dissolution appears to be enhanced by dilute solutions of low S:L ratios. The very high affinity of clay till for Ca2+ resulted in the majority of exchange sites occupied by Ca2+ despite very high Na+ pore-water concentrations at the 3.8 m BG. Nevertheless, increasing Na+ pore-water concentrations proximal to the ground surface resulted in an increase of bNa from 0.04 to 0.18 at 21.8 to 3.8 m BG. The exchange coefficient KNa/Ca varied from 0.35 to 0.05 at 21.8 and 3.8 m BG,

Table 4

respectively (Table 3). The observed two orders of magnitude variation is attributed to decreasing Ca/Na and increasing ionic strength towards the shallow oxidized zone. We also determined exchange coefficients at 3.8 and 13.5 m BG using the batch technique. This technique was attractive because it minimized the effects of calcite dissolution by using a S:L ratio of 1 and used Na+ rather than Ca2+ data for subsequent calculations. Batch test results indicate a non-linear type isotherm for loss of Na+ from solution by cation exchange (Table 4; Fig. 3). The distribution coefficient Kd decreased with increasing CEC (Table 3) and increasing solution concentration (Fig. 4). No significant ex-

Results of NaCl batch test experiment and calculated Na exchange and Kd values

NaCl (M)

Initial Na (mg/L)

Final Na (mg/L)

Na deficit (mg/L)

Volume (mL)

Solid (g)

Na deficit (mg)

Na to exchange (mg/kg)

0.2 0.5 2.0 5.0

4300 10,250 35,900 97,280

3720 ± 30 9100 ± 50 34,400 ± 500 97,300 ± 800

577 ± 30 1142 ± 50 1522 ± 500 BDL

20.2 20.2 20.6 20.1

20.7 20.7 20.8 21.0

11.7 23.1 31.3 0.0

560 1110 1500 BDL

Kd (mL/g)

0.102 0.016 BDL

The batch experiment was conducted with a solid:liquid ratio of 1.0. Uncertainties are ±1 standard deviation; BDL is below detection limit.

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

117

Na deficit

Na deficit (mM)

80

40

0

-40

0

1000

2000

3000

4000

5000

NaCl (mM)

Na to exchange sites (mg/kg)

1600

1200

0.016 mL/g

800

0.102 mL/g

400

0 0

10000

20000

30000

40000

Initial Na (mg/L)

Figure 3 Exchangeable sodium determined by batch testing: (a) Na+ deficit analyzed by neutron activation analysis and (b) distribution coefficients for exchangeable Na+.

change occurred at very high Na+ concentration (5.0 M). Exchange coefficients were derived from batch test Kd values using (Appelo and Postma, 1996): K Na=Ca ¼ K d  ½Ca2þ 0:5  100=CEC

ð4Þ 2+

where CEC is expressed as meq/100 g and Ca is in mM. For example, a KNa/Ca of 0.005 was calculated from a Kd of 0.102 at a depth of 3.8 m, assuming a CEC of 38.1 meq/100 g, Ca2+ of 12.8 mM and bCa of 0.68 KNa/Ca calculated from batch tests (Tables 3 and 4) suggest the exchange sites have a very high affinity for Ca2+ (low affinity for Na+), and support the selection of a relatively high CEC. The higher affinity of exchange sites for Ca2+ was expected for non-saline clay (prior to permeation of dilute brine), and is an intrinsic property of smectite clay (Griffioen and Appelo, 1993). A typical lyotropic series indicates divalent and ions with relatively small hydrated radius occupy exchange sites in

preference to Na+ in most natural systems, in the order K+ > Sr2+ > Ca2+ > Mg2+ > Na+ (Rowe et al., 1995). By comparison, the approximate lyotropic series for the King site determined from modeled exchange coefficients and the depth of background solute concentrations (Tables 1 and 3) was Ca2+ > Sr2+ > Mg2+ > Na+. (The lyotropic position of K+ could not be determined from the solute depth profile.) These differences between the site-specific series and the typical lyotropic series are attributed to the high affinity of exchange sites for Ca2+.

Simulated field-scale solute profiles Depth specific data for CEC, exchange site composition and exchange coefficients (Table 3), provided an opportunity to quantify the impact of cation exchange on the long-term impact of cation exchange properties on the distribution

118

W.A. Timms, M.J. Hendry

Ca/Na

1

0.1

0.01

Ionic strength

10

0

0.05

0.1

0

0.05

0.1

0.15

0.2

0.25

0.3

0.15

0.2

0.25

0.3

1

0.1

0.01

KNa/Ca Figure 4

Variation of exchange coefficient KNa/Ca with pore-water solute Ca/Na ratio and ionic strength.

of Ca2+, Na+, and Mg2+ as well as K+ and Sr2+ at the study site. Consequently, the pore-water solute distribution between 4 and 30 m BG was simulated using the PHREEQC

Table 5 profile

Solute input data for PHREEQC simulations of field

Parameter

Initial pore-water chemistry

Influent chemistry

Sample depth (m) Sample date Temp (C) pHc Na Ca Mg K Cl Alkalinity SO4 Sr

28.9 June-02 5.0 7.16 9.0 12.7 7.3 0.276 1.35 8.9 19.46 0.022*

3.5 June-02 5.0 7.54 516.7 9.8a 385.7 1.4b 1.53 14.72 700.4 0.16b

All concentrations are given in mmol/L unless indicated (alkalinity is presented as HCO3). a Average value from 1995 to 1998, n = 8 (Hendry and Wassenaar, 2000). b Influent solute concentration required for non-variable KNa/Ca PHREEQC simulation. c Measured at screen depth by Hydrolab (Wassenaar and Hendry, 1999).

reactive solute transport code and the pore-water chemistry data in Table 1 (Sr2+ data for the B series piezometers not presented). In the modeling exercise, we assumed a CEC of 20 meq/ 100 g (see Section 4.2) and a diffusion coefficient, De, of 2 · 1010 m2/s. This De value was determined for the conservative tracers dD and Cl by Hendry and Wassenaar (1999) and applied by Hendry and Wassenaar (2000) for non-reactive solute transport modeling. We defined the initial pore-water composition in the profile as the chemistry observed at 28.9 m BG in June 2002 (Table 5), in equilibrium with the initial cation exchange site composition. The influent solution of saline pore water diffusing downwards through the profile was assigned the composition of pore water obtained at 3.5 m BG in June 2002 (Table 5). A pCO2 of 2.4 was calculated using PHREEQC for the influent pore water at the field site, consistent with previously reported values at this depth (Hendry and Wassenaar, 2000). The PHREEQC geochemical equilibrium model was configured for saturation indicates of zero where possible for calcite, gypsum, dolomite and anhydrite (SIcalc 0.0, SIgyp 0.0, SIdol 0.0 and SIanh 0.0). The resulting best-fit simulations of concentration-depth profiles, although non unique, closely approximate the measured values with minor exceptions at shallow depths (Fig. 1). The simulated pH for the best-fit scenario decreased from 7.7 to 6.8 at 30 m BG, similar to observed field values (Table 1, Fig. 5) and the simulated pCO2 (1.3 to 2.2) and saturation indices for dolomite (0 to 1.8) were similar to those observed by Hendry and Wassenaar

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

file. Exchange sites remained dominated by Ca2+, in broad agreement with measured cation exchange composition on core samples. The proportion of sites at 3.5 m BG occupied by Ca2+ determined by PHREEQC simulation (b = 0.54) was slightly greater than CECm measurement (b = 0.45). The simulated values of KNa/Ca and KNa/Mg (0.01 and 0.1, respectively) were similar to values determined from CECm (0.005 and 0.1, respectively), but lower than those determined from CECa (0.25 and 0.27, respectively) at 3.8 m BG. However, the value of KNa/K simulated by PHREEQC (0.6) was much higher than measured values (0.005 at 3.8 m BG). These differences suggest that cation exchange analysis under static flow conditions and at differing S:L ratios may not provide realistic exchange parameters for in situ conditions. Although exchange coefficients are known to vary over a wide variety of aqueous compositions (Griffioen and Appelo, 1993), our simulated values are lower than those compiled by Appelo and Postma (1996) (KNa/Mg 0.4–0.6 and KNa/K 0.15–0.25, and KNa/Sr = 0.3– 0.6), with the exception of KNa/K. A KNa/Ca of 0.05 was cited by Appelo and Postma (1996) and a value of 0.02 was reported by Reardon et al. (1983) for calcareous sand. Curtin et al. (1998) noted that because the factors influencing Ca exchange have not been adequately defined, models commonly assume a Ca–Mg selectivity of one. Variations in cation selectivity are commonly noted over an order of magnitude and can differ from the generally accepted lyotropic series (Baeyens and Bradbury, 2004; Endo et al., 2002). The findings of this study are therefore not inconsistent with the available literature on cation selectivity in clay sediments. We attribute chromatographic separation in this clay aquitard to high CEC relative to groundwater velocity and the thick aquitard sequence (Appelo et al., 1990). However, the development of a chromatographic pattern also depends on the role of diffusion in blurring the separation of cations under very low flow conditions (Appelo and Postma, 1996). Observations and simulations of diffusive saline intrusion by Appelo and Willemsen (1987) demonstrate the impact of cation exchange is muted by diffusion due to no

(2000). These simulated SI were consistent with field observations (Fig. 5) with the exception of calcite (field SIcalc of 0.3). The sensitivity of modeled solute profiles to variable CEC values (Fig. 6) and variable KNa/Ca values (Fig. 7) for the base case presented in Fig. 1 suggest that the ‘‘best-fit’’ model is unlikely to be unique. Relative values of exchange coefficients were found to be more important than absolute values, however, available evidence indicates very low exchange coefficients associated with saline porewater. Exchange coefficients (based on solute activity) varied over at least an order of magnitude whereas CEC values were within 17–25 meq/100 g throughout the field profile. These findings are consistent with Bolt et al. (1982) who reported that CEC was relatively constant over a large salinity range. Poor fits were observed between simulated and observed concentration-depth profiles for Ca2+ at depths <8 m BG and, to a lesser extent, for K+ and Sr2+ at depths <6 m BG. These differences for Ca2+, Sr2+ and K+ were attributed to variable exchange coefficients at very low Ca/Na ratios and high ionic strengths towards the top of the profile. The 1D geochemical models do not describe the development of solute profiles at shallow depth and do not take into account salinization processes due to pyrite oxidation. We noted subtle but distinct differences in the simulated Mg2+ and Na+ profiles based on diffusion including ion exchange (KNa/Mg 0.1) and diffusion excluding ion exchange (Fig. 1). A delay of Mg2+ and Na+ for diffusion plus exchange relative to diffusion without ion exchange reflected the effects of ion exchange on the profile development. At the field site, the separation of the Mg2+ and Na+ solute fronts was attributed to ion exchange was in the order of two meters at a depth of about 10 m. In the case of K+ and Sr2+, adequate simulations of the measured profiles required the incorporation of ion exchange reactions (KNa/K 0.6, KNa/Sr 0.08; Fig. 1). However, the exchange coefficient for K+ must be regarded as estimates given analytical error at very low concentrations. The composition of simulated exchange sites varied slightly due to exchange reactions near the top of the pro-

pH

0

119

SIgypsum

SIdolomite

SIcalcite

5

Depth (m)

10

15

20

25

A

30

6

Figure 5

7

8

C

B 9

0

1

2

3

0 1 2 3 4 5 6

D 0

0.5

1

1.5

2

Modeled pH and SIcalcite, SIdolomite and SIgypsum for field profiles (see Fig. 1 for model parameters).

120

W.A. Timms, M.J. Hendry 0

Ca

Na

Mg

Depth (m)

5

10

15 CEC (meq/100g) 25 20.5 17 10 2

20

25

A

30 0

100 200 mM

300

B 0

200 400 mM

C 0

4

8

12 mM

16

20

Figure 6 Sensitivity of modeled Ca2+, Na+ and Mg2+ profiles at field site to CEC variability (for KNa/Ca 0.01, KNa/Mg 0.1). The lines show PHREEQC model profiles with constant exchange coefficient values and variable CEC of 2–25 meq/100 g.

0

Ca

Na

Mg

5 1 0.0 2 0.0 05 0 0.

Depth (m)

10

15

20

25

A

30

0

100

200 mM

300

C

B 0

200 400 mM

4

8

12 mM

16

20

Figure 7 Sensitivity of modeled Ca2+, Na+ and Mg2+ profiles at field site to KNa/Ca variability (for CEC 20.5 meq/100 g). The lines show PHREEQC model profiles with constant cation exchange capacity and variable KNa/Ca from 0.005 to 0.02.

increased aqueous cation concentration, as occurs during advective flow. The chromatographic separation of Mg2+ and Na+ at the study site was shown to be limited to two meters. Similar observations were made at another detailed research site 160 km north of the King site (Hendry and Wassenaar, 2004). In this case, a source of Ca2+ and a minor loss of Mg2+ at depth were evident in deviations between field observations and simulated diffusive mixing profiles. In

both of these two cases, definition of chromatographic separation in the field was shown to be difficult using conventional groundwater instrumentation. Accurate definition of this chromatographic separation would require the collection of more detailed concentration-depth profiles. In spite of the limited chromatographic separation observed and simulated, this study is the first to document chromatographic separation during salinization in an aquitard.

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

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 Na+. This subtle chromatographic separation was attributed to smoothing of the concentration profiles by diffusive transport and the apparent high affinity of exchange sites for Ca2+. Our data suggests that it is possible to approximate the migration of Mg2+ and Na+ in this aquitard by considering them as non-reactive solutes dominated by diffusive transport. The delayed downwards migration of Sr2+ and K+ by 8–9.8 m relative to Na+ was attributed to cation exchange. Our findings suggest that the distribution of species between the aqueous phase and exchange sites is not intuitive, being controlled by complex multi-component exchange, competition between solutes for exchange sites, and high exchange affinity for Ca2+. Further investigation of the microscopic structure of interlayer exchange sites for a variety of porewater compositions and ionic strengths is warranted to improve our understanding of apparent high Ca2+ affinity within these clay tills.

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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