Surfactants are widely used in household products, industrial pro-100 g L Ϫ1) of the formulated herbicide product and 0.1 cesses and as adjuvants to improve the delivery and effectiveness of to 0.5% (1 to 5 g L Ϫ1) of the spray tank solution. As a agrochemicals. Due to their amphiphilic nature, surfactants tend to accumulate at gas-liquid and solid-liquid interfaces, and thus, have result, the amount of surfactant applied to agricultural the potential to influence water flow and retention in unsaturated lands can be substantial, particularly if repeated herbisoils. The objective of this study was to investigate the effects of a cide applications are required throughout the growing nonionic surfactant, Triton X-100, on the interfacial properties and season. capillary pressure-water content relationships of F-70 Ottawa sand Due to their amphiphilic properties, possessing both and Appling soil. In the presence of surfactant, soil water contents hydrophilic and lipophilic moieties, surfactants tend to decreased incrementally as the surfactant concentration was increased accumulate or adsorb at gas-liquid and solid-liquid interfrom 0 g L Ϫ1 up to the critical micelle concentration (CMC) of Triton faces. Below the surfactant concentration at which miX -100 (0.15 g L Ϫ1). Over the same surfactant concentration range, celles begin to form, the critical micelle concentration the surface tension of water decreased from 7.2 ϫ 10 Ϫ2 J m Ϫ2 to 3.2 ϫ (CMC), interfacial adsorption of surfactants typically 10 Ϫ2 J m Ϫ2 while solid-liquid contact angle decreased from 40؇ to 10؇. No further changes in interfacial properties or soil water characteris-results in a lowering of both the surface tension of water tics were observed at surfactant concentrations above the CMC. The and the contact angle between the solid and aqueous experimental results were used to develop and evaluate alternative phases (Rosen, 1989). Therefore, the water holding cascaling approaches to describe concentration dependent changes in pacity of soils is likely to decrease in the presence of soil water characteristics based on the van Genuchten model. A scaling surfactants. This phenomenon could increase the depth factor that incorporated both surface tension and content angle relaof infiltration during spray events, and create a positive tionships provided accurate predictions of soil water retention curves pressure gradient between regions of surfactant-rich and over a range of surfactant concentrations. A simplified form of the surfactant-free soil water (Tschapek et al., 1981, 1991; scaling factor also was developed, on the basis of a single fitting Karkare et al., 1993; Karkare and Fort, 1993). Karkare parameter without the need for surface tension and contact angle data. Although further validation of the simplified scaling factor will and Fort (1993) reported that soil water content was be required, this approach offers an efficient means to describe the substantially altered in the presence of surfactants, with effects of concentration dependent change...

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Surfactants can reduce soil water retention by changing the surface tension of water and the contact angle between the liquid and solid phases. As a result, water flow and solute transport in unsaturated soil may be altered in the presence of surfactants. In this study, the effects of a representative nonionic surfactant, Triton X-100, on coupled water flow and nonreactive solute transport during unsaturated flow conditions were evaluated. Batch reactor experiments were conducted to measure the surfactant sorption characteristics, while unsaturated transport experiments were performed in columns packed with 40-270 mesh Ottawa sand at five initial water contents. Following the introduction of surfactant solution, the rate of water percolation through the sand increased; however, this period of rapid water drainage was followed by decreased water percolation due to the reduction in soil water content and the corresponding decrease in unsaturated hydraulic conductivity behind the surfactant front. The observed changes in water percolation occurred sequentially, and resulted in faster nonreactive solute transport than was observed in the absence of surfactant. A one-dimensional mathematical model accurately described coupled water flow, surfactant, and solute transport under most experimental conditions. Differences between model predictions and experimental data were observed in the column study performed at the lowest water content (0.115 cm 3 /cm 3 ), which was attributed to surfactant adsorption at the air-water interface. These findings demonstrate the potential influence of surfactants additives on unsaturated water flow and solute transport in soils, and demonstrate a methodology to couple these processes in a predictive modeling tool.

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