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Bypass flow, salinization and sodication in a cracking clay soil
Geoderma 121 (2004) 307 – 321 www.elsevier.com/locate/geoderma
Bypass flow, salinization and sodication in a cracking clay soil Giuseppina Crescimanno *, Antonio De Santis Universita` degli Studi di Palermo, Dipartimento ITAF-Sezione Idraulica, Viale delle Scienze 90128 Palermo, Italy Received 5 December 2002; received in revised form 28 October 2003; accepted 25 November 2003 Available online 22 January 2004
Abstract In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline – sodic waters, thus enhancing the risk of soil secondary salinization and sodication. Sustainable management strategies are urgently needed in Sicily to prevent extent of salinization and sodication, thus preserving soil quality. Since irrigation is performed in cracking soils using irrigation systems involving high application rates, bypass flow of water and solutes occurs during irrigation. The objectives of this paper were (i) to investigate the process of Na – Ca exchange and subsequent salinization/sodication during bypass flow and (ii) to explore possibilities of using cyclic strategies, based on alternating good-quality waters to saline solutions, to prevent salinization. Six NaCl – CaCl2 solutions (named 1 – 6) with the following values of sodium adsorption ratio (SAR, [mmolc/l]0.5) and of cationic concentration (C, mmolc/l)—(1) SAR = 5, C = 20; (2) SAR = 5, C = 1; (3) SAR = 15, C = 25; (4) SAR = 15, C = 2; (5) SAR = 30, C = 30; and (6) SAR = 30, C = 5—were prepared in laboratory. Two undisturbed soil columns were sampled in a Sicilian irrigated area where a risk of secondary salinization and sodication was indicated by previous investigations. Bypass flow experiments were performed by supplying the six solutions to each soil column in the order from 1 to 6 at an almost constant initial value of the cracking volume, using an application intensity determining occurrence of bypass flow. Results of experiments showed that in concomitance with bypass flow, (1) a process of Na – Ca exchange occurred, with Na adsorption and accumulation in the soil. In both the two columns, the exchangeable sodium percentage (ESP) was found to increase by about 100% at the end of the six bypass flow experiments. Being the volume of the supplied saline/sodic solutions much smaller than the pore volume, this result indicates that a considerable risk of sodication may occur under bypass flow conditions; (2) cyclic strategies are useful for preventing accumulation of solutes when solutions with SAR close to the soil ESP are used. Instead, when solutions with SAR higher than the initial ESP are supplied to the same soil, due to the Na – Ca exchange and subsequent sodication, alternating waters having not only lower C but also lower SAR could be necessary to prevent salinization and sodication. D 2003 Elsevier B.V. All rights reserved. Keywords: Bypass flow; Cation exchange; Salinization; Sodication
1. Introduction Conventional sources of good-quality waters in arid and semiarid regions are scarce, which has led * Corresponding author. Tel.: +39-91-7028109; fax: +39-91484035. E-mail address: [email protected] (G. Crescimanno). 0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2003.11.014
to increased use of saline/sodic waters for irrigation (Feigin et al., 1991). In addition, pressure to avoid the disposal of nutrient-rich effluents into freshwater bodies has led to a rapid expansion of effluent reuse by irrigation (Balks et al., 1998). Use and/or reuse of waters with high levels of salinity and sodicity enhances the risk of salinization and/or sodication of irrigated lands (Szabolcs, 1994),
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as well as the hazard of deterioration of soil structural and hydraulic properties (Crescimanno et al., 1995). Typically, the greater the sodium adsorption ratio (SAR) and the lower the concentration (C) of the irrigation water, the greater the potential for aggregate slaking, soil swelling, clay dispersion and thus, reduction in hydraulic conductivity, HC (Cass and Sumner, 1982). In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline – sodic waters, thus enhancing the risk of secondary salinization and sodication. Adequate management practices are urgently needed for sustainable use of saline/sodic waters (Crescimanno, 2001a,b). These management practices depend on an understanding of the physicochemical processes occurring as water moves into the soil and displaces the salts (Marwan and Rowell, 1995). The relevance that bypass flow (BF), i.e., the rapid transport of water and solutes via macropores or cracks to subsoil and to groundwater (Bouma, 1991), may have on solute and/or pesticide transport is becoming increasingly recognized (Ahuja et al., 1991). In structured soils, BF is dominated by soil hydrological processes, such as rain intensity, initial pressure head of the soil, surface storage of rain, and hydraulic conductivity of the soil matrix (Crescimanno and Provenzano, 2000). BF occurs when, during a rain event or during irrigation, rainfall intensity is higher than the absorption capacity on the soil surface (Booltink et al., 1993). The mobility of chemicals in soils is affected by continuity as well as by the size of the macropores and/or cracks (Steenhuis et al., 1995). Instead, total macroporosity is not a very relevant property in relation to water flow processes such as BF, as very few macropores, contributing little to total macroporosity, may dominate transport of water and solutes (Bouma and Dekker, 1978). When irrigation with saline/sodic waters is performed in cracking soils using high-intensity systems (Huang et al., 2000) determining the occurrence of bypass flow, quantitative evaluation of cation exchange (Tan, 1993) and Na adsorption (Bond et al., 1982, 1984) during BF is necessary to predict the risk of salinization and sodication as well as for soil reclamation (Keren and Miyamoto, 1990).
However, although a large number of investigations focusing on transport of nonexchanging solutes or tracers during BF are reported in literature (Kung et al., 2000; Schwartz et al., 2000; Collis-George, 2001), few studies deal with the transport of reactive solutes during BF (Schoen et al., 1999; Li and Ghodrati, 1997). In particular, no investigations exploring the process of cation exchange, with Na adsorption or release under BF conditions, can be found in the current literature. Crescimanno et al. (2002) investigated the effect of alternating different water qualities on accumulation and leaching of solutes in a Mediterranean cracking soil under the occurrence of bypass fluxes. Their study indicated that leaching of solutes occurred during bypass flow, with greater efficiency of removal of soluble salts when the soil was at a considerable degree of cracking. Their results confirmed that solute transport during bypass flow is not a merely convective process, and that diffusion and dispersion play a relevant role (Hillel, 1980). The objective of this paper was to explore the process of cation exchange occurring in a cracking clay soil when saline/sodic waters with SAR progressively higher than the initial exchangeable sodium percentage (ESP) were supplied to the soil at application intensities determining occurrence of bypass flow. Our specific objectives were to investigate, under the indicated conditions, (i) if Na – Ca exchange with subsequent Na adsorption occurred in the soil, thus determining a process of sodication and (ii) if cyclic strategies for salt reclamation (Grattan and Rhoades, 1990), based on alternating waters with different C values, can be efficiently used to prevent salt accumulation.
2. Materials and methods Undisturbed soil cores having different sizes according to the physical and hydraulic characteristics to be measured (Iwata et al., 1995) were sampled from the top layer (C1 horizon, 15 –40 cm) in two very close locations (sites 1 and 2) of a Sicilian vineyard (37j40V55WN; 12j38V50WE). In this field, irrigation with saline/sodic waters is carried out using water stored in the Trinita` reservoir
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by using perforated pipes which allow high application rates at the soil surface. The soil was classified as a Typic Chromoxerert (Soil Survey Staff, 1992). Since irrigation is used during the dry season, when cracks are visible, the high intensity of water application leads to bypass flow (Crescimanno, 2001a). A large soil column was sampled in each site (column 1, column 2, diameter D = 20 cm, height H = 20 cm) for measuring the saturated hydraulic conductivity of the soil matrix, K(sat), by the suction crust infiltrometer method (Booltink et al., 1991). Four undisturbed soil cores (D = 8.5 cm, H = 5 cm), two for each site, were sampled for determination of the unsaturated soil hydraulic parameters. Multi-step outflow experiments were performed on these soil cores in pressure cells by applying three successive steps with pneumatic pressures ranging from 1 to 4, from 4 to 7, and from 7 to 80 kPa (Crescimanno and Iovino, 1995). The van Genuchten – Mualem hydraulic parameters (van Genuchten, 1980) were determined by a previously validated parameter estimation procedure (Crescimanno and Baiamonte, 1999). Four undisturbed soil cores (D = 8.5 cm, H = 11.5 cm), two for each site, were sampled for determining the soil shrinkage characteristic curve (SSCC) by measuring vertical and horizontal shrinkage (Crescimanno and Provenzano, 1999). Based on the water retention curve, h(h), and on the SSCC, the crack volume, DVcr, as percentage of the volume at saturation (V), was calculated (Bronswijk, 1989) as a function of matrix potential (h). In order to evaluate the soil susceptibility to cracking (Parker et al., 1977), the coefficient of linear extensibility, COLE, was also calculated (Grossman et al., 1968). Six NaCl – CaCl2 solutions with three different SAR values and six different cationic concentrations,
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C, were prepared in the laboratory. Table 1 reports the SAR, the C, the electrical conductivity (EC) corresponding to C, the concentration of sodium (Na) and calcium (Ca) in the six solutions. To perform the bypass flow measurements, each undisturbed soil column used for measurement of the K(sat) was placed on a funnel connected with an outflow collector equipped with an analytical scale, which made it possible to measure the weight of the outflow solution. These measurements were stored in a computer. Using a small needle rain simulator, six bypass flow experiments were performed on each soil core by supplying the six solutions in the order from solution 1 to solution 6. To ensure bypass flow conditions, an application intensity (Ir = 11.00 cm/h) higher than the saturated hydraulic conductivity of the soil matrix, K(sat), was used (Bouma, 1991). A constant volume of solution, equal to 1.0 103 cm3, was applied in all the experiments. Application of each solution was performed at an almost constant initial value of the cracking volume (DVcr/V 100), corresponding to an initial matrix potential value (hi) determined, for the two columns, by reading two tensiometers installed at 8 and at 12 cm from the top of the column. Between application of two solutions, the core was brought again to the initial DVcr/V 100 and hi, by an air-drying process which lasted 2 days for each experiment. The three bypass flow experiments performed with solutions 1, 3 and 5 (solutions with increasing SAR) had the purpose to explore the process of Na – Ca exchange and to check occurrence of sodication. To avoid reductions in soil hydraulic conductivity at increasing SAR (Cass and Sumner, 1982), which could affect flow behaviour and chemical exchanges, concentration (C) of solutions 2, 4 and 6 was not kept
Table 1 Composition of the six solutions: sodium adsorption ratio (SAR), concentration (C), electrical conductivity (EC), sodium (Na) and calcium (Ca) Solution
SAR [mmolc l 1]0.5
C [mmolc l 1]
EC [dS m 1]
Na [mmolc l 1]
Ca [mmolc l 1]
1 2 3 4 5 6
5 5 15 15 30 30
20.00 1.00 25.00 2.00 30.00 5.00
2.200 0.156 2.710 0.280 3.150 0.570
10.74 0.91 20.87 1.96 28.17 4.91
9.25 0.07 3.95 0.03 1.80 0.05
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constant, but was slightly increased at increasing SAR (Crescimanno et al., 1995). The three bypass flow experiments performed by alternating the low-salinity solutions (solutions 2, 4 and 6) to the high-salinity waters (solutions 1, 3 and 5) were carried out in order to investigate if cyclic strategies, based on alternating low-salinity solutions to high-salinity waters (Grattan and Rhoades, 1990; Crescimanno et al., 2002), are efficient to prevent salinization when a soil is progressively sodicized. The electrical conductivity (EC) of the effluent solution was measured at fixed time intervals by a conductivimeter (Crison, Micro CM 2002). Sodium (Na) and calcium (Ca) concentrations were determined at the same time intervals by ion-selective electrodes (Orion Research). In order to convert the measured EC (dS/m) into cationic concentration C (mmolc/l), NaCl – CaCl2 solutions of known concentration (C) were prepared, and the EC corresponding to C was measured. The following relationship between C and EC was found: C ¼ 9:514EC
ð1Þ
To check the degree of sodication, 12 soil samples were randomly extracted from each column, and the ESP was determined (by using ammonium chloride) at the end of the bypass flow experiment performed with solution 6.
3. Results and discussion Table 2 reports some physical and chemical properties of the soil at the two sites; the table also reports the K(sat) measured on columns 1 and 2 before the bypass flow measurements. Fig. 1 shows the water retention curve, h(h), as well as the relationship between crack volume (DVcr/ V 100) and matrix potential (h), obtained for sites 1 and 2. The initial (DVcr/V 100) value, at which the bypass flow experiments were performed in both the two columns, was about 4.30%. 3.1. Bypass flow of water and solutes Fig. 2a – f illustrates the cumulative outflow curves obtained for column 1 by the bypass flow measurements performed using the six solutions in the order
Table 2 Physical and chemical soil properties of soil at the two sites and saturated hydraulic conductivity of the two columns Horizon
C1 (15 – 40 cm)
Sand (2 – 0.02 mm) (%) Silt (0.02 – 0.002 mm) (%) Clay ( < 0.002 mm) (%) Texturea COLEb Shrink – swell potentialc EC(1:5) [dS m 1] ESP [%] CEC [mmolc kg 1] Exchangeable cations Na+ [mmolc kg 1] K+ [mmolc kg 1] Ca2 + [mmolc kg 1] Mg2 + [mmolc kg 1]
1
K(sat) [cm h
]
Site 1
Site 2
44 24 32
35 33 32 Clay
0.131 high 0.32 3.4 213.0
0.090 high 0.34 3.6 220.0
7.24 12.8 167.8 25.2
7.9 12.7 171.2 28.2
Column 1
Column 2
1.41
1.21
EC(1:5): electrical conductivity of the 1:5 extract; ESP: exchangeable sodium percentage; CEC: cationic exchange capacity; K(sat): hydraulic conductivity of soil matrix. a ISSS. b COLE values calculated between saturation and air-dry conditions. c Parker et al., 1977.
from 1 to 6. The figures report the inflow and the outflow volume, V(t), as a function of time (t), the total concentration, C (mmolc/l), of the effluent solution (Cout), and the concentration of the incoming solution (Cin). The cumulative V(t) curves all showed identical behaviour. After the time when outflow started, td, an exponential progression between the times td and tcs (start of linear outflow) could be observed. Between tcs and tce (end of linear outflow), there was a nearly linear outflow pattern, and after tce, an exponential fade out pattern was evident. The linear outflow occurring between tcs and tce (i.e., the time derivative of outflow is constant) has been observed in earlier measurements (van Stiphout et al., 1987; Booltink et al., 1993; Crescimanno et al., 2002) and denotes occurrence of bypass flow (BF). In the experiment performed with solution 1, reported in Fig. 2a, we can see that the Cout values are initially lower than the Cin, with a tendency to
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Fig. 1. Water retention curve, h(h), and relationship between DVcr /V 100 and matrix potential (h) for the two sites.
reach the Cin value when water application stops (time te, end of the linear outflow), when the Cout reaches the maximum value. This maximum Cout, also observed in previous experiments (Crescimanno et al., 2002), proves that bypass flow of solutes is associated with bypass flow of water. The decrease in Cout after the end of water application (end of BF), when the macropores stop contributing to outflow and solutes are released from the matrix only, is a consequence of solute redistribution and concentration gradients within the micropores (redistribution phase, RED). With reference to the measurements performed with solutions 3 and 5, reported in Fig. 2c and e, no maximum Cout can be identified in concomitance with the end of the linear outflow. Instead, a continuous increase in Cout at increasing t can be observed after termination of water application, with Cout exceeding Cin. Release of some cations from the soil matrix during bypass flow and also after termination of bypass flow could be the reason for this continuous increase in Cout. Since solutions with SAR higher (SAR = 15 and SAR = 30) than SAR of solution 1 (SAR = 5), as well
as of initial ESP (ESP = 3.4%), were used in the experiments reported in Fig. 2c and e, the reason for the observed increase in Cout could be the occurrence of cationic exchanges between the incoming solution and the soil matrix, with sodium (Na) adsorbed into the exchange phase and calcium (Ca) and/or magnesium (Mg) released from the exchange phase. As the exchangeable Mg content in this soil is much smaller than the exchangeable Ca content (Table 2), we consider Ca to be responsible for the increase in Cout. In the experiments performed with solutions 2, 4 and 6, reported in Fig. 2b, d and f, the measured Cout decreases after the start of rainfall, reaching a minimum value at the end of bypass flow. As in this case Cout >Cin, the decrease in Cout following water application proves that leaching of solutes from the matrix occurs in concomitance with bypass flow of water. The subsequent increase in Cout occurring when bypass flow has terminated confirms solute redistribution and concentration gradients within the micropores (Cote et al., 1999), with solutes previously adsorbed by the soil matrix partly flowing in the outcoming solution. Fig. 3a – f illustrates the cumulative outflow curves obtained for column 2 by the bypass flow
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Fig. 2. (a – f) Outflow volume V(t) and concentration (Cout) of the effluent solution as a function of time, t; concentration of the incoming solution (Cin), during the six bypass flow experiments for column 1.
measurements performed using the six solutions in the order from 1 to 6. The figures show a similar behaviour of column 2 compared to that of column 1, with no maximum Cout in concomitance with the end of the linear outflow (Fig. 3c and e) and with a continuous increase in Cout at increasing t
after termination of water application, with Cout exceeding Cin. Fig. 3b, d and f also confirm the similar behaviour of the two columns. Replication of the bypass flow experiments on column 2 confirmed results and speculations made for column 1.
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Fig. 3. (a – f) Outflow volume V(t) and concentration (Cout) of the effluent solution as a function of time, t; concentration of the incoming solution (Cin), during the six bypass flow experiments for column 2.
3.2. Na –Ca exchange, salinization and sodication during bypass flow Fig. 4 reports for columns 1 and 2 the Na concentration (mmolc/l) measured in the outcoming solution (Naout) during the six experiments as a function of
time, together with the concentration of Na in the incoming solution (Nain). As can be seen in the figure, Naout is lower than Nain for the experiments with solutions 1, 3 and 5; this indicates that part of the Na entering the soil with the incoming solution is adsorbed by the exchange phase.
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Fig. 4. Concentration of sodium (Naout) in the outcoming solution as a function of time, t, and concentration of sodium in the incoming solution (Nain), for the six bypass flow experiments.
Fig. 5 reports for columns 1 and 2 the Ca concentration (mmolc/l) measured in the outcoming solution (Caout) as a function of time during the six experiments, together with the concentration of Ca in the incoming solution (Cain). Ca appears to be released from the soil in all six experiments. In experiments with solutions 1, 3 and 5, release of Ca can be the consequence of Na – Ca
exchange and is in agreement with the observed Na adsorption. In experiments with solutions 2, 4 and 6, the observed Ca release is a consequence of the decrease in the cationic concentration of the applied solutions, at constant SAR, which determines a process of salt leaching. Table 3 reports the quantities (mmolc) of Na and Ca supplied to the soil columns (Nain, Cain), leaving
Fig. 5. Concentration of calcium (Caout) in the outcoming solution as a function of time, t, and concentration of calcium in the incoming solution (Cain), for the six bypass flow experiments.
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Table 3 Quantity of sodium (Na) and calcium (Ca) entering the soil (Nain, Cain), of Na and Ca bypassing the soil matrix (Nabf, Cabf) and leaving the soil core during the whole outflow process (Naout, Caout) Solution
1 2 3 4 5 6
Nain [mmolc]
Cain [mmolc]
10.74 0.91 20.87 1.96 28.17 4.91
9.25 0.07 3.95 0.03 1.80 0.50
Column 1
Column 2
Nabf [mmolc]
Cabf [mmolc]
Naout [mmolc]
Caout [mmolc]
Nabf [mmolc]
Cabf [mmolc]
Naout [mmolc]
Caout [mmolc]
2.07 1.23 2.03 2.29 2.98 2.90
6.04 4.37 8.21 5.93 7.28 5.93
3.05 1.82 3.12 3.21 4.38 4.20
9.19 6.54 12.45 8.48 10.61 8.90
1.60 1.45 1.89 2.16 2.75 2.20
3.30 2.84 5.27 4.11 5.44 4.18
2.43 2.50 3.02 3.38 4.82 3.87
5.05 4.68 8.15 5.78 9.38 7.17
the columns during BF (Nabf, Cabf) and during the whole outflow process (Naout, Caout). The quantities of Na and of Ca were determined by integrating the Na and Ca concentrations (mmolc/l) vs. the cumulative outflow volume, V(t) (l). To explore the process of Na – Ca exchange during bypass flow (BF), the positive and the negative values of the difference (DNabf = Nain Nabf), which represents the quantity of Na accumulated or released during BF, were calculated and are reported in Table 4 for columns 1 and 2. The table also reports the quantity of Na released during the redistribution (RED) phase, represented by DNared = (Naout Nabf). For both the two columns, positive DNabf values, indicating Na adsorption by the soil during BF, were found for the experiments with solutions 1, 3 and 5; negative but negligible DNared values, indicating a nonsignificant release of Na during the RED stage, were found for the same experiments. Negative but negligible DNabf, values, indicating a nonsignificant release of Na during BF, were also Table 4 Na accumulated or released during BF (DNabf), and Na released during the redistribution phase (DNared) Solution
1 2 3 4 5 6
Column 1
Column 2
DNabf [mmolc]
DNared [mmolc]
DNabf [mmolc]
DNared [mmolc]
8.67 0.32 18.84 0.34 25.19 2.01
0.98 0.59 1.09 0.92 1.40 1.30
9.14 0.54 18.98 0.20 25.42 2.71
0.84 1.05 1.12 1.21 2.07 1.67
found during application of solutions 2 and 4. This result appears to be in agreement with the fact that during application of these solutions, only C was decreased at a constant SAR. Instead, an unexpected positive DNabf was found for both the columns in the experiment with solution 6, probably due to the considerable increase in the SAR, which determined further adsorption of Na into the soil. Table 4 shows that for both the two columns, increasing values of DNabf, indicating a progressive Na adsorption in the soil, were found for experiments 1, 3 and 5. Since nonsignificant amounts of Na were released during the RED phases as well as during application of leaching solutions (2, 4), with further Na adsorption during application of solution 6, this result indicates that sodication occurred during the bypass flow events. To analyse accumulation or leaching of Ca, the difference (DCabf = Cain Cabf,), which represents the quantity of Ca accumulated or released during BF, was calculated and is reported in Table 5 for the two columns. The same table also reports the quantity of Ca released during the RED phase, represented by DCared = (Caout Cabf). For both columns, a positive DCabf was found only for the experiment performed with solution 1, while negative values, indicating release of Ca, were found for all the other experiments either during BF or during the RED phases. In experiments with solutions 3 and 5, this Ca release is in agreement with the continuous increase in Cout illustrated in Figs. 2c and e and 3c and e, being a consequence of the Na – Ca exchange occurring dur-
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Table 5 Ca accumulated or released during BF (DCabf), and Ca released during the redistribution phase (DCared) Solution
1 2 3 4 5 6
Column 1
Column 2
DCabf [mmolc]
DCared [mmolc]
DCabf [mmolc]
DCared [mmolc]
3.21 4.30 4.26 5.89 5.48 5.88
3.15 2.17 4.24 2.55 3.33 2.97
5.95 2.77 1.32 4.08 3.64 4.13
1.75 1.84 2.88 1.67 3.93 2.99
ing BF. In experiments with solutions 2, 4 and 6, the Ca release is a consequence of the decrease in C at constant SAR. To analyse accumulation and/or leaching of solutes in the two columns, the quantity (mmolc) of solutes entering the soil (Sin), leaving the soil during bypass flow (Sbf) and during the whole outflow process (Sout), was calculated and is reported in Table 6. Determination of S (mmolc) was performed by integrating the concentration C (mmolc/l), obtained by using the relationship (1), vs. the cumulative outflow volume, V(t), (l). The value of the difference (DSbf = Sin Sbf), which represents the amount of solutes accumulated or released during BF, is represented in Fig. 6a and b for the six experiments and for the two columns. The positive DSbf values found for the two columns for experiments with solutions 1, 3 and 5 indicate a process of solute accumulation occurring during BF; the negative DSbf found for the experiments with solutions 2, 4 and 6 indicate occurrence of leaching. Fig. 6a and b also report the values of DSred = (DSout DSbf), which represents the amount of solutes leached from the column during the RED phases. As the absolute values of DSred were always lower than the DSbf values, a process of salinization evidently occurred in both the two columns between application of solutions 1 and 6. To get further insight in the process of Na – Ca exchange occurring during BF, the values of the difference (D Sbf D Cabf ) between the quantity (mmolc) of solutes accumulated during bypass flow
(DSbf) and the quantity of Ca released during BF (DCabf) were calculated and represented in Fig. 7a and b for columns 1 and 2. The figures also report the quantity (DNabf) of Na accumulated during BF. The figures show that the difference (DSbf DCabf) quantitatively corresponds to DNabf, except for very small discrepancies which could be attributed to experimental errors in the measurement of Na and Ca or to release of small amounts of other ions not measurable by the electrodes. This analysis indicates that the Na replacing the Ca in the soil is progressively accumulated, thus determining a process of sodication. These results prove that cation exchange occurs during bypass flow, confirming that solute diffusion (Cote et al., 2000; Rowell and Pateras, 2002) and hydrodynamic dispersion (Forrer et al., 1999; Vanderborght et al., 2000) both play a relevant role in solute exchange and in the process of salt leaching (Crescimanno et al., 2002). However, unexpected high ESPs value were found in both the two columns at the end of the six experiments especially if we consider that a small volume of solution (1.0 103 cm3) was supplied to the soil during each bypass flow experiments compared to the pore volume of the soil (3.4 103 cm3 for column 1, 3.7 103 cm3 for column 2). An average ESP of 7.02% was found for column 1 and an ESP of 7.4 was found for column 2. Since the initial ESPs were 3.4% for column 1 and 3.6% for column 2, in both the two columns, the ESP increased by about 100% or even more at the end of the six bypass flow measurements.
Table 6 Quantity of solutes (S) entering the soil (Sin), of solutes bypassing the soil matrix (Sbf) and leaving the soil columns during the whole outflow process (Sout) Solution
1 2 3 4 5 6
Sin [mmolc]
Column 1 Sbf [mmolc]
Sout [mmolc]
Sbf [mmolc]
Sout [mmolc]
20.00 1.00 25.00 2.00 30.00 5.00
8.21 5.92 9.87 7.95 9.93 8.57
12.64 8.84 14.91 11.30 15.42 13.27
5.61 5.10 7.77 7.13 9.28 7.66
8.84 8.55 12.04 10.57 16.33 13.39
Column 2
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Fig. 6. Solutes accumulated or released during BF (DSbf), and solutes released during the redistribution phase (DSred), for the six bypass flow experiments.
These results indicate that a considerable increase in the soil ESP may occur when application of a saline/sodic solution is performed at a considerable degree of cracking using water application intensities determining bypass flow. In addition, since our results indicated that cation exchange mainly occurred during bypass flow, we can speculate that the process of cationic exchange occurs at the contact surfaces between the incoming solution
and the cracks in the walls. This would lead to the conclusion, which needs further experimental verification, that higher salinization and sodication might be associated with bypass flow than with matrix flow conditions. Collis-George (2001) also found that during drainage of a soil, the effluent solution was mainly from the external surfaces. He observed that in the field, restraints on the free expansion of the double layer
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Fig. 7. Difference (DSbf DCabf) between the quantity of the solutes accumulated during bypass flow (DSbf) and the quantity of Ca released during BF (DCabf); quantity of Na accumulated or released during BF (DNabf) for the experiments with solutions 1, 3 and 5.
are imposed by the aggregate structure at all sizes as well as by the matrix potential, and only those surfaces adjacent to pores and/or cracks can develop thick double layers when ponded. Considerations of Collis-George and results of our experiments confirm previous results indicating that the mechanism of Na release during BF depended on the magnitude of the contact surfaces between the crack walls and the incoming solution (Crescimanno et al., 2002).
3.3. The effect of alternating waters with different concentrations at increasing SAR-ESP Fig. 8a and b report for columns 1 and 2 the quantity of solutes (DS) accumulated or released during each outflow experiment (including either the BF or the RED phases) together with the quantity of solutes (DSseq = SDS) accumulated or released during each sequence (seq) of alternating solution 2 to 1 (seq 1), solution 4 to 3 (seq 2) and solution 6 to 5 (seq 3).
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Fig. 8. Quantity of solutes (DS) accumulated or released during each bypass flow experiment and accumulated or released during sequence 1 (DSseq1), sequence 2 (DSseq2) and sequence 3 (DSseq3).
Each sequence represents a process of alternating a low C to a higher C at a constant SAR; the three sequences altogether represent a process of alternating waters with different C at increasing SAR and with SARs progressively higher than the initial ESP. This situation may occur when cyclic strategies for use or reuse of saline/sodic waters are adopted to keep salinity under levels tolerable for crops (Maas, 1990).
Fig. 8a and b show that a negative DSseq value, proving no accumulation of solutes in column 1, and a positive but negligible DSseq, proving a nonsignificant accumulation of solutes in column 2, were associated to seq 1. A significant leaching evidently occurred when solution 2 was alternated to solution 1. This result confirms previous findings of Crescimanno et al. (2002), indicating that release of
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solutes occurs under bypass flow conditions when a low-salinity water is alternated to a higher salinity solution. Instead, the positive and increasing values of DSseq represented in Fig. 8a and b indicate that a progressive accumulation of solutes and thus, a reduced efficiency of salt leaching was associated to seq 2 and especially to seq 3 for both the two columns. The decreasing efficiency of leaching found for seq 2 and seq 3 is due to the process of Na –Ca exchange and subsequent sodication occurring at increasing SAR. In seq 1, since a solution with SAR close to the initial ESP was used, alternating different C values was efficient to remove the accumulated solutes. Instead, in seq 2 and seq 3, not only C but also SAR should have been be probably decreased to avoid a process of progressive salinization.
should be probably alternated to solutions with higher C and SAR to prevent salinization and sodication. Further research is underway to investigate how flow conditions (bypass or matrix flow) may affect cation exchange, salinization and sodication. The effect of alternating waters with different C and SAR will be also investigated in order to find management strategies preventing salinization and sodication and thus minimizing the increasing risk of desertification in Sicily.
Acknowledgements Research funded by MURST (Rome, Italy), PRIN 2000: ‘‘Processi di trasporto dell’acqua e di soluti in terreni argillosi irrigati con acque salino/sodiche’’.
References 4. Conclusions The experiments described in this paper showed that in a cracking soil, under bypass flow conditions, a process of Na –Ca exchange occurred at the contact surfaces between the incoming solutions and the cracks walls. This exchange determined Na adsorption in the soil when solutions with increasing SAR were supplied to the soil. Measurement of the exchangeable sodium percentage (ESP), performed at the end of the six bypass flow experiments, proved an unexpected considerable increase in the soil ESP compared to the initial ESP value. As the supplied volume of saline/sodic solutions was very small compared to the pore volume, this result indicated that a considerable risk of sodication may occur under bypass flow conditions. In addition, the experiments performed showed that alternating a low-salinity water to a high-saline solution is a management practice preventing accumulation of solutes when solutions having SAR values close to the soil ESP are applied to the soil. Instead, when waters having SAR values higher than the initial soil ESP are progressively supplied to the same soil, sodication occurring as a consequence of Na – Ca exchange prevents leaching of solutes. In this case, solutions having both lower C and SAR
Ahuja, L.R., El-Swaify, S.A., Rahman, A., 1991. Characteristics and importance of preferential macropore transport studied with the ARS root zone water quality models. In: Gish, T.H., Shirmohammadi, A. Proceedings of National Symposium on Preferential Flow, Chicago. 16 – 17 December. ASAE, St. Joseph, MI, pp. 32 – 49. Balks, M.R., Bond, W.J., Smith, C.J., 1998. Effects of sodium accumulation on soil physical properties under an effluent-irrigated plantation. Aust. J. Soil Res. 36, 821 – 830. Bond, W.J., Gardiner, B.N., Smiles, D.E., 1982. Constant-flux adsorption of tritiated calcium chloride solution by a clay soil with anion exclusion. Soil Sci. Soc. Am. J. 46, 1133 – 1137. Bond, W.J., Gardiner, B.N., Smiles, D.E., 1984. Movement of CaCl2 solutions in an unsaturated clay soil: the effect of solutions concentration. Aust. J. Soil Res. 22, 43 – 58. Booltink, H.W.G., Bouma, J., Gimenez, D., 1991. A suction crust infiltrometer for measuring hydraulic conductivity of unsaturated soil near saturation. Soil Sci. Soc. Am. J. 55, 566 – 568. Booltink, H.W.G., Hatano, R., Bouma, J., 1993. Measurement and simulation of bypass flow in a structured clay soil: a physicomorphological approach. J. Hydrol. 148, 16 – 149. Bouma, J., 1991. Influence of soil macroporosity on environmental quality. Adv. Agron. 46, 1 – 37. Bouma, J., Dekker, L.W., 1978. A case study on infiltration into dry clay soil: I. Morphological observations. Geoderma 20, 27 – 40. Bronswijk, J.J.B., 1989. Prediction of actual cracking and subsidence in clay soil. Soil Sci. 148, 87 – 93. Cass, A., Sumner, M.E., 1982. Soil pore structural stability and irrigation water quality: I. Empirical sodium stability model. II. Sodium stability data. Soil Sci. Soc. Am. J. 46, 503 – 512. Collis-George, N., 2001. The application of double-layer theory to drainage, drying and wetting, and the Gapon Exchange
G. Crescimanno, A. De Santis / Geoderma 121 (2004) 307–321 constant in a soil with mono and divalent cations. Eur. J. Soil Sci. 52, 1 – 12. Cote, C.M., Bristow, K.L., Ross, P.J., 1999. Quantifying the influence of intra-aggregate concentration gradients on solute transport. Soil Sci. Soc. Am. J. 63, 759 – 767. Cote, C.M., Bristow, K.L., Ross, P.J., 2000. Increasing the efficiency of solute leaching: impacts of flow interruption with drainage of the ‘‘preferential flow path’’. J. Contam. Hydrol. 43, 191 – 209. Crescimanno, G., 2001a. An integrated approach for sustainable management of irrigated land susceptible to degradation/desertification. Final Report ENV4-CT97-0681. Crescimanno, G., 2001b. Irrigation practices affecting land degradation in Sicily. PhD dissertation thesis, Wageningen University (NL), ISBN 90-5808-426-4. Crescimanno, G., Baiamonte, G., 1999. Hydraulic characterization of swelling/shrinking soils by a combination of laboratory and optimization techniques. Procs. of the International Workshop ‘‘Modeling of Transport Processes in Soils at Various Scales in Time and Space’’. Leuven, Belgium, pp. 24 – 26. Crescimanno, G., Iovino, M., 1995. Parameter estimation by inverse method based on one-step and multi-step outflow experiments. Geoderma 68, 257 – 277. Crescimanno, G., Provenzano, G., 1999. Soil shrinkage characteristic in clay soils: measurement and prediction. Soil Sci. Soc. Am. J. 63, 25 – 32. Crescimanno, G., Provenzano, G., 2000. Hydrological processes affecting land degradation in the Mediterranean environment. Third International Congress of the European Society for Soil Conservation (ESSC). Valencia, Spain, 28 March – 1 April. Crescimanno, G., Iovino, M., Provenzano, G., 1995. Influence of salinity and sodicity on soil structural and hydraulic characteristics. Soil Sci. Soc. Am. J. 59, 1701 – 1708. Crescimanno, G., Provenzano, G., Booltink, H.W.G., 2002. The effect of alternating different water qualities on accumulation and leaching of solutes in Mediterranean cracking soils. Hydrol. Process. 16 (3), 130 – 717. Feigin, A., Ravina, I., Shalhevet, J., 1991. Irrigation with treated sewage effluent. Management for Environmental Protection. Springer-Verlag, Berlin. Forrer, I., Kasteel, R., Flury, M., Flu¨hler, H., 1999. Longitudinal and lateral dispersion in an unsaturated field soil. Water Resour. Res. 35, 3049 – 3060. Grattan, S.R., Rhoades, J.D., 1990. Irrigation with saline grind water and drainage water. In: Tanji, K.K.Agricultural Salinity Assessment and Management. ASCE Manual and Reports on Engineering Practices, vol. 71. ASCE, NY, pp. 432 – 449. Grossman, R.B., Brasher, B.R., Franzmeier, D.P., Walker, J.L., 1968. Linear extensibility as calculated from natural-clod bulk density measurements. Soil Sci. Soc. Am. Proc. 32, 570 – 573. Hillel, D., 1980. Fundamental of Soil Physics Academic Press, New York, USA. Huang, Z.B., Assouline, S., Zilberman, J., Ben Hur, M., 2000. Tillage and saline irrigation effects on water and salt distribution in a sloping field. Soil Sci. Soc. Am. J. 64, 2096 – 2112.
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Iwata, S., Tabuchi, T., Warkentin, B.P., 1995. Soil – Water Interaction. Marcel Dekker, New York, pp. 333 – 346 Chap. 6. Keren, R., Miyamoto, S., 1990. Reclamation of saline, sodic, and boron-affected soilsTanji, K.K.Agricultural Salinity Assessment and Management. ASCE Manual and Reports on Engineering practices, vol. 71. ASCE, NY, pp. 410 – 431. Kung, K.J.S, Steenhuis, T.S., Kladivko, E.J., Gish, T.J., Bubenzer, G., Helling, C.S., 2000. Impact of preferential flow on the transport of adsorbing and non-adsorbing tracers. Soil Sci. Soc. Am. J. 64, 1290 – 1296. Li, Y., Ghodrati, M., 1997. Preferential transport of solute through soil columns containing constructed macropores. Soil Sci. Soc. Am. J. 61, 1308 – 1317. Maas, E.V., 1990. Crop salt tolerance. In: Tanji, K.K. Agricultural Salinity Assessment and Management Manual. ASCE, New York, pp. 262 – 304. Marwan, M.M., Rowell, D.L., 1995. Cation exchange, hydrolysis and clay movement during the displacement of saline solutions from soils by water. Irrig. Sci. 16, 81 – 87. Parker, J.C., Amos, D.F., Kaster, D.L., 1977. An evaluation of several methods of estimating soil volume change. Soil Sci. Soc. Am. J. 41, 1059 – 1064. Rowell, D., Pateras, D., 2002. Diffusion and action exchange during the reclamation of saline-structured soil. Geoderma 107, 271 – 279. Schoen, R., Gaudet, J.P., Bariac, T., 1999. Preferential flow and solute transport in a large lysimeter, under controlled boundary condition. J. Hydrol. 215, 70 – 81. Schwartz, R.C., Juo, A.S.R., McInnes, K.J., 2000. Estimating parameters for a dual-porosity model to describe nonequilibrium, reactive transport in a fine-textured soil. J. Hydrol. 229, 149 – 167. Soil Survey Staff, K.J., 1992. Keys to soil taxonomy, 8th ed.SMSS Technical Monograph, vol. 19. Pocahontas Press, Blacksbourg, VA (556 pp.). Steenhuis, T.S., Parlange, J.Y., Andreini, M.S., 1995. Preferential flow in structured and sandy soils. Consequences for modeling and monitoring. In: Everett, L.et al., Handbook of Vadose Zone Characterization and Monitoring. Lewis Publ., Ann Arbor, MI, pp. 61 – 71. Szabolcs, I., 1994. Prospects of soil salinity for the 21st century. 15th International Congress of Soil Science, Acapulco, Mexico. Tan, K.H., 1993. Principles of Soil Chemistry, 2nd ed. Dekker, New York. Vanderborght, J., Timmerman, A., Feyen, J., 2000. Solute transport for steady-state and transient flow in soils with and without macropores. Soil Sci. Soc. Am. J. 64, 1305 – 1317. van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892 – 898. van Stiphout, T.P.J., van Lanen, H.A.J., Boersma, O.H., Bouma, J., 1987. The effect of bypass flow and internal catchment of rain on the water regime in a clay loam grassland. J. Hydrol. 95, 1 – 11.
Bypass flow, salinization and sodication in a cracking clay soil Giuseppina Crescimanno *, Antonio De Santis Universita` degli Studi di Palermo, Dipartimento ITAF-Sezione Idraulica, Viale delle Scienze 90128 Palermo, Italy Received 5 December 2002; received in revised form 28 October 2003; accepted 25 November 2003 Available online 22 January 2004
Abstract In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline – sodic waters, thus enhancing the risk of soil secondary salinization and sodication. Sustainable management strategies are urgently needed in Sicily to prevent extent of salinization and sodication, thus preserving soil quality. Since irrigation is performed in cracking soils using irrigation systems involving high application rates, bypass flow of water and solutes occurs during irrigation. The objectives of this paper were (i) to investigate the process of Na – Ca exchange and subsequent salinization/sodication during bypass flow and (ii) to explore possibilities of using cyclic strategies, based on alternating good-quality waters to saline solutions, to prevent salinization. Six NaCl – CaCl2 solutions (named 1 – 6) with the following values of sodium adsorption ratio (SAR, [mmolc/l]0.5) and of cationic concentration (C, mmolc/l)—(1) SAR = 5, C = 20; (2) SAR = 5, C = 1; (3) SAR = 15, C = 25; (4) SAR = 15, C = 2; (5) SAR = 30, C = 30; and (6) SAR = 30, C = 5—were prepared in laboratory. Two undisturbed soil columns were sampled in a Sicilian irrigated area where a risk of secondary salinization and sodication was indicated by previous investigations. Bypass flow experiments were performed by supplying the six solutions to each soil column in the order from 1 to 6 at an almost constant initial value of the cracking volume, using an application intensity determining occurrence of bypass flow. Results of experiments showed that in concomitance with bypass flow, (1) a process of Na – Ca exchange occurred, with Na adsorption and accumulation in the soil. In both the two columns, the exchangeable sodium percentage (ESP) was found to increase by about 100% at the end of the six bypass flow experiments. Being the volume of the supplied saline/sodic solutions much smaller than the pore volume, this result indicates that a considerable risk of sodication may occur under bypass flow conditions; (2) cyclic strategies are useful for preventing accumulation of solutes when solutions with SAR close to the soil ESP are used. Instead, when solutions with SAR higher than the initial ESP are supplied to the same soil, due to the Na – Ca exchange and subsequent sodication, alternating waters having not only lower C but also lower SAR could be necessary to prevent salinization and sodication. D 2003 Elsevier B.V. All rights reserved. Keywords: Bypass flow; Cation exchange; Salinization; Sodication
1. Introduction Conventional sources of good-quality waters in arid and semiarid regions are scarce, which has led * Corresponding author. Tel.: +39-91-7028109; fax: +39-91484035. E-mail address: [email protected] (G. Crescimanno). 0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2003.11.014
to increased use of saline/sodic waters for irrigation (Feigin et al., 1991). In addition, pressure to avoid the disposal of nutrient-rich effluents into freshwater bodies has led to a rapid expansion of effluent reuse by irrigation (Balks et al., 1998). Use and/or reuse of waters with high levels of salinity and sodicity enhances the risk of salinization and/or sodication of irrigated lands (Szabolcs, 1994),
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as well as the hazard of deterioration of soil structural and hydraulic properties (Crescimanno et al., 1995). Typically, the greater the sodium adsorption ratio (SAR) and the lower the concentration (C) of the irrigation water, the greater the potential for aggregate slaking, soil swelling, clay dispersion and thus, reduction in hydraulic conductivity, HC (Cass and Sumner, 1982). In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline – sodic waters, thus enhancing the risk of secondary salinization and sodication. Adequate management practices are urgently needed for sustainable use of saline/sodic waters (Crescimanno, 2001a,b). These management practices depend on an understanding of the physicochemical processes occurring as water moves into the soil and displaces the salts (Marwan and Rowell, 1995). The relevance that bypass flow (BF), i.e., the rapid transport of water and solutes via macropores or cracks to subsoil and to groundwater (Bouma, 1991), may have on solute and/or pesticide transport is becoming increasingly recognized (Ahuja et al., 1991). In structured soils, BF is dominated by soil hydrological processes, such as rain intensity, initial pressure head of the soil, surface storage of rain, and hydraulic conductivity of the soil matrix (Crescimanno and Provenzano, 2000). BF occurs when, during a rain event or during irrigation, rainfall intensity is higher than the absorption capacity on the soil surface (Booltink et al., 1993). The mobility of chemicals in soils is affected by continuity as well as by the size of the macropores and/or cracks (Steenhuis et al., 1995). Instead, total macroporosity is not a very relevant property in relation to water flow processes such as BF, as very few macropores, contributing little to total macroporosity, may dominate transport of water and solutes (Bouma and Dekker, 1978). When irrigation with saline/sodic waters is performed in cracking soils using high-intensity systems (Huang et al., 2000) determining the occurrence of bypass flow, quantitative evaluation of cation exchange (Tan, 1993) and Na adsorption (Bond et al., 1982, 1984) during BF is necessary to predict the risk of salinization and sodication as well as for soil reclamation (Keren and Miyamoto, 1990).
However, although a large number of investigations focusing on transport of nonexchanging solutes or tracers during BF are reported in literature (Kung et al., 2000; Schwartz et al., 2000; Collis-George, 2001), few studies deal with the transport of reactive solutes during BF (Schoen et al., 1999; Li and Ghodrati, 1997). In particular, no investigations exploring the process of cation exchange, with Na adsorption or release under BF conditions, can be found in the current literature. Crescimanno et al. (2002) investigated the effect of alternating different water qualities on accumulation and leaching of solutes in a Mediterranean cracking soil under the occurrence of bypass fluxes. Their study indicated that leaching of solutes occurred during bypass flow, with greater efficiency of removal of soluble salts when the soil was at a considerable degree of cracking. Their results confirmed that solute transport during bypass flow is not a merely convective process, and that diffusion and dispersion play a relevant role (Hillel, 1980). The objective of this paper was to explore the process of cation exchange occurring in a cracking clay soil when saline/sodic waters with SAR progressively higher than the initial exchangeable sodium percentage (ESP) were supplied to the soil at application intensities determining occurrence of bypass flow. Our specific objectives were to investigate, under the indicated conditions, (i) if Na – Ca exchange with subsequent Na adsorption occurred in the soil, thus determining a process of sodication and (ii) if cyclic strategies for salt reclamation (Grattan and Rhoades, 1990), based on alternating waters with different C values, can be efficiently used to prevent salt accumulation.
2. Materials and methods Undisturbed soil cores having different sizes according to the physical and hydraulic characteristics to be measured (Iwata et al., 1995) were sampled from the top layer (C1 horizon, 15 –40 cm) in two very close locations (sites 1 and 2) of a Sicilian vineyard (37j40V55WN; 12j38V50WE). In this field, irrigation with saline/sodic waters is carried out using water stored in the Trinita` reservoir
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by using perforated pipes which allow high application rates at the soil surface. The soil was classified as a Typic Chromoxerert (Soil Survey Staff, 1992). Since irrigation is used during the dry season, when cracks are visible, the high intensity of water application leads to bypass flow (Crescimanno, 2001a). A large soil column was sampled in each site (column 1, column 2, diameter D = 20 cm, height H = 20 cm) for measuring the saturated hydraulic conductivity of the soil matrix, K(sat), by the suction crust infiltrometer method (Booltink et al., 1991). Four undisturbed soil cores (D = 8.5 cm, H = 5 cm), two for each site, were sampled for determination of the unsaturated soil hydraulic parameters. Multi-step outflow experiments were performed on these soil cores in pressure cells by applying three successive steps with pneumatic pressures ranging from 1 to 4, from 4 to 7, and from 7 to 80 kPa (Crescimanno and Iovino, 1995). The van Genuchten – Mualem hydraulic parameters (van Genuchten, 1980) were determined by a previously validated parameter estimation procedure (Crescimanno and Baiamonte, 1999). Four undisturbed soil cores (D = 8.5 cm, H = 11.5 cm), two for each site, were sampled for determining the soil shrinkage characteristic curve (SSCC) by measuring vertical and horizontal shrinkage (Crescimanno and Provenzano, 1999). Based on the water retention curve, h(h), and on the SSCC, the crack volume, DVcr, as percentage of the volume at saturation (V), was calculated (Bronswijk, 1989) as a function of matrix potential (h). In order to evaluate the soil susceptibility to cracking (Parker et al., 1977), the coefficient of linear extensibility, COLE, was also calculated (Grossman et al., 1968). Six NaCl – CaCl2 solutions with three different SAR values and six different cationic concentrations,
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C, were prepared in the laboratory. Table 1 reports the SAR, the C, the electrical conductivity (EC) corresponding to C, the concentration of sodium (Na) and calcium (Ca) in the six solutions. To perform the bypass flow measurements, each undisturbed soil column used for measurement of the K(sat) was placed on a funnel connected with an outflow collector equipped with an analytical scale, which made it possible to measure the weight of the outflow solution. These measurements were stored in a computer. Using a small needle rain simulator, six bypass flow experiments were performed on each soil core by supplying the six solutions in the order from solution 1 to solution 6. To ensure bypass flow conditions, an application intensity (Ir = 11.00 cm/h) higher than the saturated hydraulic conductivity of the soil matrix, K(sat), was used (Bouma, 1991). A constant volume of solution, equal to 1.0 103 cm3, was applied in all the experiments. Application of each solution was performed at an almost constant initial value of the cracking volume (DVcr/V 100), corresponding to an initial matrix potential value (hi) determined, for the two columns, by reading two tensiometers installed at 8 and at 12 cm from the top of the column. Between application of two solutions, the core was brought again to the initial DVcr/V 100 and hi, by an air-drying process which lasted 2 days for each experiment. The three bypass flow experiments performed with solutions 1, 3 and 5 (solutions with increasing SAR) had the purpose to explore the process of Na – Ca exchange and to check occurrence of sodication. To avoid reductions in soil hydraulic conductivity at increasing SAR (Cass and Sumner, 1982), which could affect flow behaviour and chemical exchanges, concentration (C) of solutions 2, 4 and 6 was not kept
Table 1 Composition of the six solutions: sodium adsorption ratio (SAR), concentration (C), electrical conductivity (EC), sodium (Na) and calcium (Ca) Solution
SAR [mmolc l 1]0.5
C [mmolc l 1]
EC [dS m 1]
Na [mmolc l 1]
Ca [mmolc l 1]
1 2 3 4 5 6
5 5 15 15 30 30
20.00 1.00 25.00 2.00 30.00 5.00
2.200 0.156 2.710 0.280 3.150 0.570
10.74 0.91 20.87 1.96 28.17 4.91
9.25 0.07 3.95 0.03 1.80 0.05
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constant, but was slightly increased at increasing SAR (Crescimanno et al., 1995). The three bypass flow experiments performed by alternating the low-salinity solutions (solutions 2, 4 and 6) to the high-salinity waters (solutions 1, 3 and 5) were carried out in order to investigate if cyclic strategies, based on alternating low-salinity solutions to high-salinity waters (Grattan and Rhoades, 1990; Crescimanno et al., 2002), are efficient to prevent salinization when a soil is progressively sodicized. The electrical conductivity (EC) of the effluent solution was measured at fixed time intervals by a conductivimeter (Crison, Micro CM 2002). Sodium (Na) and calcium (Ca) concentrations were determined at the same time intervals by ion-selective electrodes (Orion Research). In order to convert the measured EC (dS/m) into cationic concentration C (mmolc/l), NaCl – CaCl2 solutions of known concentration (C) were prepared, and the EC corresponding to C was measured. The following relationship between C and EC was found: C ¼ 9:514EC
ð1Þ
To check the degree of sodication, 12 soil samples were randomly extracted from each column, and the ESP was determined (by using ammonium chloride) at the end of the bypass flow experiment performed with solution 6.
3. Results and discussion Table 2 reports some physical and chemical properties of the soil at the two sites; the table also reports the K(sat) measured on columns 1 and 2 before the bypass flow measurements. Fig. 1 shows the water retention curve, h(h), as well as the relationship between crack volume (DVcr/ V 100) and matrix potential (h), obtained for sites 1 and 2. The initial (DVcr/V 100) value, at which the bypass flow experiments were performed in both the two columns, was about 4.30%. 3.1. Bypass flow of water and solutes Fig. 2a – f illustrates the cumulative outflow curves obtained for column 1 by the bypass flow measurements performed using the six solutions in the order
Table 2 Physical and chemical soil properties of soil at the two sites and saturated hydraulic conductivity of the two columns Horizon
C1 (15 – 40 cm)
Sand (2 – 0.02 mm) (%) Silt (0.02 – 0.002 mm) (%) Clay ( < 0.002 mm) (%) Texturea COLEb Shrink – swell potentialc EC(1:5) [dS m 1] ESP [%] CEC [mmolc kg 1] Exchangeable cations Na+ [mmolc kg 1] K+ [mmolc kg 1] Ca2 + [mmolc kg 1] Mg2 + [mmolc kg 1]
1
K(sat) [cm h
]
Site 1
Site 2
44 24 32
35 33 32 Clay
0.131 high 0.32 3.4 213.0
0.090 high 0.34 3.6 220.0
7.24 12.8 167.8 25.2
7.9 12.7 171.2 28.2
Column 1
Column 2
1.41
1.21
EC(1:5): electrical conductivity of the 1:5 extract; ESP: exchangeable sodium percentage; CEC: cationic exchange capacity; K(sat): hydraulic conductivity of soil matrix. a ISSS. b COLE values calculated between saturation and air-dry conditions. c Parker et al., 1977.
from 1 to 6. The figures report the inflow and the outflow volume, V(t), as a function of time (t), the total concentration, C (mmolc/l), of the effluent solution (Cout), and the concentration of the incoming solution (Cin). The cumulative V(t) curves all showed identical behaviour. After the time when outflow started, td, an exponential progression between the times td and tcs (start of linear outflow) could be observed. Between tcs and tce (end of linear outflow), there was a nearly linear outflow pattern, and after tce, an exponential fade out pattern was evident. The linear outflow occurring between tcs and tce (i.e., the time derivative of outflow is constant) has been observed in earlier measurements (van Stiphout et al., 1987; Booltink et al., 1993; Crescimanno et al., 2002) and denotes occurrence of bypass flow (BF). In the experiment performed with solution 1, reported in Fig. 2a, we can see that the Cout values are initially lower than the Cin, with a tendency to
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Fig. 1. Water retention curve, h(h), and relationship between DVcr /V 100 and matrix potential (h) for the two sites.
reach the Cin value when water application stops (time te, end of the linear outflow), when the Cout reaches the maximum value. This maximum Cout, also observed in previous experiments (Crescimanno et al., 2002), proves that bypass flow of solutes is associated with bypass flow of water. The decrease in Cout after the end of water application (end of BF), when the macropores stop contributing to outflow and solutes are released from the matrix only, is a consequence of solute redistribution and concentration gradients within the micropores (redistribution phase, RED). With reference to the measurements performed with solutions 3 and 5, reported in Fig. 2c and e, no maximum Cout can be identified in concomitance with the end of the linear outflow. Instead, a continuous increase in Cout at increasing t can be observed after termination of water application, with Cout exceeding Cin. Release of some cations from the soil matrix during bypass flow and also after termination of bypass flow could be the reason for this continuous increase in Cout. Since solutions with SAR higher (SAR = 15 and SAR = 30) than SAR of solution 1 (SAR = 5), as well
as of initial ESP (ESP = 3.4%), were used in the experiments reported in Fig. 2c and e, the reason for the observed increase in Cout could be the occurrence of cationic exchanges between the incoming solution and the soil matrix, with sodium (Na) adsorbed into the exchange phase and calcium (Ca) and/or magnesium (Mg) released from the exchange phase. As the exchangeable Mg content in this soil is much smaller than the exchangeable Ca content (Table 2), we consider Ca to be responsible for the increase in Cout. In the experiments performed with solutions 2, 4 and 6, reported in Fig. 2b, d and f, the measured Cout decreases after the start of rainfall, reaching a minimum value at the end of bypass flow. As in this case Cout >Cin, the decrease in Cout following water application proves that leaching of solutes from the matrix occurs in concomitance with bypass flow of water. The subsequent increase in Cout occurring when bypass flow has terminated confirms solute redistribution and concentration gradients within the micropores (Cote et al., 1999), with solutes previously adsorbed by the soil matrix partly flowing in the outcoming solution. Fig. 3a – f illustrates the cumulative outflow curves obtained for column 2 by the bypass flow
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Fig. 2. (a – f) Outflow volume V(t) and concentration (Cout) of the effluent solution as a function of time, t; concentration of the incoming solution (Cin), during the six bypass flow experiments for column 1.
measurements performed using the six solutions in the order from 1 to 6. The figures show a similar behaviour of column 2 compared to that of column 1, with no maximum Cout in concomitance with the end of the linear outflow (Fig. 3c and e) and with a continuous increase in Cout at increasing t
after termination of water application, with Cout exceeding Cin. Fig. 3b, d and f also confirm the similar behaviour of the two columns. Replication of the bypass flow experiments on column 2 confirmed results and speculations made for column 1.
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Fig. 3. (a – f) Outflow volume V(t) and concentration (Cout) of the effluent solution as a function of time, t; concentration of the incoming solution (Cin), during the six bypass flow experiments for column 2.
3.2. Na –Ca exchange, salinization and sodication during bypass flow Fig. 4 reports for columns 1 and 2 the Na concentration (mmolc/l) measured in the outcoming solution (Naout) during the six experiments as a function of
time, together with the concentration of Na in the incoming solution (Nain). As can be seen in the figure, Naout is lower than Nain for the experiments with solutions 1, 3 and 5; this indicates that part of the Na entering the soil with the incoming solution is adsorbed by the exchange phase.
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Fig. 4. Concentration of sodium (Naout) in the outcoming solution as a function of time, t, and concentration of sodium in the incoming solution (Nain), for the six bypass flow experiments.
Fig. 5 reports for columns 1 and 2 the Ca concentration (mmolc/l) measured in the outcoming solution (Caout) as a function of time during the six experiments, together with the concentration of Ca in the incoming solution (Cain). Ca appears to be released from the soil in all six experiments. In experiments with solutions 1, 3 and 5, release of Ca can be the consequence of Na – Ca
exchange and is in agreement with the observed Na adsorption. In experiments with solutions 2, 4 and 6, the observed Ca release is a consequence of the decrease in the cationic concentration of the applied solutions, at constant SAR, which determines a process of salt leaching. Table 3 reports the quantities (mmolc) of Na and Ca supplied to the soil columns (Nain, Cain), leaving
Fig. 5. Concentration of calcium (Caout) in the outcoming solution as a function of time, t, and concentration of calcium in the incoming solution (Cain), for the six bypass flow experiments.
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Table 3 Quantity of sodium (Na) and calcium (Ca) entering the soil (Nain, Cain), of Na and Ca bypassing the soil matrix (Nabf, Cabf) and leaving the soil core during the whole outflow process (Naout, Caout) Solution
1 2 3 4 5 6
Nain [mmolc]
Cain [mmolc]
10.74 0.91 20.87 1.96 28.17 4.91
9.25 0.07 3.95 0.03 1.80 0.50
Column 1
Column 2
Nabf [mmolc]
Cabf [mmolc]
Naout [mmolc]
Caout [mmolc]
Nabf [mmolc]
Cabf [mmolc]
Naout [mmolc]
Caout [mmolc]
2.07 1.23 2.03 2.29 2.98 2.90
6.04 4.37 8.21 5.93 7.28 5.93
3.05 1.82 3.12 3.21 4.38 4.20
9.19 6.54 12.45 8.48 10.61 8.90
1.60 1.45 1.89 2.16 2.75 2.20
3.30 2.84 5.27 4.11 5.44 4.18
2.43 2.50 3.02 3.38 4.82 3.87
5.05 4.68 8.15 5.78 9.38 7.17
the columns during BF (Nabf, Cabf) and during the whole outflow process (Naout, Caout). The quantities of Na and of Ca were determined by integrating the Na and Ca concentrations (mmolc/l) vs. the cumulative outflow volume, V(t) (l). To explore the process of Na – Ca exchange during bypass flow (BF), the positive and the negative values of the difference (DNabf = Nain Nabf), which represents the quantity of Na accumulated or released during BF, were calculated and are reported in Table 4 for columns 1 and 2. The table also reports the quantity of Na released during the redistribution (RED) phase, represented by DNared = (Naout Nabf). For both the two columns, positive DNabf values, indicating Na adsorption by the soil during BF, were found for the experiments with solutions 1, 3 and 5; negative but negligible DNared values, indicating a nonsignificant release of Na during the RED stage, were found for the same experiments. Negative but negligible DNabf, values, indicating a nonsignificant release of Na during BF, were also Table 4 Na accumulated or released during BF (DNabf), and Na released during the redistribution phase (DNared) Solution
1 2 3 4 5 6
Column 1
Column 2
DNabf [mmolc]
DNared [mmolc]
DNabf [mmolc]
DNared [mmolc]
8.67 0.32 18.84 0.34 25.19 2.01
0.98 0.59 1.09 0.92 1.40 1.30
9.14 0.54 18.98 0.20 25.42 2.71
0.84 1.05 1.12 1.21 2.07 1.67
found during application of solutions 2 and 4. This result appears to be in agreement with the fact that during application of these solutions, only C was decreased at a constant SAR. Instead, an unexpected positive DNabf was found for both the columns in the experiment with solution 6, probably due to the considerable increase in the SAR, which determined further adsorption of Na into the soil. Table 4 shows that for both the two columns, increasing values of DNabf, indicating a progressive Na adsorption in the soil, were found for experiments 1, 3 and 5. Since nonsignificant amounts of Na were released during the RED phases as well as during application of leaching solutions (2, 4), with further Na adsorption during application of solution 6, this result indicates that sodication occurred during the bypass flow events. To analyse accumulation or leaching of Ca, the difference (DCabf = Cain Cabf,), which represents the quantity of Ca accumulated or released during BF, was calculated and is reported in Table 5 for the two columns. The same table also reports the quantity of Ca released during the RED phase, represented by DCared = (Caout Cabf). For both columns, a positive DCabf was found only for the experiment performed with solution 1, while negative values, indicating release of Ca, were found for all the other experiments either during BF or during the RED phases. In experiments with solutions 3 and 5, this Ca release is in agreement with the continuous increase in Cout illustrated in Figs. 2c and e and 3c and e, being a consequence of the Na – Ca exchange occurring dur-
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Table 5 Ca accumulated or released during BF (DCabf), and Ca released during the redistribution phase (DCared) Solution
1 2 3 4 5 6
Column 1
Column 2
DCabf [mmolc]
DCared [mmolc]
DCabf [mmolc]
DCared [mmolc]
3.21 4.30 4.26 5.89 5.48 5.88
3.15 2.17 4.24 2.55 3.33 2.97
5.95 2.77 1.32 4.08 3.64 4.13
1.75 1.84 2.88 1.67 3.93 2.99
ing BF. In experiments with solutions 2, 4 and 6, the Ca release is a consequence of the decrease in C at constant SAR. To analyse accumulation and/or leaching of solutes in the two columns, the quantity (mmolc) of solutes entering the soil (Sin), leaving the soil during bypass flow (Sbf) and during the whole outflow process (Sout), was calculated and is reported in Table 6. Determination of S (mmolc) was performed by integrating the concentration C (mmolc/l), obtained by using the relationship (1), vs. the cumulative outflow volume, V(t), (l). The value of the difference (DSbf = Sin Sbf), which represents the amount of solutes accumulated or released during BF, is represented in Fig. 6a and b for the six experiments and for the two columns. The positive DSbf values found for the two columns for experiments with solutions 1, 3 and 5 indicate a process of solute accumulation occurring during BF; the negative DSbf found for the experiments with solutions 2, 4 and 6 indicate occurrence of leaching. Fig. 6a and b also report the values of DSred = (DSout DSbf), which represents the amount of solutes leached from the column during the RED phases. As the absolute values of DSred were always lower than the DSbf values, a process of salinization evidently occurred in both the two columns between application of solutions 1 and 6. To get further insight in the process of Na – Ca exchange occurring during BF, the values of the difference (D Sbf D Cabf ) between the quantity (mmolc) of solutes accumulated during bypass flow
(DSbf) and the quantity of Ca released during BF (DCabf) were calculated and represented in Fig. 7a and b for columns 1 and 2. The figures also report the quantity (DNabf) of Na accumulated during BF. The figures show that the difference (DSbf DCabf) quantitatively corresponds to DNabf, except for very small discrepancies which could be attributed to experimental errors in the measurement of Na and Ca or to release of small amounts of other ions not measurable by the electrodes. This analysis indicates that the Na replacing the Ca in the soil is progressively accumulated, thus determining a process of sodication. These results prove that cation exchange occurs during bypass flow, confirming that solute diffusion (Cote et al., 2000; Rowell and Pateras, 2002) and hydrodynamic dispersion (Forrer et al., 1999; Vanderborght et al., 2000) both play a relevant role in solute exchange and in the process of salt leaching (Crescimanno et al., 2002). However, unexpected high ESPs value were found in both the two columns at the end of the six experiments especially if we consider that a small volume of solution (1.0 103 cm3) was supplied to the soil during each bypass flow experiments compared to the pore volume of the soil (3.4 103 cm3 for column 1, 3.7 103 cm3 for column 2). An average ESP of 7.02% was found for column 1 and an ESP of 7.4 was found for column 2. Since the initial ESPs were 3.4% for column 1 and 3.6% for column 2, in both the two columns, the ESP increased by about 100% or even more at the end of the six bypass flow measurements.
Table 6 Quantity of solutes (S) entering the soil (Sin), of solutes bypassing the soil matrix (Sbf) and leaving the soil columns during the whole outflow process (Sout) Solution
1 2 3 4 5 6
Sin [mmolc]
Column 1 Sbf [mmolc]
Sout [mmolc]
Sbf [mmolc]
Sout [mmolc]
20.00 1.00 25.00 2.00 30.00 5.00
8.21 5.92 9.87 7.95 9.93 8.57
12.64 8.84 14.91 11.30 15.42 13.27
5.61 5.10 7.77 7.13 9.28 7.66
8.84 8.55 12.04 10.57 16.33 13.39
Column 2
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Fig. 6. Solutes accumulated or released during BF (DSbf), and solutes released during the redistribution phase (DSred), for the six bypass flow experiments.
These results indicate that a considerable increase in the soil ESP may occur when application of a saline/sodic solution is performed at a considerable degree of cracking using water application intensities determining bypass flow. In addition, since our results indicated that cation exchange mainly occurred during bypass flow, we can speculate that the process of cationic exchange occurs at the contact surfaces between the incoming solution
and the cracks in the walls. This would lead to the conclusion, which needs further experimental verification, that higher salinization and sodication might be associated with bypass flow than with matrix flow conditions. Collis-George (2001) also found that during drainage of a soil, the effluent solution was mainly from the external surfaces. He observed that in the field, restraints on the free expansion of the double layer
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Fig. 7. Difference (DSbf DCabf) between the quantity of the solutes accumulated during bypass flow (DSbf) and the quantity of Ca released during BF (DCabf); quantity of Na accumulated or released during BF (DNabf) for the experiments with solutions 1, 3 and 5.
are imposed by the aggregate structure at all sizes as well as by the matrix potential, and only those surfaces adjacent to pores and/or cracks can develop thick double layers when ponded. Considerations of Collis-George and results of our experiments confirm previous results indicating that the mechanism of Na release during BF depended on the magnitude of the contact surfaces between the crack walls and the incoming solution (Crescimanno et al., 2002).
3.3. The effect of alternating waters with different concentrations at increasing SAR-ESP Fig. 8a and b report for columns 1 and 2 the quantity of solutes (DS) accumulated or released during each outflow experiment (including either the BF or the RED phases) together with the quantity of solutes (DSseq = SDS) accumulated or released during each sequence (seq) of alternating solution 2 to 1 (seq 1), solution 4 to 3 (seq 2) and solution 6 to 5 (seq 3).
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Fig. 8. Quantity of solutes (DS) accumulated or released during each bypass flow experiment and accumulated or released during sequence 1 (DSseq1), sequence 2 (DSseq2) and sequence 3 (DSseq3).
Each sequence represents a process of alternating a low C to a higher C at a constant SAR; the three sequences altogether represent a process of alternating waters with different C at increasing SAR and with SARs progressively higher than the initial ESP. This situation may occur when cyclic strategies for use or reuse of saline/sodic waters are adopted to keep salinity under levels tolerable for crops (Maas, 1990).
Fig. 8a and b show that a negative DSseq value, proving no accumulation of solutes in column 1, and a positive but negligible DSseq, proving a nonsignificant accumulation of solutes in column 2, were associated to seq 1. A significant leaching evidently occurred when solution 2 was alternated to solution 1. This result confirms previous findings of Crescimanno et al. (2002), indicating that release of
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solutes occurs under bypass flow conditions when a low-salinity water is alternated to a higher salinity solution. Instead, the positive and increasing values of DSseq represented in Fig. 8a and b indicate that a progressive accumulation of solutes and thus, a reduced efficiency of salt leaching was associated to seq 2 and especially to seq 3 for both the two columns. The decreasing efficiency of leaching found for seq 2 and seq 3 is due to the process of Na –Ca exchange and subsequent sodication occurring at increasing SAR. In seq 1, since a solution with SAR close to the initial ESP was used, alternating different C values was efficient to remove the accumulated solutes. Instead, in seq 2 and seq 3, not only C but also SAR should have been be probably decreased to avoid a process of progressive salinization.
should be probably alternated to solutions with higher C and SAR to prevent salinization and sodication. Further research is underway to investigate how flow conditions (bypass or matrix flow) may affect cation exchange, salinization and sodication. The effect of alternating waters with different C and SAR will be also investigated in order to find management strategies preventing salinization and sodication and thus minimizing the increasing risk of desertification in Sicily.
Acknowledgements Research funded by MURST (Rome, Italy), PRIN 2000: ‘‘Processi di trasporto dell’acqua e di soluti in terreni argillosi irrigati con acque salino/sodiche’’.
References 4. Conclusions The experiments described in this paper showed that in a cracking soil, under bypass flow conditions, a process of Na –Ca exchange occurred at the contact surfaces between the incoming solutions and the cracks walls. This exchange determined Na adsorption in the soil when solutions with increasing SAR were supplied to the soil. Measurement of the exchangeable sodium percentage (ESP), performed at the end of the six bypass flow experiments, proved an unexpected considerable increase in the soil ESP compared to the initial ESP value. As the supplied volume of saline/sodic solutions was very small compared to the pore volume, this result indicated that a considerable risk of sodication may occur under bypass flow conditions. In addition, the experiments performed showed that alternating a low-salinity water to a high-saline solution is a management practice preventing accumulation of solutes when solutions having SAR values close to the soil ESP are applied to the soil. Instead, when waters having SAR values higher than the initial soil ESP are progressively supplied to the same soil, sodication occurring as a consequence of Na – Ca exchange prevents leaching of solutes. In this case, solutions having both lower C and SAR
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