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Reductions in hydraulic conductivity and infiltration rate in relation to aggregate stability and irrigation water turbidity
Agricultural water management Agricultural
Water Management
29 ( 1995) 53-62
Reductions in hydraulic conductivity and infiltration rate in relation to aggregate stability and irrigation water turbidity T.M. Abu-Sharar
*, A.S. Salameh
Department of Soils and Irrigation, The Univer.siry of Jordan, Amman, Jordan Accepted 25 January 1995
Abstract Arid and semiarid soils are characterized by reductions in hydraulic conductivity (HC) and infiltration rate (IR) when employing low salinity/sodicity waters. Such reductions are further exacerbated when irrigating with turbid water. Critical salt coagulation concentrations (CCCs) were determined at constant SAR 5, 10, 15, or 20 for clay fractions of a semiarid surface sample of the Muaq’qar soil (fine, mixed thermic, Typic Calciorthid) and for turbid runoff rain water, containing clay particles, which was generated from the same area. The results showed similar values for both types of clay (predominated by mica and kaolinite). When conducting field measurements of IR or permeating duplicate columns of 1:l soiLsand mixtures with solution series of decreasing concentration (50 to 0 mol, m-l) at any of the above SAR levels, IR and HC decreased with decreasing electrolyte concentration. Substantial losses in IR and HC ( -40-60%) were observed when permeating solutions with electrolyte concentration equal to or higher than the respective CCC of the soil-clay. However, IR and HC dropped to approximately E-20% of their respective maximum values with the permeation of turbid runoff water (0.07% fine clay) adjusted to electrolyte concentration slightly lower than the corresponding CCC of the soil-clay (5, 10, 15, or 20 mol, me3 at SAR 5, 10, 15, or 20, respectively). Such reductions were larger than the corresponding reductions in HC and IR when permeating clear solutions of equivalent SAR and electrolyte concentration (A HC = 50-60% and AIR = 5 1 + 3%). This result indicated the ability of suspended fine clay particles in the permeating water to flocculate in the soil conducting pores causing their clogging. Keywords:
Salinity; Sodicity; Critical coagulation
concentration;
Surface crust
1. Introduction
Arid and semiarid soils are generally characterized by poor aggregate stability and crust formation at the surface (Shainberg and Letey, 1984). Deterioration in soil structure may * Corresponding
author.
037%3774/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved XSDIO378-3774(95)01184-6
54
T.M. Abu-Sharar, A.S. Salameh /Agricultural Water Management 29 (1995) 5362
take place even when irrigating non-sodic soils with waters of low SAR and salinity (Sumner, 1993). Because of crust formation, the resultant infiltration rate (IR) tends to decrease to a minimum value irrespective of the initial soil moisture content (Morin et al., 1989). Therefore extensive surface runoff and thus soil erosion may be generated from rainstorms of low quantity and intensity. In general, most of the suspended material in the surface runoff water would be made of silt and clay fractions ( Abu-Sharar, 1993). This is mainly due to the fragile nature of soil structure which initiates substantial aggregate slaking and clay dispersion even at predominately low rainfall intensity. Provided that the harvested turbid runoff water may be stored for some time before being utilized in irrigation, the dispersed silt and coarse clay particles will settle down and only fine clay particles may remain unflocculated and suspended. If the resultant turbid runoff water is destined for future irrigation, the concurrent low water salinity and suspended fine clay particles can cause considerable difficulty in the management of such water like crust formation, substantial reduction in IR, water ponding on soil surface, poor seedling emergence, and extensive soil erosion. Although sufficient data on critical coagulation concentration (CCC) of pure clay suspensions are available in the literature, non-correspondence between CCC of these clay specimens and clays extracted from runoff water was reported. For example, Goldberg and Forster ( 1990) found CCC values for soil clays to be between two- and ten-fold higher than for pure clay systems. Therefore, the objectives of this study were to (i) determine mineralogy and CCC of low and intermediate SAR levels for clay fractions separated from surface soil samples and turbid surface runoff water, both collected from the semiarid area of Muaq’qar (Jordan), and (ii) evaluate reductions in IR and hydraulic conductivity (HC) of that soil when employing clear (synthetic) and turbid (runoff) solutions of decreasing salinity at constant SAR level. 2. Materials and methods 2.1. Soil and s&ace
runoff water
A turbid surface-runoff water sample was obtained in June, 1991 from a small reservoir in the Muaq’qar Experiment Station (30 km to the east of the capital Amman). Suspended clay in sub-sample of that water was first flocculated using NaCl salt. Soluble salt was then washed by successive shaking and centrifugation using distilled water. The salt-free clay slurry was air-dried, ground to pass 2 mm sieve, and stored for future analysis. A composite soil sample taken from the surface (o-0.25 m depth) was collected from the Station. The soil is classified as fine, mixed thermic, Typic Calciorthid (Staff Soil Survey, 1978). For both samples of soil and runoff-extracted clay, the following determinations were made: particle size distribution, OM, CaCO,, CEC, EC, and pH. X-ray diffraction analysis (wavelength of Cu K, = 1.5405 nm) for oriented clay samples was carried out according to the standard methods of analysis (Klute, 1986; Page, 1986). 2.2. Critical coagulation
concentration
Standard Na-clay suspension was obtained from a sub-sample of the soil or the turbid surface runoff water. Each sub-sample was successively suspended in a 1-L beaker con-
T.M. Abu-Sham-,
AS. Sulatneh /Agricultural
Water Munagement 29 (1995) 53-62
55
taining 1 mol,me3 NaCl solution, then decanted to a predetermined depth after standing still for a calculated settling time of the < 2 pm clay particles using Stokes’ law. Sodiumclay suspensions ( 1% w/v) in deionized water (working suspensions) were prepared from each standard clay suspension using successive suspension in water, centrifugation at approximately 15 - 000 rev min- ‘, and decantation of the supernatant solutions. These saltfree Na-clay suspensions were employed in the determination of CCC at SAR 0 (Ca-clay), 5, IO, 15, and 20. The determination of CCC at the respective SAR was carried out by pipetting 5 ml volumes from each working clay suspension into a series of test tubes of identical optical transmission (%T). To each test tube, appropriate volumes of deionized water, NaCl, and CaCl, standard solutions totalling 5 ml volume were added at 1 mol, me3 increments in total electrolyte concentration. The clay suspensions were then shaken for 30 min and left still for additional 24 h after which %T was measured at 420 nm wavelength using a spectrophotometer. The CCC was evaluated as the electrolyte concentration which caused a sharp rise in %T. To obtain a more accurate estimate of each CCC, a new series of ten clay suspensions was similarly tested using solutions of electrolyte concentration ranging between CCC - 0.5 and CCC + 0.5 mol, rnp3 with 0.1 mol, mm3 increments. 2.3. Hydraulic conductivity Because of the relatively fine sizes of dry soil aggregates and the resulting low HC, a soil sub-sample was mixed thoroughly with an equivalent weight of HCl-washed sand ( < 1 mm diameter). Portions of the mixture (300.0 g) were then packed in glass cylinders to 15 cm depth at 1.3 Mg rnp3 bulk density. Each soil column was wetted from below using NaCl/CaCl, solutions of 50 mol,me3 total electrolyte concentration at the respective SAR 5, 10, 15, or 20. Steady-state HC of duplicate soil columns was measured for each of the successively permeated solutions of decreasing electrolyte concentration (50, 25, 20, 15, 10, 7.5,5, 2.5, and 0 mol,m-3) using constant head method of Marriotte bottle device and measuring the time for a fraction collector to collect 8 ml volumes of the effluent solutions. The permeation of each electrolyte solution continued until reaching steady-state HC as evidenced by reporting coefficient of variation less than 5% for the last ten HC measurements. For one of each duplicate soil column, electrolyte solutions of 5, 10, 15, or 20 mol,m - 3 and the respective SAR 5, 10, 15, or 20 were replaced by the turbid surface runoff water adjusted to the corresponding SAR and electrolyte concentration. If dispersed clay appeared in the effluent solutions, the %T was measured for each successive effluent fraction as long as the %T did not increase again to a stable level. 2.4. Infiltration rate Duplicate field measurements of IR were carried out using the same solution series of constant SAR as employed in the measurement of HC. The measurements were conducted employing double ring infiltrometer with a constant head (few millimeters above the soil surface) device comprised a 1-L graduated cylinder connected to the central ring through a rubber tube. For each SAR solution series, measurement of IR at a given electrolyte concentration continued every 5 min until steady-state value was observed as described
56
T.M. Abu-Sharar, AS. Salameh /Agricultural
Water Management 29 (1995) 5342
earlier in the measurement of HC. In one series of each IR duplicate measurements, solutions having total electrolyte concentrations of 5, 10, 15, or 20 mol, rnd3 at the respective SAR 5, 10, 15, or 20 were replaced by sub-samples of the turbid surface runoff water adjusted to the corresponding SAR-electrolyte concentration combination in a manner analogous to that formerly employed in the HC measurements.
3. Results and discussion 3.1. Critical coagulation
concentration
The Muaq’qar soil is a calcareous silt loam of low organic matter content (Table 1) . Clay mineralogy of the soil sample and of the suspended clay fraction in the turbid surface runoff water was dominated by mica and kaolinite. Because of its poor content of organic matter and the micaceous mineralogy (Na-mica has the highest CCC of 48 f 11 mol, me3 (Sposito, 1984) ), the Muaq’qar soil exhibited poor aggregate stability. Fig. 1 shows sample results for the %T of clay suspensions from the Muaq’qar soil and the turbid surface runoff water as influenced by increasing electrolyte concentration at constant SAR values of 5 or 20. Because of space limitation and similarity of results, other results of SAR 10 and 15 were not presented. The figure also indicates a sharp increase in the %T with increasing electrolyte concentration by one unit beyond a certain limiting value. Such an increase in the %T reflected colloidal instability and, thus, corresponded to the respective CCC value. For both clay suspensions, CCCs at a given SAR were close to each other and increased linearly with increasing solution SAR to 20 (Fig. 2). The slightly higher CCC of the dispersed clay in the surface runoff water can be attributed to the relatively finer particle sizes of that clay as compared to the <2 pm soil-clay fraction. The runoff water was sampled in June after a period of suspension long enough to permit settling of sizeable clay Table 1 Selected physical, chemical Experiment Station Soil Sand (%)
Silt (%)
and mineralogical
Clay (%)
OM (%)
properties
of the soil and surface runoff water of the Muaq’qar
CaCO,
CEC (cmol( + )
Mineralogy
kg-‘)
EC, (dS m-‘)
pH
(%) 18.3 55.0 Surface runoflwater Sand (g Silt (g 1-1) 1-1)
26.1
0.42
21.5
14.5
0.40
8.2
K,M”
Clay (g I_‘)
OM (g per 100 g)
CaCO, (g per 100 g)
CEC (cmol( + )
EC (dS m-‘)
pH
Mineralogy
kg-‘))
0.00
0.70
0.30
8.0
49.8
0.23
8.0
K,M
0.10
Soluble cations (mol, ,n ‘) Ca Na Mg 1.00
0.30
‘K, kaolinite; M, mica.
0.8.5
K 0.17
T.M. Abu-Sharar, A.S. Salameh /Agricultural Water Management 29 (1995) 5362
80-
. , - ‘w?5
.
,
.
,
.
,
57
. +
60k 40 ;Jq
* Suspend-d cloy in runoff woter . 20
3
.
,
.
5
,
.
7
, 9
.
,
.
11
-
. , . , . , . , . , . 14
13
. , . , , , . , . , . ,
80
Suspended cloy In runoff water 20 80
18
20
22
.,.,.,‘,.,.
.
sAR5
16
24
26 _
sAR20
80-
40
i
j_J<2 J&m soil-cloy
20-, . , . . , . , . , , , 0 2 4 6 8 10 12
Electrolyte
Concentration
(molcm-3)
2oih3377d6
Electrolyte
Concentration
Fig. 1. Effect of increasing electrolyte concentration on %T of clay suspensions soil and the turbid surface runoff water generated from that area.
fractionated
(molcm-
.3)
from the Muaq’qar
particles. However, CCCs for both clay fractions were higher than the corresponding values for illite suspensions of equivalent SAR, e.g. 0.25, 9.0, and 17.5 mol, me3 at SAR 0, 10,
Sodium
Adsorption
Ratio
Fig. 2. Relation between SAR and critical coagulation concentrations for the clay fraction of the Muaq’qar clay and the suspended clay in the turbid surface runoff water generated from that area.
soil
58
T.M. Abu-Sharar, A.S. Salameh/Agricultural
Wuter Management 29 (1995) 5342
and 20, respectively (Oster et al., 1980). Such differences could probably be due to the weathering of the soil-mica that may have rendered particle surfaces more irregular. Upon close approach of such particles, the unavoidable mismatch of the terraces can lead to poor contact between the edges and surfaces, causing a smaller edge-to-face attraction force and, consequently, a higher CCC (Greene et al., 1978). 3.2. Hydraulic conductivity Permeating soil columns with electrolyte solutions of decreasing concentration down to 0 moI, m-3 at constant SAR 5, 10, 15, or 20 showed a reduction in HC (expressed as relative values of the maximum HC reported when permeating the most concentrated electrolyte solution of 50 mol, m-‘) with decreasing electrolyte concentration, especially at higher SAR levels. Fig. 3 shows selected results for solutions SAR 5 and 20. At any SAR level, electrolyte concentration at which HC started to decrease was much higher than the corresponding CCC of the clay fraction. Subsequently, the major cause for such reductions was only due to aggregate slaking rather than to clay dispersion, migration and clogging of conducting pores as previously reported (Felhendler et al., 1974; Frenkel et al., 1978;
0) T; uz 5 ‘Z e g r
10
10
5
5
0
0 0
Electrolyte
Concentration
(mol,m -3)
Electrolyte
20
40
60
Concentration
00
100
(molcm-3)
Fig. 3. Effect of decreasing electrolyte concentration on reduction in HC (upper) and IR (lower) of the Muaq’qx soil. Circles mark permeating only electrolyte solutions but triangles mark permeating electrolyte solutions interrupted by the permeation of the turbid surface runoff waters of 5 or 20 mol, m-j at SAR 5 or 20, respectively.
T.M. Abu-Sharar.
AS. Salameh /Agricultural
59
Water Management 29 (1995) 53-62
Shainberg et al., 1981). This result was similar to former findings for semiarid soils of similar mineralogy from Jordan and California ( Abu-Sharar, 1993; Abu-Shararet al., 1987a, respectively). At a given SAR, decreasing electrolyte concentration of the permeating solutions to values slightly lower than the corresponding CCC of the soil-clay (5, 10, 15, and 20 mol, m-l compared with the CCCs of 6.5, 11.5, 16.5, and 21.5 mol, m-l for the respective SAR 5, 10, 15, and 20) caused HC to drop to 41%, 55.6%, 52%, and 57%, respectively. However, permeation of the turbid surface runoff water adjusted to either one of the above combinations of SAR-electrolyte concentration caused HC to drop to 19%, 21.4%, 17.5%, and 15%, respectively. Such large differences between comparable HC values (22, 34.2, 34.5, and 42%, respectively) were mainly due to the clogging of the conducting pores by the suspended clay particles in the surface runoff water. This, in fact, showed that little dispersion (0.07%) of fine clay particles in the permeating solution can effectively reduce HC to minimum values most probably owing to flocculation of these particles as the turbid water flows in the vicinity of exchangeable cations atmosphere of electrolyte concentration greater than the corresponding CCC. However, this hypothesis can be applied only when electrolyte concentration of the permeating solution drops to levels slightly lower than the corresponding CCC of the soil-clay. Abu-Sharar (1988) argued that at a given SAR, CCC could be slightly higher than the electrolyte concentration at which clay starts to disperse from soil aggregates. This is mainly due to the fact that additional force is needed to break down soil aggregates. Soil clays are covered with a variety of organic and inorganic films and coatings often bonded together into aggregates by various cementing agents (Sumner, 1993). These aggregates are recognized as the product of Bradfield’s ‘flocculation plus’ (Bradfield, 1936). On the other hand, if the electrolyte concentration of the permeating solution drops to levels much lower than the corresponding CCC, the soil-clay may disperse as the ultimate product of aggregate breakdown ( Abu-Sharar et al., 1987b). For all SAR levels, a substantial drop in %T of the effluent solutions was only observed when permeating deionized water (Fig. 4). At the final stage, HC dropped almost to levels equivalent of these observed when permeating the respective turbid surface runoff water. 100.
100. _t_
solutions 80
--
80.
I
sol.lsus. I8
$2
60. 40.
solutions
--
sol.lsus. A
x&&&A-&AL
60.
fls
20 t y=
SAR 5 40
-
0
SAR 20
07
+
250 Effluent
500 volume
750 (ml)
0
250 Effluent
500 volume
750 (ml)
Fig. 4. The %T of effluent solutions from the Muaq’qar soil columns permeated with distilled water following permeation with solution series of decreasing electrolyte concentration (circles) or solutions interrupted by permeation of the turbid surface runoff water (triangles) of 5 or 20 mol, me3 at SAR 5 or 20, respectively.
TM Abu-Sharar, AS. Salarneh /Agricultural
Water Management 29 (1995) 5362
0
0
10
20
30
Reduction
40
in
50
60
relative
Fig. 5. Relation between reduction
in relative
concentration
electrolyte
00
HC
solutions
of equivalent
70
20
40
Reduction
HC and the corresponding
reduction
60
in relative
80
HC
in relative IR when permeating
and SAR.
The %T of the effluent solutions used to peak after permeation of about one pore volume ( 128 ml). The minimum %T reported when permeating deionized water decreased with increasing SAR of the previously permeated electrolyte solutions and, thus, indicated more clay dispersion (Fig. 4). In addition, the %T of the effluent solutions from soil columns previously permeated with the turbid surface runoff waters did not drop to levels comparable with these pertaining to the corresponding soil columns permeated only with the electrolyte solutions. This observation further supports the above conclusion that the dispersed clay in the turbid surface runoff water can clog the conducting pores of the soil columns and consequently restricts the mobility of clay particles dispersed from the soil aggregates. 3.3. Infiltration rate At a given SAR, IR decreased with decreasing electrolyte concentrations of the permeating solution in a manner analogous to that of the HC (Fig. 3). When the soil was permeated with solutions of electrolyte concentration slightly lower than the corresponding CCC of the soil-clay (as these employed in the HC measurements), the IR dropped to similar value
TM. Abu-Sharar, AS. Salameh/Agricultural
Water Management 29 (1995) 5342
61
(50.5 + 3.4%) irrespective of the SAR level. However, with the replacement of the former solutions by turbid surface runoff waters of equivalent SAR and electrolyte concentration (as in the HC study), the IR dropped to a much lower level ( 15.2 + 1.4%) irrespective of the SAR value. This may be explained by the sensitive nature of IR to any disturbance in surface soil structure. Moreover, the former decline in IR was also lower than the common minimum value reported when permeating distilled water following the permeation of only electrolyte solutions of any SAR (20.7 + 4.0%). This result conforms to the conclusion that suspended clay particles in the permeating surface runoff water were effective in clogging the conducting pores of the surface soil and reducing IR to a minimum value. The relation between reductions in HC and IR when employing only electrolyte solutions of decreasing salinity at constant SAR was almost 1: 1 (Fig. 5). Therefore, early reductions in IR, i.e. when electrolyte concentration of the permeating solution was higher than the corresponding CCC of the soil-clay, can be attributed to slaking of the surface soil aggregates. Clay dispersion and the subsequent surface crust formation play an effective role in the reductions of IR only when electrolyte concentration of the permeating solution drops well below the corresponding CCC of the soil-clay.
4. Conclusion Substantial losses in HC and IR ( =40-60%) of the semiarid Muaq’qar soil may take place when permeating solutions with electrolyte concentrations not lower that the CCC of the soil-clay at equivalent SAR. Clay dispersion requires permeation of solutions having electrolyte concentration well below the respective CCC of the soil-clay. For a given SAR, substantial clay dispersion in effluent solutions from the soil columns was observed only after the permeation of deionized water. At that limit, HC and IR dropped to approximately 1520% of their respective maximum values, measured before any structural decay. Such large reductions were similar to these reported when permeating turbid runoff waters adjusted to SAR and electrolyte concentration slightly lower than the corresponding CCC of the soil-clay. Therefore one major feature of the proper management of turbid irrigation waters should be an early Aocculation of suspended clay particles.
References Abu-Sharar, T.M., 1988. Stability of soil aggregates as inferred from optical transmission of soil suspensions. Soil Sci. Sot. Am. J., 52: 951-954. Abu-Sharar, T.M., 1993. Effects of sewage sludge treatments on aggregate slaking, clay dispersion and hydraulic conductivity of a semiarid soil sample. Geoderma, 59: 327-343. Abu-Sharar, T.M., Bingham, F.T. and Rhoades, J.D., 1987a. Reduction in hydraulic conductivity in relation to clay dispersion and disaggregation. Soil Sci. Sot. Am. .I., 51: 342-346. Abu-Sharar, T.M., Bingham, F.T. and Rhoades, J.D., 1987b. Stability of soil aggreagtes as affected by electrolyte concentration and composition. Soil Sci. Sot. Am. J., 5 I : 309-3 14. Bradtield, R., 1936. The value and limitations of calcium in soil texture. Am. Soil Survey Assoc. Bull., XVII: 3132. Felhendler, R., Shainberg, 1. and Frenkel, H., 1974. Dispersion and hydraulic conductivity of soils in mixed solutions. Trans. hit. Congr. Soil Sci. IOth, I: 103-I 12.
62
TM. Abu-Sharar, AS. Salameh /Agricultural
Water Management 29 (1995) 53.62
Frenkel, H., Goerzen, J.O. and Rhoades, J.D., 1978. Effect of clay type and content, exchangeable sodium percentage, and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Sot. Am. J., 42: 32-39. Goldberg, S. and Forster, H.S., 1990. Flocculation of reference clays and arid-zone soil clays. Soil Sci. Sot. Am. J., 54: 714-718. Greene, R.S.B., Posner, A.M. and Quirk, J.P., 1978. A study of the coagulation of montmorillonite and illite suspensions by CaCl, using the electromicroscope. In: W.W. Emerson et al. (Editors), Modification of Soil Structure. Wiley, NY, pp. 3540. Klute, A. (Editor), 1986. Methods of Soil Analysis (Part 1). Physical and Mineralogical Methods. 2nd Edn. ASA Publication No. 9. Madison, WI. Morin, J., Keren, R., Benjamini, Y., Ben-Hur, M. and Shainberg, I., 1989. Water infiltration as affected by soil crust and moisture profile. Soil Sci., 148: 53-59. Oster, J.D., Shainberg, 1. and Wood, J.D., 1980. Flocculation value and gel structure of sodium/calcium montmorillonite and illite suspensions. Soil Sci. Sot. Am. I., 44: 955-959. Page, A.L. (Editor), 1986. Methods of Soil Analysis (Part 2). Chemical and Microbiological Properties, 2nd Edn. ASA Publication No. 9. Madison, WI. Shainberg, 1. and Letey, J., 1984. Response of soils to sodic and saline conditions. Hilgardia, 52: l-57. Shainberg, I., Rhoades, J.D. and Prather, R.J., 1981. Effect of low electrolyte concentration on clay dispersion and hydraulic conductivity of a sodic soil. Soil Sci. Sot. Am. J., 45: 2733277. Sposito, G., 1984. The surface chemistry of soils. Clarendon Press, Oxford. Staff Soil Survey, 1978. Soil Taxonomy. Agric. Hdbk 436. Soil Conservation Services, USDA, Washington, DC. Sumner, M.E., 1993. Sodic soils: New perspectives. Aust. J. Soil Res., 31: 683-750.
Water Management
29 ( 1995) 53-62
Reductions in hydraulic conductivity and infiltration rate in relation to aggregate stability and irrigation water turbidity T.M. Abu-Sharar
*, A.S. Salameh
Department of Soils and Irrigation, The Univer.siry of Jordan, Amman, Jordan Accepted 25 January 1995
Abstract Arid and semiarid soils are characterized by reductions in hydraulic conductivity (HC) and infiltration rate (IR) when employing low salinity/sodicity waters. Such reductions are further exacerbated when irrigating with turbid water. Critical salt coagulation concentrations (CCCs) were determined at constant SAR 5, 10, 15, or 20 for clay fractions of a semiarid surface sample of the Muaq’qar soil (fine, mixed thermic, Typic Calciorthid) and for turbid runoff rain water, containing clay particles, which was generated from the same area. The results showed similar values for both types of clay (predominated by mica and kaolinite). When conducting field measurements of IR or permeating duplicate columns of 1:l soiLsand mixtures with solution series of decreasing concentration (50 to 0 mol, m-l) at any of the above SAR levels, IR and HC decreased with decreasing electrolyte concentration. Substantial losses in IR and HC ( -40-60%) were observed when permeating solutions with electrolyte concentration equal to or higher than the respective CCC of the soil-clay. However, IR and HC dropped to approximately E-20% of their respective maximum values with the permeation of turbid runoff water (0.07% fine clay) adjusted to electrolyte concentration slightly lower than the corresponding CCC of the soil-clay (5, 10, 15, or 20 mol, me3 at SAR 5, 10, 15, or 20, respectively). Such reductions were larger than the corresponding reductions in HC and IR when permeating clear solutions of equivalent SAR and electrolyte concentration (A HC = 50-60% and AIR = 5 1 + 3%). This result indicated the ability of suspended fine clay particles in the permeating water to flocculate in the soil conducting pores causing their clogging. Keywords:
Salinity; Sodicity; Critical coagulation
concentration;
Surface crust
1. Introduction
Arid and semiarid soils are generally characterized by poor aggregate stability and crust formation at the surface (Shainberg and Letey, 1984). Deterioration in soil structure may * Corresponding
author.
037%3774/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved XSDIO378-3774(95)01184-6
54
T.M. Abu-Sharar, A.S. Salameh /Agricultural Water Management 29 (1995) 5362
take place even when irrigating non-sodic soils with waters of low SAR and salinity (Sumner, 1993). Because of crust formation, the resultant infiltration rate (IR) tends to decrease to a minimum value irrespective of the initial soil moisture content (Morin et al., 1989). Therefore extensive surface runoff and thus soil erosion may be generated from rainstorms of low quantity and intensity. In general, most of the suspended material in the surface runoff water would be made of silt and clay fractions ( Abu-Sharar, 1993). This is mainly due to the fragile nature of soil structure which initiates substantial aggregate slaking and clay dispersion even at predominately low rainfall intensity. Provided that the harvested turbid runoff water may be stored for some time before being utilized in irrigation, the dispersed silt and coarse clay particles will settle down and only fine clay particles may remain unflocculated and suspended. If the resultant turbid runoff water is destined for future irrigation, the concurrent low water salinity and suspended fine clay particles can cause considerable difficulty in the management of such water like crust formation, substantial reduction in IR, water ponding on soil surface, poor seedling emergence, and extensive soil erosion. Although sufficient data on critical coagulation concentration (CCC) of pure clay suspensions are available in the literature, non-correspondence between CCC of these clay specimens and clays extracted from runoff water was reported. For example, Goldberg and Forster ( 1990) found CCC values for soil clays to be between two- and ten-fold higher than for pure clay systems. Therefore, the objectives of this study were to (i) determine mineralogy and CCC of low and intermediate SAR levels for clay fractions separated from surface soil samples and turbid surface runoff water, both collected from the semiarid area of Muaq’qar (Jordan), and (ii) evaluate reductions in IR and hydraulic conductivity (HC) of that soil when employing clear (synthetic) and turbid (runoff) solutions of decreasing salinity at constant SAR level. 2. Materials and methods 2.1. Soil and s&ace
runoff water
A turbid surface-runoff water sample was obtained in June, 1991 from a small reservoir in the Muaq’qar Experiment Station (30 km to the east of the capital Amman). Suspended clay in sub-sample of that water was first flocculated using NaCl salt. Soluble salt was then washed by successive shaking and centrifugation using distilled water. The salt-free clay slurry was air-dried, ground to pass 2 mm sieve, and stored for future analysis. A composite soil sample taken from the surface (o-0.25 m depth) was collected from the Station. The soil is classified as fine, mixed thermic, Typic Calciorthid (Staff Soil Survey, 1978). For both samples of soil and runoff-extracted clay, the following determinations were made: particle size distribution, OM, CaCO,, CEC, EC, and pH. X-ray diffraction analysis (wavelength of Cu K, = 1.5405 nm) for oriented clay samples was carried out according to the standard methods of analysis (Klute, 1986; Page, 1986). 2.2. Critical coagulation
concentration
Standard Na-clay suspension was obtained from a sub-sample of the soil or the turbid surface runoff water. Each sub-sample was successively suspended in a 1-L beaker con-
T.M. Abu-Sham-,
AS. Sulatneh /Agricultural
Water Munagement 29 (1995) 53-62
55
taining 1 mol,me3 NaCl solution, then decanted to a predetermined depth after standing still for a calculated settling time of the < 2 pm clay particles using Stokes’ law. Sodiumclay suspensions ( 1% w/v) in deionized water (working suspensions) were prepared from each standard clay suspension using successive suspension in water, centrifugation at approximately 15 - 000 rev min- ‘, and decantation of the supernatant solutions. These saltfree Na-clay suspensions were employed in the determination of CCC at SAR 0 (Ca-clay), 5, IO, 15, and 20. The determination of CCC at the respective SAR was carried out by pipetting 5 ml volumes from each working clay suspension into a series of test tubes of identical optical transmission (%T). To each test tube, appropriate volumes of deionized water, NaCl, and CaCl, standard solutions totalling 5 ml volume were added at 1 mol, me3 increments in total electrolyte concentration. The clay suspensions were then shaken for 30 min and left still for additional 24 h after which %T was measured at 420 nm wavelength using a spectrophotometer. The CCC was evaluated as the electrolyte concentration which caused a sharp rise in %T. To obtain a more accurate estimate of each CCC, a new series of ten clay suspensions was similarly tested using solutions of electrolyte concentration ranging between CCC - 0.5 and CCC + 0.5 mol, rnp3 with 0.1 mol, mm3 increments. 2.3. Hydraulic conductivity Because of the relatively fine sizes of dry soil aggregates and the resulting low HC, a soil sub-sample was mixed thoroughly with an equivalent weight of HCl-washed sand ( < 1 mm diameter). Portions of the mixture (300.0 g) were then packed in glass cylinders to 15 cm depth at 1.3 Mg rnp3 bulk density. Each soil column was wetted from below using NaCl/CaCl, solutions of 50 mol,me3 total electrolyte concentration at the respective SAR 5, 10, 15, or 20. Steady-state HC of duplicate soil columns was measured for each of the successively permeated solutions of decreasing electrolyte concentration (50, 25, 20, 15, 10, 7.5,5, 2.5, and 0 mol,m-3) using constant head method of Marriotte bottle device and measuring the time for a fraction collector to collect 8 ml volumes of the effluent solutions. The permeation of each electrolyte solution continued until reaching steady-state HC as evidenced by reporting coefficient of variation less than 5% for the last ten HC measurements. For one of each duplicate soil column, electrolyte solutions of 5, 10, 15, or 20 mol,m - 3 and the respective SAR 5, 10, 15, or 20 were replaced by the turbid surface runoff water adjusted to the corresponding SAR and electrolyte concentration. If dispersed clay appeared in the effluent solutions, the %T was measured for each successive effluent fraction as long as the %T did not increase again to a stable level. 2.4. Infiltration rate Duplicate field measurements of IR were carried out using the same solution series of constant SAR as employed in the measurement of HC. The measurements were conducted employing double ring infiltrometer with a constant head (few millimeters above the soil surface) device comprised a 1-L graduated cylinder connected to the central ring through a rubber tube. For each SAR solution series, measurement of IR at a given electrolyte concentration continued every 5 min until steady-state value was observed as described
56
T.M. Abu-Sharar, AS. Salameh /Agricultural
Water Management 29 (1995) 5342
earlier in the measurement of HC. In one series of each IR duplicate measurements, solutions having total electrolyte concentrations of 5, 10, 15, or 20 mol, rnd3 at the respective SAR 5, 10, 15, or 20 were replaced by sub-samples of the turbid surface runoff water adjusted to the corresponding SAR-electrolyte concentration combination in a manner analogous to that formerly employed in the HC measurements.
3. Results and discussion 3.1. Critical coagulation
concentration
The Muaq’qar soil is a calcareous silt loam of low organic matter content (Table 1) . Clay mineralogy of the soil sample and of the suspended clay fraction in the turbid surface runoff water was dominated by mica and kaolinite. Because of its poor content of organic matter and the micaceous mineralogy (Na-mica has the highest CCC of 48 f 11 mol, me3 (Sposito, 1984) ), the Muaq’qar soil exhibited poor aggregate stability. Fig. 1 shows sample results for the %T of clay suspensions from the Muaq’qar soil and the turbid surface runoff water as influenced by increasing electrolyte concentration at constant SAR values of 5 or 20. Because of space limitation and similarity of results, other results of SAR 10 and 15 were not presented. The figure also indicates a sharp increase in the %T with increasing electrolyte concentration by one unit beyond a certain limiting value. Such an increase in the %T reflected colloidal instability and, thus, corresponded to the respective CCC value. For both clay suspensions, CCCs at a given SAR were close to each other and increased linearly with increasing solution SAR to 20 (Fig. 2). The slightly higher CCC of the dispersed clay in the surface runoff water can be attributed to the relatively finer particle sizes of that clay as compared to the <2 pm soil-clay fraction. The runoff water was sampled in June after a period of suspension long enough to permit settling of sizeable clay Table 1 Selected physical, chemical Experiment Station Soil Sand (%)
Silt (%)
and mineralogical
Clay (%)
OM (%)
properties
of the soil and surface runoff water of the Muaq’qar
CaCO,
CEC (cmol( + )
Mineralogy
kg-‘)
EC, (dS m-‘)
pH
(%) 18.3 55.0 Surface runoflwater Sand (g Silt (g 1-1) 1-1)
26.1
0.42
21.5
14.5
0.40
8.2
K,M”
Clay (g I_‘)
OM (g per 100 g)
CaCO, (g per 100 g)
CEC (cmol( + )
EC (dS m-‘)
pH
Mineralogy
kg-‘))
0.00
0.70
0.30
8.0
49.8
0.23
8.0
K,M
0.10
Soluble cations (mol, ,n ‘) Ca Na Mg 1.00
0.30
‘K, kaolinite; M, mica.
0.8.5
K 0.17
T.M. Abu-Sharar, A.S. Salameh /Agricultural Water Management 29 (1995) 5362
80-
. , - ‘w?5
.
,
.
,
.
,
57
. +
60k 40 ;Jq
* Suspend-d cloy in runoff woter . 20
3
.
,
.
5
,
.
7
, 9
.
,
.
11
-
. , . , . , . , . , . 14
13
. , . , , , . , . , . ,
80
Suspended cloy In runoff water 20 80
18
20
22
.,.,.,‘,.,.
.
sAR5
16
24
26 _
sAR20
80-
40
i
j_J<2 J&m soil-cloy
20-, . , . . , . , . , , , 0 2 4 6 8 10 12
Electrolyte
Concentration
(molcm-3)
2oih3377d6
Electrolyte
Concentration
Fig. 1. Effect of increasing electrolyte concentration on %T of clay suspensions soil and the turbid surface runoff water generated from that area.
fractionated
(molcm-
.3)
from the Muaq’qar
particles. However, CCCs for both clay fractions were higher than the corresponding values for illite suspensions of equivalent SAR, e.g. 0.25, 9.0, and 17.5 mol, me3 at SAR 0, 10,
Sodium
Adsorption
Ratio
Fig. 2. Relation between SAR and critical coagulation concentrations for the clay fraction of the Muaq’qar clay and the suspended clay in the turbid surface runoff water generated from that area.
soil
58
T.M. Abu-Sharar, A.S. Salameh/Agricultural
Wuter Management 29 (1995) 5342
and 20, respectively (Oster et al., 1980). Such differences could probably be due to the weathering of the soil-mica that may have rendered particle surfaces more irregular. Upon close approach of such particles, the unavoidable mismatch of the terraces can lead to poor contact between the edges and surfaces, causing a smaller edge-to-face attraction force and, consequently, a higher CCC (Greene et al., 1978). 3.2. Hydraulic conductivity Permeating soil columns with electrolyte solutions of decreasing concentration down to 0 moI, m-3 at constant SAR 5, 10, 15, or 20 showed a reduction in HC (expressed as relative values of the maximum HC reported when permeating the most concentrated electrolyte solution of 50 mol, m-‘) with decreasing electrolyte concentration, especially at higher SAR levels. Fig. 3 shows selected results for solutions SAR 5 and 20. At any SAR level, electrolyte concentration at which HC started to decrease was much higher than the corresponding CCC of the clay fraction. Subsequently, the major cause for such reductions was only due to aggregate slaking rather than to clay dispersion, migration and clogging of conducting pores as previously reported (Felhendler et al., 1974; Frenkel et al., 1978;
0) T; uz 5 ‘Z e g r
10
10
5
5
0
0 0
Electrolyte
Concentration
(mol,m -3)
Electrolyte
20
40
60
Concentration
00
100
(molcm-3)
Fig. 3. Effect of decreasing electrolyte concentration on reduction in HC (upper) and IR (lower) of the Muaq’qx soil. Circles mark permeating only electrolyte solutions but triangles mark permeating electrolyte solutions interrupted by the permeation of the turbid surface runoff waters of 5 or 20 mol, m-j at SAR 5 or 20, respectively.
T.M. Abu-Sharar.
AS. Salameh /Agricultural
59
Water Management 29 (1995) 53-62
Shainberg et al., 1981). This result was similar to former findings for semiarid soils of similar mineralogy from Jordan and California ( Abu-Sharar, 1993; Abu-Shararet al., 1987a, respectively). At a given SAR, decreasing electrolyte concentration of the permeating solutions to values slightly lower than the corresponding CCC of the soil-clay (5, 10, 15, and 20 mol, m-l compared with the CCCs of 6.5, 11.5, 16.5, and 21.5 mol, m-l for the respective SAR 5, 10, 15, and 20) caused HC to drop to 41%, 55.6%, 52%, and 57%, respectively. However, permeation of the turbid surface runoff water adjusted to either one of the above combinations of SAR-electrolyte concentration caused HC to drop to 19%, 21.4%, 17.5%, and 15%, respectively. Such large differences between comparable HC values (22, 34.2, 34.5, and 42%, respectively) were mainly due to the clogging of the conducting pores by the suspended clay particles in the surface runoff water. This, in fact, showed that little dispersion (0.07%) of fine clay particles in the permeating solution can effectively reduce HC to minimum values most probably owing to flocculation of these particles as the turbid water flows in the vicinity of exchangeable cations atmosphere of electrolyte concentration greater than the corresponding CCC. However, this hypothesis can be applied only when electrolyte concentration of the permeating solution drops to levels slightly lower than the corresponding CCC of the soil-clay. Abu-Sharar (1988) argued that at a given SAR, CCC could be slightly higher than the electrolyte concentration at which clay starts to disperse from soil aggregates. This is mainly due to the fact that additional force is needed to break down soil aggregates. Soil clays are covered with a variety of organic and inorganic films and coatings often bonded together into aggregates by various cementing agents (Sumner, 1993). These aggregates are recognized as the product of Bradfield’s ‘flocculation plus’ (Bradfield, 1936). On the other hand, if the electrolyte concentration of the permeating solution drops to levels much lower than the corresponding CCC, the soil-clay may disperse as the ultimate product of aggregate breakdown ( Abu-Sharar et al., 1987b). For all SAR levels, a substantial drop in %T of the effluent solutions was only observed when permeating deionized water (Fig. 4). At the final stage, HC dropped almost to levels equivalent of these observed when permeating the respective turbid surface runoff water. 100.
100. _t_
solutions 80
--
80.
I
sol.lsus. I8
$2
60. 40.
solutions
--
sol.lsus. A
x&&&A-&AL
60.
fls
20 t y=
SAR 5 40
-
0
SAR 20
07
+
250 Effluent
500 volume
750 (ml)
0
250 Effluent
500 volume
750 (ml)
Fig. 4. The %T of effluent solutions from the Muaq’qar soil columns permeated with distilled water following permeation with solution series of decreasing electrolyte concentration (circles) or solutions interrupted by permeation of the turbid surface runoff water (triangles) of 5 or 20 mol, me3 at SAR 5 or 20, respectively.
TM Abu-Sharar, AS. Salarneh /Agricultural
Water Management 29 (1995) 5362
0
0
10
20
30
Reduction
40
in
50
60
relative
Fig. 5. Relation between reduction
in relative
concentration
electrolyte
00
HC
solutions
of equivalent
70
20
40
Reduction
HC and the corresponding
reduction
60
in relative
80
HC
in relative IR when permeating
and SAR.
The %T of the effluent solutions used to peak after permeation of about one pore volume ( 128 ml). The minimum %T reported when permeating deionized water decreased with increasing SAR of the previously permeated electrolyte solutions and, thus, indicated more clay dispersion (Fig. 4). In addition, the %T of the effluent solutions from soil columns previously permeated with the turbid surface runoff waters did not drop to levels comparable with these pertaining to the corresponding soil columns permeated only with the electrolyte solutions. This observation further supports the above conclusion that the dispersed clay in the turbid surface runoff water can clog the conducting pores of the soil columns and consequently restricts the mobility of clay particles dispersed from the soil aggregates. 3.3. Infiltration rate At a given SAR, IR decreased with decreasing electrolyte concentrations of the permeating solution in a manner analogous to that of the HC (Fig. 3). When the soil was permeated with solutions of electrolyte concentration slightly lower than the corresponding CCC of the soil-clay (as these employed in the HC measurements), the IR dropped to similar value
TM. Abu-Sharar, AS. Salameh/Agricultural
Water Management 29 (1995) 5342
61
(50.5 + 3.4%) irrespective of the SAR level. However, with the replacement of the former solutions by turbid surface runoff waters of equivalent SAR and electrolyte concentration (as in the HC study), the IR dropped to a much lower level ( 15.2 + 1.4%) irrespective of the SAR value. This may be explained by the sensitive nature of IR to any disturbance in surface soil structure. Moreover, the former decline in IR was also lower than the common minimum value reported when permeating distilled water following the permeation of only electrolyte solutions of any SAR (20.7 + 4.0%). This result conforms to the conclusion that suspended clay particles in the permeating surface runoff water were effective in clogging the conducting pores of the surface soil and reducing IR to a minimum value. The relation between reductions in HC and IR when employing only electrolyte solutions of decreasing salinity at constant SAR was almost 1: 1 (Fig. 5). Therefore, early reductions in IR, i.e. when electrolyte concentration of the permeating solution was higher than the corresponding CCC of the soil-clay, can be attributed to slaking of the surface soil aggregates. Clay dispersion and the subsequent surface crust formation play an effective role in the reductions of IR only when electrolyte concentration of the permeating solution drops well below the corresponding CCC of the soil-clay.
4. Conclusion Substantial losses in HC and IR ( =40-60%) of the semiarid Muaq’qar soil may take place when permeating solutions with electrolyte concentrations not lower that the CCC of the soil-clay at equivalent SAR. Clay dispersion requires permeation of solutions having electrolyte concentration well below the respective CCC of the soil-clay. For a given SAR, substantial clay dispersion in effluent solutions from the soil columns was observed only after the permeation of deionized water. At that limit, HC and IR dropped to approximately 1520% of their respective maximum values, measured before any structural decay. Such large reductions were similar to these reported when permeating turbid runoff waters adjusted to SAR and electrolyte concentration slightly lower than the corresponding CCC of the soil-clay. Therefore one major feature of the proper management of turbid irrigation waters should be an early Aocculation of suspended clay particles.
References Abu-Sharar, T.M., 1988. Stability of soil aggregates as inferred from optical transmission of soil suspensions. Soil Sci. Sot. Am. J., 52: 951-954. Abu-Sharar, T.M., 1993. Effects of sewage sludge treatments on aggregate slaking, clay dispersion and hydraulic conductivity of a semiarid soil sample. Geoderma, 59: 327-343. Abu-Sharar, T.M., Bingham, F.T. and Rhoades, J.D., 1987a. Reduction in hydraulic conductivity in relation to clay dispersion and disaggregation. Soil Sci. Sot. Am. .I., 51: 342-346. Abu-Sharar, T.M., Bingham, F.T. and Rhoades, J.D., 1987b. Stability of soil aggreagtes as affected by electrolyte concentration and composition. Soil Sci. Sot. Am. J., 5 I : 309-3 14. Bradtield, R., 1936. The value and limitations of calcium in soil texture. Am. Soil Survey Assoc. Bull., XVII: 3132. Felhendler, R., Shainberg, 1. and Frenkel, H., 1974. Dispersion and hydraulic conductivity of soils in mixed solutions. Trans. hit. Congr. Soil Sci. IOth, I: 103-I 12.
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TM. Abu-Sharar, AS. Salameh /Agricultural
Water Management 29 (1995) 53.62
Frenkel, H., Goerzen, J.O. and Rhoades, J.D., 1978. Effect of clay type and content, exchangeable sodium percentage, and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Sot. Am. J., 42: 32-39. Goldberg, S. and Forster, H.S., 1990. Flocculation of reference clays and arid-zone soil clays. Soil Sci. Sot. Am. J., 54: 714-718. Greene, R.S.B., Posner, A.M. and Quirk, J.P., 1978. A study of the coagulation of montmorillonite and illite suspensions by CaCl, using the electromicroscope. In: W.W. Emerson et al. (Editors), Modification of Soil Structure. Wiley, NY, pp. 3540. Klute, A. (Editor), 1986. Methods of Soil Analysis (Part 1). Physical and Mineralogical Methods. 2nd Edn. ASA Publication No. 9. Madison, WI. Morin, J., Keren, R., Benjamini, Y., Ben-Hur, M. and Shainberg, I., 1989. Water infiltration as affected by soil crust and moisture profile. Soil Sci., 148: 53-59. Oster, J.D., Shainberg, 1. and Wood, J.D., 1980. Flocculation value and gel structure of sodium/calcium montmorillonite and illite suspensions. Soil Sci. Sot. Am. I., 44: 955-959. Page, A.L. (Editor), 1986. Methods of Soil Analysis (Part 2). Chemical and Microbiological Properties, 2nd Edn. ASA Publication No. 9. Madison, WI. Shainberg, 1. and Letey, J., 1984. Response of soils to sodic and saline conditions. Hilgardia, 52: l-57. Shainberg, I., Rhoades, J.D. and Prather, R.J., 1981. Effect of low electrolyte concentration on clay dispersion and hydraulic conductivity of a sodic soil. Soil Sci. Sot. Am. J., 45: 2733277. Sposito, G., 1984. The surface chemistry of soils. Clarendon Press, Oxford. Staff Soil Survey, 1978. Soil Taxonomy. Agric. Hdbk 436. Soil Conservation Services, USDA, Washington, DC. Sumner, M.E., 1993. Sodic soils: New perspectives. Aust. J. Soil Res., 31: 683-750.