CSIRO PUBLISHING www.publish.csiro.au/journals/sr
Soil Research, 2011, 49, 56–64
Evaluation of the effects of cation combinations on soil hydraulic conductivity N. S. Jayawardane A,C, E. W. Christen A,B, M. Arienzo A, and W. C. Quayle A A
CSIRO Land and Water, PMB 3, Hanwood, Griffith, NSW 2680, Australia. University of New England, Armidale, NSW 2350, Australia. C Corresponding author. Email:
[email protected] B
Abstract. Effects of soil solution cation concentrations and ratios on hydraulic properties must be understood in order to model soil water flow in reactive soils or develop guidelines for sustainable land application of wastewater. We examined effects of different ratios and concentrations of the cations Ca2+, Mg2+, Na+, and K+, using hydraulic conductivity measurements in repacked soil cores, as an indicator of soil structural stability. We examined widely used indices— sodium, potassium, and monovalent cation absorption ratios (SAR, PAR, MCAR)—which assume that the flocculating effects of Ca2+ and Mg2+ are the same, and the dispersive effects of Na+ and K+ are the same. Our laboratory measurements indicate that at any given values of MCAR, the reductions in soil hydraulic conductivity with decrease in electrolyte concentration are not identical for different cation combinations in solution. The hydraulic conductivity curves showed a marked lateral shifting for both the surface and subsurface soils from a winery wastewater application site. This indicates that MCAR is inadequate as a soil stability parameter in soil solutions containing a mixture of Na+, K+, Ca2+, and Mg2+. We employed an unpublished equation that was proposed by P. Rengasamy as a modified index of soil stability for mixed cation combinations, using calculated relative flocculating powers of different cations (‘CROSS’, cation ratio of structural stability). Our observation of lateral shift in hydraulic conductivity measurements at any value of MCAR appears to relate to changes in CROSS values for all cation combinations tested, except for K–Mg solutions, for which a more generalised CROSS equation with modified parameters seems more suitable for calculating the CROSS value. Appropriate modified parameters for use in this generalised CROSS equation were determined empirically, using the experimental data. We derived a combination of threshold electrolyte concentration and CROSS values required to maintain high hydraulic conductivity for the soils at a winery wastewater application site. The potential use of this relationship in developing management practices for sustainable wastewater management at the site is discussed. Further research on the applicability of CROSS and generalised CROSS equations for other soils in the presence of different mixed cation combinations is needed. Additional keywords: salinity, sodicity, CROSS, MCAR, SAR, PAR, TEC.
Introduction Modern computers and sophisticated models allow hydraulic modelling of complex surface and subsurface water flow scenarios. With ground waters, wastewaters, and irrigation waters containing varying levels of salts that affect soil hydraulic properties, the accuracy of such flow modelling depends on the predictability of the changes in hydraulic properties, including the effects of varying cation combinations. Such predictability is also needed in developing guidelines for sustainable management of land application sites for wastewaters, which often show wide variations in cation composition, including significant amounts of K and Mg. Previous studies (Richards 1954; Quirk and Schofield 1955; McNeal and Coleman 1966) on soils with reactive clay have shown that the structural stability of the soils on application of water containing combinations of different cations depends on CSIRO 2011
the interaction between soil sodicity and total cation concentration in the soil water. The primary processes responsible for soil structural degradation are swelling and clay dispersion. Clays will disperse spontaneously at a given soil exchangeable sodium percentage (ESP) when the salt concentration in the soil water is below a critical electrolyte concentration (Quirk and Schofield 1955). The soil ESP is closely related to the sodium absorption ratio (SAR) of the applied water. SAR is a measure of sodium content in applied waters relative to the divalent cations and is expressed as: SAR ¼ ½Naþ =ðð½Ca2þ þ ½Mg2þ Þ=2Þ1=2
ð1Þ
where [Na+], [Ca2+], and [Mg2+] are concentrations in milliequivalents per 100 mL (m.e./100 mL). Detailed laboratory studies with salt solutions in which the cations consist of Na+ and Ca2+ have shown that a reduction in 10.1071/SR09222
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Soil Research
electrolyte concentration or an increase in SAR of a percolating solution results in an increase in reactive clay swelling and dispersion (McNeal et al. 1966), a change in pore size distribution (Jayawardane and Beattie 1979), and a decrease in saturated conductivity of soils (Quirk and Schofield 1955; McNeal and Coleman 1966; Jayawardane 1979, 1983, 1992). Quirk and Schofield (1955) defined the threshold electrolyte concentration (TEC) as the concentration at which a 20% reduction in the soil hydraulic conductivity occurs, at any given ESP. This concept is widely applied and is very useful in soils where the deterioration in soil structure and hydraulic properties occurs very rapidly with decrease in electrolyte concentration. Rengasamy et al. (1984) measured the threshold cation concentration for spontaneous and mechanical dispersion in red-brown earths. In most soils, however, the deterioration in soil structure and hydraulic properties occurs gradually with decrease in electrolyte concentration (McNeal and Coleman 1966; McNeal et al. 1966; Jayawardane 1979, 1983; Jayawardane and Beattie 1979; Jayawardane and Blackwell 1991). To predict the more gradual changes in soil hydraulic properties with changes in salt solution composition in such soils, Jayawardane (1977, 1979) extended the idea of the threshold concentration to the equivalent salt solutions or equivalence concept. Saturated hydraulic conductivities of soils from Tasmania, Australia, for salt solutions of SAR 20 and 10 and different electrolyte concentrations predicted by this method were close to measured values (Jayawardane 1977, 1979). Subsequent studies have shown that the equivalence concept could be used to accurately predict the changes in both saturated and unsaturated hydraulic conductivity of soils due to different salt solutions (Jayawardane 1979, 1983, 1992; Jayawardane and Blackwell 1991), where the cations were Na+ and Ca2+. Where the monovalent cation is K+ instead of Na+, the corresponding potassium absorption ratio (PAR) can be used instead of SAR, together with the total cation concentration, to define soil stability. Where the cations in wastewater or irrigation water consist of Ca2+, Mg2+, Na+, and K+, the use of the term monovalent cation absorption ratio (MCAR) has been suggested as an alternative index (Smiles and Smith 2004). MCAR is a summation of SAR and PAR, expressed as: MCAR ¼ ð½Na þ ½KÞ=ðð½Ca þ ½MgÞ=2Þ1=2 ¼ SAR þ PAR
ð2Þ
where [Na+], [K+], [Ca2+], and [Mg2+] are concentrations in m.e./100 mL. Equation 2 could be used to evaluate the amounts of the monovalent cations on the exchange complex, in a manner similar to the use of SAR to estimate ESP. If the flocculating effects of Ca2+ and Mg2+ are the same, and the dispersive effects of Na+ and K+ are the same, the effects of MCAR on soil stability and hydraulic properties will be the same as those described above for Na–Ca salt solutions. Thus, in mixed cation systems, it has been suggested that MCAR and total cation concentration could potentially be used to characterise soil stability (Smiles and Smith 2004, 2008), in a manner similar to the use of SAR
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and total cation concentration in systems where Na+ and Ca2+ are the dominant cations. There are limited studies on the effects of Mg2+ and K+ on soil structural stability, relative to Ca2+ and Na+ effects. Generally, the beneficial cation effects on soil stability and hydraulic properties relate to the type of exchangeable cations in the following order Ca2+ > Mg2+ > K+ > Na+ (Quirk and Schofield 1955), although there have been some exceptions to this general pattern. For instance, while significant differences have been observed between the different cations in solutions of high MCAR, at relatively low MCAR these differences appear to markedly decrease or become insignificant (M. Arienzo, E. W. Christen, W. Quayle, N. S. Jayawardane, unpubl.). In order to accurately predict the effect of the mixture of different cations on soil stability and hydraulic properties, the equivalence-type approaches could be used to quantify the relative flocculating effect of Ca2+ and Mg2+, and the relative dispersive effect of Na+ and K+. Rengasamy and Sumner (1998) derived the flocculating power of different cations on the basis of the Misono softness parameter responsible for hydration reactions and the ionic valence. Rengasamy (2002) found that the relative values of flocculating power of cations are: Na+ = 1, K+ = 1.8, Mg2+ = 27, and Ca2+ = 45. Flocculating power gives the reverse of dispersive effects. P. Rengasamy (pers. comm.) used these relative values of flocculating power to quantify the relative flocculating effect of Mg2+ compared to Ca2+ as given by the ratio 27/45, which equates to 0.56. Similarly, he defined the relative dispersive effect of K+ compared to Na+ as given by the ratio 1/1.8, which equates to 0.6. Substituting these values of 0.56 and 0.60 in Eqn 2, he defined the CROSS equation (Eqn 3) as follows: CROSS ¼ ½Naþ þ 0:56½Kþ =ðð½Ca2þ þ 0:6½Mg2þ Þ=2Þ1=2
ð3Þ
where the concentrations of these ions (Na+, K+, Ca2+, and Mg2+) are expressed in m.e./100 mL. It should be noted that CROSS is not based on the exchange isotherm and cannot predict the degree of adsorption of Na+ and K+. However, it can potentially be used to predict the dispersive effects on soil stability and hydraulic properties, depending on the relative amounts of the four cations present in the equilibrium soil solution. The assumptions for the use of Eqn 3 are that the theoretically derived values of the relative flocculating power of cations accurately reflect their effects on soil stability and hydraulic properties. This study examines the potential use of Eqn 3 to evaluate the relative effects of different cation combinations on hydraulic conductivity of a soil for mixed cation solutions. We employ a dataset collected as described below, which was used previously to compare the relative effects of Na+ and K+ on soil hydraulic conductivity (M. Arienzo, E. W. Christen, W. Quayle, N. S. Jayawardane, unpubl.) at a land site for winery wastewater application. Materials and methods Experimental site and procedures Soil samples were taken at 0–0.10 and 0.60–0.90 m depths from a wastewater utilisation area at a winery in Griffith in central
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N. S. Jayawardane et al.
New South Wales, Australia. The winery currently crushes 80 000 t of grapes and has recently developed a farm for the disposal of wastewater by irrigation. The soil was classified as a Sodosol according to the Australian Soil Classification (Isbell 1996), and details of soil characteristics are provided in Table 1 (M. Arienzo, E. W. Christen, W. Quayle, N. S. Jayawardane, unpubl.). The surface soil is mildly alkaline (pH 8.1), becoming strongly alkaline (pH 9.2) at depth. The soil is ~60% clay in the surface samples and ~50% in the subsurface samples. Organic carbon content is low, ranging between 0.5 and 1.6 g/kg, decreasing with depth. The salinity level in surface soil is low, 0.14 dS/m, and increases moderately with increasing profile depth to reach a moderate level of 0.49 dS/m at 0.60–0.90 m. This salinity trend with soil depth is mirrored in the increase in ESP, from 3.1 to 12.8%, suggesting that the soil becomes sodic with depth and is potentially likely to have poor internal drainage resulting from the dispersion of clay colloids. The exchangeable potassium percentage (EPP) decreases with depth, from 7.1 to 5.5%. The soil clay mineralogical composition is a mixture of smectite, kaolin, and illite. The smectite content decreases from 56% in the surface soil to 51% in the subsoil.
Solutions of different cation combinations used in the study The experimental approach was to measure changes in saturated hydraulic conductivity due to combinations of SAR and different electrolyte concentrations where the divalent cation is Ca2+ and the monovalent cation is Na+, and then make comparisons with saturated hydraulic conductivities to solutions with other cation combinations (Table 2), namely Na–Mg, Na–Ca–Mg, K–Ca, K–Mg, and K–Ca–Mg. Such comparisons were made with solutions having SAR or PAR values of 40, 20, and 5. Both surface and subsoil layers were used in these comparisons. Since we did not use any solutions which contain both Na+ and K+, the MCAR values equate to the SAR or the PAR of that solution. Table 2 also shows that for Na–Ca solutions, the value of MCAR is equal to the value of CROSS calculated using Eqn 3. With partial or total substitution of K+ for Na+ and the partial or total substitution of Mg2+ for Ca2+, the value of CROSS deviates from the MCAR value for that particular solution (Table 2). With each of the six cation combinations used, solutions of electrolyte concentrations of 64, 16, 8, 4, 2, 1, 0.5, and 0.25 m.e./100 mL were used as the percolating solution. The sequencing procedure for how the solutions were applied to the soil cores is given below.
Soil core preparation and use in hydraulic conductivity measurements with solutions of varying cation compositions Repacked soil cores for soil hydraulic conductivity measurement were prepared as follows. The soil was air-dried, crushed, and sieved to give a sample of 2–0.2 mm diameter. This was then mixed thoroughly and subsamples were used for repacking soil cores (50 mm diameter and 50 mm long) to a bulk density of 1.2 g/cm3. Triplicate repacked soil cores were used. The cores were wetted slowly through a filter paper placed at the bottom of the core with Na–Ca salt solutions at the highest electrolyte concentration (64 m.e./100 mL) and SAR 40. The solutions were made up from calcium chloride and sodium chloride. Once the soil core was saturated, it was leached with the same solution by establishing a constant head above the soil using a mariotte bottle to give a hydraulic gradient of 2. After chemical equilibrium and, hence, hydraulic equilibrium is reached as indicated by the outflow rate reaching a steady rate, the saturated hydraulic conductivity was calculated. At least 10 pore volumes were leached before measurement of hydraulic conductivity. The soil cores were then leached with solutions of the same SAR of 40 and the next highest electrolyte concentration (16 m.e./100 mL) until the soil reached chemical equilibrium with the salt solution and, hence, a new hydraulic conductivity. This process was continued by successively leaching with solutions of the same SAR of 40 and progressively lower electrolyte concentrations of 8, 4, 2, 1, 0.5, and 0.25 m.e./100 mL, and the hydraulic conductivity was measured in each solution. Thus, for any given SAR and cation mix, the same soil cores were used to measure hydraulic conductivity at progressively lower electrolyte concentrations, using an identical approach adopted in previous, related studies (McNeal and Coleman 1966; McNeal et al. 1966; Jayawardane 1979; Jayawardane and Beattie 1979). Using new soil cores, the experiment was then repeated with Na–Ca solutions of SAR 20 and also SAR 5 solutions, using the experimental procedure described above for SAR 40 solutions. Thus, at each SAR and cation mix, the same soil cores were used to measure hydraulic conductivity for the same selected series of progressively lower electrolyte concentrations (64, 16, 8, 4, 2, 1, 0.5, and 0.25 m.e./100 mL). The experiment was also then repeated for K–Ca solutions (Table 2) with corresponding combinations of PAR (40, 20, and 5) and the same series of electrolyte concentrations as used for the SAR solutions, as described above. These PAR solutions were made up using potassium chloride instead of sodium chloride. New soil cores were used with each of the PAR (40, 20, and 5) solutions, for measuring hydraulic conductivity at progressively lower electrolyte concentrations.
Table 1. Soil properties of the surface (0–0.10 m) and subsurface (0.60–0.90 m) soil of the winery site before the leaching experiment (data are means of three replicates) EC, Electrical conductivity; CEC, cation exchange capacity Depth (m)
pH
EC (dS/m)
Sand
Silt (%)
Clay
OC (g/kg)
CEC
Ca
0–0.10 0.60–0.90
8.1 9.2
0.14 0.49
34.0 35.0
9.4 16.3
56.2 48.7
1.6 0.5
28.3 39.9
17.0 20.5
Mg (cmol(+)/kg) 8.35 12.1
Na
K
0.90 5.1
2.1 2.2
Cation combinations and soil hydraulic conductivity
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Table 2. Salt solutions used in the laboratory hydraulic conductivity measurements and their values of soil stability parameters SAR, PAR, MCAR: Sodium, potassium, and monovalent cation absorption ratios. With each of the six cation combinations used, solutions of electrolyte concentrations of 64, 16, 8, 4, 2, 1, 0.5, and 0.25 m.e./100 mL were used as the percolating solution
1.8
Cations
SAR
0.6
Na–Ca
40 20 5 40 20 5 40 20 5
PAR
MCAR
CROSS
40 20 5 40 20 5 40 20 5
40 20 5 40 20 5 40 20 5 40 20 5 40 20 5 40 20 5
40 20 5 52 26 7 45 22 6 22 11 3 29 14 4 25 12 3
Na–Ca K–Ca Na–Ca–Mg K–Ca–Mg Na–Mg K–Mg CROSS values
(a)
1.6 1.4 1.2 1.0 0.8
59
52
40
45
22
25
0.4
Na–Ca–Mg
K–Ca
K–Mg
K–Ca–Mg
0.0
Relative hydraulic conductivity
Na–Mg
29
0.2 1.8
(b)
1.6 1.4 1.2 1.0 22
26
0.8
20
12
11
0.6 0.4 14
0.2 0.0 1.8
(c)
1.6 1.4
5
1.2
3
1.0 0.8
Using the methodology described above for Na–Ca solutions, the experiments to measure hydraulic conductivity were then repeated with Na–Mg and K–Mg solutions (Table 2), substituting magnesium chloride for calcium chloride in preparing the leaching solutions of the corresponding SAR and PAR values. Similar experiments to measure hydraulic conductivity were also carried out with Na–Ca–Mg and K–Ca–Mg solutions (Table 2), with solutions made up of equal amounts of Ca and Mg when making the corresponding solutions of SAR or PAR. For each salt solution combination, the equilibrium hydraulic conductivity value was taken as the hydraulic conductivity measured towards the end of each leaching cycle when the flow rate had reached approximately steady-state. Data analysis procedure adopted for calculation of relative hydraulic conductivities In order to minimise the effects of variability in core packing on the soil hydraulic conductivity, the following data analysis procedure was adopted. The average of the three replicate cores was used as the representative value for a given salt solution combination of SAR or PAR and electrolyte concentration. Dividing the equilibrium hydraulic conductivity value at lower electrolyte concentrations by the equilibrium hydraulic conductivity value of the highest electrolyte concentration (64 m.e./100 mL) solution of a given cationic mix and at any given SAR or PAR, the relative hydraulic conductivity values were calculated for each salt solution. The relative hydraulic conductivities of the surface soil (0–0.1 m) when leached with solutions containing different cation combinations are shown in Fig. 1. Similar data, for the subsurface soil (0.6–0.9 m) are shown in Fig. 2.
3
0.6
6 4
0.4
7
0.2 0.0 2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
Log electrolyte concentration (m.e./100 mL) Fig. 1. Relative hydraulic conductivities in the surface soil layer, for solutions with sodium or potassium absorption ratios of (a) 40, (b) 20, (c) 5, and different cation combinations. The shift to the right of the lines corresponds to decreasing values of CROSS indicated on the respective lines for each solution (in boxes). CROSS values for K–Mg solutions show a different trend.
Results and discussion Relative hydraulic conductivities of surface soil, in the presence of different cation combinations in MCAR 40, 20, and 5 solutions If MCAR provides an accurate index of soil stability, at any given MCAR the curves representing the change in relative hydraulic conductivity with decreasing electrolyte concentration should be identical. However, the decreases in relative hydraulic conductivities in the surface soil layer in solutions with MCAR of 40 with reduction in electrolyte concentrations show marked differences (Fig. 1a) for solutions with different cation combinations. These wide differences in the hydraulic conductivity curves at the MCAR value of 40 clearly indicate that MCAR is not an accurate predictor of soil stability, or of resultant changes in hydraulic properties in the presence of solutions with varying mixed cation combinations.
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1.8
N. S. Jayawardane et al.
Na–Ca K–Ca Na–Ca–Mg K–Ca–Mg Na–Mg K–Mg CROSS values
(a)
1.6 1.4 1.2 1.0 0.8
52
45
0.6
40
25
22
0.4 29
0.2
Relative hydraulic conductivity
0.0 1.8
(b)
1.6 1.4 1.2 1.0 0.8 0.6
22
26
12
20
0.4
11
0.2
14
0.0 1.8
(c)
1.6 1.4
3
1.2
3 5
1.0
6
0.8
4
0.6
7
0.4 0.2 0.0 2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
Log electrolyte concentration (m.e./100 mL) Fig. 2. Relative hydraulic conductivities in the subsurface soil layer, for solutions with sodium or potassium absorption ratios of (a) 40, (b) 20, (c) 5, and different cation combinations. The shift to the right of the lines corresponds to decreasing values of CROSS indicated on the respective lines for each solution (in boxes). CROSS value for K–Mg solutions show a different trend.
The changes in relative hydraulic conductivities with decreasing electrolyte concentrations at SAR 40 in different cation solutions (Fig. 1a) generally show a sigmoid shape as observed in previous studies (Quirk and Schofield 1955; McNeal and Coleman 1966; Jayawardane 1979, 1983, 1992), with a marked drop as the electrolyte concentration is reduced below the threshold concentration. The hydraulic conductivity values for the Ca–K solution, however, showed an initial increase with decreasing electrolyte concentration to 2 m.e./100 mL, and then decreased. Compared with the curve for Na–Ca solutions, the curves for Na–Ca–Mg and Na–Mg solutions show a progressive shift to the left (Fig. 1a), indicating an increase in threshold concentration values. This shift is in keeping with the relative flocculating powers of Ca2+ and Mg2+ of 45 and 27, respectively. Similarly, compared with the curve for K–Ca solutions, the curves for K–Ca–Mg and K–Mg also show a progressive shift to the left (Fig. 1a). The curve for K–Ca shows a shift to the right (Fig. 1a) compared with the Na–Ca curve, indicating a decrease in threshhold concentration values, in keeping with the relative
flocculating powers of Na+ and K+ of 1 and 1.8, respectively. Similarly, the change in monovalent cation from Na+ to K+ also results in a shift of the relative hydraulic conductivity curves to the right for solutions where the divalent cation is Mg2+ or a mixture of Ca2+ and Mg2+ in equal concentrations (Fig. 1a). Figure 1a shows that, with the exception of K–Mg solution, the relative positions of the family of hydraulic conductivity curves appear to be consistent with the values of CROSS for different cation combinations, calculated using Eqn 3. Thus, compared with the Na–Ca solution, which has a CROSS value of 40, the curves for progressively higher CROSS values show a progressive shift to the left, indicating higher instability. Similarly, the curves for progressively lower CROSS values show a progressive shift to the right, indicating greater stability. Thus, the CROSS values appear to be a better indicator of soil stability in solutions with mixed cations than the currently used MCAR values. The relative hydraulic conductivities in the surface soil layer in solutions with MCAR of 20 and electrolyte concentrations for the different cation combinations also show marked differences (Fig. 1b). This also clearly indicates that the values of the MCAR are not accurate predictors of changes in soil hydraulic properties in the presence of different mixed cation combinations. The patterns of shifts in the relative hydraulic conductivity curves in MCAR 20 solutions of different cation combinations (Fig. 1b) are very similar to those described above for MCAR 40 solutions (Fig. 1a). For instance, compared with the curve for Na–Ca solutions, the curves for Na–Ca–Mg and Na–Mg solutions show a progressive shift to the left (Fig. 1b), in keeping with the higher flocculating powers of Ca than of Mg. Similarly, compared with the curve for K–Ca solutions, the curves for K–Ca–Mg and K–Mg show a progressive shift to the left (Fig. 1b). The curve for K–Ca shows a shift to the right (Fig. 1b) compared with the Na–Ca curve, in keeping with the higher flocculating powers of K+ than of Na+. Similarly, the change in monovalent cation from Na+ to K+ also results in a shift of the relative hydraulic conductivity curves to the right for solutions where the divalent cation is Mg2+ or a mixture of Ca2+ and Mg2+ in equal concentrations (Fig. 1b). As was observed for the MCAR 40 solutions, the relative position of curves in Fig. 1b for MCAR 20 solutions are related to the values of CROSS calculated using Eqn 3, except for K–Mg solutions. This also indicates that the CROSS values are a better indicator of soil stability in most solutions with mixed cations than the currently used MCAR values. The relative hydraulic conductivities in the surface soil layer in solutions with the low MCAR value of 5 and different cation combinations (Fig. 1c) show that the relative hydraulic conductivity remains ~1 even at low electrolyte concentrations, especially at CROSS values
