Soil Transport Processes Research Articles

Multi‐Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7‐cm‐diameter multi‐functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27‐mm‐o.d. stainless‐steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8‐s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

Multi-Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

Robust Estimation of the Generalized Solute Transfer Function Parameters

A method is presented to mathematically characterize a wide range of solute transport processes in soils from time‐domain‐reflectometry (TDR)‐based breakthrough curve (BTC) measurements. To do this, we combined the flexible generalized transfer function model (GTF) with the definition of the time normalized resident concentrations Crt* The GTF is a four‐parameter flexible transfer function able to describe both the convection‐dispersive (CD) and the stochastic‐convective (SC) process of dispersion in a soil. In addition, it allows other dispersion processes typical for heterogeneous soils to be modeled. To obtain robust estimations of the GTF model parameters, closed‐form expressions of the GTF time normalized resident concentrations Crt* and temporal moments of Crt* were defined. These expressions allow the transport parameters to be estimated from TDR‐based BTCs without relying on problematic probe calibrations and mass recovery assumptions. Since GTF is a four‐parameter model, the robustness of the parameterization procedure was tested by fitting the Crt* of the GTF model to numerically generated, error‐contaminated BTCs. Using the least‐square optimization technique, robust estimates of the GTF parameters could be obtained from TDR observations at two different soil depths. Application of the proposed method was also tested for a real undisturbed sandy subsoil and compared with the convective lognormal transfer (CLT) and CD models. For this case, it was shown that the GTF model improved greatly the goodness of prediction of the BTCs. The proposed method offers a new powerful tool for analyzing transport processes of nonreactive solutes in soil.

Robust Estimation of the Generalized Solute Transfer Function Parameters

A method is presented to mathematically characterize a wide range of solute transport processes in soils from time-domain-reflectometry (TDR)-based breakthrough curve (BTC) measurements. To do this, we combined the flexible generalized transfer function model (GTF) with the definition of the time normalized resident concentrations Crt* The GTF is a four-parameter flexible transfer function able to describe both the convection-dispersive (CD) and the stochastic-convective (SC) process of dispersion in a soil. In addition, it allows other dispersion processes typical for heterogeneous soils to be modeled. To obtain robust estimations of the GTF model parameters, closed-form expressions of the GTF time normalized resident concentrations Crt* and temporal moments of Crt* were defined. These expressions allow the transport parameters to be estimated from TDR-based BTCs without relying on problematic probe calibrations and mass recovery assumptions. Since GTF is a four-parameter model, the robustness of the parameterization procedure was tested by fitting the Crt* of the GTF model to numerically generated, error-contaminated BTCs. Using the least-square optimization technique, robust estimates of the GTF parameters could be obtained from TDR observations at two different soil depths. Application of the proposed method was also tested for a real undisturbed sandy subsoil and compared with the convective lognormal transfer (CLT) and CD models. For this case, it was shown that the GTF model improved greatly the goodness of prediction of the BTCs. The proposed method offers a new powerful tool for analyzing transport processes of nonreactive solutes in soil.

Multi-Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

NMR in Environmental Science

NMR in Environmental Science

Imaging Fluorescent Dye Concentrations on Soil Surfaces

To investigate transport processes in soils, detailed information about the spatial distribution of solutes is required. We describe a method to obtain concentration maps of fluorescent tracers on cross sections of soil cores with a high spatial resolution. The fluorescence signal of two dye tracers, brilliant sulfaflavine (BF; 1H‐Benz(de)isoquinoline‐5‐sulfonic acid, 2,3‐dihydro‐6‐amino‐1,3‐dioxo‐2‐(p‐tolyl)‐, monosodium salt) and sulforhodamine B (SB; xanthylium, 3,6‐bis(diethylamino)‐9‐(2,4‐disulfophenyl)‐, inner salt, sodium salt), was imaged on the exposed cross sections. The fluorescence signal was corrected for variable illumination light intensity and optical properties of the soil across the exposed surface. Correction factors for varying optical soil properties were derived from the image of the reflected excitation light at the exposed surface. Linear calibration relations related the corrected fluorescence image to the total tracer concentration (Ct) map, that is, mass of dye dissolved in the soil solution and sorbed to the soil particles per unit volume bulk soil. Corrections for varying optical properties of the soil surface were important to reduce the uncertainty of the concentration that was estimated from the fluorescence signal. For BF, the calibration relations were different for different soil materials and a soil specific calibration had to be used. Variations in background fluorescence were an important source of uncertainty of the BF concentration estimates but can be overcome by applying higher concentrations. For SB, variations in calibration relations and in the background fluorescence were considerably smaller, and so is the uncertainty of the estimated SB concentrations.

Imaging Fluorescent Dye Concentrations on Soil Surfaces

To investigate transport processes in soils, detailed information about the spatial distribution of solutes is required. We describe a method to obtain concentration maps of fluorescent tracers on cross sections of soil cores with a high spatial resolution. The fluorescence signal of two dye tracers, brilliant sulfaflavine (BF; 1H-Benz(de)isoquinoline-5-sulfonic acid, 2,3-dihydro-6-amino-1,3-dioxo-2-(p-tolyl)-, monosodium salt) and sulforhodamine B (SB; xanthylium, 3,6-bis(diethylamino)-9-(2,4-disulfophenyl)-, inner salt, sodium salt), was imaged on the exposed cross sections. The fluorescence signal was corrected for variable illumination light intensity and optical properties of the soil across the exposed surface. Correction factors for varying optical soil properties were derived from the image of the reflected excitation light at the exposed surface. Linear calibration relations related the corrected fluorescence image to the total tracer concentration (Ct) map, that is, mass of dye dissolved in the soil solution and sorbed to the soil particles per unit volume bulk soil. Corrections for varying optical properties of the soil surface were important to reduce the uncertainty of the concentration that was estimated from the fluorescence signal. For BF, the calibration relations were different for different soil materials and a soil specific calibration had to be used. Variations in background fluorescence were an important source of uncertainty of the BF concentration estimates but can be overcome by applying higher concentrations. For SB, variations in calibration relations and in the background fluorescence were considerably smaller, and so is the uncertainty of the estimated SB concentrations.

Soil transport driven by biological processes over millennial time scales

Downslope soil transport in the absence of overland flow has been attributed to numerous mechanisms, including particle-by-particle creep and disturbances associated with biological activity. Process stochasticity and difficulties associated with field measurement have obscured the characterization of relevant long-term soil transport rates and mechanisms. In a series of incised fluvial terraces along the Charwell River, South Island, New Zealand, we documented vertical profiles of tephra concentration and topographic derivatives along a hillslope transect to quantify soil transport processes. Along the undissected hilltop, we observed a thin primary tephra layer (ca. 22.6 ka) within loess deposits ∼80 cm below the landscape surface. In the downslope direction, the depth to the highly concentrated tephra layer decreases, coincident with an increase in hillslope convexity (which is proportional to landscape lowering rate if soil flux varies linearly with hillslope gradient). Exhumation of the tephra layer results from landscape lowering due to disturbance-driven soil transport. Approximately 20 m downslope of the interfluve, the depth to the tephra layer declines to 40–50 cm, peak tephra concentrations decrease by a factor of 4, and tephra is distributed uniformly within the upper 40 cm of soil. The transition from a thin, highly concentrated tephra layer at depth to less concentrated, widely distributed tephra in the upper soil may result from soil mixing and transport by biological disturbances. Along our transect, the depth to this transition is ∼50 cm, coincident with the rooting depth of podocarp and Nothofagus trees that populated the region during much of the Holocene. Our observations can be used to calibrate the linear transport model, but, more important, they suggest that over geomorphic time scales, stochastic bioturbation may generate a well-mixed and mobile soil layer, the depth of which is primarily determined by flora characteristics.

An Unsaturated Transient Flow Method for Determining Solute Adsorption by Variable‐Charge Soils

The amount of reactive ions adsorbed during transport in variable‐charge soils is often smaller than those predicted from batch adsorption experiments. This overestimation of adsorption for invading ions by the batch method is likely to be caused by excessive desorption of indigenous, strongly adsorbed ions. We propose an unsaturated transient flow method by which equilibrium adsorption of weakly reactive ions can be determined without causing appreciable desorption of indigenous ions. In our method, water is imbibed into horizontal columns packed with soil that has been mixed with a salt solution at different concentrations to attain adsorption equilibrium. We make use of the observation that during imbibition of water, the antecedent solution is pushed ahead by the invading solution and accumulates in the region beyond the plane of separation, with the solution concentration unchanged. From linear plots of the solute content vs. soil water content in this region, the amount of solutes adsorbed by soil and the equilibrium solution concentration prior to the water imbibition are obtained. The advantages of the method are that adsorption isotherms can be determined under conditions similar to those expected during transport processes in soil, and that it is exempt from uncertainty about attainment of adsorption equilibrium as in the miscible displacement methods. The proposed method is best suited for variable‐charge soils where adsorption of weakly reactive ions is largely due to an increase in the exchange capacity.

Detailed characterization of solute transport in a heterogeneous field soil.

There is a necessity for improved physical understanding of solute transport processes in heterogeneous soil systems. In situ nondestructive techniques like time domain reflectometry (TDR) and fiber optic miniprobes (FOMPs) permit the collection of unique measurements of solute transport processes in soils for the purposes of model development and validation. This study examined the application of TDR and FOMPs to measure solute transport at various points laterally and at two depths in a heterogeneous clay-loam soil. A miscible displacement experiment was performed at a constant irrigation flux to examine the applicability of these probes to field soils. In their first application to a field soil, the FOMPs were successfully calibrated and performed well in measuring solute breakthrough curves. Two flow regimes were identified in the soil profile, the first where lateral spreading of the solute occurred in the surface horizon, followed by convergence into preferential flow pathways in the second transport zone. The measured transport response was heterogeneous with at least two identifiable vertical flow phases. It was demonstrated using transfer function modeling and data from a corresponding laboratory study that the FOMPs were measuring the slower phase, while the TDR probes captured a composite of the fast and slow phases. The combination of these two techniques may be a means to separate solute transport phases in heterogeneous media and relate laboratory column results to field studies.

Quantitative morphology and network representation of soil pore structure

Pore-network models are attractive to relate pore geometry and transport processes in soil. In this contribution a `morphological path' is presented to generate a network model based on quantitative morphological investigations of the 3D pore geometry in order to predict soil hydraulic properties. The 3D-geometry of pores larger than 0.04 mm in diameter is obtained using serial sections through impregnated samples. Beside pore-size distribution an important topological aspect of pore geometry is the spatial connectivity of the pore space which is difficult to measure. A connectivity function is proposed defined by the 3D-Euler number. The goodness in the estimation of the Euler number using serial sections is investigated in subsamples of different sizes and shapes. Then, a simple network model is generated which can be adapted to a predefined pore-size distribution and connectivity function. Network simulations of hydraulic properties are compared to independent measurements at the same soil material and the effect of topology on water flow and solute transport is investigated. It is concluded that a rough estimation of pore-size distribution and topology defined by the connectivity function might be sufficient to predict hydraulic properties.

Method for In Situ Field Calibration of Fiber Optic Miniprobes

A fiber optic miniprobe (FOMP) system, based on remote fluorometry, has been recently developed for in situ real‐time measurement of solute transport processes in soil. In order for the FOMP output light intensity measurements to be converted to fluorescent tracer concentration, an in situ calibration is necessary for each probe. The conventional calibration method consists of leaching the entire soil unit of interest with several pore volumes of tracer solution for each calibration step. The steady signal measured by the probes at each step is then related to tracer concentration. This procedure works well for typical laboratory soil columns but is impractical for fiber optic calibration in longer soil columns or field conditions because of the large amount of time and tracer required. As a result, a “point” calibration method has been developed that consists of injecting standard solutions directly into the small soil volume surrounding the measurement tip of the probe. Comparisons of the conventional leaching method with the injection calibration techniques confirmed the new procedures work well in both silica sand and clay loam soil columns. As a result, the new calibration technique was tested in a heterogeneous field soil and provided a simple and accurate approach to calibrating FOMPs for in situ real‐time measurement of solute transport processes in the field.

Nitric Oxide Emissions from Engineered Soil Systems

Sophisticated laboratory equipment and procedures are developed and used in controlled experi- ments to measure nitric oxide (NO) emissions ranging from 42 to 75 ng N/m 2 ? s from sludge-amended soil of concern to environmental engineers because nitric oxide emitted to the troposphere is a precursor to troublesome ozone formation and also of concern to agricultural engineers because valuable nitrogen as fertilizer is lost from the soil. Water-filled pore space is confirmed to be of critical importance to NO flux, and the upper layers of soil are determined to contribute the larger portion of the NO fluxing from the soil to the troposphere. More than 42% of the total NO flux comes from the top 1 cm of soil, with NO contributions decreasing exponentially with soil depth and very little if any tropospheric NO contributed from soil at a depth of 20 cm or greater. The results are discussed in terms of microbiological, chemical, and soil transport processes that influence NO flux from sludge-amended soil. INTRODUCTION Nitrogen oxide (NO) emissions from unamended and en- gineered soils can adversely impact local and regional air qual- ity in the lower troposphere and can directly impact public health and vegetation as subsequent ozone (O3) formation; ac- cumulation leads to pulmonary congestion, disorientation, al- tered breathing, headaches in humans, and decreased crop yields. In addition, these emissions are a direct economic con- cern because nitrogen valued as fertilizer is lost from the soil. Consequently, these emissions are studied here to gain a better understanding of their net formation, transport, and transfor- mation in the form of NO flux from the soil. With the ability to measure nitric oxide emissions under controlled conditions in the laboratory, it can be possible in the future to gain a better understanding of the mechanisms responsible for NO formation, transport, and transformation with a view to the control of NO emissions from soil into the lower troposphere. Whether or not increased NO emissions lead to ozone prob- lems in the lower troposphere is dependent on other important atmospheric variables, most notably the NO x and volatile or-

Measuring and modelling the vertical distribution of semi-volatile organic compounds in soils. II: model development

A dynamic air-soil exchange model which divides the soil into multiple soil layers has been developed and used to simulate the vertical distribution of PCBs in an undisturbed grassland soil. An historical emission profile for PCBs to the UK atmosphere was used to deduce concentrations in the air compartment. Model simulations were tuned by varying the most uncertain model input parameters, namely soil turnover rates by bioturbation and emission estimates. Comparison of model output and reported PCB concentration profiles agreed within a factor of six. Insights were gained into the relative importance of the soil transport processes and specifically bioturbation was shown to be the dominant transport mechanism for PCBs in this soil. It was concluded that the model is sufficiently accurate for incorporation into regional mass balance models, although vertical layering of the soil may be unnecessary in models describing the long-term air-soil interaction of organic compounds.

Characterization of Macropore Flow Mechanisms in Soil by Means of a Split Macropore Column

Naturally occurring macropores such as root channels and earthworm holes have been shown to affect water flow and solute transport processes in soils. To further our understanding of the specific processes that are involved in macropore flow, we have developed a Split Macropore Column (SMC) which consists of a semi‐cylindrical soil column with a semi‐cylindrical macropore at the main axial of its planer surface. The SMC allows one to control a number of important physical parameters such as the size, density, and continuity of the macropores for miscible displacement studies, as well as an opportunity to observe visually the macropore's impact on flow processes at the critical matrix‐macropore interface. After rigorous testing, we found that this system is a useful instrument to study macropore flow mechanisms. Using these SMCs, we performed a series of miscible displacement experiments to study the effects of soil type, flux rate, macropore size, and macropore to matrix size ratio. Our initial results show three zones of flow through the column: (i) film flow at the matrix–macropore interface, (ii) in the matrix directly adjacent to the macropore, and (iii) in the remaining matrix. The initial data suggest that macropore flow is influenced more by the matrix–macropore size ratio than by the macropore size.

Point Measurement of Solute Transport Processes in Soil Using Fiber Optic Sensors

Many of the mechanistic studies of solute transport in soil require nondestructive measurement of water flow and solute transport parameters at temporal and spatial resolutions far beyond the capabilities of our existing methodologies. Using remote fiber optic fluorometry (RFF) techniques, a new methodology has been developed which allows in situ measurement of solute transport processes in soil in real time and on a continuous basis. The system consists of transmitting a constant beam of light through the input leg of a bifurcated fiber optic miniprobe to a location of interest within the soil matrix. At the probe's tip, the incoming light interacts with the soil matrix where it is partially absorbed and partially reflected back into the probe. The reflected signal is transmitted through the output leg to a photodetector and quantified. The intensity of the output signal, which is constant under steady conditions, changes when a plume of fluorescent water tracer passes through the soil matrix in front of the probe, allowing for in situ measurement of a solute breakthrough curve (BTC) at the point' of observation. Using this system, a series of miscible displacement studies was performed in 20-cm-long, 5-cm-diameter soil columns in which a fiber optic miniprobe (3.00 mm in diameter) was horizontally inserted at a column's midpoint. Details of the fiber optic system, including its components, setup, and operation, and the results of more than 15 BTCs in different soils will be discussed, along with an overview of the advantages and disadvantages of this new methodology.

Environmental soil and water chemistry: principles and applications

WATER CHEMISTRY AND MINERAL SOLUBILITY. Physical Chemistry of Water and Some of Its Constituents. Solution/Mineral-Salt Chemistry. SOIL MINERALS AND SURFACE CHEMICAL PROPERTIES. Soil Minerals and Their Surface Properties. Sorption and Exchange Reactions. ELECTROCHEMISTRY AND KINETICS. Redox Chemistry. Pyrite Oxidation Chemistry. Reaction Kinetics in Soil-Water Systems. SOIL DYNAMICS AND AGRICULTURAL-ORGANIC CHEMICALS. Organic Matter, Nitrogen, Phosphorus and Synthetic Organics. COLLOIDS AND TRANSPORT PROCESSES IN SOILS. Soil Colloids and Water-Suspended Solids. Water and Solute Transport Processes. The Chemistry and Management of Salt-Affected Soils and Brackish Waters. LAND-DISTURBANCE POLLUTION AND ITS CONTROL. Acid Drainage Prevention and Heavy Metal Removal Technologies. SOIL AND WATER: QUALITY AND TREATMENT TECHNOLOGIES. Water Quality. Soil and Water Decontamination Technologies. Appendix. Suggested and Cited References. Index.

2D numerical simulation for simultaneous heat, water and gas migration in soil bed under different environmental conditions

A general mathematical model for investigating simultaneous heat, water and gas (air plus vapor) transfer in unsaturated porous soil under different environmental conditions is presented based on the volume-averaging method. Two-dimensional numerical simulation in steady state is conducted for obtaining accurate images of field characteristics in a confined soil bed, which might be useful to provide necessary information for agricultural applications. Various effects of environmental parameters, such as ambient temperature, relative humidity, radiative heat flux and wind speed, on transport processes in soil without plant roots are analyzed through the calculating results in the present paper.

The origin and loss of fractal scaling in simulated soil aggregates

SummaryA growing body of evidence supports the use of fractals to approximate the heterogeneous structure of soil. Fractal models of transport processes in soil indicate that the fractal dimension and the upper limit to fractal scaling (denoted ξ) determine flow rates and scaling properties. A two‐dimensional Monte Carlo simulation of soil aggregation is used to identify the key processes which may lead to fractal‐like scaling in the soil matrix. A bonding law is developed which parameterizes the relative importance of chemical bonding to binding by microbial by‐products and fungi. We investigate the sensitivity of the fractal dimension and ξ to the nature of the bonding law, and to aggregate fragmentation, initial particle size distribution and mechanical restructuring. Fractal‐like structures result from the aggregation process. However, the measured fractal dimension is insensitive to the detailed form of the bonding law. The factor that most sensitively influences the fractal dimension is found to be mechanical restructuring such as would arise from the action of roots, soil fauna and natural weathering. By contrast, ξ is most sensitive to the bonding law. The results suggest that the fractal dimension and characteristic diameter of aggregates should be uncorrelated, and that increased binding by microbial by‐products and fungi should produce larger, less dense, aggregates.