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Soil & Water Management & Conservation

Effects of Effluent Irrigation on Seal Formation, Infiltration, and Soil Loss during Rainfall

Institute of Soil, Water and Environmental Sciences, the Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan, 50250, Israel

^* Corresponding author (marcos{at}volcani.agri.gov.il)

	ABSTRACT

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

 
The use of effluent for irrigation could affect the chemical and hydraulic properties of soils due to its high salt and organic matter (OM) content, and, consequently, the rainfall–infiltration–runoff–erosion relationships during the subsequent rainy season. This study investigates the effects of long-term effluent irrigation on soil chemical properties, seal formation, infiltration, and soil loss under rainfall. Simulated rainfall (85 mm) was applied to (i) air-dried or (ii) prewetted clay and sandy soils from plots that had been irrigated with fresh water (FW) or effluent for >10 yr. Effluent irrigation increased the total OM content and the exchangeable sodium percentage (ESP) of the soils. The cumulative infiltration in FW- and effluent-irrigated clay soils was 6.5 and 5.6 mm, respectively, in the initially dry soils, and 52.3 and 51.5 mm, respectively, in the prewetted soils. In the FW- and effluent-irrigated sandy soils, the corresponding values of cumulative infiltration were 79.5 and 44.7 mm, and 85.0 and 56.3 mm, respectively. In the sandy soil, the higher sodium adsorption ratio (SAR) values in the leachate of effluent-irrigated soil led to greater clay dispersion, which enhanced seal formation, reduced infiltration, and increased soil loss. In the clay soil, slaking was the main process involved in seal formation, neglecting the possible deleterious effect of effluent irrigation. When slaking was prevented, the SAR values in the leachate of the effluent-irrigated soil decreased during rainfall and were similar to those of the FW-irrigated soil at the end of the applied rainfall amount. This was probably due to the exchange of adsorbed Na with soluble Ca, which minimized the differences in clay dispersion, infiltration, and soil loss. Therefore, in the clay soil, aggregate slaking might be the main process involved in seal formation and affecting infiltration and erosion. These results show that the effect of effluent irrigation on infiltration, runoff, and soil loss depends on the soil type (amount of clay and CaCO₃) and the dominant mechanisms of seal formation. Therefore, to prevent a possible deleterious effect on soil structure, it is necessary to identify sensitive areas and soils before the application of effluents for irrigation.

Abbreviations: DOM, dissolved organic matter • EC, electrical conductivity • ESP, exchangeable sodium percentage • FW, fresh water • IR, infiltration rate • OM, organic matter • SAR, sodium adsorption ratio

	INTRODUCTION

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

 
SEMIARID AND ARID REGIONS are characterized by long dry seasons followed by short rainy seasons with high rainfall intensity events, inducing high runoff and erosion rates. Therefore, profitable agriculture in these regions relies on irrigation. Because of the shortage of FW in these regions, and to maintain agriculture that satisfies the demands for food and to combat desertification, the use of treated domestic sewage water (effluent) for irrigation is becoming a common practice. Moreover, the pressure to avoid the disposal of nutrient-rich effluents into FW bodies or the sea has contributed to the rapid expansion of effluent reuse by irrigation (Halliwell et al., 2001).

Sewage water comprises 0.1% suspended or dissolved organic and inorganic compounds (Feigin et al., 1991). To prevent contamination of crops and adverse effects on soil, crops, and water resources, sewage water intended for irrigation, usually, undergoes secondary (biological) treatment. However, because this treatment does not decrease the concentrations of the major salts, and does not eliminate organic matter completely (Ben-Hur, 2004), long-term irrigation with effluent could lead to changes in the chemical and physicochemical properties of the irrigated soils, which, in turn, could affect the soil hydraulic properties. Consequently, prolonged use of these effluents for irrigation could increase the ESP and OM content of irrigated soils (Balks et al., 1998; Mamedov et al., 2001; Agassi et al., 2003; Ben-Hur, 2004), and changes in these two parameters could affect the soil structure and its stability.

Infiltration is one of the important processes in the hydrologic cycle, since it determines the supply of water to the soil profile as well as the amount of runoff and soil erosion, which have a direct effect on soil and water degradation. In arid and semiarid regions, the main factor that controls the soil infiltration rate (IR) under water-drop impact is the formation of a structural seal at the soil surface (Morin et al., 1981; Ben-Hur et al., 1985, 1987). This seal is relatively thin, and is characterized by greater density, higher strength, finer pores, and lower saturated hydraulic conductivity than the underlying soil (McIntyre, 1958; Gal et al., 1984; Wakindiki and Ben-Hur, 2002; Assouline, 2004), leading to a drastic decrease of IR during its formation (Morin and Benyamini, 1977; Assouline and Mualem, 1997; Assouline, 2004).

Agassi et al. (1981) suggested that the formation of a structural seal is a result of two complementary mechanisms: (i) physical disintegration of surface soil aggregates, caused by the impact energy of the raindrops; and (ii) the physicochemical dispersion of soil clays, which migrate into the soil with the infiltrating water and clog the pores immediately beneath the surface to form the "washed-in" zone. The relative importance of the latter mechanism depends on the electrical conductivity (EC) of the soil solution, and the ESP of the soil surface. As the EC decreases and the ESP increases, the reduction in IR due to seal formation is usually more pronounced (Kazman et al., 1983; Shainberg and Letey, 1984). Hence, no significant clay dispersion is expected during effluent irrigation, because the relatively high EC of the effluent is above the flocculation value of the soil clay. However, under rainfall conditions, extensive clay dispersion and significant IR reduction would be expected in soils that have received prolonged effluent irrigation, due to the relatively high ESP of the soil surface and the fast leaching of the electrolyte during the rainstorm. It should be noted that this expected result considers the salinity factor of effluent but disregards possible effects of the OM content factor on soil stability and hydraulic properties. Total OM content in the soil is an important soil property affecting aggregate stability of the soil (Tisdall and Oades, 1982). An increase of OM in the soil reduces soil susceptibility to slaking and dispersion, and decreases seal formation and IR reduction during a rainstorm (Lado et al., 2004b). In contrast, dissolved organic matter (DOM) can reduce the hydraulic conductivity of soils due to the presence of humic substances that enhance clay dispersion (e.g., Durging and Chaney, 1984; Frenkel et al., 1992; Tarchitzky et al., 1999). This effect was attributed to the interaction of negative charged OM with the positive charged edges of the clay, which prevents the edge to face association of clay particles responsible for flocculation (Tarchitzky et al., 1999).

In addition to the mechanisms related to the impact energy of the raindrops and the physicochemical dispersion of the soil clay, aggregate disintegration at the soil surface caused by a slaking process could be another important mechanism of seal formation (Bresson et al., 2004). Slaking occurs when the aggregate is not strong enough to withstand the stresses produced by differential swelling, entrapped air, rapid release of heat during wetting, and the mechanical action of moving water (Emerson, 1977; Collis-George and Green, 1979; Kay and Angers, 1999). These stresses are described as slaking forces. The slaking process is controlled by the wetting rate of the soil: the faster the wetting rate the stronger the slaking forces and the greater the proportion of aggregates that undergo slaking. In spite of the importance of the slaking process in aggregate disintegration, few studies have addressed its effects on seal formation, IR and soil loss under effluent irrigation conditions.

Despite the increasing interest of the use of effluents for irrigation, few studies have investigated their effects on seal formation, IR, runoff, and soil loss. Mamedov et al. (2000) analyzed the effect of effluent irrigation on seal formation and IR in four calcareous soils from Israel. The FW- and effluent-irrigated soils were subjected to simulated rainfall (distilled water) with kinetic energy ranging from 3.6 to 15.9 kJ m^–3. For all the soils and kinetic energies, the IR values of the effluent-irrigated soils were lower than those of the FW-irrigated ones. These results were attributed to the relatively high ESPs of the effluent-irrigated soils, which enhanced the breakdown of aggregates and the consequent seal formation at the soil surface. However, when the kinetic energy of the rain was high, the differences of IR between FW- and effluent-irrigated samples of clay soils with >400 g kg^–1 were negligible. These results differ from those of Agassi et al. (2003), who analyzed the effects of effluent irrigation on seal formation under simulated rainfall in two calcareous soils with clay contents of 185 and 406 g kg^–1. In these soils, the ESP increased after effluent irrigation from 3.5 to 6.4% for the lower clay content, and from 2.3 to 6.1%, for the higher. No differences in runoff between effluent- and FW-irrigated soils were found in this study. Agassi et al. (2003) suggested that the absence of differences was due to a rapid decrease of the ESP of the soil surface during the rainstorm, caused by the displacement of adsorbed Na by soluble Ca.

Mamedov et al. (2001) studied the effects of wetting rate on runoff and soil loss, under simulated rainfall, from five Israeli soils with clay contents ranging from 80 to 620 g kg^–1. The soils were sampled from fields irrigated with FW or effluent for >15 yr. The total runoff and soil loss were higher in the effluent- than in the FW-irrigated treatment only for the soil with 80 g kg^–1clay content. In general, the effects of the various wetting rates on the total runoff and the soil loss were similar in the effluent- and in the FW-irrigated soils.

This short literature review indicates that the impact of long-term effluent irrigation on the rainfall-runoff-erosion relationships during rainfall depends on a wide range of factors, including clay, CaCO₃ and OM contents, salinity, wetting rates or initial conditions, and seal formation. Unlike most of the studies cited above that generally focused on only a limited number of factors, the present study suggest an integrative analysis of all these factors to gain a better insight on the interaction between long-term effluent irrigation and rainfall–runoff–erosion relationships during rainfall.

	MATERIALS AND METHODS

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

 
Soil Samples
Two Israeli soils with different textures were chosen for this study: a sandy soil from Kibbutz Ramat-Hacovech in the coastal plain, and clay soil from Kibbutz Mizra in the Yizre'el Valley. The dominant clay in both soils was smectite, and their general properties are presented in Table 1. In the two sites, a composite sample was collected from the surface (0.0–0.25 m) of adjacent experimental plots with orchards of orange trees (Citrus sinensis) in Ramat Hacovech and of grapefruit trees (Citrus paradisii) in Mizra, both under the same management. The plots were irrigated with FW or effluent using a minisprinkler system during 10 and 12 yr for the sandy and clay soils, respectively. The qualities of the FW and effluents used for irrigation at these two experimental sites are presented in Table 2.

Rainfall Simulator Study
The FW- and effluent-irrigated samples from the sandy and clay soils were crushed and sieved to obtain samples with aggregates <4 mm. The disturbed soil samples were packed in perforated trays measuring 0.30 by 0.50 m and 0.02 m deep, in bulk densities of 1.4 and 1.2 Mg m^–3 for the sandy and clay soils, respectively. The packed soil trays were placed on an 8-cm layer of coarse sand, in a box positioned under a rotary disc rainfall simulator (Morin et al., 1967) at a slope of 9%.

The typical mechanical parameters of the simulated rainfall were: raindrop mean diameter, 1.9 mm; median drop velocity, 6.2 m s^–1; and kinetic energy, 18.1 J mm^–1 m^–2. The duration of each simulated rainstorm was approximately 120 min, and 85 mm of deionized water were precipitated at a rainfall intensity of 42 mm h^–1. This intensity was used in many rainfall simulation studies (Agassi et al., 2003; Lado and Ben-Hur, 2004a), and allows reaching a complete seal formation after 2 h of rainfall.

Two initial soil conditions were studied. In the first one, air-dry soil was exposed directly to the rainstorm. This initial condition induces a relatively fast wetting of the aggregates at the soil surface at the beginning of the rainstorm. In the second initial condition, the air-dry soil was prewetted by means of a mist of deionized water at a mean rate of 1 mm h^–1, which was obtained by cyclic exposure of the soil trays to a mist at 30 mm h^–1 for 6 s in every 3 min. The duration of prewetting varied between 2.5 h for the sandy soil and 16.1 h for the clay soil. This initial condition was expected to induce a much slower wetting of the aggregates during the rainfall simulation.

Water percolating through the soil during the rainstorm was collected and measured every 160 s to determine the IR. Likewise, the runoff during the entire rainstorm was collected and measured. The soil that was eroded from each soil tray was measured by drying the runoff samples and weighing the dry material. The measured soil loss could be considered as being due to interrill erosion, because the trays in the rainfall simulator were short (0.5 m) (Meyer and Harmon, 1984). The cumulative leachate was collected three times during the rainfall, every 40 min, and the EC, SAR, DOM and clay concentration in these three samples were measured. The clay concentration was determined according to Gupta et al. (1984) by measuring the absorbance of the suspensions at 420 nm with a spectrophotometer Uvikon 933 (Kontron Instruments, Milano, Italy).

Statistical Analysis
Each of the eight cases studied (2 soil types x 2 irrigation water qualities x 2 initial conditions) was conducted in three replicates, and the differences of the means were subjected to analysis of variance (ANOVA) as a complete randomized design. Separation of means was subjected to Tukey's honestly significant difference test (Steel and Torrie, 1960). Regression analysis was conducted to identify relationship between the measured parameters. All tests were performed at the 0.05 significance level.

	RESULTS AND DISCUSSION

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

 
The chemical properties of the FW- and effluent-irrigated samples from the sandy and clay soils are presented in Table 3. The effluent-irrigated samples showed higher values than the FW-irrigated ones for the total OM content in the sandy soil, the ESP in both soils, and the EC and SAR in their saturated pastes (Table 3). In contrast, no significant differences were found between the total OM content in the clay soil, and the pH values and the DOM concentrations in the saturated paste extracts of the FW- and the effluent-irrigated soils.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Mean infiltration rates as functions of cumulative rainfall for the two soils irrigated with fresh-water or effluent at two initial conditions. Bars indicate one standard deviation.

Slaking
The responses of the two soils to FW and effluent irrigation with respect to IR reduction during rainfall (Fig. 1) should reflect the effects of the physicochemical dispersion of the clay and the slaking mechanism on seal formation. When the initially dry soil was exposed to rainfall, the wetting rate of the aggregates at the soil surface was fast, and the aggregate disintegration at the soil surface during the rainstorm would involve extensive slaking. In contrast, when the soil was prewetted, the resulting wetting rate of the soil aggregates was much slower, and therefore the slaking forces would have been weaker. The relative impact of slaking on seal formation can be estimated by comparing the cumulative infiltration for each soil at the two initial conditions (Fig. 1 and Table 4). Analysis of variance showed a significant interaction between initial condition and irrigation water quality in the sandy soil, with a significant effect of both factors on the cumulative infiltration. On the contrary, such interaction was not found in the clay soil, significant differences being found for initial conditions but not for irrigation water qualities. The amounts of the infiltrated water in the initially dry soils were significantly lower than those in the prewetted soils in all the cases (Table 4), but these differences were much greater in the clay soil than in the sandy one. These results suggest that the effect of the slaking process on seal formation was more pronounced in the clay than in the sandy soil for both irrigation water qualities. This resulted from the higher clay content of the Mizra soil, in agreement with Lado et al. (2004a), who showed that in montmorillonitic soils, aggregate slaking increased with increasing clay content.

Clay Dispersion
The clay concentrations in the leachates from the FW- and effluent-irrigated soils under the two initial conditions are presented in Fig. 2 as functions of the cumulative leachate during the rainstorm. For the clay soil, only the results of the prewetting treatment are shown in Fig. 2, since insufficient leachate was collected in the initially dry soil to enable the determination of the clay concentration or other parameters. In both soils the clay concentration in the leachate increased with the cumulative leachate depth (Fig. 2). The presence of clay particles in the leachate indicates that clay dispersion occurred in the soil during the rainstorm; the higher the clay concentration, the more the dispersion. Therefore, the clay concentration in the leachate from the clay soil (Fig. 2B) was greater than in that from the sandy soil (Fig. 2A), as expected from the differences between the initial clay contents in these two soils (Table 1). In the sandy soil, no significant differences in the leachate clay concentrations were found between the two initial conditions for either irrigation water quality. However, the clay concentrations in the leachate from the effluent-irrigated soil were significantly higher than in those from the FW-irrigated soil, indicating that in this soil long-term effluent irrigation provided better conditions for clay dispersion than FW irrigation (Fig. 2A). In contrast, in the clay soil, the differences between the clay concentrations in the leachates from the FW- and effluent-irrigated soils were not significant (Fig. 2B).

View larger version (22K): [in this window] [in a new window]   Fig. 2. Mean concentrations of clay in the leachate for the two soils irrigated with fresh-water and effluent at two initial conditions vs. the cumulative leachate depth. Bars indicate one standard deviation.

Effect of Dissolved Organic Matter
The differences in clay concentrations in the leachate between the FW- and effluent-irrigated soil samples for the sandy soil case could have resulted from the higher ESP or DOM concentration. The DOM concentrations in the leachates from the clay and sandy soils under both initial conditions and irrigation water qualities are presented in Fig. 3 as functions of the cumulative leachate depth. For both soils, the DOM concentrations in the leachates decreased with the cumulative leachate depth and no significant differences in the DOM concentrations were found between the FW- and effluent-irrigated soils, independently of their type or the initial condition. These results indicate that the differences in clay dispersion between the FW- and effluent-irrigated sandy soil were not caused by differences in the soil DOM.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Mean concentration of dissolved organic matter (DOM) in the leachate for the two soils irrigated with fresh-water and effluent at two initial conditions vs. cumulative leachate depth. Bars indicate standard deviation.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Sodium adsorption ratio (SAR) in the leachate for the two soils irrigated with fresh-water and effluent at two initial conditions vs. cumulative leachate depth. Bars indicate one standard deviation.

Soil Loss
The total soil losses measured during the entire rainstorm for the various cases are presented in Fig. 5. Soil loss was not measured in the prewetted sandy soil under the FW–irrigation since no runoff was formed (Fig. 1). In general, the results of the soil loss are closely related to those of the runoff: in the cases when IR was low, runoff was high and hence soil loss was enhanced. In the clay soil, the slaking process significantly increased the soil loss under both irrigation water qualities: the total soil loss in the initially dry soil ranged from 102.3 to 164.8 g m^–2, while that in the prewetted soil ranged from 7.9 to 37.9 g m^–2. In contrast, no significant differences in total soil loss were found between the effluent- and the FW-irrigated soils (Fig. 5). This result may be attributed to the small differences in soil dispersion and runoff volumes between the two irrigation treatments for the clay soil. In the sandy soil, the slaking process had little effect on soil loss, which ranged from 7.29 to 28.08 g m^–2 in the initially dry soil and from 0.0 (no runoff) to 28.48 g m^–2 in the prewetted soil. In this case, the SAR (Fig. 4A) and the clay dispersion (Fig. 2A) were greater under effluent than under FW irrigation. This led to a lower IR and greater runoff volume and sediment transport capacity, thus increasing the relative effect of irrigation water quality on soil loss (Fig. 5).

View larger version (17K): [in this window] [in a new window]   Fig. 5. Total soil loss during the entire rainstorm for the two soils irrigated with effluent or fresh-water at two initial conditions. Bars indicate standard deviation. Different lowercase letters indicate statistically significant (at 0.05% level) differences between irrigation water quality within soil type and initial condition, and different uppercase letters indicate statistically significant (at 0.05% level) differences between initial conditions within soil type and irrigation water quality.

	CONCLUSIONS

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

The use of effluent is expected to increase in the coming years and to become the main source of water for irrigation in Israel. However, extreme care must be taken to identify sensitive soils and areas for its application. Most of the soils in the coastal plain of Israel, which is the recharge area of the coastal aquifer, are of the same type than the sandy soil of the present study. The use of effluents for irrigation in this area could enhance seal formation, increasing runoff and soil loss with the consequent environmental problems, as well as reducing the amount of infiltrating water suitable to recharge the aquifer.

	ACKNOWLEDGMENTS

 
This work was funded by: (i) the EU Marie Curie fellowship under contract No: EVK1-CT-202-50020; (ii) Middle East Regional Cooperation (USAID-MERC) Program, project No. M22-006; and (iii) a grant (GLOWA-Jordan River) from the Israeli Ministry of Science and Technology and the German Bundesministerium fuer Bildung und Forschung (BMBF).

	NOTES

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

 
Contribution from the Agricultural Research Organization, the Volcani Center, 602/05.

Received for publication December 13, 2004.

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

 

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