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Published online 8 September 2005
Published in J Environ Qual 34:1910-1920 (2005)
DOI: 10.2134/jeq2005.0073
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Soil Water Repellency Induced By Long-Term Irrigation with Treated Sewage Effluent

R. Wallacha,*, O. Ben-Ariea and E. R. Graberb

a Department of Soil and Water Sciences, Faculty of Agricultural, Food, and Environmental Quality Sciences, Hebrew University of Jerusalem, Rehovot 76100 Israel
b Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization (ARO), Bet Dagan 50250 Israel

* Corresponding author (wallach{at}agri.huji.ac.il)

Received for publication March 1, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This study describes soil water repellency developed under prolonged irrigation with treated sewage effluent in a semiarid environment. Soil surface layer (0–5 cm) and soil profile (0–50 cm) transects were sampled at a high resolution at the close of the irrigation season and rainy winter season. Samples from 0- to 5-cm transects were subdivided into 1-cm slices to obtain fine scale resolution of repellency and organic matter distribution. Extreme to severe soil water repellency in the 0- to 5-cm soil surface layer persisted throughout the 2-yr study period in the effluent-irrigated Shamouti orange [Citrus sinensis (L.) Osbeck cv. Shamouti] orchard plot. Nearby Shamouti orange plots irrigated with tap water were either nonrepellent or only somewhat repellent. Repellency was very variable spatially and with depth, appearing in vertically oriented "repellency tongues." Temporal and spatial variability in repellency in the uppermost 5-cm soil surface layer was not related to seasonality, soil moisture content, or soil organic matter content. Nonuniform distribution of soil moisture and fingered flow were observed in the soil profile after both seasons, demonstrating that the repellent layer had a persistent effect on water flow in the soil profile. A lack of correlation between bulk density and volumetric water content in the soil profile demonstrates that the observed nonuniform spatial distribution of moisture results from preferential flow and not heterogeneity in soil properties. Soil water repellency can adversely affect agricultural production, cause contamination of underlying ground water resources, and result in excessive runoff and soil erosion.

Abbreviations: OM, organic matter • WDPT, water drop penetration time


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN MANY ARID and semiarid regions such as Israel, water demand has exceeded the reliable supply of surface water and renewable ground water due to rapid growth in municipal and industrial use. The agricultural sector is the major consumer of water in Israel, using two-thirds of available resources. The growing competition for scarce water resources, coupled with laws limiting ground water pumping, has led to utilization of low quality water in irrigated agriculture. In particular, the exploitation of treated sewage effluent has been growing over the previous 30 yr, and by the year 2010, effluent is expected to provide approximately half of all water used in irrigated agriculture in Israel. However, applying effluent to arable lands also involves certain environmental and agricultural risks.

Effluent differs from fresh water by higher contents of electrolytes, dissolved organic matter, suspended solids, and biochemical and chemical oxygen demand (BOD, COD). These varied constituents in effluent can affect soil physical and hydraulic properties. Adverse effects leading to decreased hydraulic conductivity and infiltration rates include physical blockage of pores by suspended material (de Vries, 1972; Vinten et al., 1983; Levy et al., 1999), and clay dispersion by organic acids or electrolytes (Durgin and Chaney, 1984; Frenkel et al., 1992; Tarchitzky et al., 1999). Positive effects of effluent on soil physical structure involve the role of dissolved organic matter in improving aggregate stability (Chaney and Swift, 1986; Fortun et al., 1989; Piccolo et al., 1997).

The current study was motivated by a soil structure problem encountered over recent years in certain Israeli fields and orchards that have been under prolonged irrigation with treated effluent. Farmers found that the wetting zone at the soil surface around drippers was shrinking, resulting in dry spots along drip irrigation lines. This problem was not observed in adjacent tap water–irrigated fields. The ad hoc solutions employed by the farmers were to increase the amount of water applied at each irrigation event to increase the lateral expansion of the wetting front, or to decrease the distance between drippers along the line. Investigating this phenomenon, we found that prolonged use of effluent has caused some soils to become water repellent, the subject of this study.

It has been recognized for many years that certain organic matter (OM) compounds can induce water repellency in soils by several means. Predominant factors influencing soil hydrophobicity include: (i) fire (DeBano and Krammes, 1966; Savage, 1974; Shakesby et al., 1993); (ii) various plant species (McGhie and Posner, 1980; Doerr, 1998; Franco et al., 2000); (iii) assorted OM components, mainly aliphatic hydrocarbons and amphiphilic substances (Hudson et al., 1994; Franco et al., 1995; Roy and McGill, 1998; Roy and McGill, 2000); and (iv) fungal hyphae and bacterial activity (Bond and Harris, 1964; Chan, 1992; Garkaklis et al., 2000; York and Canaway, 2000). Despite years of research (reviewed in Wallis and Horne, 1992; DeBano, 2000; Doerr et al., 2000), a true understanding of the repellency phenomenon from a chemical and biological perspective is still elusive.

The main impacts of soil water repellency are reduced infiltration capacity, increased overland flow and soil erosion, development of fingered flow in structural or textural preferential flow paths, and creation of unstable, irregular wetting fronts (Hendrickx et al., 1993; Dekker and Ritsema, 1994; Ritsema and Dekker, 1994a, 1998; Robinson, 1999; McLeod et al., 2001). Hydrophobicity-induced fingered flow can lead to considerable variations in water content in a water-repellent soil, which can lead to poor seed germination and plant growth (Wallis and Horne, 1992). Additionally, water flow in preferential pathways has been found to cause accelerated leaching of surface-applied agrochemicals and salts, resulting in increased risks to underlying ground water (Hendrickx et al., 1993; Ritsema and Dekker, 1998; Blackwell, 2000; Graber et al., 2001).

The need to manage effluent irrigation–induced repellency prompted this study, with the long-range perspective being effective agro-management. We were specifically interested in examining: (i) the vertical and lateral distribution of soil water repellency in the undisturbed surface soil layer at actual field moisture content in advance of irrigation events; (ii) the distribution of water in the profiles of repellency-affected soils immediately following an irrigation event and before water redistribution via evapotranspiration; (iii) the recurrence of repellency in the upper soil surface layer following the winter rainy season; and (iv) the distribution of water in the soil profile following the winter rainy season. We were also interested in discerning any relationships between soil organic matter content and repellency in the surface soil layer, and between soil organic matter content and water distribution in the deeper profile.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
General
The study was conducted in a long-term effluent-irrigated citrus orchard. The irrigation season of orchards in Israel extends about eight months (usually April through November), during which time no significant rain falls. Irrigation water is typically applied in a regular, periodic fashion, and, due to the hot and dry weather prevailing throughout the irrigation period, the soil surface is usually dry before each irrigation event.

Study Site
The study site was the Safariya commercial orchards located in the central part of the Israel Coastal Plain near Ben-Gurion airport. The current study concentrated on Plot 123, which was effluent-irrigated for over 20 yr, and Plots 140 and 142, both of which were tap water–irrigated continuously. The three orchard plots are relatively flat topographically. Soil in all three plots is sandy: sand/silt/clay in g kg–1 of Plot 123 is 840/20/140; Plot 140, 830/20/150; and Plot 142, 820/40/140 (average of duplicate samples of 0- to 5-cm layer measured by hygrometer method). Shamouti oranges are grown on the three examined plots. Irrigation is applied once weekly at about 35 mm wk–1 by minisprinkler, for a total rate of about 700 mm over the irrigation season. Typical rainfall over the winter months is about 500 mm. Two types of transects were obtained: soil surface layer (0–5 cm) transects, and soil profile (0–50 cm) transects. Transect locations were carefully chosen with regard to uniform canopy coverage to minimize differences related to litter amount. Transect details are tabulated in Table 1; effluent quality data are reported in Table 2.


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  		Table 1. Sampling details for plots with Shamouti orange trees.

 

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  		Table 2. Some chemical characteristics of treated sewage effluent used for irrigation. Data provided by the orchard operators for the years 2002–2004.

 
0- to 5-cm Soil Surface Layer Transects
Water repellency, moisture content, and OM content of the soil surface layer and its spatial variability were studied by intensive soil sampling. Sampling along transects between two trees coincided with the closing stages of the two seasons of interest: the hot, dry irrigation season (sampling events in October or November), and the winter rainy season (sampling events in March). For the October–November events, sampling took place the day before the weekly irrigation. For the March events, sampling took place after a dry spell. Sharpened stainless steel cylinders (Eijkelkamp, Giesbeek, the Netherlands) of 5-cm i.d. and 5-cm height (100-cm3 volume) were used for undisturbed sampling by pressing them carefully into the soil. Samples were obtained about 3 cm apart, for a total of 12 to 20 across each transect. Dry leaves, stems, and other vegetal residuals at the soil surface were removed gently by hand before sampling.

After lifting the cylinders out of the soil, the upper and lower cylinder faces were sealed immediately by fitted plastic caps to prevent changes in soil water content. The soil samples were stored in the laboratory in special aluminum cases resistant to humidity and heat (Eijkelkamp) until soil water repellency determination in the laboratory. There, the caps were removed and water repellency of the soil surface determined by the water drop penetration time (WDPT) test. Water drops that penetrated in the soil maintained a well-defined, spherical shape and were easily removed by spatula. A wooden disc of 4.9-cm diameter and 1-cm thickness was then inserted into the bottom of the stainless-steel cylinder, pushing the soil sample upward. The exposed 1-cm soil slice at the top of the cylinder was removed by a knife, and the WDPT of the next 1-cm layer in the cylinder was tested. This process was repeated for the entire 5-cm cylinder of soil. Undisturbed soil samples at field moisture content were specifically tested for WDPT because a major goal of the study was to examine the distribution of soil water repellency as encountered by irrigation and rain events. We have found that WDPT measurements of disturbed soil samples may not be representative of WDPT measurements of undisturbed soil samples (Graber et al., unpublished data, 2005). Following the WDPT tests, water content and organic matter content of each 1-cm slice were determined.

0- to 50-cm Soil Profile Transects
To examine whether, and in what manner, a repellent surface soil layer affects water distribution in the deeper soil profile, two transects to 50-cm depth were obtained. Transects were sampled between two adjacent minisprinklers located between two trees, such that water application rates across the transects are expected to be relatively uniform. The first was obtained in October 2002, the morning following a late afternoon irrigation event, and the second, the following spring (March 2003), after a rain event (Table 1). The rationale behind the timing was to avoid moisture redistribution by root uptake and to minimize the effect of evaporation from the soil surface on water content. Sampling took place at close intervals (about 6 to 11 cm between cylinder centers for October 2002 and March 2003, respectively) along transects between adjacent trees over a distance of about 140 to 145 cm. To keep the soil properties as close as possible for the two 0- to 50-cm sampling campaigns, the two transects were obtained about 20 cm apart between the same two trees. The stainless steel cylinders were pressed vertically into the soil using a rod assembly and emptied into plastic bags for sample storage. Samples were collected from depths: 0 to 5, 5 to 10, 15 to 20, 25 to 30, 35 to 40, and 45 to 50 cm, and analyzed for moisture and OM content in the laboratory.

Repellency Determination by Water Drop Penetration Time Test
Soil water repellency was determined using the WDPT test (Doerr, 1998). Two 200-µL drops of distilled water were placed on the surface of the soil samples, and the time that elapsed before the drops were absorbed was determined. The average time for duplicate drops is reported as the WDPT. In the vast majority of cases, WDPT for duplicate drops differed by no more than 10%.

In general, a soil is considered to be water repellent if WDPT exceeds 5 s (DeBano, 1981; Dekker et al., 1998). In the present study, five classes were distinguished: Class I, wettable, not water repellent (infiltration within 5 s); Class II, slightly water repellent (5 < WDPT ≤ 60 s); Class III, strongly water repellent (60 < WDPT ≤ 600 s); Class IV, severely water repellent (600 < WDPT ≤ 3600 s); and Class V, extremely water repellent (WDPT > 3600 s) (Bisdom et al., 1993). Test time was extended to a maximum of five hours (18000 s), after which time, if the drops still had not penetrated, a reading of 18000 was recorded.

Moisture and Organic Matter Content Determinations
Moisture content was determined gravimetrically by drying the soil samples at 105°C for 24 h. Since the volume of the cylinder is known (100 cm3), it was possible to convert gravimetric moisture content to volumetric moisture content for samples from the 0- to 50-cm transects. This conversion could not be performed for samples from the 0- to 5-cm transects, as the samples in each cylinder were subdivided and the exact volume of each slice could not be determined. Following drying, total OM content of sieved (2 mm), 1-g subsamples was determined by weight loss-on-ignition at 400°C for 8 h.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
0- to 5-cm Soil Surface Layer Transects
Effluent-Irrigated
The WDPT results for undisturbed 1-cm slices across transects obtained from Plot 123 (effluent-irrigated for more than 20 yr) are shown for four sampling events (October 2002, March 2003, October 2003, March 2004) in Fig. 1 . The abscissa represents WDPT (in log scale), and the ordinate represents the average depth of each 1-cm slice below ground surface. Individual results are denoted by small solid markers, while the average WDPT of the layer is denoted by a large, open similar marker. The vertical dotted lines indicate the boundaries between the different repellency classes. A very high degree of water repellency, persisting throughout the two years of sampling, can be observed in the entire 0- to 5-cm surface layer. Average repellency class varies from severely (Class IV) to extremely (Class V) repellent. The variability in WDPT within a given layer across each transect is generally at least an order of magnitude, and frequently two or more orders of magnitude. High variability in WDPT in the vertical direction is also apparent. Despite this great variability in individual results, the average WDPT in each layer shows a clear decreasing trend with depth in three of the four transects (with the exception of October 2003).



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  		Fig. 1. The abscissa represents water drop penetration time (WDPT) in log scale, and the ordinate represents the average depth of each 1-cm slice below ground surface. Individual results are denoted by small solid markers, while the average WDPT of the layer is denoted by a large, open similar marker. The squares represent 0 to 1 cm, circles 1 to 2 cm, triangles 2 to 3 cm, inverted triangles 3 to 4 cm, and diamonds 4 to 5 cm. The vertical dotted lines indicate the boundaries between the different repellency classes as defined in the text.

 
The spatial distribution of WDPT measurements along two transects (October 2003 and March 2004) is represented as contour plots in Fig. 2 . Note that the vertical scale in Fig. 2 is expressed in mm and the horizontal in cm. While the general decrease in repellency with depth can be observed in both cross-sections, it is also possible to discern a tendency toward vertically trending "repellency tongues." Such "tongues" in the 0- to 5-cm soil surface layer could be expected to direct flow into less hydrophobic regions between them, contributing to the development of preferential flow pathways.



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  		Fig. 2. Spatial distribution of water drop penetration time (WDPT) in seconds for two 0- to 5-cm transects from Plot 123 in (a) October 2003 and (b) March 2004.

 
Gravimetric moisture content and OM content across the same 0- to 5-cm-thick transects from Plot 123 are shown in Fig. 3 in terms of mean and standard deviation ({sigma}). Moisture content was relatively homogeneous across each transect (small standard deviations), essentially invariant with depth, and nearly the same in all four transects (about 10–20 g kg–1). In contrast, OM distribution with depth is not uniform (Fig. 3). Also, OM content in each layer is very variable, with greater variability in the upper layers than in the lower layers (compare standard deviations in Fig. 3). There is some apparent difference (although not statistically significant) in OM distribution with depth as a function of season: in the fall sampling events (October 2002 and 2003), mean OM content decreased with increasing depth, while in the spring events (March 2003 and 2004), mean OM content was more uniform with depth (Fig. 3).



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  		Fig. 3. Moisture content (MC) and organic matter content (OM) for Plot 123 (effluent-irrigated) transects.

 
The temporal and spatial variability in repellency as measured by WDPT cannot be explained by seasonality, soil moisture content, or soil organic matter content. Concerning seasonal differences, the October 2002 transect (obtained at the close of the dry summer irrigation season) exhibited by far the greatest degree of water repellency at all depths (Fig. 1), yet the transect obtained the following October exhibits the lowest measured repellency. The two March transects (the close of the rainy winter season) exhibit intermediate repellency. Given that moisture content was very low (10–20 g kg–1), nearly identical in all soil samples, and essentially invariant with depth (Fig. 3), the observed differences in WDPT cannot be related to differences in moisture content. Nor can the WDPT results be rationalized by OM content. By comparing Fig. 1 and 3, it is possible to see that in spite of the fact that the October 2002 event had the highest OM content, and the March 2004 event the lowest, these two transects display the highest overall degree of repellency. The lack of relationship between OM content and undisturbed WDPT can be seen directly in the scatter plots of Fig. 4 , where OM content is plotted vs. WDPT for all 1-cm slices for the four different sampling events. Such lack of correspondence between WDPT and total OM content is a common observation in repellent soils (e.g., Doerr et al., 2000).



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  		Fig. 4. Organic matter content (OM) versus water drop penetration time (WDPT) for the different layer slices.

 
Tap Water–Irrigated
Of the two tap water–irrigated plots sampled with detailed 0- to 5-cm transects, one of the plots (Plot 140; March 2004) was completely wettable in all samples (total 35 samples; not shown). The second plot (Plot 142; November 2003) exhibited repellency, but to a lesser degree than even the least repellent effluent-irrigated transect obtained in October 2003. Repellency results for Plot 142 are presented in Fig. 5a as a histogram of relative frequency of WDPT class for each 1-cm layer. A similar histogram for the October 2003 transect from the effluent-irrigated plot (the least repellent of the four sampling campaigns, Fig. 1) is shown in Fig. 5b for comparison. Note that the degree of repellency in the tap water–irrigated transect is relatively elevated only in the upper 3 cm and decreases considerably below that, in contrast to the very high degree of repellency maintained throughout the 5-cm profiles for all the effluent-irrigated transects (Fig. 1, Fig. 5b).



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  		Fig. 5. Relative frequency of water drop penetration time (WDPT) repellency class histograms for (a) Plot 142, tap water–irrigated, November 2003, and (b) Plot 123, effluent-irrigated, October 2003.

 
Gravimetric moisture content and OM content for surface soil transects of Plots 140 and 142 are shown in Fig. 6 . Moisture content in Plot 140 (average 30–40 g kg–1) was relatively uniform across layers and with depth. Moisture content in Plot 142 was somewhat greater (average 50–60 g kg–1), and more variable in each layer and with depth. Seeing as all samples from the drier Plot 140 were wettable, and that the slightly moister Plot 142 exhibited certain repellency, it is clear that the lack of repellency in Plot 140 cannot be attributed to moisture content. A comparison of moisture content and degree of repellency for the extremely repellent plot (effluent-irrigated Plot 123, 10–20 g kg–1 moisture content), moderately repellent plot (tap water–irrigated Plot 142, 50–60 g kg–1 moisture content), and nonrepellent plot (tap water–irrigated Plot 140, 30–40 g kg–1 moisture content), also demonstrates that the extent of repellency in these plots is not dependent on the moisture contents of the soil samples.



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  		Fig. 6. Moisture content (MC) and organic matter content (OM) for tap water–irrigated Plots (a) 140 and (b) 142.

 
Organic matter content decreases with depth in Plot 142 (Fig. 6b), sampled March 2004, and is more uniform with depth in Plot 140 (Fig. 6a), sampled November 2003. Thus, the same seasonal distribution of OM observed for Plot 123 (decreasing in fall, uniform in winter) is also apparent in Plots 140 and 142. The generally higher OM content in Plot 140 (wettable) also demonstrates that the amount of OM is not linked to water repellency.

0- to 50-cm Soil Profile Transects
Repellency in the tested soils decreases dramatically below the uppermost 5-cm soil surface layer, and soil deeper than 10 cm is fully wettable (data not shown). Spatial distribution of volumetric water content in the soil profile for the two 0- to 50-cm soil profile transects is shown in Fig. 7 . In both cross-sections, the spatial moisture content distribution in the soil profile is nonuniform, and the development of flow fingers readily apparent. These fingers were formed owing to wetting flow instability that was formed below the water-repellent soil layer (de Rooij, 2000). Some fingers persist down to 40 cm, leaving drier regions between them. Water content differences between fingers and adjacent "drier" regions in the current study are less than those observed by Dekker and Ritsema (2000), presumably because here, the soil profile below 5 cm is naturally wettable at all water contents, while that of Dekker and Ritsema (2000) was repellent to depths of about 40 cm. To avoid moisture redistribution by root uptake, the October 2002 sampling (Fig. 7a) was performed about 16 h following the onset of irrigation (late afternoon). Therefore, it can be assumed that some fingers were not fully developed at the time of sampling. Note that "fingers" did not develop below the minisprinklers (located at the two edges of transects), in spite of the expected local higher water application rate (Fig. 7a). A different and more uniform water flow pattern was developed in the subsequent winter sampling (March 2003; Fig. 7b). The pattern of moisture content nonuniformity in this transect includes drier patches near the soil surface that guide incipient preferential flow pathways, and a more fully developed finger toward the right-hand side of the transect. Note that the two transects were obtained about 20 cm apart.



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  		Fig. 7. Volumetric moisture content across 0- to 50-cm-deep transects from Plot 123 in (a) October 2002 and (b) March 2003.

 
Organic matter content decreases as depth increases in both transects (Fig. 8) . Distribution of OM is more regular across the March 2003 transect as compared with the October 2002 transect.



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  		Fig. 8. Organic matter content (OM) distribution in 0- to 50-cm-deep transects from Plot 123 in (a) October 2002 and (b) March 2003.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Development of severe to extreme soil water repellency under effluent irrigation in a Shamouti orange orchard (Plot 123) is clearly demonstrated in the results reported herein. This repellency is developed mainly in the uppermost 5-cm soil surface layer, as is common for other repellent soils (Doerr et al., 2000). In contrast, nearby Shamouti orange plots irrigated with tap water were either nonrepellent, or only somewhat repellent. Given that the four sampling campaigns of the effluent-irrigated site each gave the same extent of repellency and repellency variation (Fig. 1), it can be assumed that a single campaign at each of the tap water–irrigated sites is representative of the large differences in repellency between the three sites. To the best of our knowledge, soil water repellency developed or exacerbated under effluent irrigation has not been previously reported in the scientific literature for field soils. Preliminary observations in other effluent-irrigated orchard plots (in the Safariya area and others) demonstrate that development of soil water repellency under effluent irrigation is not unique to Plot 123 (Wallach, unpublished data).

Our observation that repellency was nonexistent or not well-developed in tap water–irrigated citrus orchards corresponds to information available in the scientific literature. There seems to be a common misapprehension that soil water repellency is widespread in citrus, which could lead to the conclusion that the observed water repellency in Plot 123 is related to the citrus crop rather than to effluent irrigation. Yet, soil water repellency in citrus was reported only in the 1940s in certain Florida citrus groves (Jamison, 1942, 1945, 1947; Wander, 1949). The repellency was attributed to specific fertilizer and lime treatments, with repellency appearing in up to 30% of fertilized and limed soils, and in fewer than 8% of untreated soils (Wander, 1949). Jamison later reported that repellency in citrus had ceased to be a problem because of changes in cultivation and irrigation regimes (Jamison, 1969). The lack of repellency in citrus corresponds with our observations that repellency is weakly expressed, or not at all, in tap water–irrigated plots (Plots 142 and 140, respectively). Regarding the effect of citrus-derived OM on repellency, it is worthwhile to note that the OM content of Plot 140 (wettable tap water–irrigated plot) is even greater than of Plot 142 (somewhat repellent tap water–irrigated plot).

Spatial variability in soil water repellency both laterally and in depth is very large, a phenomenon which has been previously observed in other repellent soils (Wallis et al., 1993; Doerr et al., 1998; Dekker et al., 2000). From the results, it is clear that the variability cannot be explained by either water content or OM content. Given the vast spatial variability in repellency observed, there do not appear to be any temporal significant differences in the effluent-irrigated plot, even after rinsing by winter rains. Thus, repellency induced by effluent irrigation may prove to be quite persistent. We have not yet examined the extent and variability of repellency after wetting, and whether there is a relationship between critical water content and seasonality.

The mechanism by which effluent irrigation water increases repellency in orchards is thus far not understood, and further investigation is currently underway. The relationship between repellency degree and different amphiphilic and surface active organic compounds, such as are common in treated effluents, is inconsistent at best (Adhikari and Chakrabarti, 1975; Wallis and Horne, 1992; Doerr et al., 2000). Surface active humic materials were generally found to increase soil hydrophobicity (Adhikari and Chakrabarti, 1975; Wallis et al., 1993; Lichner et al., 2002), while various cationic, anionic, and nonionic surfactants were found to enhance wettability (Miyamoto, 1985; Cisar et al., 2000; Feng et al., 2002). Perhaps of relevance to repellency development under effluent irrigation, Jamison (1942) found that dilute soap solutions improved wetting rate of repellent soil initially, but that on drying, the surfactant-treated soil exhibited even greater repellency. An added complication is that effluents used for irrigation tend to promote soil biological activity and abundance (Monnett et al., 1995; Barkle et al., 2000; Sparling et al., 2001), which may contribute to development of repellency (Bond and Harris, 1964; Savage et al., 1969; White et al., 2000).

The current study shows that the repellent soil surface layer causes nonuniform water distribution in the wettable soil of the root zone, resulting in formation of preferential flow pathways. Percolation of water in such preferential flow pathways can cause a reduction in plant-available water, and an increase in leaching of dissolved chemicals. In theory, differences in dry bulk density in the soil profile could also explain the nonuniform volumetric water distribution. In such a case, there would be a direct relationship between the two. In Fig. 9 , however, a lack of correlation between bulk density and volumetric water content can be clearly observed. The extent of correlation was examined to a depth of 30 cm for the October 2002 transect and to 20 cm for the March 2003 transect, to include the entire variability of the wetting front, but to exclude dry soil at greater depths to where the wetting front may not have extended (Fig. 7). The lack of correlation between bulk density and volumetric water content in the soil profile provides additional evidence that the observed nonuniform spatial distribution of moisture results from preferential flow and not from heterogeneity in soil properties (Ritsema and Dekker, 1994b).



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  		Fig. 9. Bulk density versus volumetric water content for 0- to 50-cm-deep transects from Plot 123 in (a) October 2002 and (b) March 2003.

 
Unused amounts of water and chemicals leaving the root zone in preferential flow pathways caused by repellency can be reduced through a properly designed irrigation system and water application schedule. Bearing in mind that soil surface layer repellency was found to be very high under the low moisture contents typical before irrigation events, and considering the general conclusion of many studies that water repellency decreases as moisture content increases (Wallis and Horne, 1992; Doerr et al., 2000), a general recommendation could be to decrease the interval between subsequent irrigations to keep the soil surface moist. Such a generic recommendation, however, needs to be supported by additional field studies designed to test the persistence of effluent-induced repellency as a function of surface soil water content. Furthermore, it must be considered that maintaining a wet soil surface will increase water loss by evaporation, thus requiring greater amounts of irrigation water. An increase in irrigation frequency cannot be reasonably accompanied by a significant decrease in irrigation water depth at each irrigation event, since salt leaching from the root zone should take place frequently owing to the high salinity levels of the treated effluents (Table 2). It should be recalled that nonuniform moisture distribution in repellency-affected soil profiles can bias the results of devices installed in the soil to monitor water tension and/or moisture content for purposes of testing irrigation efficiency or for irrigation automation. Likewise, soil sampling for other purposes (e.g., chemical assays, etc.) should be properly designed according to the excessively nonuniform conditions in plots irrigated with effluents.

The persistence of preferential flow paths due to flow instability in rewetted soil (Glass et al., 1989; Nieber, 1996), is still an issue for further research in irrigated, repellent soils. Such persistent pathways may result in somewhat drier pockets in the soil profile, and, together with the relatively high salinity of effluents, could increase the potential of these pockets to become salinity sinks during irrigation periods. The current study was not designed to test whether winter rain water flowed in the same preferential flow pathways as those of the previous summer. However, the irregular distribution of infiltrated rainwater during the winter (Fig. 7) may decrease the efficiency of salinity leaching, contributing to nonuniform, small-scale soil salinization in effluent-irrigated, repellent soils.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In semiarid and arid regions that suffer from fresh water shortages, the use of marginal waters such as treated sewage effluent is indispensable for support and sustenance of agriculture. Even in areas not stressed by water shortage, land application of effluent is considered a viable disposal option, preferable over other alternatives such as release to surface water bodies. Irrigation with treated sewage effluent has been found to have positive impacts on soil fertility (Speir et al., 1999; Yadav et al., 2002; Mohammad and Mazahreh, 2003), and to improve or maintain soil physical properties (Mathan, 1994; Magesan et al., 1999; Agassi et al., 2003). Certain environmentally detrimental aspects of effluent irrigation have also been documented in field studies, including enhanced downward transportation of pesticides (Graber et al., 1995), increased soil sodicity (Balks et al., 1998), and excessive nitrate leaching (Magesan et al., 1998; Snow et al., 1999). Consequently, land application of treated effluent needs to be accompanied by sustainable soil management practices so as to maximize benefits while minimizing harm. In this study we document, apparently for the first time, the development of soil water repellency under irrigation with secondary treated sewage effluent in field soils. Soil water repellency affected the uniformity of moisture content distribution in the soil profile following irrigation events in the summer, and rainfall events in the winter. Such nonuniform wetting can adversely affect agricultural production and lead to contamination of underlying ground water resources. Other consequences of soil water repellency include excessive runoff and soil erosion (Wallis and Horne, 1992; Doerr et al., 2000). As such, additional research is required to determine the extent of soil water repellency developed under effluent-irrigated agriculture and its causes, with the ultimate goal to find ways to alleviate and avoid development of soil water repellency under effluent irrigation in the future.


   ACKNOWLEDGMENTS
 
This research was supported by the Israel Ministry of Agriculture and Rural Development, Office of the Chief Scientist.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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P. Fine and A. Hass
Role of Organic Matter in Microbial Transport during Irrigation with Sewage Effluent
J. Environ. Qual., May 25, 2007; 36(4): 1050 - 1060.
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