bluuebunny/crossref_metadata_2025 huggingface dataset:
Anion Exchange in Saprolite
Authors: McVay K. A. , Radcliffe D. E. , West L. T. , Cabrera M. L.
Published: July 2010

Many studies have shown that subsoils in the Piedmont region of the southeastern USA have anion exchange capacity (AEC) and can adsorb anions such as nitrate NO−3 Groundwater discharging to wells and streams often passes through thick layers of saprolite beneath Piedmont soils, but little is known about the AEC of saprolite. Our objective was to determine the AEC on saprolite samples collected with a split‐spoon sampler during the installation of seven groundwater monitoring wells in north Georgia. Forty four samples were collected from depths ranging from approximately one to 15 m below the surface and analyzed for AEC, extractable iron, and pH. The saprolite thickness was at least 15 m at all seven sites as verified by drilling an adjacent 7.6‐cm hole to bedrock. The average AEC for 37 saprolite samples was 0.80 cmolc kg−1 This was slightly less than half of the AEC observed in typical subsoil horizons of Piedmont soils. There was a significant positive correlation (r = 0.64) between saprolite AEC and iron oxide content. Taking into account the differences in thickness of the soil and saprolite, saprolite had about six times the AEC of subsoil. The expected adsorption coefficient (Kd) for NO−3 in saprolite was approximately 0.81 cm3 g−1, based on the relationship between AEC and Kd Using an estimated bulk density and saturated water content based on texture, this corresponds to a retardation coefficient (R) of 4.4. Models and remediation efforts involving NO−3 in shallow Piedmont groundwater need to account for anion adsorption in saprolite.
Modeling Boron Adsorption Isotherms and Envelopes Using the Constant Capacitance Model
Authors: Goldberg Sabine
Published: July 2010

Boron adsorption on 23 soil samples belonging to six different soil orders was investigated both as a function of solution B concentration (0–23.1 mmol L−1) and as a function of solution pH (4–11). Boron adsorption exhibited maxima at high solution B concentration. Boron adsorption increased with increasing solution pH, reached a maximum around pH 9, and decreased with further increases in solution pH. The constant capacitance model was able to describe B adsorption on the soil samples as a function of both solution B concentration and solution pH simultaneously by optimizing three surface complexation constants. The ability to describe B adsorption as a function of pH represents an advancement over the Langmuir and Freundlich adsorption isotherm approaches. Incorporation of these constants into chemical speciation transport models will allow simulation of soil solution B concentrations under diverse environmental and agricultural conditions.
An Integrative Approach for Monitoring Water Movement in the Vadose Zone
Authors: Liu Shuyun , Yeh Tian‐Chyi J.
Published: July 2010

Electrical resistivity tomography (ERT), during the past few years, has emerged as a potentially cost‐effective, noninvasive tool for imaging changes of moisture content in the vadose zone. The accuracy of ERT surveys, however, has been the subject of debate because of its nonunique inverse solution and spatial variability in the constitutive relation between resistivity and moisture content. In this paper, an integrative inverse approach for ERT, based on a stochastic information fusion concept of Yeh and Šimůnek, was developed to derive the best unbiased estimate of the moisture content distribution. Unlike classical ERT inversion approaches, this new approach assimilates prior information about the geological and moisture content structures in a given geological medium, as well as sparse point measurements of the moisture content, electrical resistivity, and electric potential. Using these types of data and considering the spatial variability of the resistivity–moisture content relation, the new approach directly estimates three‐dimensional moisture content distributions instead of simply changes in moisture content in the vadose zone. Numerical experiments were conducted to investigate the effect of uncertainties in the prior information on the estimate. The effects of spatial variability in the constitutive relation were then examined on the interpretation of the change in moisture content, based on the change in electrical resistivity from the ERT survey. Finally, the ability of the integrative approach was tested by directly estimating moisture distributions in three‐dimensional, heterogeneous vadose zones. Results show that the integrative approach can produce accurate estimates of the moisture content distributions and that incorporating some measurements of the moisture content is essential to improve the estimate.
Multiphase Inverse Modeling: Review and iTOUGH2 Applications
Authors: Finsterle Stefan
Published: July 2010

Calibration of a numerical process model against laboratory or field data is often referred to as “inverse modeling.” As the numerical simulation models become more complex, the number of parameters to be estimated generally increases, requiring new testing, modeling, and inversion strategies. The purpose of this survey is to review inverse modeling approaches for unsaturated and multiphase flow models. The discussion focuses on applications rather than theoretical considerations, which have been previously reviewed in the context of saturated flow and transport modeling. We also examine model parameterization issues, specifically the representation of heterogeneity through a limited number of variables that can be subjected to parameter estimation and uncertainty propagation analyses. Different parameterization strategies are illustrated using the multiphase flow simulation–optimization code iTOUGH2. A comprehensive inverse modeling package (such as iTOUGH2, which includes automatic model calibration followed by an extensive residual, error, and uncertainty propagation analysis) is an essential tool to improve test design and data analysis of complex multiphase flow systems.
Multiphase Reactive Transport Modeling of Seasonal Infiltration Events and Stable Isotope Fractionation in Unsaturated Zone Pore Water and Vapor at the Hanford Site
Authors: Singleton Michael J. , Sonnenthal Eric L. , Conrad Mark E. , DePaolo Donald J. , Gee Glendon W.
Published: July 2010

Numerical simulations of transport and isotope fractionation provide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic conditions, and infiltration. We incorporate the temperature‐dependent equilibration of stable isotopic species between water and water vapor, and their differing diffusive transport properties into the thermodynamic database of the reactive transport code TOUGHREACT. These simulations are used to illustrate the evolution of stable isotope profiles in semiarid regions where recharge during wet seasons disturbs the drying profile traditionally associated with vadose zone pore waters. Alternating wet and dry seasons lead to annual fluctuations in moisture content, capillary pressure, and stable isotope compositions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope profiles that are distinct from those observed under drying (zero infiltration) conditions. After infiltration, evaporation causes a shift to higher δ18O and δD values, which are preserved in the deeper pore waters. The magnitude of the isotopic composition shift preserved in deep vadose zone pore waters varies inversely with the rate of infiltration.
A Mountain‐Scale Model for Characterizing Unsaturated Flow and Transport in Fractured Tuffs of Yucca Mountain
Authors: Wu Yu‐Shu , Lu Guoping , Zhang Keni , Bodvarsson G. S.
Published: July 2010

We present a large‐scale modeling study characterizing fluid flow and tracer transport in the unsaturated zone (UZ) of Yucca Mountain, Nevada, the proposed underground repository site for storing high‐level radioactive waste. The modeling study is conducted using a three‐dimensional numerical model, which incorporates a wide variety of field data and takes into account the coupled processes of flow and transport in Yucca Mountain's highly heterogeneous, unsaturated, fractured porous rock. The modeling approach is based on a dual‐continuum formulation. Using different conceptual models of unsaturated flow, various scenarios of current and future climate conditions and their effects on the UZ are evaluated to aid in the assessment of the repository's system performance. These models are calibrated against field‐measured data. Model‐predicted flow and transport processes under current and future climates are discussed.
Modeling Seepage into Heated Waste Emplacement Tunnels in Unsaturated Fractured Rock
Authors: Birkholzer Jens T. , Mukhopadhyay Sumit , Tsang Yvonne W.
Published: July 2010

Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high‐level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that—for the first several hundred years after emplacement—will be heated to above‐boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above‐boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. We describe numerical analyses of the coupled thermal‐hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock‐drift interface—independent of the thermal conditions—a second barrier exists to downward percolation at above‐boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual‐permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions.
Vadose Zone Remediation of Carbon Dioxide Leakage from Geologic Carbon Dioxide Sequestration Sites
Authors: Zhang Yingqi , Oldenburg Curtis M. , Benson Sally M.
Published: July 2010

In the unlikely event that CO2 leakage from deep geologic CO2 sequestration sites reaches the vadose zone, remediation measures for removing the CO2 gas plume may have to be undertaken. Carbon dioxide leakage plumes are similar in many ways to volatile organic compound (VOC) vapor plumes, and the same remediation approaches are applicable. We present here numerical simulation results of passive and active remediation strategies for CO2 leakage plumes in the vadose zone. The starting time for the remediation scenarios is assumed to be after a steady‐state CO2 leakage plume is established in the vadose zone, and the source of this plume has been cut off. We consider first passive remediation, both with and without barometric pumping. Next, we consider active methods involving extraction wells in both vertical and horizontal configurations. To compare the effectiveness of the various remediation strategies, we define a half‐life of the CO2 plume as a convenient measure of the CO2 removal rate. For CO2 removal by passive remediation approaches such as barometric pumping, thicker vadose zones generally require longer remediation times. However, for the case of a thin vadose zone where a significant fraction of the CO2 plume mass resides within the high liquid saturation region near the water table, the half‐life of the CO2 plume without barometric pumping is longer than for somewhat thicker vadose zones. As for active strategies, results show that a combination of horizontal and vertical wells is the most effective among the strategies investigated, as the performance of commonly used multiple vertical wells was not investigated.
Coupled Processes of Fluid Flow, Solute Transport, and Geochemical Reactions in Reactive Barriers
Authors: Kim Jeongkon , Schwartz Franklin W. , Xu Tianfu , Choi Heechul , Kim In S.
Published: July 2010

A complex pattern of coupling between fluid flow and mass transport develops when heterogeneous reactions occur. For instance, dissolution and precipitation reactions can change a porous medium's physical properties, such as pore geometry and thus permeability. These changes influence fluid flow, which in turn impacts the composition of dissolved constituents and the solid phases, and the rate and direction of advective transport. Two‐dimensional modeling studies using TOUGHREACT were conducted to investigate the coupling between flow and transport developed as a consequence of differences in density, dissolution–precipitation, and medium heterogeneity. The model includes equilibrium reactions for aqueous species, kinetic reactions between the solid phases and aqueous constituents, and full coupling of porosity and permeability changes resulting from precipitation and dissolution reactions in porous media. In addition, a new permeability relationship is implemented in TOUGHREACT to examine the effects of geochemical reactions and density difference on plume migration in porous media. Generally, the evolutions in the concentrations of the aqueous phase are intimately related to the reaction‐front dynamics. Plugging of the medium contributed to significant transients in patterns of flow and mass transport.
Modeling Biodegradation of Organic Contaminants under Multiphase Conditions with TMVOCBio
Authors: Battistelli Alfredo
Published: July 2010

The existing TMVOC numerical reservoir simulator, developed to model the migration of organic mixtures in the subsurface under multiphase conditions, was improved by adding capabilities for the modeling of aerobic and anaerobic biodegradation reactions of hydrocarbons and chlorinated solvents. Reactive transport is coupled with the multiphase nonisothermal flow of multicomponent fluid mixtures containing water and sets of user‐defined noncondensible gases (NCG), volatile organic compounds (VOCs), and dissolved solids. The mathematical formulation of biodegradation reactions, a modified version of that developed for the BIOMOC computer code, is presented together with underlying assumptions. TMVOCBio allows the modeling of simultaneously occurring aerobic and anaerobic degradation processes involving multiple organic substrates, electron acceptors (EA), and nutrients, accounting for the inhibition phenomena conventionally considered by other analytical and numerical codes. Code verification against accurate numerical solutions and code validation against published laboratory and field experimental results relevant to saturated subsurface systems showed good agreement.
Modeling Three‐Dimensional Groundwater Flow and Advective Contaminant Transport at a Heterogeneous Mountainous Site in Support of Remediation
Authors: Zhou Quanlin , Birkholzer Jens T. , Javandel Iraj , Jordan Preston D.
Published: July 2010

A calibrated groundwater flow model for a contaminated site can provide substantial information for assessing and improving hydraulic measures implemented for remediation. We developed a three‐dimensional transient groundwater flow model for a contaminated mountainous site at which interim corrective measures were initiated to limit further spreading of contaminants. This flow model accounts for complex geologic units that vary considerably in thickness, slope, and hydrogeologic properties, as well as large seasonal fluctuations of the groundwater table and flow rates. Other significant factors are local recharge from leaking underground storm drains and recharge from steep uphill areas. The zonation method was employed to account for the clustering of high and low hydraulic conductivities measured in a geologic unit. A composite model was used to represent the bulk effect of thin layers of relatively high hydraulic conductivity found within bedrock of otherwise low conductivity. The inverse simulator iTOUGH2 was used to calibrate the model for the distribution of rock properties. The model was initially calibrated using data collected between 1994 and 1996. To check the validity of the model, it was subsequently applied to predicting groundwater level fluctuation and groundwater flux between 1996 and 1998. Comparison of simulated and measured data demonstrated that the model is capable of predicting the complex flow reasonably well. Advective transport was approximated using pathways of particles originating from source areas of the plumes. The advective transport approximation was in good agreement with the trend of contaminant plumes observed during the same years. The validated model was then refined to focus on a subsection of the large system. The refined model showed that most of the hydraulic measures implemented for remediation are effective.
A Statistical Technique for Interpreting Streamflow Timing Using Streambed Sediment Thermographs
Authors: Blasch Kyle W. , Ferré Ty P. A. , Hoffmann John P.
Published: July 2010

A moving standard deviation (MSD) technique is developed to infer the onset and cessation of ephemeral streamflow using temperature data from the upper 2.25 m of streambed sediments. During periods of streamflow, shifting of the predominant thermal‐transport mechanism within the sediments from conduction to advection produced changes in the amplitude of the vertically propagating diurnal temperature waves. Analytical expressions describing propagation of conductive and advective diurnal temperature waves through streambed sediments are presented for identifying depths with the largest changes in the diurnal temperature wave amplitude between periods of flow and no flow. The MSD statistical technique was developed to identify the thermal amplitude changes from bed sediment thermographs and to infer streamflow timing. The accuracy of the MSD technique is quantified using direct streamflow and streambed water content measurements. Accuracy of the technique was most sensitive to the MSD window length and the threshold parameter separating periods of conductive and advective heat transport. An alternative calibration procedure was developed using temperature measurements alone. The average error for streamflow timing was approximately 400 min for each event. The results show that temperature sensors may be deployed at a range of sediment depths depending on streamflow stage and soil thermal and hydraulic properties, and that the MSD procedure can provide an objective and repeatable means to quantify streamflow timing.
Determination of Hydraulic Properties in Sloping Landscapes from Tension and Double‐Ring Infiltrometers
Authors: Bodhinayake Waduwawatte , Si Bing Cheng , Noborio Kosuke
Published: July 2010

The majority of landscapes, natural or cultivated, are nonlevel. However, specifically designed instruments are not available for estimation of soil hydraulic properties in sloping landscapes. The objective of this study is to examine if tension and double‐ring infiltrometers are suitable for determination of soil hydraulic properties on sloping soil surfaces. A field experiment was conducted in a silt loam soil (Typic Haplustolls) in Saskatchewan, Canada to explore the usefulness of tension and double‐ring infiltrometers for the determination of soil hydraulic properties in sloping landscapes. Soil surfaces were created to represent four treatments, 0 (level), 7, 15, and 20% slopes. For each treatment, water infiltration rates were measured using a double‐ring infiltrometer and a tension infiltrometer at −3, −6, −10, −13, −17, and −22 cm water pressure heads. In addition, three‐dimensional computer simulation studies were performed for a tension infiltrometer with various disc diameters and water pressure heads for different surface slopes. Steady‐state infiltration rate, field‐saturated hydraulic conductivity, unsaturated hydraulic conductivity as a function of water pressure head, macroscopic capillary length parameter, and water‐conducting macro‐ and mesoporosity were compared for different surface slopes. These parameters were not significantly different (p < 0.05) between level and sloping lands. Experimental and numerical results of this study suggest that both tension and double‐ring infiltrometers are suitable for characterization of surface soil hydraulic properties in landscapes with slopes up to 20%.
Continuous Temperature Logging in Air across the Deep Vadose Zone
Authors: Reiter Marshall
Published: July 2010

Accurate temperature measurements with good depth resolution will be helpful in appreciating processes that influence temperatures in the vadose zone. In some areas, such as the Albuquerque Basin, groundwater flow perturbs the temperature regime in the saturated zone, making the deep vadose zone a potentially better region to investigate ground surface temperature changes. When it is not logistically or economically possible to span the vadose zone with a pipe filled with water, the temperature measurements must be made in air. Because of the very small heat capacity of air, these temperature measurements are difficult. This paper describes a new temperature sensor with a relatively fast time constant in air that provides accurate temperature measurements in a continuous logging mode. Temperature gradient characteristics below the 5‐m depth, based on a 1‐m depth measurement interval, are reproducible at a number of logging speeds. Absolute temperatures between the logs at any given depth below about 25 m typically agree to within 0.01°C. The temperature logging system should enable one to gather considerable information regarding the thermal regime in the vadose zone.
Three‐Dimensional Modeling of Hydrotropism Effects on Plant Root Architecture along a Hillslope
Authors: Tsutsumi Daizo , Kosugi Ken'ichiro , Mizuyama Takahisa
Published: July 2010

A three‐dimensional model of root system development and soil water flow is described and applied to actual conditions along a hillslope. In the model, gravitropism, hydrotropism, and circumnutation were employed as the main factors controlling root elongation. Root systems of 2‐yr‐old pine trees (Pinus massoniana Lamb.) on natural slopes in southern China were excavated and examined, and their development was simulated through the use of continuously monitored temperature and rainfall data. In the simulated root systems, angles between first‐order lateral root segments and the vertical direction on the upslope portion of a tree were larger than those on the downslope portion of the tree; hence, root systems exhibited asymmetric architectures. This asymmetry was more obvious for root systems developed on the downslope side of the hillslope. Because root systems simulated without the effect of hydrotropism did not develop asymmetric architectures, the direction of soil water flux and the effect of hydrotropism appear to be the main factors contributing to the observed architectural asymmetry, typical of root systems along hillslopes. Calculations with the proposed root system model were helpful in elucidating and understanding the predominant processes affecting root system development on hillslopes.
Ground Penetrating Radar Measurements in a Controlled Vadose Zone: Influence of the Water Content
Authors: Loeffler Olivier , Bano Maksim
Published: July 2010

Ground penetrating radar (GPR) is a nondestructive method, which, as with other geophysical methods, has been successfully used to estimate the water content or hydraulic properties of soils. We performed GPR measurements to calibrate and compare water content estimates with actual water contents in a sand box. A vadose zone was simulated by injecting water in a sand box. We obtained four GPR data sets: for dry sand, for sand with water tables at 72‐ and 48‐cm depths, and for sand after drainage. Using the reflections (or diffractions) from the bottom of the sand box (or objects buried in the sand), mean relative dielectric permittivities were determined at several depths in the sand box. These relative dielectric permittivities were used to calculate “real” mean relative dielectric permittivities of a sand box made up of three layers (dry sand, unsaturated sand, and fully saturated sand), knowing that a layer can be subdivided into more layers depending on the depth of the reflections (or diffractions) recorded. We used three relationships between relative dielectric permittivity and the water content to estimate the mean water content for each layer. From these water contents and the known volume of sand considered, we estimated the amount of water in the sand box for each water table. Subtracting the volume obtained for dry sand from the volume obtained for the different water tables gave estimates of the variations in water quantities in the sand box; these were compared with the quantities injected in the sand box. Despite uncertainties in the determination of the mean relative dielectric permittivities, the calculated variations in water quantities were very similar to those injected in the sand box.
Radar Detection of Buried Landmines in Field Soils
Authors: Miller T. W. , Hendrickx J. M. H. , Borchers B.
Published: July 2010

The contrast in the dielectric constant between a landmine and the surrounding soil is one of the most important parameters to be considered when using ground penetrating radar (GPR) for landmine detection. For most geologic materials the dielectric constant lies within a range of 3 to 30, with dry sand at the lower end of this range at about 3 to 5. Nonmetallic antitank landmines have dielectric constants within a range of about 3 to 10 depending on their composition. A model was developed to predict whether or not field conditions are appropriate for use of GPR instruments. The predictions of this model were validated using GPR profiles in field soils with different soil textures at various soil water contents. Model predictions and field measurements provide convincing evidence that increasing the soil water content around a nonmetallic landmine can improve detection in sand and silt soils. However, data for the clay soils suggest that under elevated soil water conditions detection of nonmetallic landmines are not improved; instead radar images in these soils become worse with increasing soil water content. Data suggest that detection of metallic landmines also degrades with increasing soil water content in sandy soils. The field data are in agreement with the model predictions. Our experimental and model results demonstrate the great potential and the pitfalls of landmine sensors based on GPR. Knowledge of soil texture, dry bulk density, and water content are necessary to determine or predict whether soil conditions are suitable or not for GPR mine detection. The model presented here can be useful for making this determination.
Obtaining the Spatial Distribution of Water Content along a TDR Probe Using the SCEM‐UA Bayesian Inverse Modeling Scheme
Authors: Heimovaara Timo J. , Huisman Johan A. , Vrugt Jasper A. , Bouten Willem
Published: July 2010

Time domain reflectometry (TDR) has become one of the standard methods for the measurement of the temporal and spatial distribution of water saturation in soils. Current waveform analysis methodology gives a measurement of the average water content along the length of the TDR probe. Close inspection of TDR waveforms shows that heterogeneity in water content along the probe can be seen in the TDR waveform. We present a comprehensive approach to TDR waveform analysis that gives a quantitative estimate of the dielectric permittivity profile along the length of the probe and, therefore, the distribution of water content. The approach is based on the combination of a multisection scatter function model for the TDR measurement system with the shuffled complex evolution Metropolis algorithm (SCEM‐UA). This combined approach allows for the estimation of the 40 parameters in the transmission line model using a series of simple calibration measurements. The proof of concept is given with measurements in a layered system consisting of air and water. Finally, TDR waveforms from layered soil samples were analyzed to estimate the distribution of the water content along the length of the probe. Results show that the proposed method provides much more reproducible results than obtained with the traditional travel time method. Because the proposed method can be fully automated, it increases the applicability of the TDR method, especially in applications where detailed (real‐time) data are required on heterogeneous infiltration.
Time‐Domain Reflectometry Probe for Water Content and Electrical Conductivity Measurements in Saline Porous Media
Authors: Persson Magnus , Bendz David , Flyhammar Peter
Published: July 2010

A new coated time‐domain reflectometry (TDR) probe design is described and evaluated. In contrast to previous coated TDR probes, our probe may be used to measure both the dielectric constant (Ka) and bulk electrical conductivity (σa) in saline porous media. This was made possible by attaching two coaxial cables to a 0.27‐m three‐rod probe with a coated central rod. The shield of the first cable was connected to one of the outer rods and the conductor was connected to the coated central rod. The conductor and shield of the other coaxial cable were connected to each of the two outer rods, respectively. Thus, our probe consists of two unbalanced, two‐rod probes. The probe is called coated–uncoated probe (CUP). Four prototypes with two different coating materials (i.e., polyolefin and kynar heat‐shrink tubes) were evaluated. The probes were calibrated in several fluids having different Ka and σa The Ka measurement of the coated part of the probe was successfully fitted to target Ka using a two‐phase dielectric mixing model. Due to signal attenuation, measurements of Ka were not possible for σa higher than 9 dS m−1 for the polyolefin‐coated probes whereas the upper limits for the kynar‐coated probes and the uncoated probe were 5 and 2.5 dS m−1, respectively. Measurements of σa are only possible with the uncoated part. Measurements of Ka and σa were also taken during three upward infiltration experiments in sand using soil solution electrical conductivities of 0.01, 6.31, and 12.03 dS m−1 For the uncoated part, Ka could not be measured when σa was higher than about 2 dS m−1, whereas Ka measurements were possible using the coated part even when σa was 3 dS m−1
Induced Polarization of Unsaturated Sands Determined through Time Domain Measurements
Authors: Titov K. , Kemna A. , Tarasov A. , Vereecken H.
Published: July 2010

We studied the electrical induced polarization (IP) response of simple multiphase porous systems by conducting time‐domain (TD) IP measurements on two different groups of sieved quartz samples: sands containing air in unsaturated pores and sands where the unsaturated pores were filled with kerosene. The analyzed chargeability vs. water content relationship showed an extreme behavior. The resistivity vs. water content relationship exhibited two distinct power law regions characterized by different values of the power‐law exponents. Quartz–water–air and quartz–water–kerosene samples showed similar behavior. A conceptual model of polarizing cells is proposed to explain the observed IP phenomena. We consider a sequence of large and narrow passageways for electric current as the elementary polarizing cell. The polarization of the cell is related to the difference in effective radii of the passageways. In the saturated cell, the water‐filling intergrain space is considered the large current passageway. Areas of grain contact are considered narrow passageways. In unsaturated cells, the thin water film on the grain surfaces is considered the narrow passageway. Areas of grain contact are viewed as large passageways. The polarization of unsaturated cells in this condition should be larger than that of saturated cells. With further drying, unsaturated cells start releasing water from the grain contact area, which leads to a convergence of the effective radii of large and narrow passageways and, consequently, to a decrease in cell polarization. The conceptual model is capable of explaining the observed dependence of polarization on water content. The chargeability maximum corresponds to the limit between the two distinct regions, each having a different resistivity vs. water content relationship, which we consider as the critical water content. Our model suggests that at the critical water content, pore water becomes predominantly adsorbed on the solid surface.
Topsoil Structure Influencing Soil Water Retrieval by Microwave Radiometry
Authors: Schneeberger K. , Schwank M. , Stamm Ch. , de Rosnay P. , Mätzler Ch. , Flühler H.
Published: July 2010

Many remote sensing applications, including those of future space missions, require accurate knowledge of the influence of topsoil structure on the water content as measured using L‐band radiometry. We report on field‐measured L‐band (1.4 GHz) microwave emission from a bare soil. Of special interest in this work is the procedure used to transform remotely sensed data to soil water content and its comparability with time domain reflectometer (TDR) in situ measurements. Surface roughness of the soil was characterized on a millimeter scale using an optical measurement technique. Different models for interpreting the microwave signals in terms of the water content were investigated. The agreement between in situ water contents and surface water contents estimated with radiometry data using the Fresnel equation was found to be poor. A coherent layer model, with and without considering roughness effects, was tested to compare radiometrically measured and modeled soil reflectivities. The correspondence remained unsatisfactory, even when we considered a dielectric gradient fitted to the TDR profiles and surface roughness represented by a scattering model. We developed a new air‐to‐soil transition model, which includes dielectric mixing effects due to small‐scale surface structures. This model considerably improved agreement between measured and modeled results. We conclude that small‐scale structures of the topsoil cannot be neglected in interpreting L‐band measurements.
Electrical Streaming Potential Measured at the Ground Surface: Forward Modeling and Inversion Issues for Monitoring Infiltration and Characterizing the Vadose Zone
Authors: Sailhac Pascal , Darnet Mathieu , Marquis Guy
Published: July 2010

Field estimation of the soil water flux has direct application for water resource management. Standard methods like tensiometry or time domain reflectometry are often difficult to use because of subsurface heterogeneity, whereas noninvasive tools such as electrical resistance tomography, nuclear magnetic resonance, or ground penetrating radar are limited to the estimation of the water content. We present an electrical method that provides water flux estimates: streaming potential (SP) monitoring. This cost‐effective tool may help to estimate the nature of the flow process (infiltration or evaporation) in the vadose zone. We discuss interpretation strategies in terms of numerical modeling of both hydraulic and electric processes in the vadose zone and propose an inversion scheme that allows the soil hydraulic parameters to be estimated from in situ infiltration experiments.
Determination of Solute Distributions in the Vadose Zone Using Downhole Electromagnetic Induction
Authors: Hall Laila M. , Brainard James R. , Bowman Robert S. , Hendrickx Jan M. H.
Published: July 2010

We conducted a field experiment to evaluate the ability of downhole electromagnetic induction (EM) measurements to track the migration of a salt plume in the vadose zone. A 6.9 g L−1 slug of NaCl was applied to a 3 by 3 m area for 85 d at a flux of 2.7 cm d−1 Electrical conductivity measurements were collected weekly, using 13 12‐m boreholes installed in a 15 by 15 m area. The measured bulk soil electrical conductivity (ECa) was converted to soil water electrical conductivity (ECw) using water contents from neutron probe measurements and clay contents from soil cores. The calculated mass of salt in the profile agreed well with the known mass of salt infiltrated when the appropriate immobile water content was assumed. The low water content, θw, (<15% by volume) and low bulk soil electrical conductivity, ECa, (<100 mS m−1) measured at the test site presented more resistive conditions than previous studies of this type and were a cause of uncertainty in the results. Good agreement between calculated and known ECw resulted from the ability to measure θw and ECa in the same locations. Sensitivity analyses showed that the calculated ECw was strongly dependent on the assumed upper limit of immobile water content, and less sensitive to soil temperature and clay content. The results of this study demonstrate that downhole EM methods can accurately characterize water and solute distributions in the vadose zone.
Optimization of ERT Surveys for Monitoring Transient Hydrological Events Using Perturbation Sensitivity and Genetic Algorithms
Authors: Furman Alex , Ferré Ty P. A. , Warrick Art W.
Published: July 2010

A simple yet powerful algorithm is presented for the optimal allocation of electrical resistivity tomography (ERT) electrodes to maximize measurement quality. The algorithm makes use of a definition of the sensitivity of an ERT array to a series of subsurface perturbations. An objective function that maximizes the average sensitivity of a survey comprised of a large number of arrays is defined. A simple genetic algorithm is used to find the optimal ERT survey if there is a limited time allowed for survey. We further show that this approach allows for user definition of the sensitivity distribution within the targeted area. Results show clear improvement in the sensitivity distribution. The total sensitivity of the optimized survey compared with typically used surveys composed of one array type. This improved sensitivity will allow for more accurate monitoring of static and transient vadose zone processes. Furthermore, the algorithm presented may be fast enough to allow for real‐time optimization during time‐lapse surveys.
Review of Field Methods for the Determination of the Tortuosity and Effective Gas‐Phase Diffusivity in the Vadose Zone
Authors: Werner David , Grathwohl Peter , Höhener Patrick
Published: July 2010

Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas‐phase diffusivities in the subsurface. We review methods to measure the tortuosity factor or the effective gas‐phase diffusion coefficient in situ. The strong dependency of these parameters on the structure and volume of the air‐filled pore space in the subsurface calls for an accurate and robust in situ measurement. A variety of approaches have been proposed during the last decades, each based on the observation and interpretation of gaseous tracer diffusion in near‐surface soils or the deeper vadose zone under various initial and boundary conditions. We briefly describe the conceptual basis and experimental setup of each method and give insight into error propagation. We then discuss 115 effective diffusion coefficients De compiled from the original method papers and applications. In situ methods and laboratory measurements on undisturbed soil cores yield comparable results. The Penman relationship, De/Dm = 0.66θa, sets an upper limit for the field‐determined effective diffusion coefficient in the case of uniform porosity. The Moldrup relationship, , originally proposed for sieved and repacked soils, gave the best predictions of several porosity‐based relationships, but the relative deviation between observed and predicted De can be substantial. Therefore, the application of such a relationship for the site‐specific modeling of gas‐phase diffusion should be justified with in situ measurements, especially in heterogeneous environments.
Inverse Mobile–Immobile Modeling of Transport During Transient Flow: Effects of Between‐Domain Transfer and Initial Water Content
Authors: Köhne J. Maximilian , Köhne Sigrid , Mohanty Binayak P. , Šimůnek Jirka
Published: July 2010

Mobile–immobile models (MIM) have rarely been used for inverse simulation of measurements of variably saturated water flow and contaminant transport. We evaluated two MIM approaches with water transfer across the mobile and immobile regions either based on relative saturation (Se) differences, MIM(Se), or pressure head (h) differences, MIM(h), for inverse simulation of transient water flow and Br− transport in aggregated soil. Six undisturbed Ap soil columns (14.7‐cm length and diameter) at wet, medium, and dry initial water contents were subjected to application of 0.005 L Br− solution of 1000 mg L−1 and subsequent irrigation of 1 cm h−1 for 3 h. Two similar irrigations were applied after 7 and 14 d. Measurements comprised pressure heads at depths of 2.8 and 12.8 cm, average soil column water contents, outflow, and effluent solute concentrations. This experimental information was used for simultaneous optimization of van Genuchten soil hydraulic parameters for mobile and immobile regions, the dispersivity, and the first‐order rate coefficients for water and solute transfer. In total, eight parameters were estimated for MIM(Se) and 10 parameters for MIM(h). The inverse MIM approaches described the various hydraulic and transport data adequately. Physical nonequilibrium was more pronounced for wet and dry than for intermediate initial moisture, while initial moisture had no obvious effect on the total Br− lost. For wet and dry initial conditions, parameter estimates seemed fairly reliable, with the exception of the highly correlated saturated water contents in mobile and immobile regions. MIM(h) yielded parameters that appeared physically more consistent with observations, but required smaller time steps than MIM(Se) to overcome oscillations of pressure heads. Both MIM approaches were found to be suitable for inverse simulation of physical nonequilibrium transport during variably saturated flow.
In Situ Long‐Term Chloride Transport through a Layered, Nonsaturated Subsoil. 1. Data Set, Interpolation Methodology, and Results
Authors: Javaux M. , Vanclooster M.
Published: July 2010

A long‐term data set of Cl−concentrations in an artificial, leaking lake and in the underlying vadose zone was used to assess in situ vertical transport through a tertiary sandy deposit. Since the temporal resolution of the observed Cl−concentrations in the lake was relatively poor, a method was implemented to assess uncertainty on process identification as caused by a poor definition of the boundary condition. For this purpose we performed continuous stochastic simulations of Cl−concentrations in the lake conditioned to the discretely measured Cl−concentrations. This allows one to generate an ensemble of equiprobable time series of top boundary conditions. Each time series was subsequently used in an inverse convective–dispersive (CD) transport model based on the transfer function concept to identify the apparent transport properties of the unsaturated natural porous medium. For each simulation, one optimal parameter set in the least‐square sense is then obtained, and the variance of the optimized parameters reflects the uncertainty due to the poor sampling frequency of Cl−in the lake. For the case study presented, we show that the variance component due to poor sampling of the top boundary condition is small for estimating the transport velocities. This component increases with the hydrodynamic dispersivity. However, for this latter parameter, the sampling variance component was still small compared with the total variability between dispersivity at different depths. Physical interpretation of variability and scale dependency of this latter parameter is studied in more detail in an accompanying paper.
Estimating Uncertain Flow and Transport Parameters Using a Sequential Uncertainty Fitting Procedure
Authors: Abbaspour K. C. , Johnson C. A. , van Genuchten M. Th.
Published: July 2010

Inversely obtained hydrologic parameters are always uncertain (nonunique) because of errors associated with the measurements and the invoked conceptual model, among other factors. Quantification of this uncertainty in multidimensional parameter space is often difficult because of complexities in the structure of the objective function. In this study we describe parameter uncertainties using uniform distributions and fit these distributions iteratively within larger absolute intervals such that two criteria are met: (i) bracketing most of the measured data (>90%) within the 95% prediction uncertainty (95PPU) and (ii) obtaining a small ratio (<1) of the average difference between the upper and lower 95PPU and the standard deviation of the measured data. We define a model as calibrated if, upon reaching these two criteria, a significant R2 exists between the observed and simulated results. A program, SUFI‐2, was developed and tested for the calibration of two bottom ash landfills. SUFI‐2 performs a combined optimization and uncertainty analysis using a global search procedure and can deal with a large number of parameters through Latin hypercube sampling. We explain the above concepts using an example in which two municipal solid waste incinerator bottom ash monofills were successfully calibrated and tested for flow, and one monofill also for transport. Because of high levels of heavy metals in the leachate, monitoring and modeling of such landfills is critical from environmental points of view.
Dual Field Calibration of Capacitance and Neutron Soil Water Sensors in a Shrinking–Swelling Clay Soil
Authors: Fares A. , Buss P. , Dalton M. , El‐Kadi A. I. , Parsons L. R.
Published: July 2010

Multisensor capacitance sensors (MCS) are now popular alternatives to neutron scattering (NS) soil water–monitoring devices. The objectives of this study were to (i) quantify the effect of clay shrinkage–swelling on the soil bulk density; (ii) determine field calibration equations for an MCS and an NS device; and (iii) compare the performance of the MCS with a NS meter under field conditions. The calibration was conducted in a duplex soil with sandy clay loam overlying clay in South Australia. Six access tubes were installed in a 6 by 8 m grid. Three moisture treatments were replicated twice for every moisture level. The bulk density of the top 20 cm increased with increasing water content; this increase was more pronounced in the upper 10‐cm horizon, which could be attributed to soil compaction. However, a negative correlation was obtained between bulk density and water content in the 30‐ to 100‐cm depth layers reflecting the shrinking and swelling properties of the fine‐textured subsoil. Results also show highly significant effects of sampling depth and moisture level on NS and MCS readings. Compared with linear calibration, a three‐parameter model improved NS calibration and/or minimized the root mean square errors for 6 out of the 10 sampling depths. Except for the 10‐cm sampling depth, individual calibration for each 10‐cm soil layer improved the accuracy of the MCS as compared with the use of single calibration equation for the entire profile. Site‐specific calibration improved the accuracy of both the NS and MCS soil water–monitoring devices.