Soil Science Society of America Journal 63:1554-1560 (1999)
© 1999 Soil Science Society of America
DIVISION S-1-SOIL PHYSICS
Gaseous Tracer Technique for Estimating Air–Water Interfacial Areas and Interface Mobility
Heonki Kima,
P.Suresh C. Raob and
Michael D. Annablea
a Dep. of Environmental Engineering Sciences, Univ. of Florida, P.O. Box 116450, Gainesville, FL 32611-6450 USA
b School of Civil Engineering, Purdue Univ., West Lafayette, IN 47907-1284 USA
pscr{at}purdue.edu
 |
ABSTRACT
|
|---|
A series of gaseous miscible displacement experiments were conducted to estimate specific air–water interfacial areas (ai) and water contents in an unsaturated sand column. A straight-chain hydrocarbon (n-decane) was used as the gaseous interfacial tracer and methylene chloride and chloroform were used as the water-partitioning gaseous tracers. A gas chromatographic technique was employed for the tracer experiments conducted at room temperature using nitrogen as the mobile phase and water as the immobile liquid. Tracer experiments covered a water saturation (Sw) range of 1.5 to 56%. The largest ai value (
1500 cm2 cm-3), measured at the lowest Sw (1.5%), was somewhat smaller than the solid surface area (2000 cm2 cm-3) determined using the nitrogen-sorption technique. As Sw increased, ai values decreased exponentially to 80 cm2 cm-3 at Sw of 56%. Within a limited Sw range (0.29 < Sw < 0.55), where both aqueous and gaseous interfacial tracer data were measured, the ai values measured using a gaseous tracer (n-decane) were 2 to 3 times larger than those measured in a previous study using an aqueous interfacial tracer (sodium dodecylbenzene sulfonate [SDBS]). The velocity of the air–water interface was estimated to be between 23 and 36% of the bulk pore-water velocity. The water contents measured using water-partitioning tracers were within ±5% of those based on gravimetric measurements.
|
INTRODUCTION
|
|---|
MANY ENVIRONMENTALLY important physical and chemical processes occurring in the vadose zone are strongly influenced by the quantity and morphology of the air–water interface. The air–water interface is one of the major controlling factors of mass and energy transfer processes, such as evaporation, volatilization, gas exchange, and heat flow in water-unsaturated soils (e.g., Skopp, 1985). The air–water interface often affects the distributions of organic contaminants under conditions where the interface itself acts as a significant pool of organic compounds (Posner et al., 1952; Dorris and Gray, 1981; Hauxwell et al., 1992; Pennell et al., 1992; Valsaraj, 1994) and microorganisms and colloidal particles (Powelson et al., 1990; Wan and Wilson, 1994a, b; Wan et al., 1994; Conklin et al., 1995; Powelson and Mills, 1998). Although water retention and flow in soils has been studied extensively over the past three decades (Bear, 1972; Hillel, 1998; Iwata et al., 1988), a thorough understanding of the behavior of air–water interfaces has not been achieved because methods for experimental measurement of air–water interfacial area in porous media have been lacking.
Significant advances have been made in developing models for predicting fluid–fluid interfacial areas (including the air–water interface) and pore-scale distributions of immiscible fluids in porous media. These models were based on either physical simplification of complex pore-scale geometry (Skopp, 1985; Miller et al., 1990; Gvirtzman and Roberts, 1991; Cary, 1994; Reeves and Celia, 1996) or thermodynamic interpretation of multifluid porous media (Leverett, 1941; Gray and Hassanizadeh, 1991; Bradford and Leij, 1997). Lack of experimental data has inevitably led to questions regarding model validity for prediction of interfacial areas between immiscible fluids. Experimental methods for measuring fluid–fluid interfacial areas in porous media have appeared only recently (Karkare and Fort, 1996; Brusseau et al., 1997; Kim et al., 1997; Saripalli et al., 1997; Saripalli et al., 1998a, b).
Saripalli et al. (1997, 1998a, b) and Kim et al. (1997) reported on the use of a water-soluble anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), to measure oil–water and air–water interfacial areas in porous media. A field-scale application of this technique to measure oil–water interfacial areas in an aquifer contaminated with fuel hydrocarbons was described by Annable et al. (1998). Karkare and Fort (1996) proposed the use of a water-insoluble surfactant to measure air–water interfacial areas. Their method involves quantifying the water movement induced from the addition of a water-insoluble surfactant and relating it to changes in interfacial tension resulting from surfactant adsorption. Brusseau et al. (1997) reported an experimental technique, based on the use of n-heptane as the gaseous interfacial tracer, to determine the air–water interfacial areas in water-unsaturated soil columns.
The air–water interfacial areas (ai), determined by different experimental methods such as using aqueous tracers and gaseous tracers, may not quantify the same physical interfacial domain. For example, an aqueous interfacial tracer may not be able to detect all of the air–water interface present because of limitations in hydrodynamic access. Alternatively, aqueous tracers may underestimate the accessible interfacial areas if the interface itself has an advective velocity that is significant compared to the velocity of the bulk mobile water. The air–water interfacial areas measured by gaseous interfacial tracers may not be close to the actual values for the media if the interfaces are isolated from the mobile gaseous phase by a liquid. Regardless of these possible limitations, both types of tracers can provide information important for predicting liquid and gaseous transport of surface-active volatile organic compounds (VOCs) in the vadose zone.
A number of methods, for field- and lab-scale applications, have been developed for the estimation of water contents in water-unsaturated porous media. Conventional methods include gravimetric techniques, neutron activation, gamma-ray attenuation, time-domain reflectometry, and electric resistance blocks (Gardner, 1986). In unsaturated soils with a continuous gaseous phase, water-partitioning gaseous tracers have been shown to successfully measure water saturation; however, this assessment was limited at a single water content (Brusseau et al., 1997). Water-partitioning gaseous tracers used for water content measurement can be any organic or inorganic compounds that have appropriate Henry's constant values at the temperature of interest and minimal adsorption by the solid matrix.
In this paper we present the results of a series of column experiments for measuring ai values as a function of the degree of water saturation (Sw) using a gaseous interfacial tracer (n-decane). The objectives of this study are: (i) to compare measured ai values with the measurements made in a previous study using aqueous interfacial tracers within the same porous medium (Kim et al., 1997; 1998) and (ii) based on the difference between ai values measured using gaseous and aqueous tracers, estimate the velocity of water at the air–water interface (interfacial velocity) relative to that of the bulk pore water. A secondary objective was to estimate water contents using two water-partitioning tracers (chloroform, methylene chloride) and compare them to those determined gravimetrically.
|
Materials and methods
|
|---|
Gaseous Tracers
Reagent-grade n-decane was used as the gaseous interfacial tracer and chloroform and methylene chloride were used as the gaseous water-partitioning tracers; these compounds were purchased from Fisher Scientific Co. (Hampton, NH) and had a purity of greater than 99%. Methane gas (99% purity; Aldrich, Milwaukee, WI) was used as the nonreactive gaseous tracer. Ultra-pure nitrogen (>99.999%) was used as the carrier gas for the gaseous miscible displacement experiments. The compounds used as tracers were selected because they have favorable Henry's law constants and air–water interfacial adsorption coefficients (Table 1)
. A flame ionization detector (FID) with an adequate detection limit was used to measure the tracers. Compound toxicity was not considered here but it is clearly critical for tracer selection in field tests.
Porous Medium
The porous medium used in all tracer studies was sand packed into a stainless steel column (30 cm long, 1 cm i.d.). The sand used in this study was the same as that used in previous studies (see Kim et al., 1997, 1998). Prior to dry packing, the sand was washed several times with tap water, dried in an oven at 110°C, and then baked at 500°C for 24 hours to remove organic matter. The sand had a mean grain-size diameter of 250 µm with a particle size range of 105 µm to 1 mm. The porosity and bulk density of the packed sand column were 0.36 and 1.72 g cm-3, respectively.
A small amount of water (100 to 200 µL) was introduced at both ends of the column; the column was then sealed with stainless steel plugs and heated to 150°C for more than 24 hours to promote a uniform distribution of the water throughout the column. This procedure caused the water to slowly condense, which was assumed to minimize the formation of trapped air pockets. This sequence was repeated until the desired water content was achieved. A similar procedure was used to increase the water content between the tracer experiments. The heating–cooling cycles did not result in any measurable water loss, as determined by gravimetric measurements.
Gaseous Displacement Experiments
After cooling to room temperature (22°C), the sand column was installed in a modified gas chromatograph (Shimadzu, GC-14A) for gaseous miscible displacement experiments. A schematic diagram of the experimental setup is shown in Fig. 1
. The carrier gas (nitrogen) was saturated with water by bubbling through a 1-L flask filled with water to preclude water loss from the sand column during the gaseous tracer experiments. Water saturation of the sand-packed column, measured gravimetrically before and after each gas tracer experiment, did not change. The temperature of the sand column and all other GC components was maintained at room temperature (22°C), while the temperature of the GC detector (FID) was set at 230°C.
About 3 to 10 µL of gas from the head space of a 40-mL vial containing 5 to 10 mL of the organic liquid used as the tracer was withdrawn using a gas-tight syringe and injected into the GC injection port. The net weights of injected chloroform and methylene chloride were approximately 2 to 5 µg and the net weight for n-decane was 0.1 µg. Methane gas was brought to ambient atmospheric pressure before injection. About 1 to 5 µL of methane gas was injected using the same process described earlier. Carrier gas-pore velocity during the tracer displacement experiments was maintained at about 10 to 17 cm min-1. During each displacement experiment, tracer concentration in the column effluent was continuously monitored using an FID and a strip chart recorder (Fisher Scientific Co., Recordall, Series 5000). The FID response to the injected tracers was linear. The FID voltage response was normalized to the 0th moment (equivalent to scaling tracer concentration to the total mass of applied tracer) for all tracer experiments.
Data Analysis
Using the total retardation factors (Rt) determined from the n-decane displacement experiments, ai values were calculated based on the following equation
 | (1) |
where 
w and a are the dimensionless volumetric water and air contents, respectively, 
(g cm-3) is the bulk density of the sand column, H is the Henry's law constant (dimensionless), Kd (cm3 g-1) is the linear isotherm constant for sorption of a chemical by the sand, Kia (cm) is the adsorption coefficient for accumulation of a chemical at the air–water interface, and ai (cm2 cm-3) is the specific air–water interfacial area.
Values of all terms in Eq. [1] except ai were determined independently. The Rt values for n-decane displacements were estimated from the normalized, first, temporal moments (Valocchi, 1985) of methane and n-decane breakthrough curves (BTCs). The water-partitioning term (w/aH) and the soil sorption term (Kd/aH) were not included in estimating ai because the aqueous solubility of n-decane is extremely low (Table 1). A published value for the n-decane vapor-phase adsorption coefficient
at the air–water interface (Hoff et al., 1993) was used for the ai calculation. The gas-phase pore volume measured using methane agreed well with values determined gravimetrically, supporting the assumption that methane acted as a nonreactive tracer.
The measured Rt values for the water partitioning tracers (methylene chloride and chloroform) were used in Eq. [1] to estimate the volumetric water contents (w). The soil sorption term, (Kd/aH), and interfacial adsorption term (aiKia/a) were not used for the w estimation since soil sorption and interfacial adsorption were negligible for the water-partitioning tracers. Background sorption of both water-partitioning tracers on soil were measured and determined to be insignificant.
|
Results and discussion
|
|---|
Water-Partitioning Tracer Tests
Experiments with the water-partitioning gas tracers were conducted over an Sw range of 1.5 to 56%. The BTCs for methylene chloride and chloroform at different water saturations are shown in Fig. 2 . It is evident from the delayed breakthrough, compared with the nonreactive tracer (methane), that retardation of the water-partitioning tracers increases as Sw increases. This increased retardation is due to the tracer partitioning into the bulk water phase. The water content values estimated using both tracers were in excellent agreement with those measured directly by gravimetric techniques (Fig. 3)
. The relative error of the measured water contents with respect to the gravimetrically determined water contents was <5% for all of the experiments.
View larger version (16K):
[in this window]
[in a new window]
|
Fig. 3 A comparison of the volumetric water contents determined gravimetrically and estimated using the gas-partitioning tracers. The solid line is for ideal 1:1 line
|
|
Interfacial Tracer Tests
The BTCs for n-decane (Fig. 4)
demonstrate that the gas-phase retardation of n-decane decreases with increasing Sw. This is a direct result of decreasing ai as Sw increases (Fig. 5)
. Also plotted in Fig. 5 are data from aqueous interfacial tracer experiments we reported earlier (Kim et al., 1997, 1998). Note that the porous medium (sand) is the same as that used in the aqueous tracer experiments. The ai values measured using n-decane vapor as the gaseous interfacial tracer increase exponentially as Sw decreases while the ai measured using the aqueous tracers increases linearly with decreasing Sw. At a very low water content (w = 0.005; water saturation Sw = 1.5%), the measured ai (1500 cm2 cm-3) is much larger than that predicted (780 cm2 cm-3) from an exponential function fit to the rest of the data (Fig. 5). This value is also larger than the solid-surface area of 180 cm2 cm-3 estimated from a simple geometric analysis assuming that the sand grains can be represented as perfect, uniform, smooth spheres. However, the solid-surface area for the sand sample, determined using the nitrogen sorption technique, is about 1200 cm2 g-1 which translates to about 2000 cm2 cm-3 for the packed sand column. Therefore, the sharp increase of ai at very low Sw can be attributed to microscopic surface roughness of the sand (see scanning electron microscope photographs in Fig. 6)
that is reflected at the air–water interface as the water film becomes very thin (but not accounted for in a simple geometric analysis). Also, the surface area represented by this surface roughness would be covered by water films and becomes increasingly less important at higher water contents.
View larger version (34K):
[in this window]
[in a new window]
|
Fig. 4 Breakthrough curves of the gaseous interfacial tracer (n-decane) at different water saturations. Inset graph shows methane BTCs at two water saturations
|
|
View larger version (25K):
[in this window]
[in a new window]
|
Fig. 5 Air–water interfacial areas (ai) measured using gaseous and aqueous interfacial tracers. Data for SDBS are from Kim et al. (1997); data for the alcohols are from Kim et al. (1998). The solid line through the n-decane data is based on regression
, while the dotted line is based on Eq. [9]
|
|
View larger version (153K):
[in this window]
[in a new window]
|
Fig. 6 Scanning electron microscope (SEM) photographs of the sand sample used in this study. Photographs of three sand grains are shown in (A), (C), and (D). The sand grains had several "craters," a close-up of which is shown in (B) for the grain in (A)
|
|
A significant difference was found in the ai values measured using the gaseous and aqueous tracers in the Sw range of 29 to 55% (Fig. 5). The difference between the two measurements increases as Sw decreases. Three possible causes for this difference are: (i) adsorption of the gaseous tracers at soil surfaces, (ii) accessibility of the air–water interface to the tracers introduced either through the gaseous or liquid mobile phase, and (iii) mobility of the adsorbed tracers at the air–water interface during the aqueous tracer experiments. We discounted the first possibility since thin water films are likely to cover the sand grains even at a very low Sw and also because the aqueous solubility of n-decane is very low. Both of these factors would preclude significant sorption of n-decane by the sand. Rhue et al. (1989) have shown that water can effectively out-compete aromatic hydrocarbons for vapor-phase adsorption on predominantly mineral surfaces even at low relative humidities. Differences in accessibility of the air–water interfaces to the two types of tracers can be an important factor. At lower Sw, some fraction of water (thus, the air–water interface) may not be accessible to the aqueous phase tracer; however, this hydrodynamically isolated water in the porous medium may still be accessible to the gaseous tracer because the gas is the dominantly continuous phase. The opposite is true at higher Sw since the aqueous tracer may detect most of the air–water interface, while the gaseous tracer may see only a fraction of that interface. Finally, the assumption that the tracer sorbed at the air–water interface is immobile may be not valid under all flow conditions. The adsorbed aqueous interfacial tracers at the air–water interface may move in the same direction as the bulk water flow with a velocity that is somewhere between zero (stagnant) and slightly higher than the pore-water velocity as in film flow down an inclined plane. We assume that the shear forces imparted on the water by the gas flow do not induce significant water flow at the air–water interface in comparison to the gas-flow velocity.
Interface Mobility
Assuming that all of the air–water interface is accessible to both aqueous and gaseous tracers in the middle range of Sw (29 to 55%) where both tracer techniques are applicable, the interface mobility term should be incorporated for the aqueous transport of surface-active chemicals. The modified one-dimensional advective transport (equilibrium adsorption, steady water flow) model with the interfacial mobility terms is
 | (2) |
Neglecting the dispersive transport terms (for both bulk fluid and the interface) is not likely to significantly alter our interpretations here because the present analysis is based on the mean arrival time (first temporal moments). Rearrangement of Eq. [2] yields
 | (3) |
where
 | (4) |
and
 | (5) |
where Rc is the corrected retardation factor (dimensionless) of an aqueous interfacial tracer excluding other sorption terms (e.g., soil sorption, gas-phase partitioning) for a hypothetical system with a nonmoving interface, C (mol cm-3) is the tracer concentration in the bulk water phase, 
is the ratio of average, linear advective water velocities between the bulk water and the interface (dimensionless), Kiw (cm) is the adsorption coefficient of the aqueous interfacial tracer based on the aqueous equilibrium tracer concentration, ai is measured by the gaseous interfacial tracer, vi and vb are the average linear pore-water and interfacial velocities (cm min-1), respectively, t is time (min), and x is distance (cm).
Based on the difference in the ai values measured using gaseous and aqueous tracers, it is possible to estimate the velocity ratio, defined in Eq. [5]. Since we assumed that there is no detectable mobility at the air–water interface during gaseous flow, and that all the air–water interface in the porous medium is accessible to both aqueous and gaseous tracers, the ai measured using the gaseous tracer is assumed to represent all of the air–water interface formed in the medium. Therefore, a hypothetical retardation factor (Rc) for the aqueous tracer can be calculated based on the ai value from the gaseous tracer experiments, assuming no mobility at the air–water interface. The difference between the experimentally observed retardation factor, R*, of the aqueous tracer and Rc is the result of mobility of the air–water interface.
Based on Eq. [2], the observed velocity (v*, cm min-1) of an interfacial tracer is
 | (6) |
The observed advective velocity of an interfacial tracer (v*) is the same as that of the bulk water when the mean velocities at the interface (vi) and in the bulk water (vb) are the same; that is, = 1. When the interface is immobile (vi = 0, = 0), the observed velocity of the interfacial tracer is
 | (7) |
Using Eqs. [2] through [6], the velocity ratio () between interface and bulk water is
 | (8) |
In order to calculate , retardation factors (R*) were calculated using an empirical equation that was based on experiments using SDBS as an aqueous interfacial tracer and a Kiw value of 5.06 x 10-3 cm (Kim et al., 1997, 1998)
 | (9) |
where
is the effective air–water interfacial area measured by the aqueous tracer SDBS. The corrected retardation factors (Rc) with the nonmoving interface were estimated based on the air–water interfacial areas measured by the gaseous tracer (n-decane).
The relationship between and Sw is shown in Fig. 7
along with the observed (R*) and hypothetical (Rc) retardation factors. Within the Sw range where both aqueous and gaseous tracer experiments data are available, the velocity of the air–water interface was estimated to be about 23 to 36% of the bulk pore water velocity (Fig. 7). Note that decreases in a linear fashion
as Sw decreases even though the difference between observed and hypothetical retardation factors increases with decreasing Sw. This result implies that at lower Sw a greater fraction of water is associated with the immobile water domain. Others have shown that the immobile water fraction in unsaturated soils increases with decreasing water content (Skopp and Warrick, 1974; Gaudet et al., 1977). Since molecular diffusion is the only process by which the interfacial tracer can reach the air–water interface in the immobile water region, more nonequilibrium effects, manifested as early breakthrough and extensive tailing of BTCs, are expected. This excessive broadening of the aqueous tracer BTCs was observed for the column displacement experiments with SDBS as the aqueous interfacial tracer (Kim et al., 1997).
View larger version (17K):
[in this window]
[in a new window]
|
Fig. 7 The ratio of interfacial velocity to the bulk pore-water velocity (), based on the observed sodium dodecylbenzene sulfonate (SDBS) retardation factors (Kim et al., 1997) and the estimated hypothetical retardation factors of SDBS based on the air–water interfacial areas (ai) measured by gaseous interfacial tracer (n-decane) experiments
|
|
|
Summary
|
|---|
The use of gaseous interfacial and partitioning tracers to determine the air–water interfacial area and volumetric water content in unsaturated porous media is described. In the miscible displacement experiments the discontinuous water phase is the immobile liquid and the continuous gas phase is the mobile fluid. Retardation of a gaseous tracer, n-decane, resulting from adsorption at the air–water interface, was measured at several water contents and the corresponding interfacial areas were determined. Starting with a large ai at very low Sw, ai values decreased exponentially with increasing Sw. At high Sw, gas-phase experiments had to be terminated since gas was now the discontinuous phase. Air–water interfacial areas measured with n-decane as the gaseous tracer were compared with previously reported ai determined using SDBS as the aqueous tracer. In the Sw range where both tracers provided ai estimates, experiments with n-decane as the tracer always yielded larger values for the interfacial areas. This was attributed to an under-estimation of the true tracer retardation due to advective mobility of the air–water interface during aqueous displacement. Analysis of the data suggests that, within an Sw range of 29 to 55%, the advective velocity of the interface may be 20 to 40% of that of the bulk aqueous phase. An alternative explanation is that the interfacial area that is hydrodynamically accessible to gaseous tracer is much larger than that which is accessible to aqueous tracers.
|
ACKNOWLEDGMENTS
|
|---|
This material is based on doctoral dissertion research by the senior author. Research was sponsored in part by the Air Force Office of Scientific Research, Air Force Material Command, U.S. Air Force, under grant F49620-95-1-0321. The SEM photographs of the sand and nitrogen surface area measurements were provided by Mr. Masa Rao, Univ. of California, Santa Barbara. This paper was approved for publication as Florida Agricultural Experimental Station Journal No. R-06809.
Received for publication June 24, 1998.
|
REFERENCES
|
|---|
- Annable M.D., Jawitz J.W., Rao P.S.C., Dai D.P., Kim H., Wood A.L. Field evaluation of interfacial and partitioning tracers for characterization of effective NAPL-water contact areas. Ground Water 1998;36:495-502.
- Bear J. Dynamics of Fluids in Porous Media. Mineola, NY: Dover, 1972.
- Bradford S.A., Leij F.J. Estimating interfacial areas for multi-fluid soil systems. J. Contam. Hydrol. 1997;27:83-106.
- Brusseau M.L., Popovicova J., Silva J.A.K. Characterizing gas-water interfacial and gas-phase transport of organic contaminants in unsaturated porous media. Environ. Sci. Technol. 1997;31:1645-1649.
- Cary J.W. Estimating the surface area of fluid phase interfaces in porous media. J. Contam. Hydrol. 1994;15:243-248.
- Conklin M.H., Corley T.L., Roberts P.A., Davis J.H., van de Water J.G. Nonequilibrium processes affecting forced ventilation of benzene and xylene in a desert soil. Water Resour. Res. 1995;31:1355-1365.
- Dorris G.M., Gray D.G. Adsorption of hydrocarbons on silica supported water surfaces. J. Phys. Chem. 1981;85:3628-3635.
- Gardner W.H. Water content. In: Klute A., ed. Method of soil analysis. Madison, WI: Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, 1986:493-544.
- Gaudet J.P., Jegat H., Vachaud G., Wierenga P.J. Solute transfer with exchange between mobile and stagnant water through unsaturated soil. Soil Sci. Soc. Am. J. 1977;41:665-671.[Abstract/Free Full Text]
- Gray W.G., Hassanizadeh S.M. Paradoxes and realities in unsaturated flow theory. Water Resour. Res. 1991;27:1855-1863.
- Gvirtzman H., Roberts P.V. Pore scale spatial analysis of two immiscible fluids in porous media. Water Resour. Res. 1991;27:1165-1176.
- Hauxwell R., Pallas N.R., Pethica B.A. Adsorption of n-alkane vapors on water. Langmuir 1992;8:602-603.
- Hillel D. Environmental soil physics. San Diego, CA: Academic, 1998.
- Hoff J.T., Mackay D., Shiu W.Y. Partitioning of organic chemicals at the air–water interface in environmental systems. Environ. Sci. Technol. 1993;27:2174-2180.
- Iwata S., Tabuchi T., Warkentin B.P. Soil–water interactions. NY: Marcel Dekker, 1988.
- Karkare M.V., Fort T. Determination of the air–water interfacial area in wet "unsaturated" porous media. Langmuir 1996;12:2041-2044.
- Kim H., Rao P.S.C., Annable M.D. Determination of effective air–water interfacial area in partially saturated porous media using surfactant adsorption. Water Resour. Res. 1997;33:2705-2711.
- Kim H., Rao P.S.C., Annable M.D. Influence of air–water interfacial adsorption and gaseous partitioning on the transport of organic chemicals in unsaturated porous media. Environ. Sci. Technol. 1998;32:1253-1259.
- Leverett M.C. Capillary behavior in porous solids. Trans. Am. Inst. Min. Metall. Tet. Eng. 1941;142:152-169.
- Mackay D., Shiu W.Y. A critical review of Henry's Law constants for chemicals of environmental interest. J. Phys. Chem. Ref. Data 1981;10:1175-1179.
- Miller C.T., Poirier-McNeill M.M., Mayer A.S. Dissolution of trapped nonaqueous phase liquids: mass transfer characteristics. Water Resour. Res. 1990;26:2783-2796.
- Pennell K.D., Rhue R.D., Rao P.S.C., Johnston C.T. Vapor phase sorption of p-xylene and water on soils and clay minerals. Environ. Sci. Technol. 1992;26:756-763.
- Posner A.M., Anderson J.R., Alexander A.E. The surface tension and surface potential of aqueous solutions of normal aliphatic alcohols. J. Colloid Sci. 1952;7:623-644.
- Powelson D.K., Simpson J.R., Gerba C.P. Virus transport and survival in saturated and unsaturated flow through columns. Jour. Environ. Qual. 1990;19:396-401.[Abstract/Free Full Text]
- Powelson, D.K., and A.I. Mills. 1998. Bacterial adsorption to gas–water interface and bacterial transport in unsaturated sand. p. 305. In Agronomy Abstracts. American Society of Agronomy, Madison, WI.
- Reeves P.C., Celia M.A. A functional relationship between capillary pressure, saturation, and interfacial area as revealed by a pore-scale network model. Water Resour. Res. 1996;32:2345-2358.
- Rhue R.D., Pennell K.D., Rao P.S.C., Reve W.H. Competitive adsorption of alkylbenzene and water vapors on predominantly mineral surfaces. Chemosphere 1989;18(9/10):1971-1986.
- Saripalli K.P., Kim H., Rao P.S.C., Annable M.D. Measurement of specific fluid-fluid interfacial areas in porous media. Environ. Sci. Technol. 1997;31:932-936.
- Saripalli K.P., Rao P.S.C., Annable M.D., Hatfield K. Measurement of specific interfacial areas of immiscible fluids in flow systems. U.S. patent no. 1998;5,744,709(April 28, 1998. U.S. Patent Office, Washington, DC) a.
- Saripalli K.P., Rao P.S.C., Annable M.D. Determination of specific NAPL-water interfacial areas of residual NAPLs in porous media using the interfacial tracers technique. J. Contam. Hydrol. 1998;30:375-391 b.
- Skopp J. Oxygen uptake and transport in soils: Analysis of the air–water interfacial area. Soil Sci. Soc. Am. J. 1985;49:1327-1331.[Abstract/Free Full Text]
- Skopp J., Warrick A.W. A two-phase model for the miscible displacement of reactive solutes in soil. Soil Sci. Soc. Am. Proc. 1974;38:525-550.
- Valocchi A.J. Validity of the local equilibrium assumption for modeling sorbing solute transport through homogeneous soils. Water Resour. Res. 1985;21:808-820.
- Valsaraj K.T. Hydrophobic compounds in the environment: Adsorption equilibrium at the air–water interface. Water Res. 1994;28:819-830.
- Wan J., Wilson J.L. Visualization of the role of the gas–water interface on the fate and transport of colloids in porous media. Water Resour. Res. 1994;30:11-23 a.
- Wan J., Wilson J.L. Colloid transport in unsaturated porous media. Water Resour. Res. 1994;30:857-864 b.
- Wan J., Wilson J.L., Kieft J.L. Influence of the gas–water interface on transport of microorganisms through unsaturated porous media. Appl. Environ. Microbiol. 1994;60:509-516.[Abstract/Free Full Text]
- Yaws C., Yang H.-C., Pan X. Henry's law constants for 362 organic compounds in water. Chem. Eng. Sci. 1991;98:179-185.
TOP
ABSTRACT
INTRODUCTION
