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Published online 12 March 2007
Published in Soil Sci Soc Am J 71:314-321 (2007)
DOI: 10.2136/sssaj2006.0169
© 2007 Soil Science Society of America
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SOIL PHYSICS

Clay Mineralogy, Ionic Composition, and pH Effects on Hydraulic Properties of Depositional Seals

M. Ladoa,*, M. Ben-Hurb and I. Shainbergb

a Faculty of Sciences, Univ. da Coruña, A Zapateira s/n 15071, Spain
b Inst. of Soil, Water and Environ. Science, The Volcani Centre, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel

* Corresponding author (mlado{at}udc.es).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Formation of a depositional seal is common in soils subjected to rainfall or sprinkler or surface irrigation; however, studies on its hydraulic properties are scarce. In this study, the effects of clay mineralogy, exchangeable sodium percentage (ESP), electrolyte concentration, and pH on the hydraulic properties of depositional seals were investigated. A silt loam packed in columns was leached with 5 g L–1 suspensions of reference montmorillonite, illite, and kaolinite with various ESPs (0–100), electrolyte concentrations (<2–20 mmol L–1), and pH (7–11). Deposition of the clay particles on the soil surface formed seals, and their hydraulic resistance (Rc), which is the ratio between the thickness of the seal and its hydraulic conductivity, was measured. The Rc values of the seals developed by all clays with ESP 0 was low (<10.1 h). Conversely, at ESPs ≥10 and electrolyte concentrations ≤20 mmol L–1, the Rc values of illitic seals ranged from 0.3 to 9.5 h and that of montmorillonitic seals ranged from 0.6 to 443.7 h. In both clays, increasing the ESP of the clay increased the seal Rc. A decrease of the electrolyte concentration increased the Rc of the montmorillonitic seals but had no effect on the Rc of the illitic seal. The Rc of Ca- and Na-kaolinitic seals was low at pH <9.0. Increasing the pH to 11 increased the Rc of the Na-kaolinite seal due to greater clay edge negative charge and associated dispersion. Conversely, in the Ca-kaolinite seal, the increase in pH decreased Rc due to CaCO3 precipitation and clay cementation.

Abbreviations: ESP, exchangeable sodium percentage • SAR, sodium adsorption ratio


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seal formation at the soil surface decreases the infiltration capacity of the soil (Agassi et al., 1981; Morin et al., 1981; Shainberg and Singer, 1985). The seal exhibits greater density, higher strength, finer pores, and lower saturated hydraulic conductivity (Ks) than the underlying soil (McIntyre, 1958; Gal et al., 1984; Fox et al., 1998b; Assouline, 2004), all of which drastically reduce the infiltration rate (Morin and Benyamini, 1977; Assouline and Mualem, 1997; Assouline, 2004).

Depending on their formation mode, seals (or crusts) are classified as structural or depositional (Chen et al., 1980; Tarchitzky et al., 1984; Southard et al., 1988). Structural seal formation is due to two complementary mechanisms (Agassi et al., 1981): (i) physical disintegration of surface soil aggregates, caused by wetting and raindrop impact energy, and (ii) physicochemical dispersion of clay particles, which migrate into the soil with the infiltrating water and clog the pores immediately beneath the surface, so forming the "washed-in" layer. In contrast, depositional seals are formed by settling of detached particles transported across the soil surface by runoff, when the sediment load exceeds the runoff transport capacity. In soils exposed to rainfall, the spatial distribution of the different seal types is related to field microtopography (Bielders et al., 1996; Fox et al., 1998a, 1998b), with the depositional seal located in surface depressions. With surface irrigation, depositional seal is predominant.

The formation and hydraulic properties of structural seals have been studied extensively (McIntyre, 1958; Morin and Benyamini, 1977; Agassi et al., 1981; Assouline and Mualem, 1997), as well as its relationships with soil properties, such as organic matter content (Lado et al., 2004), clay content (Ben-Hur et al., 1985), ESP and soil solution electrolyte concentration (Shainberg and Letey, 1984), and clay mineralogy (Stern et al., 1991; Ben-Hur et al., 1992; Wakindiki and Ben-Hur, 2002; Ben-Hur and Wakindiki, 2004; Lado and Ben-Hur, 2004).

In contrast to the extensive research conducted on structural seals, studies focusing on depositional seals are scarce, and most of them concentrated on their micromorphology (Southard et al., 1988; Bresson and Boiffin, 1990; Valentin and Bresson, 1992; West et al., 1992). Thus, little information is available on the formation dynamics or hydraulic properties of depositional seals. Roulier et al. (2002) measured the hydraulic conductivity of structural and depositional seals that were formed on a silt loam under natural rainfall, and found that structural seals had higher hydraulic conductivity than depositional seals. Similar results were reported by Fox et al. (1998b), who compared the hydraulic properties of structural and depositional seals formed on a silt loam and a silty clay loam exposed to simulated rainfall of 22.8 mm h–1. For each of these soils, the hydraulic resistance of the depositional seal, the ratio of seal thickness to hydraulic conductivity, was around six times that of the structural one. No information about the mineralogy of these soils was provided.

Shainberg and Singer (1985) studied the effects of electrolyte concentration on the hydraulic properties of laboratory-prepared depositional seals on two montmorillonitic soils: a sandy loam and a loam, with ESP of 4.8 and 0.8, respectively. Shainberg and Singer (1985) found that a depositional seal significantly reduced the permeability of the soils, and its effect depended on the electrolyte concentration of the leaching solution. When the electrolyte concentration of the suspension was below the flocculation value of the clay, the hydraulic conductivities of the depositional seals decreased markedly, to <0.5% of those of the bulk soils. The effect of electrolyte concentration and sodium adsorption ratio (SAR) could be different in soils with different mineralogy, however.

Clay mineralogy has been found to play a key role in aggregate stability and in the susceptibility of soils to structural seal formation (Stern et al., 1991; Lado and Ben-Hur, 2004). Whereas smectitic soils and soils with some smectite content are unstable and prone to structural seal formation, kaolinitic and illitic soils with no smectite are stable and less susceptible to seal formation (Stern et al., 1991). Singer (1994) reviewed the effects of clay mineralogy on soil dispersivity, and found that smectitic soils were the most dispersive, kaolinitic soils the least dispersive, and illitic soils exhibited intermediate dispersivity. Other studies with reference clays and with clays extracted from soils (Arora and Coleman, 1979; Goldberg and Forster, 1990; Chorom and Rengasamy, 1995) also demonstrated that 2:1 clays were more dispersive than 1:1 clays. Wakindiki and Ben-Hur (2002), studying structural seal formation in soils with different mineralogies, also concluded that montmorillonitic soils were more susceptible to seal formation than kaolinitic soils. These differences in the aggregate stability and structural seal formation between the various soils were due mainly to the differences in the clay structure and morphology and their behavior in suspension.

Whereas the effects of clay mineralogy on the formation and properties of structural seals have been studied extensively, no studies have addressed the effects of clay mineralogy and its interactions with the ionic composition (electrolyte concentration, SAR, ESP, and pH) of the clay suspension on the hydraulic properties of depositional seals. Therefore, the objective of our study was to investigate the effects of reference clay minerals and their interaction with electrolyte concentration, cation composition (Na/Ca ratio), and pH of the clay suspensions on the hydraulic properties of depositional seals. Reference clays do not reflect the behavior of soil clays exactly (Goldberg and Forster, 1990), but they can be used as a useful model to understand the behavior of different clay types in the absence of interactions with organic matter and sesquioxides (Chorom and Rengasamy, 1995), before study of the properties of depositional seals in natural soils.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A smectitic calcareous silt loam (Calcic Haploxeralf) from Be'er Sheva, Israel, was used as a base soil on whose surface the depositional seals were formed. The mechanical composition of the soil was 230 g kg–1 clay, 360 g kg–1 silt, and 410 g kg–1 sand. The soil CaCO3 content was 180 g kg–1; the organic matter content, 21 g kg–1; the cation exchange capacity, 17.7 cmolc kg–1; and the ESP was 2.1. This soil was selected as a mechanical support for the formation of depositional seals due to its pores of small average size, which limits the movement of clay particles to below the soil surface, and allows the formation of depositional seals clearly differentiated from the underlying bulk soil (Chen et al., 1980).

Preparation of Clay Suspensions
Three reference clays were used in this study: Wyoming montmorillonite and Fithian illite (Ward's NSE, Rochester, NY), and Supreme kaolinite (English China clays, Austell, UK). Homoionic clays of Na or Ca were prepared by leaching the reference clays with 1 M solutions of NaCl or CaCl2, respectively. After the clay was saturated with the desired cation, to obtain Cl–-free clay it was washed with deionized water followed by ethanol–water mixtures and then freeze-dried (Goldberg and Forster, 1990). In preparing clay with a desired ESP, the appropriate amounts of the homoionic Na and Ca clays were mixed together in aqueous suspension.

Montmorillonite, illite, and kaolinite suspensions with a concentration of 5 g L–1 and various cationic compositions were prepared as follows. Montmorillonite and illite with ESPs of 0 (Ca clay), 10, 20, 60, and 100 (Na clay) were mixed with deionized water. In addition, the same clays with ESPs of 0, 10, 20, and 100 were mixed with solutions of electrolyte concentration of 10 and 20 mmol L–1 and SARs similar to the ESP of the clay. This procedure is justified because these two parameters (SAR and ESP) are usually interchangeable (Shainberg and Letey, 1984). The pH of all the montmorillonite and illite suspensions was maintained at around 7. In contrast, since dispersion of kaolinite is affected mainly by the pH of the equilibrium solution (Goldberg and Glaubig, 1987), Na- and Ca-kaolinite suspensions were prepared in deionized water, and appropriate amounts of NaOH or Ca(OH)2 were added to the Na- and Ca-kaolinite suspensions, respectively, to obtain Ca-kaolinite suspensions with pH 7 and 11, and Na-kaolinite suspensions with pH 7, 9, and 11. All the clay suspensions, with the various clay types, ESPs, pH values, and electrolyte concentrations, were used in the formation of the depositional seals in column experiments.

Column Experiments
In the column experiments, the rate of solution or suspension penetration into soil columns was measured. Two sets of experiments were conducted: (i) soil without a depositional seal—in this experiment, the flow rates of the various solutions (without clay) through the silt loam were measured; and (ii) soil with a seal—in this experiment, the flow rate of the various clay suspensions through the soil was measured.

In the first set of experiments, 120 g of air-dried soil with aggregate size <2 mm was packed to a bulk density of 1.22 g cm–3 on top of a 2-cm layer of acid-washed coarse quartz sand in a 5-cm-diameter Plexiglas column with a metal screen in its bottom. The thickness of the soil layer in the columns was 5.0 cm. The soil columns were then wetted from below until saturation by means of a peristaltic pump, at a wetting rate of 35 mm h–1 for approximately 45 min, with a 20 mmol L–1 solution of electrolytes having an SAR of 2.1 (which was equal numerically to the soil ESP). Following saturation, the flow direction was reversed and the soil column was then leached from the top with solutions having electrolyte concentrations and SAR values similar to those of the various clay suspensions. Each solution percolated through the soil column until steady state was achieved with respect to the electrolyte concentration and pH of the leachate, and the flow rate. In all the leaching runs, the flow rates were measured under a constant hydraulic head (the height difference between the inlet and the outlet of the water in the column system) of 0.41 m, controlled by means of a constant-head device (i.e., a Mariotte bottle).

In the second set of column experiments, new columns were packed with the soil and saturated from below with a 20 mmol L–1 solution of electrolytes having SAR 2.1, as in the previous set. Following saturation, the prepared clay suspensions were placed into Mariotte bottles, and were allowed to percolate through the soil in the columns. The suspensions in the Mariotte bottles were stirred with a magnetic stirrer throughout this operation. During the percolation of the clay suspension through the soil column, the clay particles settled on the soil surface and formed a depositional seal. Each suspension was percolated through a separate soil column until steady state was maintained with respect to the electrolyte concentration and pH of the effluent, and until changes in the flow rate were minor. In general, steady state was obtained after leaching 90 mm of effluent. Even when steady state was obtained at <90 mm of effluent, leaching until 90 mm continued to obtain the same amount of deposited clay in all treatments so that the hydraulic properties of the depositional seal could be compared. Flow rates were measured under the same hydraulic head that was used in the first set of experiments. The duration of each leaching run varied between 80 and 4050 min, depending on the type of clay and the ESP and ionic composition of the solution. In both sets of experiments, the effluent was collected using a fraction collector, and its volume, electrical conductivity, and pH were measured.

Statistical Analysis
Each of the experiments and treatments was conducted in three replicates (three soil columns), and the differences of the means were subjected to ANOVA as a complete randomized design. The variability of relative values was calculated by considering only the variations in the numerator of the ratio. Separation of means was subjected to Tukey's Honestly Significant Difference test (Steel and Torrie, 1960). All tests were performed at the 0.05 significance level.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Results
The relative flow rates of the solutions with different compositions through the silt loam as functions of cumulative leachate are shown in Fig. 1 . The relative flow rate of each percolating solution was calculated by relating its flow rate to the average flow rate obtained with solution containing an electrolyte concentration of 20 mmol L–1 and SAR of 2.1 (similar to the ESP of the soil). Since the hydraulic head of the columns and the thickness of the soil were the same in all the leaching experiments, the differences in relative flow rates obtained in the various treatments (Fig. 1) reflected changes in the Ks of the soil caused by the solution composition and concentration (Shainberg and Letey, 1984).


Figure 1
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  		Fig. 1. Relative flow rate of the silt loam leached with detrimental and nondetrimental solutions, as functions of cumulative leachate. Bars indicate one standard deviation.

 
When the silt loam was leached with nondetrimental solutions (i.e., solutions with SAR 0, 10, and 20 and electrolyte concentrations of 10 and 20 mmol L–1), the relative flow rates were high and steady (Fig. 1), and there was no change in the Ks of the soil. In these cases, the electrolyte concentration in the leaching solution prevented dispersion and swelling of the clay, even under high Na concentration (SAR 20), and this, in turn, kept the soil Ks at its initial value throughout the leaching run (Shainberg and Letey, 1984). The average Ks of the soil under these conditions was 8.2 mm h–1. In contrast, when the soil was leached with detrimental solutions (i.e., solutions with SAR 0, 10, 20, 60, and {infty} and electrolyte concentration of <2 mmol L–1, and solutions with SAR {infty} and electrolyte concentrations of 10 and 20 mmol L–1), the flow rates decreased slightly during leaching (Fig. 1). This decrease in the flow rate was mainly due to in situ clay swelling and clay dispersion, which decreased the volume of the conducting pores in the soil, which, in turn, decreased the Ks (Shainberg and Letey, 1984).

The relative flow rates through the silt loam columns during their leaching with montmorillonite and illite suspensions with various ESPs and electrolyte concentrations of <2, 10, and 20 mmol L–1 are presented in Fig. 2 as a function of cumulative leachate. The relative flow rates for each clay suspension, as shown in Fig. 2, were calculated as the ratios between the flow rate obtained during leaching with that clay suspension and the average flow rate obtained during leaching with a clear solution (i.e., with no clay; Fig. 1) that had the same electrolyte concentration and SAR as that of the corresponding clay suspension. Hence, the decrease of the relative flow rates during soil leaching with the clay suspensions (Fig. 2) resulted from the formation of a depositional seal at the soil surface. It is evident that the effect of a depositional seal on the flow rate through the sealed soil depended on the type of clay, the composition of the cations adsorbed on the clay and on the electrolyte concentration in the suspension (Fig. 2). In the case of montmorillonite, for each electrolyte concentration in the percolating suspension (<2, 10, and 20 mmol L–1), the decrease of the flow rate became, in general, more pronounced as the ESP of the montmorillonite in the suspension increased (Fig. 2A). Likewise, the decrease in the relative flow rate became less pronounced as the electrolyte concentration in the montmorillonite suspensions increased (Fig. 2A). In the case of illite suspensions, the reductions in flow rate during leaching with suspensions having an electrolyte concentrations of <2, 10, and 20 mmol L–1 and ESP 0, 10, and 20 were, in general, not significant and inconsistent (Fig. 2B). In contrast, leaching the silt loam soil with illite suspensions having ESP 60 or 100 decreased the relative flow rate for every electrolyte concentration, although this decrease was significant only in the suspension with an electrolyte concentration of <2 mmol L–1 (Fig. 2B). The relative flow rates during leaching with the illitic suspensions with ESP 60 and 100 and an electrolyte concentration <2 mmol L–1 were, in general, similar (Fig. 2B).


Figure 2
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  		Fig. 2. Relative flow rates through the columns during leaching with montmorlillonite and illite suspensions with various exchangeable sodium percentages and electrolyte concentrations. Bars indicate one standard deviation.

 
The relative flow rates of Ca-kaolinite (ESP 0) suspensions prepared in deionized water, at both pH 7 and 11, and those of Na-kaolinite (ESP 100) suspensions prepared in deionized water, at pH 7, 9, and 11, are presented in Fig. 3 as functions of the cumulative leachate. Leaching the silt loam with clear solutions (i.e., no clay) having pH values of 7, 9, and 11 had no effect on the pH of the leachate (Fig. 4 ), probably because of the buffering capacity of the calcareous soil. Therefore, it was assumed that leaching the studied soil with solutions of different pH did not change the Ks of the bulk soil. Thus, the relative flow rates shown in Fig. 3 for the kaolinite suspensions with pH 7, 9, and 11, were calculated as the ratio between the flow rate obtained under leaching with each clay suspension and the average flow rate obtained under leaching with a clear solution (i.e., with no clay) having an electrolyte concentration <2 mmol L–1 and the same SAR value as the corresponding clay suspension (Fig. 1). For Na-kaolinite (ESP 100), increasing the pH of the percolated suspension from 7 to 11 decreased the relative flow rate through the sealed soil (Fig. 3). In contrast, for Ca-kaolinite (ESP 0), decreasing the pH of the percolated suspension from 11 to 7 decreased the relative flow of the sealed soil.


Figure 3
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  		Fig. 3. Relative flow rates through the columns during leaching with kaolinite suspensions with various exchangeable sodium percentages (ESP) and pHs as functions of cumulative leachate. Bars indicate one standard deviation.

 

Figure 4
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  		Fig. 4. The pH of the leachate as a function of cumulative leachate during leaching of the soil with kaolinite suspensions of various pH levels. Bars indicate one standard deviation.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Since the clay concentrations in all the studied suspensions were similar, the amount of clay deposited at the soil surface for each cumulative volume was also similar for all the percolating suspensions. These seals were formed by sedimentation of flocculated or dispersed clay particles at the soil surface and their thickness and Ks determined the percolation rates of the clay suspensions (Southard et al., 1988). To determine the effects of the chemical and mineralogical properties of the clay suspensions on depositional seal formation, the hydraulic properties of the seals should be determined under the various conditions.

Water flow in a saturated soil occurs in accordance with Darcy's law:

Formula 1[1]
where q is the flux, Ks is the saturated hydraulic conductivity, {Delta}h is the hydraulic head, and {Delta}L is the soil thickness. The formation of a depositional seal at the surface of the studied silt loam resulted in a two-layer system with an upper layer that is less permeable than the lower one. Steady-state conditions require that the flux through the depositional seal (qc) should be equal to that through the underlying soil (qb), therefore,

Formula 2[2]
where Kc and Kb are the hydraulic conductivities of the seal and the bulk soil, respectively, {Delta}hc and {Delta}hb are the hydraulic heads across the seal and the bulk soil, respectively, and {Delta}Lc and {Delta}Lb are the thicknesses of the seal and the bulk soil, respectively.

Although the hydraulic heads through the soil and seal are not known, their sum is equal to the total hydraulic head ({Delta}h), thus

Formula 3[3]
where {Delta}hb is defined in Eq. [4] and Kc in Eq. [5]:

Formula 4[4]

Formula 5[5]
Posting Eq. [4] in Eq. [5] leads to

Formula 6[6]
In this study, the depositional seals that were formed on the soil surface were very thin, generally <1 mm, so that accurate measurement was practically impossible. Therefore, calculation of the hydraulic conductivity of the seal (Kc) was also not possible. The hydraulic resistance (Rc) of the seal, however, which is defined in Eq. [7], can be used as an indicator of the hydraulic properties of the seal (Fox et al., 1998b; Freebairn et al., 1991): a higher Rc indicates a greater density, narrower pores, and consequently lower Kc of the seal, or a greater thickness of the seal. Changes in the two properties Kc and Lc affect the resistance of the seal to water infiltration into the soil column.

Formula 7[7]
Posting Eq. [7] in Eq. [6] leads to

Formula 8[8]
The Rc values of the depositional seals were calculated by means of Eq. [8] from the measured values of the soil Ks under leaching with the clear solution and the q values that were obtained when leaching with the various clay suspensions. The Rc values of the depositional seals formed by montmorillonite or illite suspensions are presented in Fig. 5 as functions of ESP for various electrolyte concentrations, and the Rc values of the depositional seals formed by Ca-kaolinite or Na-kaolinite are presented in Fig. 6 as functions of the pH of the percolated clay suspensions. These Rc values were calculated for the steady-state fluxes obtained after leaching with 90 mm of the various clay suspensions (Fig. 2 and 3). The differences in Rc of the depositional seal between the various clays and their dependence on electrolyte concentration and cationic composition (Fig. 5 and 6) resulted from their mineralogical structure, their electrochemical properties, and the mechanisms of the clay packing in the depositional seal as discussed below.


Figure 5
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  		Fig. 5. Hydraulic resistance of the depositional seal as a function of the exchangeable sodium percentage for the suspensions prepared with various electrolyte concentrations: (A) montmorillonite; (B) illite. Bars indicate one standard deviation.

 

Figure 6
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  		Fig. 6. Hydraulic resistance of the depositional seal as a function of pH of the suspension for Ca- and Na-kaolinite. Bars indicate one standard deviation. Values for each saturating cation followed by the same lowercase letter are not significantly different at P < 0.05. Values for each pH followed by the same uppercase letter are not significantly different at P < 0.05.

 
The Rc values of the depositional seals formed by Ca-montmorillonite, Ca-illite, and Ca-kaolinite suspensions with various electrolyte concentrations were relatively low, <10.1 h, and did not significantly differ from each other (Fig. 5 and 6, Table 1). The electrolyte concentration in the various Ca-clay suspensions prepared in deionized water was approximately 0.5 mmol L–1, which is above the flocculation values of Ca-montmorillonite, Ca-illite, and Ca-kaolinite (Schofield and Samson, 1954; Shainberg and Letey, 1984). Thus, the seals in these cases were formed by the settling of flocculated, relatively large clay particles (Shainberg and Singer, 1985; Southard et al., 1988). When such flocculated particles are deposited at the soil surface, relatively large pores are formed, which result in depositional seals having high Ks and low Rc (Fig. 5 and 6, Table 1). Similar flocculation conditions prevailed in the various Ca-clay suspensions with electrolyte concentrations of 10 and 20 mmol L–1, and therefore, no significant differences in Rc values were obtained (Table 1).


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  		Table 1. Hydraulic resistance of depositional seals that were developed after leaching with 90 mm of montmorillonite, illite, or kaolinite suspension with pH approximately 7, exchangeable sodium percentages (ESP) of 0 to 100, and different electrolyte concentrations. The variability was calculated considering only the variation of the flux through the depositional seal (qc) from Eq. [8].

 
In the case of Na- or Ca-montmorillonite suspensions with an electrolyte concentration <2 mmol L–1, the Rc of the depositional seals increased slightly between ESP 0 and 10, increased sharply between ESP 10 and 60, and remained unchanged between ESP 60 and 100 (Fig. 5A, Table 1). This phenomenon could be attributed to the demixing distribution of cations in Na- and Ca-montmorillonite systems (Shainberg and Letey, 1984). In Ca-montmorillonite suspensions, the clay platelets are organized in tactoids or packets with 5 to 10 platelets in each packet, with a c-spacing of 1.89 nm between the Ca platelets. In montmorillonite with ESP 10, most of the adsorbed Na cations concentrate on the external surfaces of the clay packets, whereas the internal surfaces are occupied by Ca cations (Shainberg and Letey, 1984). Consequently, the depositional seal formed by a suspension of montmorillonite with ESP 10 was probably made of packets with thicknesses of 10 to 20 nm, having external surfaces saturated mainly with exchangeable Na and a flocculation value of approximately 5 mmol L–1 (Oster et al., 1980). These packets are about as thick as Ca packets, and therefore, seals formed by them would have an Rc that would be similar to that of a Ca-montmorillonite seal (Fig. 5A, Table 1). As the ESP increases to 20, Na+ penetrates into the internal surfaces of the packets and causes their breakdown (Shainberg and Letey, 1984). Therefore, leaching the soil with montmorillonite suspensions with ESP >10 resulted in the deposition of broken, smaller clay packets at the soil surface, and in a seal with higher Rc (Fig. 5A, Table 1). At ESP 60, complete breakdown and dispersion of the packets occurred (Shainberg and Letey, 1984), enabling the completely dispersed platelets to settle in a more uniform fashion with an orientation parallel to the surface of the soil (Southard et al., 1988), and to form a seal with a dense structure, a high-tortuosity path for water flow, and high Rc (Fig. 5A, Table 1). With an electrolyte concentration <2 mmol L–1, the Rc of the depositional seal that was formed by the montmorillonite suspension with ESP 60 was similar to that of the seal formed by Na-montmorillonite (Fig. 5A, Table 1). This was, probably, because in both systems the seals are made of dispersed single platelets.

The Rc of the seals that were formed by suspensions of montmorillonite with ESP ≥10 were very sensitive to the electrolyte concentrations of the percolating suspensions (Fig. 5A, Table 1). This is explained by the effect of ESP on the flocculation values of montmorillonite. The flocculation values of montmorillonite with ESP ≥10 are <12 mmol L–1 (Shainberg and Letey, 1984, Goldberg and Forster, 1990). Thus, increasing the electrolyte concentration in the percolating suspensions from 10 and 20 mmol L–1 caused flocculation of the montmorillonite, and the formation of depositional seals with a relatively more "open" structure (van Olphen, 1977; Shainberg and Singer, 1985) and larger pores, which decreased their Rc values (Fig. 5, Table 1).

The Rc of the illitic seals were, in general, significantly lower than those of montmorillonitic seals for each ESP in the range between 10 and 100, and these differences increased as the ESP of the clay increased and the electrolyte concentration in the suspension decreased (Fig. 5A and 5B, Table 1). For example, for the various electrolyte concentrations in the clay suspensions, the Rc of illite seals with ESP 10 ranged from 0.3 to 0.4 h and that of montmorillonite with the same ESP from 0.6 to 46.8 h; in contrast, the Rc of Na-illite seals ranged from 7.8 to 9.5 h and that of Na-montmorillonite seals ranged from 21 to 391.1 h (Fig. 5B, Table 1). These lower Rc values of the illitic depositional seals compared with the montmorillonitic seals (Fig. 5) probably resulted from the differences in the sizes of the illite and montmorillonite particles that formed these respective seals. For the above ESP values, the particles of illite are bigger than those of montmorillonite (van Olphen, 1977; Chorom and Rengasamy, 1995) and these larger illite particles formed seals with more open and porous structures and, therefore, with lower Rc values than those of the montmorillonitic seals (Fig. 5, Table 1).

The responses of the illitic seals to changes in the ESP and electrolyte concentration in the clay suspension differed from those of montmorillonitic seals (Fig. 5). Electrolyte concentrations in the illite suspensions had a negligible effect on the Rc of the seals (Fig. 5B, Table 1). The flocculation value of illite with ESP ≥20 is >25 mmol L–1 (Arora and Coleman, 1979; Oster et al., 1980). Thus, it can be assumed that particles with ESP ≥20 were dispersed in suspensions with electrolyte concentrations of <2, 10, and 20 mmol L–1. In contrast, the illite clay with ESP ≤10 was, in general, under flocculation conditions for all the electrolyte concentrations studied. Consequently, no significant effect of electrolyte concentration in the suspension on the Rc of the illitic seals was found (Fig. 5B, Table 1).

While the Rc of montmorillonitic seals increased sharply as the ESP increased from 20 to 60 and did not change with a further increase in ESP (Fig. 5A), the Rc of the illitic seals increased linearly and significantly with increasing ESP across the whole ESP range (Fig. 5B). These differences between illitic and montmorillonitic seals can be attributed to the fact that no demixing takes place in Na- and Ca-illite (Shainberg and Letey, 1984). In illite, the changes in ESP occur only on the external surface of the illitic crystals and, as a result, the Rc of the illitic seals increased linearly with increasing clay ESP (Fig. 5B).

The Rc of Ca- and Na-kaolinitic seals at the various pH values ranged from 2.4 to 7.7 h, except for the Na-kaolinite seal with pH 11, where the Rc climbed to 15.2 h (Fig. 6, Table 1). This range was similar to that of the illitic seals (Fig. 5B, Table 1). These two clays have relatively big particles and, therefore, the seals formed had relatively large pores, which ensured relatively low Rc values.

The Rc of Na-kaolinite seals increased with increases in the suspension pH, whereas the Rc of Ca-kaolinite seals decreased with increases in the suspension pH (Fig. 6). At pH 7, the Rc of the Na-kaolinitic seal was significantly lower than that of the Ca-kaolinitic seal (Fig. 6, Table 1). Conversely, at pH 11, the Rc of the Na-kaolinite seal was significantly higher than that of the Ca-kaolinite seal. At pH 7, the net charge on the edges of the kaolinitic packages is mainly positive (van Olphen, 1977), and edge-to-face flocculation could occur. Edge-to-face flocculation of Na-kaolinite produced relatively bigger clay particles, open structures, and larger pores. Such structures maintained the relatively low Rc values of the Na-kaolinite seals (Fig. 6). Conversely, Ca-kaolinite particles are more compacted and the flocculation of these particles produced depositional seals with smaller pores and higher Rc values (Fig. 6).

Increasing the pH of the kaolinite suspension to 11 increased the negative charge at the edges of the kaolinite packages (van Olphen, 1977). In this case, the edge-to-face interaction was not possible, and the repulsion between the clay particles increased. Consequently, in Na-kaolinite systems, clay dispersion increased and the depositional seals that were formed by the sedimentation of dispersed clay particles had lower porosities and higher Rc values (Fig. 6, Table 1).

In the Ca-kaolinite system, increasing the pH to 11 by addition of Ca(OH)2 led also to the formation of CaCO3 due to the dissolution of atmospheric CO2 in the system (Chorom and Rengasamy, 1995). This CaCO3 promoted the cementation of the Ca-kaolinite particles and the formation of larger Ca-kaolinite particles, which produced depositional seals with larger pores and lower Rc values (Fig. 6, Table 1).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The effect of clay mineralogy (montmorillonitic, illitic, and kaolinitic), electrolyte concentration (<2, 10, and 20 mmol L–1) and cationic composition (SAR of 0, 10, 20, 60, and Na solutions) on the hydraulic properties of a seal deposited on a silt loam were studied. The Rc values of seals formed by clays with ESP 0 was low, suggesting high hydraulic conductivities of depositional seals when saturated with Ca. The Rc values of montmorillonitic seals increased steeply with increases in clay ESP. Conversely, the Rc values of illitic seals increased only moderately with an increase in ESP and those of kaolinite were affected more by pH than by ESP. The dependence of Rc on clay mineralogy, ionic composition, and pH was found to be consistent with the effect of these parameters on the colloidal properties of the clay suspensions and the size of the flocculi.


   ACKNOWLEDGMENTS
 
This work was partly funded by the European Union Marie Curie fellowship under contract no. EVK1-CT-202-50020.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution from the Agricultural Research Organization, the Volcani Center, no.609/062006 series.

Received for publication April 25, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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