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TECHNICAL REPORTS
Surface Water Quality

Effects of Prolonged Irrigation with Treated Municipal Effluent on Runoff Rate

^a Soil Erosion Research Station, Soil Conservation and Drainage Division, Ministry of Agriculture, c/o Rupin Inst. Post 40250 Israel
^b Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization (ARO), Israel
^c Dep. of Soil and Water Sciences, The Hebrew Univ. of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Israel

^* Corresponding author (menahema{at}moag.gov.il)

Received for publication May 12, 2002.

	ABSTRACT

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

 
The use of domestic effluents for the irrigation of crops has been widespread in Israel for the past 30 years. The sodium adsorption ratio (SAR) of the standardized domestic effluents ranges between 4 and 6. According to the literature, when soils with SAR levels of 4 to 6 are exposed to direct raindrop impact they are subjected to enhanced aggregate disintegration, leading to sealing processes of the soil surface and subsequent increased runoff and soil erosion. However, these phenomena were not observed in the laboratory and field experiments of this study. On the other hand, a rapid decrease of the soil SAR to its initial values was observed, in laboratory and fieldwork, once the soil was subjected to a simulated rainstorm of distilled water (laboratory) or natural rainstorms (field plots). We can conclude that the process of SAR increase during irrigation with standardized effluent water is reversible. Further investigation in this direction can lead to recommendations regarding the necessary levels of domestic sewage water purification in correlation with soil types, climatic conditions, and hazards to tap water aquifers.

	INTRODUCTION

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

 
THE USE OF DOMESTIC EFFLUENTS for the irrigation of crops has been widespread in Israel for the past 30 years. The shortage of tap water has made it necessary to use unconventional water sources for agricultural uses. Today, domestic effluents supply at least 30% of the water for irrigation of crops and this portion will increase considerably in the coming years. The main increase in the substitution of tap water with effluents has occurred in the last decade.

The use of domestic effluents for irrigation of crops is a very efficient method for the disposal of effluents from economical and environmental perspectives. The main obstacle for extending the use of effluents for the irrigation of crops has resulted from their quality. The standardized domestic effluents of Israel contain relatively high concentrations of suspended solids, dissolved organic matter (DOM), and dissolved chemicals (e.g., NaCl, B, nutrients, and some heavy metals). Sodium chloride is not removed during the purification process and the only way to reduce its concentration is to control NaCl contribution from the source. Chlorine is toxic to many crops; avocado (Persea americana Mill.), for example, is sensitive to levels of 90 mg L¹, while for citrus, levels greater than 200 mg L^-1 are harmful. Generally, Cl levels in the effluent are over 200 mg L^-1.

The deleterious effect of Na cations on the hydraulic properties of soil has already been presented by Richards (1954). Usually, sodium adsorption ratio (SAR) levels in the Israeli effluents range between 4 and 6. Prolonged irrigation with such water will probably increase the exchangeable sodium percentage (ESP) of the soil to about 4 to 6. During the irrigation season such ESP levels are not harmful to the infiltration rate of the soil since the electrical conductivity of the effluent water (approximately 1.9 dS m^-1) is commonly high enough to prevent the chemical dispersion of the soil aggregates (e.g., Agassi et al., 1981, 1985). However, during the rainy season, when bare soil surfaces with ESP levels of 4 to 6 are subjected to the impact of the raindrops (distilled water), seal formation and subsequent decrease in the infiltration rate and increase in soil erosion often occur (Agassi et al., 1981, 1985). Seal formation is caused by two complementary mechanisms: (i) physical breakdown of the soil aggregates caused by the mechanical impact of water drops on the bare soil surface, and (ii) physico–chemical dispersion of soil aggregates and the downward movement of fine particles to where they lodge and clog the conducting pores (e.g., Mcintyre, 1958; Agassi et al., 1981; Levy et al., 1986). These processes are enhanced by the increase in the soil ESP and the decrease in the electrical conductivity of the soil solution (Agassi et al., 1981; Katzman et al., 1983).

The effect of DOM content in the effluent on the stability of the soil aggregates is still questionable. Some researchers suggest that DOM increases the stability of the soil aggregates against dispersion processes caused by raindrop impact, relatively high ESP levels, and relatively low ionic strength of the soil solution. However, works by Tarchitzky et al. (1993)(1999) and Levy et al. (1999) found that the presence of DOM in the effluents enhanced soil–clay dispersivity, increased the clay flocculation value, and decreased the hydraulic conductivity of sandy soil.

Irrigation with effluents began in Israel about 30 years ago. Although ESP levels in such fields increased from approximately 2 to 3 to approximately 4 to 6, no evidence has been presented by the farmers or extension service officers regarding a decrease in infiltration rates and a subsequent increase of runoff and erosion during the rainy season.

Two hypotheses are proposed to explain this contradiction with the "state of the art," regarding the deleterious effects of ESP levels of 4 to 6 on infiltration, runoff, and soil erosion:

• The deleterious effect of SAR in the effluent water is neutralized by the beneficial effect of DOM on aggregate stability. We assume that the net result of the SAR and DOM effects will be no change in the infiltration or runoff rates.
  
• The increasing soil ESP at the soil surface is reversible. We assume that the ESP is rapidly decreasing to its initial values during the rainy season.
  

There were three objectives of this work:

1. Study the effects of prolonged irrigation with standardized effluents on the runoff of rainfall water (reciprocal of infiltration) from Israeli soils commonly irrigated with effluents.
   
2. Study the specific effect of DOM and SAR on the runoff of rainfall water.
   
3. Study whether changes in the chemical properties of the soil, as a result of the irrigation with effluent water, are reversible during the rainy season.
   

	MATERIALS AND METHODS

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

 
The soils used in this study are the only soils commonly used for irrigation with effluents in Israel for field crops: typical loamy loess (Calcic Haploxeralf) from Gilat in the Central Negev and a dark brown sandy clay grumosol (Chromic Haploxerert) from Hafetz Haim in the southern Coastal Plain (Table 1). The predominant clay mineral in these soils is montmorillonite. The irrigation season is between April and the end of August. After harvesting of the crop (September–October) the soil is cultivated for seedbed preparation.

The experimental setup included: two water types (tap water and effluent), two soils (loess and grumosol), two soil preparation methods (disturbed and undisturbed), and four replications for each combination of the unmeasured variables.

Part B Experiments
Part B of the experiments was designed to study Objective 2. The soils used were the same as in Part A and they were collected at the same site and at the same time.

To study the second objective, we needed to distinguish between the effects of DOM and the SAR on the runoff. To accomplish this we used soil samples that had always been irrigated with local tap water. One group of the soil samples was irrigated with the standardized effluent water commonly used in the region, the second group was irrigated with synthesized water of the same SAR and electrical conductivity as the standardized effluent water, and a third group was irrigated with the local tap water.

When the effluent–water soil samples are subjected to simulated rainfall of distilled water, the DOM and the SAR levels of the soil samples are supposed to simultaneously affect the runoff rate. When the synthesized water soil samples are subjected to the simulated rainfall of distilled water, only the SAR will affect the runoff rate. When the tap water soil samples are subjected to the simulated rainfall of distilled water, neither the DOM nor the SAR levels of the effluent water will affect the runoff rate. Thus, this set of experiments enabled us to study the specific effects of DOM and SAR levels, in standardized effluent water, on the runoff rate.

Two sets of soil samples were prepared for the experiments: disturbed and undisturbed. For the disturbed set, the soil was packed into a bed surrounded by a 100- x 200- x 4-cm metal frame. The undisturbed set comprised of the aforementioned soil baskets. The soil baskets were embedded in the soil near the soil beds and were filled with the same soil.

The two sets were each divided into three groups (as indicated above) and each was irrigated for seven months with either type of water, effluent water, synthesized water or tap water. The amount of applied water was designed to simulate three years of irrigation (approximately 1100 mm). The interval between irrigations was 3 to 4 d.

The experimental setup included: three water types (tap water, effluent water, and synthesized water), two soils (loess and grumosol), two soil preparations methods (disturbed and undisturbed), and four replications for each combination of unmeasured variables.

At the end of the irrigation season the soils were left to dry and the soil characteristics were determined (Table 1). The baskets were removed and placed in a set of soil boxes. The disturbed samples were prepared and packed using the same procedure as in Part A. The boxes (from each set) were subjected to a simulated rainfall using the same procedure as in Part A. The simulated rainfall continued until an equilibrium runoff rate (ER) was achieved (after 50 mm of rain). During the experiments, samples of runoff were collected to determine runoff rates. The cumulative runoff (CR) calculations and the statistical analyses were the same as for Part A.

Part C Experiments
Part C of the experiments was designed to study Objective 3; therefore, only the effluent soil was used in this part of the study. The soils used were the same as those in Part A. They were collected at the same site and at the same time. The soil samples were prepared and packed using the same procedure as in Part A. The boxes were subjected to a fog-type (zero energy) simulated rainfall of distilled water (0.01 dS m^-1). Rainfall intensity was 35 mm h^-1. The simulated rainfall continued until 5, 10, 15, 20, 40, 60, 80, and 100 mm of rain had accumulated. As soon as each of the above measures of rain accumulated, one soil box was removed from the carousel. The interval between simulated rainfall events was 3 d. Thirty minutes after the box was removed from the carousel, the upper 1 mm of soil was very carefully removed from each of the soil boxes and the soil ESP was determined.

We preferred to work with rain of zero energy, to avoid the problem of water and soil particles splashing out of the soil boxes, a common phenomenon when the soil is subjected to a rain with energy. In a field plot subjected to natural rain, there is an equilibrium between splash in and splash out of the plot. However, in the laboratory rainfall simulation studies, with 30- x 50-cm soil boxes, the predominant mechanism is splash out. Therefore, the soil particles at the soil surface are constantly splashing out and one cannot study the dynamics of changes in ESP levels at the soil surface. The process that we studied in the laboratory with zero-energy rain did not simulate the natural processes in the field, but did provide results under controlled conditions. To verify the laboratory results soil samples were also taken, during the rainy season, from a field of a grumosol soil (approximately 50% clay) in Kibutz Yagur (Jesre'el Valley), irrigated continuously with effluent water. The samples were taken from various depths with different amounts of cumulative rain and were analyzed for SAR and ESP levels.

The objective of this set of experiments was to study the dynamics of ESP changes at the soil surface during simulated and natural rainfall.

	RESULTS AND DISCUSSION

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

 
Part A
The results for Part A are presented in Tables 3 and 4. In Table 3 it should be noted that when we compare the average of all the tap water treatments with that of the effluent water treatments, it is obvious that there is no significant difference between these two variables, with respect to equilibrium runoff rate (ER) and cumulative runoff (CR). Similar results were obtained for the disturbed and undisturbed treatments regarding the ER. However, CR was significantly greater in the disturbed than in the undisturbed treatment, suggesting that in the initial stages of the experiment, runoff rates from disturbed soil samples were higher. It can be concluded that the methodology of the soil sample preparation had a slight effect on the runoff rates at the initial stages of the rainfall. When we compare the average of all the grumosol treatments with that of the loess soil treatments, it is obvious that both ER and CR were significantly higher in the grumosol. We can conclude that these soils reacted similarly to prolonged irrigation with the standardized effluent water of Israel. A similar slight effect of soil sodicity and ionic strength of the soil solution on these soils was presented by Agassi et al. (1982)(1985). When we compare the average of all the disturbed soil–tap water combinations with that of all the disturbed soil–effluent water combinations, it is obvious that the ER of the second combination was lower compared with the first one, while they have similar results for CR. The difference was very small. However, similar results were obtained in the preliminary experiments. There was no difference in either ER or CR between the combinations, undisturbed soil–tap water compared with undisturbed soil–effluent water, loess–tap water compared, with loess–effluent water and grumosol–tap water compared with grumosol–effluent water.

Part B
The results presented in Table 5 suggested that no significant effect of irrigation water type on the runoff rate was found. A slight difference was found as a result of the effect of soil type (grumosol vs. loess) and mode of packing (disturbed vs. undisturbed) on runoff rate. We can conclude that neither the DOM content nor the SAR levels in the standardized effluent water of Israel affected the runoff and infiltration rates.

	CONCLUSIONS

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

 
Generally, standard domestic effluents in Israel have no adverse effect on the hydraulic parameters and the stability of the common arable lands, providing that there are no surface and subsurface drainage problems. These findings are supported by various field observations during the summer irrigation and rainy winter seasons in fields that have been irrigated with effluent water for at least 10 years.

We found that as a result of the effect of the winter rains the soil ESP decreased rapidly from a level of 5 to 6 at the end of the summer (during which the soils were irrigated with effluent water with SAR levels of 4–6) to its initial presummer ESP level of 2 to 3. We hypothesize that this accounts for the contradiction between the findings of the present study and the accepted concept regarding the effect of increasing the ESP from approximately 2 to 3 to approximately 5 to 6 on the stability and hydraulic properties of the soils. Another reason for the contradiction between the findings of this work and the accepted concept may be that the approximately 5 to 6 ESP level of the soils used in the preceding studies was generally the outcome of natural phenomena, and was not due to irrigation with high SAR water. We attempted in a number of experiments to reduce the natural (equilibrium) ESP level of a loess soil (approximately 10% CaCO₃) from 3 to 4 to approximately 1 by leaching the soil samples with distilled water or with a CaCl₂ solution, but this was not successful. This may be explained by the fact that the soil contained primary minerals that contains Na. Hence, the ESP level of this soil is obviously a result of the equilibrium between Na and Ca + Mg cations. It is expected that the behavior of soil samples with "artificial" and "natural" ESP levels when leached with distilled water will differ, but this assumption requires further study.

Montmorillonite was the dominant clay mineral in the soils investigated in the present study. According to Stern et al. (1991) and Wakindiki and Ben Hur (2002), montmorillonitic soils are the most sensitive to the adverse effect of the soil ESP on infiltration and runoff. We can conclude that if the ESP increase in montmorillonitic soils, as a result of prolonged irrigation with effluent water with an SAR of 4 to 6, was not harmful, then it will probably not be harmful with kaolinitic soils.

	ACKNOWLEDGMENTS

 
This research was supported by a research grant from the Chief Scientist Fund, Ministry of Agriculture, Israel.

	NOTES

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

 
Abbreviations: CR, cumulative runoff; DOM, dissolved organic matter; ER, equilibrium runoff rate; ESP, exchangeable sodium percentage; SAR, sodium adsorption ratio.

	REFERENCES

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

 

• Agassi, M., J. Morin, and I. Shainberg. 1982. Laboratory studies of infiltration and runoff control in semi-arid soils in Israel. Geoderma 28:345–356.[ISI]
• Agassi, M., J. Morin, and I. Shainberg. 1985. Effect of raindrop impact energy and water salinity on infiltration rate of sodic soils. Soil Sci. Soc. Am. J. 49:186–190.[Abstract/Free Full Text]
• Agassi, M., I. Shainberg, and J. Morin. 1981. Effect of electrolyte concentration and soil sodicity on infiltration rate and crust formation. Soil Sci. Soc. Am. J. 45:848–851.[Abstract/Free Full Text]
• Katzman, Z., I. Shainberg, and M. Gal. 1983. Effect of low levels of exchangeable Na and applied phosphogypsum on the infiltration rate of various soils. Soil Sci. 35:184–192.
• Levy, G.J., A. Rosenthal, I. Shainberg, J. Tarchitzky, and Y. Chen. 1999. Soil hydraulic conductivity changes caused by irrigation with reclaimed waste water. J. Environ. Qual. 28:1658–1664.[Abstract/Free Full Text]
• Levy, G.J., I. Shainberg, and J. Morin. 1986. Factors affecting the stability of crusts in subsequent storms. Soil Sci. Soc. Am. J. 50:196–201.[Abstract/Free Full Text]
• McIntyre, D.S. 1958. Permeability measurements of soil crusts formed by raindrop impact. Soil Sci. 85:185–189.
• Morin, J., D. Goldberg, and I. Seginer. 1967. A rainfall simulator with a rotating disk. Trans. ASAE 10:74–79.
• Richards, L.A. (ed.) 1954. Diagnosis and improvement of saline and alkaline soils. USDA Handb. 60. U.S. Salinity Lab., USDA, Washington, DC.
• Steel, R.G.D., and J.H. Torrie. 1960. Principles and procedures of statistics. McGraw-Hill, New York.
• Stern, R., M. Ben Hur, and I. Shainberg. 1991. Clay mineralogy effect on rain infiltration, seal formation and soil losses. Soil Sci. 152:455–462.
• Tarchitzky, J., Y. Chen, and A. Banin. 1993. Humic substances and pH effects on sodium and calcium montmorillonite flocculation and dispersion. Soil Sci. Soc. Am. J. 57:367–372.[Abstract/Free Full Text]
• Tarchitzky, J., Y. Golobati, R. Keren, and Y. Chen. 1999. Reclaimed wastewater effects on flocculation value of montmorillonite suspension and hydraulic properties of sandy soil. Soil Sci. Soc. Am. J. 63:554–560.[Abstract/Free Full Text]
• Wakindiki, I.I.C., and M. Ben Hur. 2002. Soil mineralogy and texture effects on crust micromorphology, infiltration and erosion. Soil Sci. Soc. Am. J. 66:897–905.[Abstract/Free Full Text]

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