Overview of inert tracer experiments in key Belgian soil types: Relation between transport and soil morphological and hydraulic properties
Water Resources Research
Abstract and Figures
To investigate relations between solute transport, soil properties, and experimental conditions, we summarize results from leaching experiments that we carried out in a range of soils, at different scales (column (0.3–1.0 m ID, 1.0 m length) and field plot scale), and using different leaching rates (0.5–30 cm d 1). The lateral mixing regime and longitudinal dispersion were derived from time series of tracer concentrations at several depths in the soil. Field-and column-scale transport were similar in loam and silt loam soils. The mixing regime was related to soil morphological features, such as vertical tongues, stratification, macropores, and a water-repellent layer. The dispersion increased in all soils more than linearly with increasing leaching rate, implying that the dispersivity is not an intrinsic soil characteristic. The change of dispersivity with leaching rate was linked to the unsaturated hydraulic conductivity using a multidomain conceptualization of the pore space.
![Breakthrough curves measured in the Regosol at 72.5 cm below the surface during steady state and transient flow. In the transient experiment, water was applied each day during a short application period whereas it was applied continuously in the steady state experiment. In both experiments the same amount of water was applied during 1 day. From Vanderborght et al. [2000a].](https://www.researchgate.net/profile/Jan-Feyen/publication/228515868/figure/fig1/AS:349460103221248@1460329189066/Breakthrough-curves-measured-in-the-Regosol-at-725-cm-below-the-surface-during-steady_Q320.jpg)
![Dispersivity L versus effective flow rate J weff in the Lamelli-Luvic Arenosol and the Eutric Regosol. (For the Regosol, L derived from BTCs at 1 m depth in the soil profile are shown.) From Vanderborght et al. [2000a].](https://www.researchgate.net/profile/Jan-Feyen/publication/228515868/figure/fig2/AS:349460103221249@1460329189087/Dispersivity-L-versus-effective-flow-rate-J-weff-in-the-Lamelli-Luvic-Arenosol-and-the_Q320.jpg)
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... Yet also at the depth of 20-30 cm, the correlation was positive (Figures 7k and 9k), contrary to those at the others. These phenomena might be attributed to a plow pan immediately below at the 30-40 cm depth resulting from tobacco cultivation and accompanied deep tillage down to 30 cm depth, which impeded downward movement, accumulated leached Br − , and encouraged lateral flow (Besson et al., 2011;Jarvis, 2007;Vanderborght et al., 2001). As a consequence, the variation of soil Br − here as implied by CV was the lowest among the five depths investigated (Table 1). ...
Knowledge about water and solute transport in the vadose zone is critical for agrochemical management, especially for the tobacco cultivation characterized by extensive fertilizer and pesticide applications. Under the field conditions, however, natural soil heterogeneity complicates and limits the analysis of water and solute transport. Applying the scale‐dependent treatment distribution in a rainfall simulation experiment, the objective was to evaluate the impact of rainfall characteristics on bromide (Br⁻) leaching in a typical tobacco field in China's Yunnan Province. On a transect established with 24 plots, rainfall amount, intensity, and time delay relative to solute application were arranged in cyclic patterns at different spatial scales. Soil samples in 10‐cm increments down to 50 cm depth were collected for Br⁻ determination after the simulated rainfall. The results display a significantly decreasing trend of Br⁻ concentration with depth, suggesting the predominance of matrix diffusion over preferential flow at the scale of investigation in the intensively cultivated tobacco field. The Pearson correlation analysis only detected significant associations of Br⁻ with rainfall intensity at the 0–10 cm depth and with rainfall amount at the 20–30 cm. With the aid of frequency‐domain analysis, by comparison, strong spatial correlations were revealed with rainfall characteristics at each depth, even with the presence of a plow pan at the depth of 30–40 cm due to deep tillage. Although the dominant control of Br⁻ distribution varied with depth, these findings demonstrate the profound effects of rainfall characteristics on solute leaching and the usefulness of scale‐dependent treatment distribution in field‐scale investigations of solute transport as well as other hydrological processes.
... The heterogeneity of soil in the vertical direction can be attributed to different soil layers. In that case cases, one-dimensional models are used to describe transport processes (Vanderborght et al., 2001;Filipovi c et al., 2019). However, spatial variation is mainly characterized by changes in hydraulic conductivity, texture, and structure (Zhang et al., 2004). ...
The terrestrial water budget is the difference between receiving precipitation (P) and outgoing evapotranspiration and discharge (D) fluxes at the earth's surface. Recent and continuing satellite missions and constellations have measured these components separately or together at various spatiotemporal resolutions. Realistic depictions of complicated hydrologic feedback loops and correct representations of the terrestrial water budget are critical for monitoring changing water cycle features and understanding land–atmosphere interactions and extremes such as floods. This chapter discusses various radar remote sensing techniques that aid in monitoring terrestrial water budget elements such as precipitation, soil moisture, and surface water levels. This information is highlighted in three main case studies. The first case study pertains to monitoring precipitation from remote sensing using the Integrated Multisatellite Retrievals product of Global Precipitation Measurement constellation satellites. The second case study discusses estimation techniques to monitor soil moisture and downscaling for regional hydrologic studies at a fine spatial resolution. The third section elucidates the remote sensing to monitor surface water levels from radar remote sensing.
... It depends on the flow rates and the related water contents establishing in the porous system. Some reported a dispersivity increasing with saturation degree [35] due to larger pores involved at higher flow rates. Others [20,23] observed a reverse behavior, ascribed to less tortuous paths, and hence decreasing dispersivity at higher water content. ...
Solute transport parameters are known to be scale-dependent due mainly to the increasing scale of het-erogeneities with transport distance and with the lateral extent of the transport field examined. Based on a transect solute transport experiment, in this paper we studied this scale dependence by distinguishing three different scales with different homogeneity degrees of the porous medium: the observation scale, transport scale and transect scale. The main objective was to extend the approach proposed by van Wes-enbeeck and Kachanoski to evaluating the role of textural heterogeneities on the transition from the observation scale to the transport scale. The approach is based on the scale dependence of transport moments estimated from solute concentrations distributions. In our study, these moments were calculated starting from time normalized resident concentrations measured by time domain reflectometry (TDR) probes at three depths in 37 soil sites 1 m apart along a transect during a steady state transport experiment. The Generalized Transfer Function (GTF) was used to describe the evolution of apparent sol-ute spreading along the soil profile at each observation site by analyzing the propagation of the moments of the concentration distributions. Spectral analysis was used to quantify the relationship between the solid phase heterogeneities (namely, texture and stones) and the scale dependence of the solute transport parameters. Coupling the two approaches allowed us to identify two different transport scales (around 4-5 m and 20 m, respectively) mainly induced by the spatial pattern of soil textural properties. The analysis showed that the larger transport scale is mainly determined by the skeleton pattern of variability. Our analysis showed that the organization in hierarchical levels of soil variability may have major effects on the differences between solute transport behavior at transport scale and transect scale, as the transect scale parameters will include information from different scales of heterogeneities.
... The laboratory studies referred to above were mostly conducted using repacked soil columns. More uncertainties are expected for fieldscale undisturbed soils because of large-scale soil heterogeneity, soil layering, the presence of fractures and macropores, spatial variations in the flow velocity within the transport domain, time-dependent boundary conditions, root water uptake, and other complexities (e.g., Vanderborght et al., 2000Vanderborght et al., , 2001Javaux et al., 2006;Vanderborght and Vereecken, 2007;Koestel et al., 2009). Carefully constructed laboratory tracer experiments permit a far more precise assessment of the effects of soil moisture content on the dispersivity, as compared to field-scale situations. ...
A major contaminant transport process in soils is hydrodynamic dispersion by affecting the spreading and arrival of surface-applied pollutants at underlying groundwater reservoirs. When a soil is unsaturated, hydrodynamic dispersion is very much affected by soil water saturation. Centimeter- and decimeter-scale column experiments were carried out to explore the effects of fluid saturation and particle size on the unsaturated solute dispersivity. Measured in-situ breakthrough curves were analyzed in terms of both classical advection-dispersion and dual-porosity (mobile-immobile) type transport equations. A clear non-monotonic relationship was found between the dispersivity and soil water saturation. The extent of non-monotonicity was more pronounced for a relatively coarse-textured sand compared to a finer sand. This finding has been reported rarely before; it explains some of the inconsistencies of saturation-dispersivity relationships in the literature.
... However, an unexpected decrease of dispersion below a certain threshold, so-called critical saturation, has also been observed (Toride et al., 2003;Raoof and Hassanizadeh 2013). Above the critical saturation, the development of preferential flow paths favours faster travel times leading to shorter times for mixing and reaction (Vanderborght et al., 2001;Ursino et al., 2001;Persson et al., 2005;Gouet-Kaplan and Berkowitz, 2011;Kapetas et al., 2014). In addition, flow channelling may also increase concentration gradients, thus enhancing diffusive mass transfer and reaction rates (Jiménez-Martínez et al., 2015. ...
The impact of phases distribution on mixing and reaction is hardly assessable experimentally. We use a multiple point statistical method, which belongs to the family of machine learning algorithms, to generate simulations of phases distributions from data out of laboratory experiments. The simulations honour the saturation of the laboratory experiments, resemble the statistical distributions of several geometric descriptors and respect the physics imposed by capillary forces. The simulated phases distributions are used to compute solute transport. The breakthrough curves reveal that different phases distributions lead to broad ranges of early arrival times and long-term tailings as saturation decreases. For a given saturation, a similar long-term scaling of mixing area, interface length, and corresponding reactivity is observed regardless of phases distribution. However, phases distribution has a clear impact on the final values (before breakthrough) of area of mixing, interface length and mass of reaction product.
... The laboratory studies referred to above were mostly conducted using repacked soil columns. More uncertainties are expected for fieldscale undisturbed soils because of large-scale soil heterogeneity, soil layering, the presence of fractures and macropores, spatial variations in the flow velocity within the transport domain, time-dependent boundary conditions, root water uptake, and other complexities (e.g., Vanderborght et al., 2000Vanderborght et al., , 2001Javaux et al., 2006;Vanderborght and Vereecken, 2007;Koestel et al., 2009). Carefully constructed laboratory tracer experiments permit a far more precise assessment of the effects of soil moisture content on the dispersivity, as compared to field-scale situations. ...
... Soil heterogeneity in natural soils in the vertical direction is mostly due to the presence of different soil horizons, which are related to soil type (Vanderborght et al., 2001;Jacques et al., 2002). In such cases, transport processes can be described using 1D models. ...
Core Ideas
The heterogeneity of soil hydraulic properties can be described with effective parameters.
Increasing model complexity can be used to represent plot‐scale soil heterogeneity.
One‐dimensional dual‐domain flow models are used to reproduce 2D preferential transport.
Local subscale variability effects are included as mass transfer in an effective model.
Agricultural soils are characterized by a structure that is strongly dependent on farming practices like tillage and trafficking. These practices can create compacted zones in the soil, thus initiating preferential flow. Two‐ or three‐dimensional models can be used to account for the spatial variability of the soil hydraulic and transport properties. Since it is challenging to obtain such data, it is logical to find simpler approaches. Our objective was to design a one‐dimensional (1D) modeling approach that effectively accounts for plot‐scale soil structure variability. A 1D dual‐permeability model was tested in which compacted soil was represented by a matrix domain and uncompacted soil by a fracture domain and eventually by assuming an additional immobile water region (MIM) in the fracture domain representing compacted clods embedded within the uncompacted soil. Models (1D) were compared with two‐dimensional single‐porosity (2D_SP) modeling results for water flow and Br⁻ transport based on a previously performed field tracer experiment. Results indicated good agreement between 1D dual‐domain approaches (1D_DPERM and 1D_DPERM_MIM) and the 2D_SP representative model simulation results with high model efficiency and with respect to the field observations. This implied that a 1D vertical model description was sufficient to represent plot‐scale variability if smaller scale soil structure heterogeneities could be accounted for as effective parameters in dual‐domain models. Variation in the mass transfer term had a large effect on the vertical Br⁻ profile distribution. The parameters describing the sizes and shapes of the domains were most relevant for estimating mass transfer between soil structural features in heterogeneous agricultural fields. Still, the calibration of the upscaling approach of two‐domain interactions in larger scale models remains challenging.
The time domain reflectometry (TDR) technique is a geophysical method that allows, in a time-varying electric field, the determination of dielectric permittivity and electrical conductivity of a wide class of porous materials. Measurements of the volumetric water content (θw) in soils is the most frequent application of TDR in Soil Science and Soil Hydrology. In last four decades several studies have sought to explore potential applications of TDR. Such studies (except those conducted on θw estimation) mainly focused on monitoring soil solute transport. In more recent times, innovative TDR approaches have also been implemented to extend current TDR fields of application to the problem of monitoring non-aqueous phase liquids (NAPLs) in variable saturated soils. NAPLs are organic compounds with low solubility in water and are characterised by a high mobility in the vadose zone. Due to their high toxicity, NAPLs constitute a severe geo-environmental problem, thus making detection and observation of such substances in soils an increasingly important issue. The present paper deals with these studies and aims to provide an up-to-date review of the main NAPL-TDR studies. To date, the literature has focused on TDR applications in three main fields: i) NAPL monitoring in homogeneous, variable saturated soils, ii) NAPL monitoring in layered variable saturated soils, and iii) NAPL monitoring during soil decontamination processes. For an exhaustive and complete overview of TDR research in this field, we also recall the basic principles of TDR signal propagation, the functioning of a typical TDR device, and the dielectric mixing models that are widely used to interpret the dielectric response of NAPL-contaminated soils.
This review discusses the causes and consequences of ‘non‐equilibrium’ water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non‐equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non‐equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non‐equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non‐equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.
We simulate flow and dispersion of tracers in three-dimensional fractured geometries obtained with Voronoi tessellations. “Fractures” are generated and discretized using a parallel in-house code. These “fractures” can also be regarded as the high-permeability flow paths through the rock or a network of the “super-k” channels. The generated geometry contains multiply-connected matrix and fracture regions. The matrix region represents a porous rock filled with solid, water, and oil. Tracers diffuse in both regions, but advection is limited only to the fractures. The lattice-Boltzmann and random-walk particle-tracking methods are employed in flow and transport simulations. Mass-transfer across the matrix–fracture interface is implemented using the specular reflection boundary condition. Tracer partitioning coefficients can vary among the tracer compounds and in space. We use our model to match a field tracer injection test designed to determine remaining oil saturation. By analyzing the time-dependent behavior of the fully resolved, three-dimensional “fracture”–matrix geometry, we show that the industry-standard approach may consistently overestimate remaining oil saturation. For a highly heterogeneous reservoir system, the relative error of the field-based remaining oil estimates may exceed 50%.
The applicability of two different steady-state flow approximations of the convection-dispersion equation (CDE) to derive transport parameters from time series of concentrations or breakthrough curves (BTCs) that are observed during transient flow leaching experiments was evaluated, in the first often-used approximation, the time coordinate was transformed to a cumulative drainage coordinate, I, assuming that the water content remained constant during the leaching experiment. In the second approximation, the time coordinate was transformed to a solute penetration depth, ζ, assuming that the flow rate and water content remained constant with depth across the solute displacement front. Comparisons of numerical solutions of the CDE for transient flow conditions with analytical solutions of the approximate steady-state models revealed that the first approximate model underestimates the dispersion of the BTC when the water content fluctuates considerably during the leaching experiment. Alternatively, fitting this model to a BTC as a function of I results in an overestimation of the dispersion coefficient D and the dispersivity λ = D/v. Since the second approximate model described the simulated BTCs well, good estimates of D and λ were obtained when this model was fitted to a BTC as a function of ζ. If λ is a function of the flow rate JW, the fitted λ could be related to an effective or flux-weighted average flow rate so that the soil specific relation λ(JW) could be defined.
The chapter presents and evaluates current experimental information and theoretical approaches used to represent chemical transport and transformations in unsaturated soil. It focuses on the field regime and discusses the current approaches used for modeling chemical transport in natural media. The field evidence regarding preferential flow is quite consistent in one respect. Preferential flow can occur under a variety of circumstances and is not restricted to clay-rich soils with significant structural voids. Fluid transport through well-defined structural voids is not predictable unless the distributions of voids, aperture sizes and shapes, depths of penetration, and interconnectivity are known. Progress is being made slowly in characterizing transport through rock fractures but there the geometry is much more stable in time than it is in the soil regime. Laboratory studies have demonstrated clearly that soil structure is almost certain to introduce mass transfer limitations to equilibrium between the dissolved and sorbed phases in soil. The chapter concludes that there are fundamental differences between the transport characteristics of the laboratory and field environments.
This article is concerned with the predominantly vertical movement of water through soils that to some degree have a network of large channels (macropores) and the consequences of this movement for the convective transport of solutes and suspended matter. Beven and Germann (1982) have reviewed the experimental evidence indicating that infiltration and redistribution of water in soils containing macropores are not adequately described by theories that treat the soil as a homogeneous medium conforming to Darcian principles of water flow. Such theories, developed for the mixing of solutions in hallow tubes (Taylor, 1953) and porous rock strata (Brigham et al., 1961), have been applied to miscible displacement experiments with columns of sand, resins, glass beads, or finely sieved and repacked soil. The underlying assumptions are that an unreactive solute moves through the medium at the same velocity as the water and all the analysis and interpretation of these experiments have been reviewed several times (Gardner, 1965; Biggar and Nielsen, 1967; Biggar and Nielsen, 1980; Wagenet, 1983) and will not be repeated here.
Flow pathways of water and solutes in soils form distinct patterns,
which are not a priori predictable. Macropore structure is a prime
cause, but other factors, such as differing initial or boundary
conditions, may also predispose a soil to produce bypassing of
infiltrating water. This study was conducted to assess the flow pathways
of water in different soils and to investigate the effect of initial
water content on the flow pattern. Dye-tracing experiments were carried
out at 14 different field sites. The sites represent a good portion of
soils used for agricultural crop production in Switzerland. Each site
consisted of two 1.4 by 1.4 m plots, one of which had been covered with
a plastic roof for two months before the experiment to achieve different
initial water contents. Forty millimeters of water containing the dye
Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto
the plots with a sprinkling apparatus. One day after irrigation the
plots were excavated, and the stained pattern was examined on a vertical
1 by l m soil profile. The spatial structure of flow patterns showed
remarkable differences. In most soils, water bypassed the soil matrix.
In some soils, dye penetrated beyond l m depth, whereas in others it
remained in the top 50 cm. Structured soils were more prone to produce
bypass flow, deep dye penetration, and pulse splitting than
nonstructured soils. The initial water content had a less pronounced
effect in some soils and no effect in others.
A column of 100 cm length and 15.4 cm diameter was filled with sand. A solute displacement experiment, using tritium as a tracer, was conducted under saturated water flow conditions. Analysis of the observed effluent concentration resulted in an estimated dispersivity of 0.094 cm. Solute displacement experiments under unsaturated water flow conditions were conducted, by leaching the column intermittedly on a daily basis with constant amounts of water. Three such experiments were conducted with respectively 100 cm3, 250 cm3 and 500 cm3 of water infiltrated daily. The parameters of the hydrodynamic dispersion equation were fitted to the observed effluent concentrations. Very large dispersion coefficients were obtained with a dispersivity of about 7.3 cm. Hence, changing from saturated to unsaturated conditions increases the dispersivity by a factor of about 80. A dispersion convection equation with mobile and immobile water phases was also fitted to the data and for all experiments it was found that about 36% of the water in the unsaturated column could be considered as immobile. The dispersivity in the mobile zone was found to be slightly larger than for saturated conditions.
The solute concentrations measured in the field experiment of G. L.
Butters et al. (this issue) are used to compare two models of vadose
zone solute transport: the deterministic one-dimensional
convection-dispersion model, which represents solute transport far from
the source of solute entry, and the stochastic-convective lognormal
transfer function model, which represents solute transport near the
source. The stochastic-convective model provided an excellent
representation of the spreading of the solute pulse to a depth of 3 m
after calibration at 0.3 m. Conversely, the deterministic model
dramatically underpredicted solute spreading beyond 0.3 m after
calibration. An analysis of the area-averaged solute concentration
revealed a nearly linear scale effect in the dispersivity to a depth of
at least 14.8 m. A change in the growth pattern of dispersion observed
in the breakthrough curve at 4.5 m was attributed to a soil texture
change near 3 m, which caused the apparent dispersivity of the pulse to
decrease between 3.0 and 4.5 m, after which it increased significantly
between 4.5 m and the final profile sampling between 0 and 25 m.