Soil Specific Surface Area Research Articles

Determination of the specific soil surface area from the hygroscopic water content

Complex parameters such as the criteria of similarity for the physical adsorption of water by soils can be developed using the methods of the similarity theory. Based on the criterial relationship, a model was developed to calculate the total specific surface area from the experimental data on the soil water content. The model was tested for the plow horizons of some soddy-podzolic soils in the northwestern regions of the Russian Federation. The proposed approach can be used for the design and organization of experiments for the routine analysis of soil samples.

Sodium hypochlorite separates an older soil organic matter fraction than acid hydrolysis

Different chemical treatments can be used to isolate an old and chemically resistant soil organic matter fraction from soils. Here, we compared the residues after acid hydrolysis (6 N HCl) with those obtained after NaOCl treatment (6 wt%) of the silt + clay fractions from 48 soil samples. The samples were taken at sites differing in climate and land use across Switzerland. To determine the influence of the two treatments on soil organic matter and mineral structures, we examined infrared spectra and isotopic signatures of carbon ( 14C and δ 13C) of the residues. Treatment with NaOCl removed more (63 to 91%) organic carbon (OC) than did treatment with HCl (35 to 66%), and it had no effect on mineral structures, whereas treatment with HCl converted crystalline minerals to more amorphous ones. Increases in specific soil surface area (SSA) did not correlate with the amount of OC removed. The amount of OC removed by each treatment was not (NaOCl) or only weakly (HCl) related to the initial OC content of the silt + clay fraction, suggesting the presence of a relatively constant fraction of chemically resistant OC. 14C activities of NaOCl-resistant residues were lower than those of HCl residues, indicating that soil organic matter residues isolated by NaOCl treatment were older than the residues obtained by acid hydrolysis. It is concluded that oxidation with NaOCl is the better way than hydrolysis with HCl to obtain an operationally-defined stable organic matter fraction from soils.

Comparison of Methods for Determining Specific Surface Area of Soils

The present study was undertaken to compare the capabilities of four of the known methods; more specifically, N2 adsorption, methylene blue (MB)-titration, MB-spot test, and ethylene glycol monoethyl ether (EGME) methods were evaluated to determine the specific surface area (SSA) of 16 different clayey soils. The study showed that N2 adsorption method underpredicts the SSA of soils, especially for smectitic soils. No significant differences were observed between N2 SSA, MB SSA-titration, or MB-spot test for kaolinitic soils. The SSA estimates of MB-titration and MB-spot test methods were highly correlated for all soils. The EGME method has a very different procedure from the MB methods; however, it was highly correlated with MB methods (r2=0.95). The N2 adsorption method had no correlation to other methods. The cation exchange capacity of tested soils was highly correlated to the SSA, as high as r2=0.77. No unique relationship was determined between the clay fraction and SSA.

Clay mineralogy, cation exchange capacity and specific surface area of loess soils with different volcanic ash contents

The loessical soils of the semiarid Argentinean Pampas (SAP) contain variable amounts of volcanic ashes. Their influence on the mineral composition and some physicochemical properties of soils like soil specific surface area (SSA) and cation exchange capacity (CEC) have been still not elucidated. Because of that we analyzed 24 topsoil samples (0 to 20 cm) of soils with high and low volcanic ashes contents for clay mineralogy and its influence on soil chemical and physical properties and CEC and SSA. Results showed that ash enriched soils were placed to the south and ash free soils to the north-east of the SAP in agreement with the volcanic ash deposition pattern of 1932 Quizapu vocano eruption. Mineralogy of the clay fraction was dominated by amorphous minerals and less crystallized and expansible smectites in ash enriched soils, and by illites in ash free soils. No differences in crystallinity or expansibility were found between montmorillonites of ash free and ash enriched soils. Fine sized clays (< 0.2 μm) were dominated by illites and illite–montmorillonite intergrades in ash free soils and by amorphous materials in ash enriched soils. Such results indicated that montmorillonites tend to form more when volcanic ashes are present but their crystallinity and expansibility do not change with ashes content. SSA was positively related with silt contents in both ash free and ash enriched soils ( r 2 = 0.70, p < 0.05). The clay fraction (< 2 μm) explained only 9% of SSA variability and the fine clay fraction (< 0.2 um) did not affect SSA. The influence of silt on SSA was attributed to the existence of 2 : 1 minerals in the silt fraction. These results also indicated that ash contents did not influence SSA. CEC correlated positively with clay ( R 2 = 0.67, p < 0.001) in ash enriched soils, and with organic matter in ash free soils ( R 2 = 0.837, p < 0.001) but it did not correlate with fine clay contents in any of both soil types. These trends were explained on the basis of the higher CEC conferred by smectites in ash enriched soils and by OM in ash free soils. It can be deduced that soil degradation processes producing losses of fine sized particles, like wind or water erosion, will decrease SSA in both the ash enriched and the ash free soils, but only CEC in ash enriched soils. Organic matter losses due to excessive cultivation will decrease CEC in ash free soils.

Water films and scaling of soil characteristic curves at low water contents

Individual contributions of capillarity and adsorptive surface forces to the matric potential are seldom differentiated in determination of soil water characteristic (SWC) curves. Typically, capillary forces dominate at the wet end, whereas adsorptive surface forces dominate at the dry end of a SWC where water is held as thin liquid films. The amount of adsorbed soil water is intimately linked to soil specific surface area (SA) and plays an important role in various biological and transport processes in arid environments. Dominated by van der Waals adsorptive forces, surface‐water interactions give rise to a nearly universal scaling relationship for SWC curves at low water contents. We demonstrate that scaling measured water content at the dry end by soil specific surface area yields remarkable similarity across a range of soil textures and is in good agreement with theoretical predictions based on van der Waals interactions. These scaling relationships are important for accurate description of SWC curves in dry soils and may provide rapid and reliable estimates of soil specific surface area from SWC measurements for matric potentials below ‒10 MPa conveniently measured with the chilled‐mirror dew point technique. Surface area estimates acquired by fitting the scaling relationship to measured SWC data were in good agreement with SA data measured by standard methods. Preliminary results suggest that the proposed method could provide reliable SA estimates for natural soils with hydratable surface areas smaller than 200 m2/g.

Kaolinite – alkali interaction and effects on basic properties

The influence of type and amount of clays present in soils on their properties is well understood. The clays exert their influence through large specific surface area and charges on them. Their effect is mostly exhibited through inter particle bonding and subsequent particle associations. The mineralogical influence of soils in water is well documented. However, the change in soil water system because of presence some of the contaminants can greatly influence the soil behaviour. Some of the changes are due to formation of new compounds due to interactions between the soil and pollutant. The paper reports the effect of interaction of kaolinite mineral with alkali on the index properties of soils from which the geotechnical behaviour can be understood. Detailed X-ray diffraction studies have shown that sodium aluminum silicate hydroxide hydrate (NASH) is formed by clay alkali reactions. The type and amount of formation of the compound is influenced by the concentration of alkali solution. While the compound formed is in smaller quantities with 1 N NaOH solution, significantly high quantity is formed with 4 N NaOH solution. Presence of alumina is shown to play a significant role. It was observed that the formation of sodium aluminum silicate hydroxide hydrate is reduced in the presence of alumina. Specific gravity of contaminated clay soil was reduced which confirms the formation of new compounds. Water adsorption and specific surface area of soil are also influenced due to soil alkali interaction. The changes in the free swell and index properties of soil in the presence of alkali have been explained by the changes in soil fabric and the formation of new compound.

Inter-particle contact heat transfer in soil systems at moderate temperatures

An inter-particle contact heat transfer model for evaluating soil thermal conductivity is analysed with respect to soils, representing different textural classes, exposed to moderate temperatures ranging from 15 to 30°C. This model is a combination of a self-consistent approximation model, enhanced with an inter-particle contact heat transfer correction coefficient. For dry and saturated soils, this coefficient is defined as a ratio of a soil harmonic mean thermal conductivity of solid and fluid (air or water) phases, to the average thermal conductivity of soil solid grains. For unsaturated soils, we assume a linear interpolation of the correction coefficient between absolutely dry and saturated states, with a Kersten function (Ke) as a proportional factor. The strongest impact of the correction coefficient (maximum reduction of heat transfer) is observed for coarse soils below a critical value of saturation degree (Sr-cr–corresponds to Ke≅0) followed by medium and fine soils. For Sr>Srcr, the reduction of heat transfer gradually diminishes as Sr approaches 1 (i.e. saturated state). Soil texture, soil specific surface area, porosity and mineralogical composition (particularly quartz content) are important factors influencing the heat transfer correction coefficient. Their influence appears to be more substantial at the lower half of the wetness range (Sr<0.5). Simulation results from the new enhanced model closely follow experimental data.

Surface reactivity of A horizons towards polar compounds estimated from water adsorption and water content

The measurement of the specific surface area of soils may be useful for ranking soils in their ability to sorb polar compounds such as some pesticides and pollutants. For A horizons with varied mineralogy, particularly those containing large amounts of soil organic matter, this is problematic. We have estimated the surface area of a range of topsoils from water adsorption using the BET equation. The values obtained were greater than those measured from the adsorption of para-nitrophenol. There was a good relationship between the BET water area and the CEC of the samples (r2 = 0.83). There was a better relationship between the BET surface area and the water content of air-dry topsoils (r 2 = 0.98). We suggest that the air-dry water contents of topsoils could be used as a quick method to rank soils in order of their surface reactivity towards polar molecules, and this is preferable to more complicated methods.

Predicting Volatile Organic Vapor Sorption from Soil Specific Surface Area and Texture

AbstractVapor sorption of a volatile organic chemical (VOC) to soil minerals at low water contents can increase the total VOC sorption capacity of the soil by several orders of magnitude. Vapor sorption can subsequently cause high VOC fluxes out of the soil during periods with fluctuating low soil water contents. Models for predicting VOC sorption, including vapor sorption, as a function of soil water content are therefore valuable for evaluating contaminant volatilization and exposure risk at polluted sites. A model for estimating trichloroethylene (TCE) surption parameters at low relative vapor pressure from ethylene glycol monoethyl ether (EGME)‐measured soil specific surface area (SA) and soil clay and organic carbon content was developed based on TCE sorption data from 26 different porous media. Significant improvement in prediction accuracy compared with a previous model was achieved. Based on data for 13 soils, an organic carbon partitioning coefficient of Koc, = 0.60 foc cm3 g−1 (R2 = 0.97) for TCE is recommended for use in the new TCE sorption model. In case SA is not available, an expression for predicting SA from soil clay content is proposed. The clay‐based TCE surption model was tested against independent data for two Japanese soils (a sand and a clay loam) measured in this study by a gas chromatography‐based soil microcolumn method. The sorption model accurately predicted TCE retardation (sorption) as a function of soil water content for both soils. Model calibration to sorption data for three different VOC's (benzene, TCE, and toluene) implies that the sorption model may also be applied to chemicals other than TCE. However, sorption measurements for more chemicals and soils at varying water contents are necessary to further generalize the VOC sorption model.

Kinetics of Simazine Sorption on Soils and Soil Particle Size Fractions

Sorption behavior of simazine in whole soils and four selected soil particle size fractions of two Hiroshima soils, a loam and a decomposed granitic (DG) soil, was measured during two-step sorption experiments. In the first sorption step, adsorption kinetics were followed for 36 hours in initially simazine-free soil samples. A rapid adsorption was observed for all soil particle size fractions after 10 minutes. Between 10 minutes and 36 hours adsorption slowly progressed in all loam size fractions while adsorption stopped in all DG size fractions. In the second sorption step, 70% of the solution in the soil samples was substituted with simazine-free solution and sorption behavior was followed for an additional 36 hours. A rapid desorption took place in all soil samples during the first 10 minutes, followed by a slow adsorption in the loam size fractions and no apparent adsorption or desorption in the DG size fractions. The amount of simazine rapidly adsorbed (step I) and desorbed (step II) was best correlated with the soil specific surface area for the low-organic DG size fractions and with soil organic matter content for the loam size fractions. For both sorption steps I and II, the adsorption kinetics in the loam size fractions were well described by a modified Freundlich type equation. Adsorption kinetics during step II were dependent on both the soil organic matter content and the total amount of simazine adsorbed during step I, likely due to diffusion-controlled sorption of simazine on the soil organic matter.

Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model

Near‐surface measurements of soil water content (θ) using time domain reflectometry (TDR) may exhibit anomalous behavior in the presence of diurnal temperature (T) fluctuations. Experimental results obtained in a companion paper led to the hypothesis that the observed bulk dielectric permittivity (ϵb) is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric permittivity of bulk water with increased T; and (2) the increase in TDR‐measured ϵb with increased T due to release of bound water. In this study we develop a physically based model for the temperature dependency of TDR‐measured soil bulk dielectric permittivity and propose practical correction factors. The model considers the modified properties of water near solid surfaces to define a layer of rotationally hindered water (within the TDR frequency bandwidth) having a temperature dependent thickness. Changes in measured ϵb(T) are thus attributed to variations in the thickness of the rotationally hindered layer which has a lower dielectric permittivity than free water and hence is less “visible” to travel‐time‐based TDR waveform analyses. The model is sensitive to the soil specific surface area and the water content, both of which determine the ratio of bound to bulk soil water. Comparisons with experimental data covering a wide range of soils, water contents, and temperatures showed good agreement. Further studies are needed to evaluate some of the model's critical parameters such as the cutoff frequency below which water is considered bound. A temperature correction approximation is based on analytical expressions for TDR‐measured bulk dielectric permitivity and requires estimates of soil specific surface area and bulk density, which may be estimated from soil texture. The thermodielectric sensitivity of TDR‐measured bulk dielectric permittivity and water content may serve as a basis for estimating soil specific surface area.

Adsorption‐desorption of zinc in mollisols and their relationship with uptake of fertilizer‐applied zinc by rice

Zinc (Zn) adsorption in mollisols conformed to the linear form of Freundlich equation. The log K values were positively correlated with silt, clay, and carbonate contents and soil pH, but negatively correlated with sand content. Sequential desorption of adsorbed Zn in 0.05M Ca(NO3)2, 0.1M Mg(NO3)2, 0.005M DTPA, and 0.1M HCl revealed that weakly and specifically bound fractions of added Zn, which could easily equilibrate with soil solution, were low and decreased with silt and carbonate contents and soil pH. Weakly bound fraction increased with sand content. Strongly bound and complexed fraction of applied Zn was the maximum and increased with clay, soil organic carbon and carbonate contents and specific surface area, but decreased with sand content. The mineral bound fraction of applied Zn was intermediate and increased with silt, clay and carbonate contents, and soil pH and specific surface area. Zinc uptake due to added Zn fertilizer by rice crop (Y) negatively correlated with log K, but positively related to Zn content in the equilibrium soil extract and Zn desorbed in 0.05M Ca(NO3)2. Both log K and l/n values together explained 59.5% of the total variation in Y, while Zn content in the equilibrium soil extract, Zn desorbed in 0.05M Ca(NO3)2, 0.005M DTPA and 0.1M HCl collectively accounted 79.6% of the total variation in Y.

The role of soil conditions in fine root ecomorphology in Norway spruce (Picea abies (L.) Karst.)

The present study is an attempt to investigate the pattern of morphological variability of the short roots of Norway spruce (Picea abies (L.) Karst.) growing in different soils. Five root parameters – diameter, length and dry weight of the root tip, root density (dry weight per water-saturated volume) and specific root area (absorbing area of dry weight unit) were studied with respect to 11 soil characteristics using CANOCO RDA analysis. The investigation was conducted in seven study areas in Estonia differing in site quality class and soil type. Ten root samples per study area were collected randomly from the forest floor and from the 20 cm soil surface layer. Eleven soil parameters were included in the study: humus content, specific soil surface area, field capacity, soil bulk density, pH (KCl and H2O dilution's), N and Ca concentrations, Ca/Al and C/N ratios, and the decomposition rate of fine roots (<2 mm dia.). Root morphological characteristics most strongly related to the measured soil characteristics in the different sites were specific root area, root density and diameter of the short roots, the means varying from 29 to 42 m2 kg−1, from 310 to 540 kg m−3 and from 0.26 to 0.32 mm, respectively; root density being most sensitive. The most favourable site and soil types resulting in fine roots with morphological characteristics for optimizing nutrient uptake (e.g. low short root density and high specific root area) were Umbric Luvisol (Oxalis), Dystric Gleysol (Oxalis) and Gleyic Luvisol (Hepatica). These soil types correspond to highly productive natural forest stands of Norway spruce in Estonia. All measured soil variables explained 28% of total variance of the root characteristics. The most important variables related to root morphology were the humus content, field capacity and specific soil surface area.

Root :soil adhesion in the maize rhizosphere : the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assessed the effects of differing soil textures and differing soil matric potentials upon the strength of the root:soil attachment. A device for growing roots upon a soil surface was designed, and was used to produce roots which were attached to the soil. In order to quantify root:soil adhesion, roots of maize seedlings, grown on the soil surface, were subsequently peeled off using a universal test machine, in conjunction with simultaneous time-lapse video observation. To clarify the partitioning of energy in the root:soil peeling test, separate mechanical tests on roots, and on two adherent remoulded topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, compared to the root:soil adhesion energy. The mechanical properties of two adherent remoulded topsoil balls were a decrease of the soil:soil adhesion energy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to separate the different processes occurring during the root:soil peeling test, in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was thus possible, enabling calculation of the root:soil interfacial rupture energy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil water suction on the value of the interfacial rupture energy was very moderate. The root:soil interfacial rupture energy was controlled mainly by a product of microscopic soil specific surface area and the macroscopic contact surface area between the root and the soil. Biological and physical interactions contributing to root:soil adhesion such as root:soil interlocking mechanics were also analysed and discussed.

ADSORPTION EQUILIBRIUM OF CARBON TETRACHLORIDE ON DRY SOILS*

Vapor phase adsorption equilibrium of carbon tetrachloride, a priority pollutant, on dry soils was studied at 288, 293, and 298 K. Using a gravimetric adsorption apparatus, adsorption /desorption isotherms of carbon tetrachloride were generated on two different soil samples. The effects of temperature and soil characteristics were examined. Isosteric heals of adsorption were calculated and heat curves were constructed. Adsorption isotherms of carbon tetrachloride on dry soil samples were Type II, indicating formation of multilayers of adsorbate on the soil particle surface. Considerable hysteresis effects, associated with capillary condensation, were observed upon desorption. Thermal data confirmed that the adsorption of carbon tetrachloride vapor on soil was primarily physical adsorption. Heat curves showed that the soil samples had energetically heterogeneous surfaces. A positive correlation between The soil's specific surface area and its sorption capacity was observed. Clay content and pore size were also dominating factors. The experimental data were correlated by the Polanyi Potential, the BET, and the GAB models in order to provide input lo fate and transport models predicting the degradation or movement of volatile organic pollutants in soil. The BET equation gave accurate data fit, within a deviation range of 2·63-5·40%, for up to 40% of the saturation pressure. The GAB equation provided superior fit of the data for the entire relative pressure range. Absolute error percentages from the GAB model ranged from 1·77 to 5·38%. Results followed the Potential Theory satisfactorily and led to a single temperature-independent characteristic curve.

Analysis of vapor phase adsorption equilibrium of 1,1,1-trichloroethane on dry soils

The migration and fate of volatile organic pollutants in soils are highly dependent on their vapor-phase sorptive behavior. The objective of the research presented in this paper was to investigate the vapor-phase adsorption/desorption equilibrium of 1,1,1-trichloroethane (TCA) on dry soils. A gravimetric adsorption apparatus was developed and used to generate adsorption/desorption isotherms of TCA at 288, 293 and 298 K on two soil samples, obtained from Visalia-California and Times Beach-Missouri, with different physical/chemical characteristics. The influences of temperature and soil properties were investigated. Isosteric heats of adsorption were calculated and heat curves were established. The experimental data were correlated by well-known vapor phase isotherm models including the Polanyi Potential, the BET, and the GAB models. Equilibrium isotherms of TCA on both soils were Type II, characterizing vapor condensation to form multilayers, and they exhibited hysteresis upon desorption. A positive correlation between the soil's specific surface area and its sorption capacity was observed. Clay content and pore size were also dominating factors. Thermal data showed that the adsorption of TCA vapor on soil was primarily due to physical forces and both samples exhibited energetically heterogeneous surfaces. Results followed the Potential Theory satisfactorily and led to a single temperature-independent characteristic curve for each soil-TCA pair. The BET model gave an accurate data fit for up to 40% of the saturation pressure, while the GAB model provided a superior fit of the data for the entire relative pressure range.

Modeling the Martian seasonal water cycle

Ever since the observations of Percival Lowell, the annual cycle of Martian water has been a fascinating topic in planetary exploration. Observations by the Viking Orbiter, supplemented by Earth‐based microwave and infrared observations, have given us a reasonable picture of this cycle. We are now also able to model the cycle using our Mars Climate Model, a simplified atmospheric general circulation model designed specifically for this purpose. We find that a thin adsorbing layer of the Martian regolith plays a fundamental role in the water cycle, limiting the lower atmospheric relative humidity and preventing the formation of widespread ice deposits at low latitudes. We are thus able to estimate a large‐scale average value of the specific soil surface area of this regolith. Water which evaporates from the permanent north polar ice cap during summer is returned by a process of repeated evaporation and precipitation on the retreating seasonal cap the following spring, so that the global inventory of water outside the polar caps ranges within narrow limits. (There is a small net annual deposition of water ice at the south polar cap which is always at dry ice temperatures.) If ice on the residual south polar cap is exposed during the summer, it rapidly sublimes, generating vapor amounts similar to those observed in northern summer. Recovery to normal dry conditions in the southern atmosphere occurs very rapidly in the next year. Such an event could explain the otherwise anomalous Earth‐based pre‐Viking observations of a wet southern summer. If southern ice deposits at lower latitudes are exposed, the vapor can be transferred irreversibly through the strong Hadley cell to the north polar cap. We therefore speculate that the asymmetry of Mars' current orbit is responsible for the asymmetry of the present water distribution (with extensive permanent water ice deposits located only in the colder, aphelion summer, northern hemisphere).

Simultaneous sorption of Cd, Cu, Ni, Zn, Pb, and Cr on soils treated with sewage sludge supernatant

Disposal of sewage sludge creates the potential for heavy metal accumulation in theenvironment. This study assessed nine soils currently used as Dedicated Land Disposal units(DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento,California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studiedby simultaneously mixing these elements in the range of 0-50 µmol L-1 with sludgesupernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH = 4.5 or 6.5, andconstant ionic strength (0.01 M Na-acetate). The concentration of metals in the supernatant wasdetermined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorptionwas linearly related to its concentration in the supernatant solution. The distribution coefficientKd (Kd = concentration on solid phase/concentration in solution phase) was computed as theslope of the sorption isotherm. The distribution coefficients were significantly correlated to soilorganic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5.There was also a correlation between Kd and soil specific surface area but no relationship to othersoil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soilorganic carbon and surface area appear to be the most important soil properties influencing metaladsorption through formation of organo-metal complexes. The Kd values for all elements werehigher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinitiesbased on their Kd values: Pb>Cu>Zn>Ni>Cd≈Cr at pH 4.5 andPb>Cu≈Zn>Cd>Ni>Cr at pH 6.5.

The Effect of Moisture and Soil Texture on the Adsorption of Organic Vapors

AbstractThe fate of volatile organic compounds (VOCs) moving as vapors in the subsurface is dependent on their interaction with the soil. Adsorption of VOC vapors is greatly influenced by soil texture and soil‐water content. The effects of differences in texture and soil‐water content on vapor partition coefficients for trichloroethylene (TCE) were examined. Batch experiments were conducted for a variety of soils and at different soil‐water contents, w, to determine the relationship between the vapor/solid partition coefficient, KD′, and w. In dry soils, KD′ was nonlinearly related to soil‐water content because, in that range, water molecules compete with VOC molecules for adsorption sites on the soil surface. Under wet conditions, KD′ became linearly related to water content according to Henry's law, indicating that adsorbed water molecules were acting as a solvent for VOC molecules. In general, KD′ under oven‐dry conditions did not relate well to total specific surface area of soils, most likely because VOC molecules adsorb only on the outside surfaces of soil particles (due to their nonpolarity), rather than the total surface area present. In the dry range, adsorption was dominated by soils with high specific areas (i.e., high clay content), while soils with higher organic carbon content manifested higher adsorption amounts in the wet moisture range. A one‐parameter, exponential model well described the log KD′‐w curve in the nonlinear region. The model parameter, a, was found to be highly dependent on the specific surface area of the soil. The proposed KD′‐(w) model incorporated in conventional VOC transport models seems promising for analyzing the effects of VOC vapor adsorption on VOC subsurface transport.

Field‐scale transport of interacting solutes through the unsaturated zone: 1. Analysis of the spatial variability of the transport properties

The effect of physicochemical interactions between the soil solution and the soil matrix on the spatial variability of soil properties pertinent to the transport of mixed Na/Ca−Cl salts in the unsaturated zone was analyzed. These properties were the soil hydraulic conductivity and the soil water retention functions and the retardation and the elution factors which account for Na/Ca exchange and chloride exclusion. On the local scale, effects of the soil solution concentration and composition (in terms of the chloride concentration C and the sodium adsorption ratio SAR, respectively) on these soil properties were derived using a theoretical approach which combined the mixed‐ion diffuse double layer theory, the structure of the clay particles, the soil's pore size distribution, and hydrodynamic principles. On the field scale the effect of the soil solution C and SAR on these soil properties was analyzed by coupling the theoretical approach with measured spatial distributions of the soil hydraulic properties at a reference “inert” state as well as of the soil cation exchange capacity and the soil specific surface area. The effect of the soil solution‐soil matrix interactions on the spatial variability of the soil hydraulic properties and the retardation and the elution factors was quantified in terms of mean values and coefficients of variation CV, expressed as functions of the soil solution C and SAR, and the degree of effective saturation Θ. Results of the analyses suggested that the spatial variability (relative to the inherent field variability in the inert reference state) of both the hydraulic conductivity K and the water content θ increased as both the SAR and Θ increased and as C decreased but decreased as C was further decreased (when C &lt; ≈ 15 meq L−1). The mean value of K and θ relative to the inert reference state increased and decreased, respectively, as C increased and as both SAR and Θ decreased. The relative variability of both the retardation factor for Na, RfNa, and the elution factor for Cl, Eƒ, increased as the SAR increased and as both C and Θ decreased. The mean value of RfNa increased and the mean value of Cƒ decreased as both C and Θ decreased and as the SAR increased.