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Department of Environmental Sciences, Soil and Water Science Unit, University of California, Riverside, CA 92521
* Corresponding author (John.letey{at}ucr.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Abbreviations: DPT, drop-penetration time
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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A water-repellent soil will be defined as one which does not wet spontaneously when a drop of water is placed upon the surface. A positive pressure (water-entry pressure head, hp) must be applied to force water into the soil. The basic soil hydraulic properties of a water repellent soil differ significantly from wettable soils. For example, in a laboratory column, the hydraulic conductivity (K) of a wettable soil is independent of the depth of ponded water. However, Carrillo et al. (2000) and Feng et al. (2001) observed that the hydraulic conductivity of a water-repellent material increased with increase in depth of ponded water. The increase in hydraulic conductivity with increased ponding depth was associated with increased water content with increased ponding depth. The hydraulic conductivity of the water-repellent sand attained the value of the wettable untreated sand if the ponded water depth (ho) was high enough. The ratio of ho/hp that resulted in maximum K was 
3.1 for two sands treated to create different values of hp. Whether the decrease in average water content at the lower values of ho was uniformly distributed in the sand or a result of minute finger flow in the laboratory column where only a fraction of the sand was wet was not conclusively determined.
Infiltration rates in wettable soils decrease with increasing time after water application. However, Feng et al. (2001) observed that the temporal infiltration rates into a water repellent sand was greatly affected by the water ponding depths. At the lower values of ho, the infiltration rate increased with time after water application. As the value of ho was increased, there was a transition from the temporal increase in infiltration rate to a decrease in infiltration rate typical of a wettable material. The transition from increasing to decreasing infiltration rates with time occurred when ho/hp was approximately equal to 2.6.
Assuming soil pores can be characterized by capillary tubes, the capillary equation
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is the liquid surface tension, 
is the liquid-solid contact angle, r is the capillary radius, 
is the liquid density, and g is the gravitational constant. Note that lowering the value of lowers the value of hp. Lowering the value of also lowers the value of , which should further reduce the value of hp. Miyamoto and Letey (1971) provided theoretical relationships between and for materials with various solid-air surface tensions.
Note that in Eq. [1] the value of hp is positive when is >90° because the cos is negative for this case. The value of hp is negative for values <90° because of the positive sign of cos . From an infiltration point of view, a high absolute numerical value of hp is desired when the values of hp are negative. Conversely, a low value of hp is desired when hp has a positive value. Thus, the consequences of lowering the value of depends upon whether the value of hp is positive or negative. When hp is positive, lowering the value of reduces the value of hp, which is desirable from an infiltration point of view. When hp is negative, lowering the value of reduces the absolute numerical value of hp. This theoretically would lower the infiltration rate. However, lowering the value of also lowers the value of which would result in a higher value for cos . Therefore, whether the decrease in surface tension is positive or negative depends on the relative effects of the increase in cos as compared with the decrease in .
Adding surfactants to waters reduces the value of , therefore applying a surfactant to water and lowering its surface tension would have a positive effect on infiltration in a water-repellent soil but could have a positive or negative effect on infiltration into a soil that is not water repellent. The beneficial or negative effects depends upon the value of . Pelishek et al. (1962) verified this hypotheses.
Surfactants increase the rewet properties of treated soils. Therefore, one of the positive features of using surfactants is to convert water-repellent soils into wettable soils with improved infiltration characteristics.
Surfactants have most extensively been used in turf areas to improve wettability of these soils. For example, Cisar et al. (2000) and Kostka (2000) reported that surfactants decreased the incidence of localized dry spots and generally improved turf quality. Development of treatment protocol on turf areas has been mostly on an empirical basis. In other words, applying a range of treatments and observing the effects has been the general approach. In as much as there are theoretical relationships between liquid surface tension and infiltration in water-repellent soils, a quantitative theoretical basis for prescribing surfactant treatment may be possible. The objective of the research reported in this paper was to investigate the feasibility of using theoretical relationships to develop guidelines in surfactant application on water repellent soils.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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The sand (mixed, hyperthermic Typic Torripsamments) sample was collected from the top 20 cm of the soil profile at the University of California Coachella Valley Research Station in Coachella, CA. The sand was made water repellent by using octadecylamine [CH3(CH2)17NH2] after particulates >2 mm were removed by sieving. A mixture of 100 kg of sand, 75 g of octadecylamine, and 50 L of tap water was shaken in a large V-mixer for 24 h and then dried in an oven at 75°C for 24 h. Any excess octadecylamine was removed by rinsing the treated sand twice with water, and then the sand was dried in the oven again for 24 h at 75°C. This treatment created a stable water repellency that did not change with time after contact with water. Therefore, the confounding effects of the temporal changes in water repellency were eliminated.
The water entry pressure, hp, was measured using the technique of Carrillo et al. (1999). The treated sand was packed into a column and the ponding depth at which the water started infiltrating into the sand was measured and recorded as the water entry pressure. A series of aqueous ethanol solutions producing various surface tensions was prepared to measure the 90° surface tension (ND) of the treated sand. Three drops (40 µL) of these solutions were applied to the sand surface with a Pasteur pipette, and the time taken for droplets to completely penetrate the sand was recorded. The lowest solution concentration (highest surface tension) that penetrated in 5 s was taken to be the solution surface tension which wets the sand at 90° (ND), as specified by Watson and Letey (1970). The contact angle was calculated by using the following relationship (Carrillo et al., 1999)
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w is surface tension of water. The surfactants were mixed with tap water to create concentrations of active ingredients between 1 x 10-4 and 5000 mg L-1. The surface tension of these solutions was measured using a surface tensiomat (Fisher Scientific, Pittsburg, PA). Only surfactant concentrations of 500, 1000, 3000, and 5000 mg L-1 were used in the infiltration experiments. The DPTs for these four surfactant concentrations were measured by recording the time required for drops placed on the treated sand surface to completely infiltrate.
Polypropylene tube with 5-cm i.d. and a wall thickness of 0.4 cm was used to measure infiltration rate (i). The length of the tube for sand sample was 20 or 40 cm. A fine wire screen cap at the bottom of the tube retained the sand and allowed air to escape. The same-size tube to hold liquids at the surface was 60 cm long. A polypropylene tube (6-cm i.d. and 2.5 cm long) was used to connect the sand column to the liquid column. This connector tube was glued to the bottom of the liquid tube. A 0.3-cm deep groove was cut in the middle of connector, and a rubber O-ring in the groove provided a seal when the upper and lower columns were forced together. A port located 2 cm above the sand surface was used for solution application. Surfactant or ethanol solutions were rapidly applied through a plastic tube from a Marriott bottle. The Marriott bottle was placed on a balance (Pennsylvania Model 7300, capacity 11.34 kg, precision 1 g) and the weight of Marriott bottle with liquids was recorded by a computer as a function of time.
To prevent water from preferentially flowing between the sand and the wall, the tube was coated with a Teflon-based dry film lubricant before packing. The packing apparatus, which connected a small tube that contained three coarse wire mesh grates at 2, 11, and 21 cm from bottom of the tube, was used to pack sand evenly (Glass et al., 1989). These grates randomized the sand as it fell through the small tube into the soil sample tube. The average bulk density of the packed sand was 1.53 g cm-3. A strip of ruler was attached along the sand column tube to measure the wetting front depth.
Infiltration measurements were made with surfactant concentrations of 1000, 3000, and 5000 mg L-1. The surface tension of all these concentrations was about 0.035 N m-1 (Fig. 1) . For comparison, infiltration measurements were done with 42% (v/v) ethanol solution that had an identical surface tension. Sand columns of 20 and 40 cm were used in the experiment and the ponding depth (ho) was 20 cm in all cases. A surfactant concentration of 500 mg L-1 was also tested in the 20-cm columns. Effluent was collected for 15-min increments for the first hour and 30-min increments thereafter from the 40-cm columns, and the surface tension of these effluent samples was measured.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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The relationship between surface tension and surfactant concentrations are shown in Fig. 1. The surface tension of Product A was greater than Product B when the active ingredient concentration was <25 mg L-1. At concentrations >25 mg L-1, the surface tensions of Product A and Product B were almost equal and constant at 0.035 N m-1. Because the 90° surface tension was 0.047 N m-1, drops of surfactant concentrations >25 mg L-1 with a surface tension equal to 0.035 N m-1 theoretically should have penetrated the soil spontaneously. However, drops of surfactant solution remained on the surface for several minutes before completely penetrating the sand (Table 1). Note that the penetration time decreased with increasing surfactant concentration; and that at a given concentration, Surfactant B penetrated more rapidly than Surfactant A. Valoras et al. (1969) reported that surfactants are adsorbed by soil materials. It is postulated that as the surfactant solution drops were placed on the sand surface, some adsorption of the surfactant occurred. The adsorption increased the surface tension of the solution to a value that exceeded ND. With time there was a reorientation of the surfactant molecules such that the drop eventually penetrated the sand. The lower DPT for Surfactant B than for Surfactant A at a given concentration is consistent with Surfactant B having a lower surface tension than Surfactant A at low surfactant concentrations (Fig. 1).
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2.6. In the present experiment, ho/hp was 2.0 and the surfactant solution behaved qualitatively as untreated water. After a period of ponding, the solution reached the bottom of the column and seeped out. Typically the solution flow becomes constant after seepage from the bottom because the hydraulic head gradient becomes stabilized. Note that the flow rate continued to increase even after seepage of solution from the column. The flow rate did ultimately stabilize for the two highest surfactant concentrations at about 5 mm min-1. The 5000 mg L-1 solution reached a steady state quicker than the 3000 mg L-1 treatment. The 1000 mg L-1 treatment continued to increase and at the end of the experiment had reached a flow rate only equal to one-half of the higher concentrations.
The hydraulic head gradient remained constant throughout the experiment. Therefore the increase in flow rate with time must be related to an increase in hydraulic conductivity with time. Hydraulic conductivity is a function of sand-water content, therefore the sand-water content must have increased with time. Carrillo et al. (2000) found that the average water content in the sand increased as the value of ho increased. The main point is that water-repellent sands do not wet up to saturation even though water is ponded on the surface and water is flowing through the column unless the depth of ponding is relatively high.
The results depicted in Fig. 2 differ from those presented by Feng et al. (2001) for water. The seepage rate of water as reported by Feng et al. (2001) was constant with time as expected for a system with a constant hydraulic head gradient. Therefore the surfactant added to the water altered the temporal flow dynamics which is probably associated with surfactant adsorption and liquid surface tension changes.
The surface tension of the effluent solutions are reported in Table 2. Note that for the 1000 mg L-1 treatment, the surface tension of the effluent was greater than the surface tension of the applied solution. Adsorption of the surfactant is evident from these data. The surface tension of the effluent during the period of experiment was not as low as the 90° surface tension. Therefore it would not be expected that the sand column would have been completely wet by the surfactant solution which is consistent with the relatively low infiltration rate throughout the run.
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The infiltration rate of the aqueous ethanol solution is presented as a function of time in Fig. 3 . The surface tension of the ethanol solution was the same as the surface tension of the surfactants which were applied. In contrast to the surfactant solution, the ethanol solution infiltration rate decreased with time and reached a steady-state value shortly after seepage at the bottom of the column. Furthermore, the surface tension of the effluent ethanol solution was identical to the applied solution (Table 2). These results provide further evidence that the flow of surfactant solution was dictated by surfactant adsorption and subsequent liquid surface tension.
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The general behavior of the surfactant solutions in the 20-cm columns was the same as in the 40-cm columns. Therefore detailed data on the 20-cm columns will not be presented. The 40-cm columns were selected for illustration because they allowed a longer time of infiltration before seepage at the bottom of the column. However, the 20-cm columns were much easier to dry and the infiltration rerun using tap water was done on these columns.
The infiltration rate of untreated water as a function of time for the various pretreated 20-cm columns are depicted in Fig. 4 . The infiltration rate decreased with time and approached a steady state after seepage occurred at the bottom of the column. These results illustrate the rewet properties of the surfactant treated material. The infiltration rate patterns are typical of a wettable soil. Apparently Surfactant B had better rewet properties than Surfactant A, particularly at the lower concentrations. The steady-state flow rate as related to treatment was in the order of 500 < 1000 < 3000 = 5000 mg L-1. Although the lower concentrations provided positive rewet properties, they were not sufficient to allow the column to be completely wet. Because of the shorter columns the hydraulic head gradient at steady state for the water was 2.0 as compared with 1.5 for the infiltration experiment data presented in Fig. 2. In comparing the steady state flow rate under equal hydraulic head gradients, the rewet flow rate was 10% higher than for the surfactant solution.
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SUMMARY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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The results of this research indicate that the quantitative effects of surfactants in changing the surface tension of applied water has relatively little value in predicting the infiltration rate of the surfactant solution. All of the surfactant concentrations used in the experiment had the identical liquid surface tension. Nevertheless, different infiltration rates were observed for the different surfactant concentrations. None of the surfactant solution concentrations had infiltration rates comparable with an aqueous ethanol solution with the same surface tension. Adsorption of the surfactant to the soil material alters the surface tension and thus the surface tension of the water as it percolates through the soil is higher than the surface tension of the applied solution.
One of the major benefits of using surfactants to overcome water repellency is the rewet properties after treating and drying the soil material. Surfactants can convert a water repellent soil material to a wettable soil material. They are most beneficial when the water repellency is associated with the surface layer and the depth of water repellency is not great because surfactant adsorption near the soil surface reduces the amount of surfactant reaching the greater depths. This condition is typical of turf and can explain the fairly common use of surfactants in managing turf.
Because the adsorption of surfactants by soil materials greatly affects infiltration, prescription of surfactant treatment on a theoretical basis is not feasible. Although some basic concepts can be established, the utility of surfactants in managing water-repellent systems will be highly dependent on empirical observations.
Received for publication September 18, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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This article has been cited by other articles:
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  M. Ishiguro and T. Fujii Upward Infiltration into Porous Media as Affected by Wettability and Anionic Surfactants Soil Sci. Soc. Am. J., May 1, 2008; 72(3): 741 - 749. [Abstract] [Full Text] [PDF]
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 R. Wallach, O. Ben-Arie, and E. R. Graber Soil Water Repellency Induced By Long-Term Irrigation with Treated Sewage Effluent J. Environ. Qual., September 8, 2005; 34(5): 1910 - 1920. [Abstract] [Full Text] [PDF]
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