Diederik Jacques

Spatial Variability Of Hydraulic Properties In A Multi-layered Soil Profile

Unsaturated hydraulic properties of field soils are needed for water flow and solute transport calculations in variably saturated soils. The purpose of this study was to quantify the spatial variability and spatial crosscorrelation of estimated parameter values of a flexible retention model that was

Overview of inert tracer experiments in key belgian soil types: Relation between transport and soil morphological and hydraulic properties

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.

Comparison of three hydraulic property measurement methods

Hydraulic functions of soils may differ depending on the different measuring methods used. The performance of three different methods for measuring soil-hydraulic properties of a heterogeneous field were evaluated. The experiments were conducted using three different sizes of undisturbed soil cores collected systematically along a 31 m long transect of a well drained sandy loam soil having three soil horizons (Ap, 0-0.25 m; C1, 0.25-0.55 m; C2, 0.55-1.00 m). The laboratory studies involved: (1) detailed unsteady drainage-flux experiments performed on fifteen columns of 1 m length and 0.3 m diameter; (2) combined crust test and hot air methods applied to thirty columns of 0.2 m length and 0.2 m diameter and to a subset of sixty cylinders of 0.1 m length and 0.045 m diameter, respectively, taken from the Ap horizon; and (3) desorption experiments carried out on a total of one hundred eighty cores of 0.051 m length and 0.05 m diameter collected evenly from the three horizons, Mean soil hydraulic properties were inferred from experimental data characterizing either selected depths or the soil profile as a whole. The results revealed considerable differences among estimated mean soil properties as obtained with different measuring techniques. Although the application of scaling theory substantially reduced variation in the measured pressure heads (h) and conductivities (K), the results revealed that scaling parameters determined from soil pressure head were not identical to scaling factors determined from hydraulic conductivity. The results also show that K scaling factors in general were much more variable than h scaling factors, and that the observed variability in scaling factors also depend upon the measurement technique used

Sensitivity analysis of physical and chemical properties affecting field-scale cadmium transport in a heterogeneous soil profile

Field-scale transport of reactive solutes depends on spatially variable physical and chemical soil properties. The quantitative importance of physical and chemical parameters required for the prediction of the field-scale solute flux is generally unknown. A sensitivity analysis is presented that ranks the importance of spatially variable water flow and solute transport parameters affecting field-scale cadmium flux in a layered sandy soil. In a Monte-Carlo simulation approach, partial rank correlation coefficients were calculated between model parameters and cadmium flux concentrations at various time steps. Data on the heterogeneity of flow and transport parameters were obtained from a 180 m-long and 1 m-deep Spodosol transect. Each soil layer was described in terms of probability density functions of five model parameters: two shape parameters of van Genuchten’s water retention curve, saturated hydraulic conductivity, dispersivity and soil ‐ water distribution coefficient. The results showed that the cadmium flux concentrations at the bottom of the soil profile were most sensitive to the cadmium deposition rate and the soil‐ water distribution coefficient of all soil horizons. The maximum cadmium flux concentrations were also affected by variations in hydraulic conductivity of the humic topsoil horizons. Variations in shape parameters of the water retention curve did not significantly affect the field-scale cadmium flux. Variations in the dispersivity of the subsoil significantly influenced the early time cadmium concentrations. Monte-Carlo simulations involving non-linear sorption showed that cadmium flux concentrations were dominated by variations in the sorption constant and in the exponent of the Freundlich isotherm. q 2002 Elsevier Science B.V. All rights reserved.

Analysis of steady state chloride transport through two heterogeneous field soils

Chloride transport was investigated in a loamy soil and a silty-loam soil at the field scale under steady state flow conditions using a water flux of 2.84 cm d−1 for the loamy and 1.5 cm d−1 for the silty-loam soil. The solute plume movement was recorded by means of horizontally installed time domain reflectrometry (TDR) probes at 5 depths up to 90 cm below the soil surface and 24 locations along a transect of 8 m. The measurements consisted of solute resident concentrations every 2 hours in the loamy soil for a total period of 42 days and every 4 hours for the silty-loam soil for a period of 65 days. Parameters of the convective-dispersive equation (CDE) and the lognormal stochastic-convective transport model (CLT) were determined using time-normalized resident concentration breakthrough curves Crt*(z, t). In addition, temporal moments of Crt*(z, t) were related to travel time moments and transport parameters for the two transport processes. At both sites the breakthrough curves at different depths were better described by the CLT than by the CDE. However, early solute breakthrough was underestimated at most depths. Mean travel time and dispersivity were estimated using the temporal moments of Crt*(z, t) with the assumption of a stochastic-convective transport process. In the loamy soil, solute was traveling from a heterogeneous, macroporous top soil toward a subsoil containing significantly fewer macropores. The flow of solutes through the macropores is not detected by the TDR probes, resulting in a larger observed mean travel time compared with the expected mean travel time based on the piston flow model and no increase in dispersivity. In contrast, in the subsoil the observed and expected mean travel times were in good agreement, and dispersivity increased with depth. In the silty-loam soil, mean travel times derived from concentration measurements were larger than the expected mean travel times based on the piston flow model, implying temporal storage of solutes in stagnant water zones. Dispersivity also showed deviations from the expected linear increase with depth, probably because of changing soil properties with depth.

INVERSE ESTIMATION OF SOIL HYDRAULIC AND SOLUTE TRANSPORT PARAMETERS FROM TRANSIENT FIELD EXPERIMENTS: HETEROGENEOUS SOIL

While inverse parameter estimation techniques for determining key parameters affecting water flow and solute transport are becoming increasingly common in saturated and unsaturated zone studies, their application to practical problems, such as irrigation, have received relatively little attention. In this article, we used the Levenberg–Marquardt optimization algorithm in combination with the HYDRUS–2D numerical code to estimate soil hydraulic and solute transport parameters of several soil horizons below experimental furrows. Three experiments were carried out, each of the same duration but with different amounts of water and solutes resulting from 6, 10, and 14 cm water depths in the furrows. Two more experiments were performed with the same amounts of applied water and solute and, consequently, for different durations, on furrows with depths of 6 and 10 cm of water. We first used a scaling method to characterize spatial variability in the soil hydraulic properties, and then simultaneously estimated the saturated hydraulic conductivity (Ks) and the longitudinal dispersivity (DL) for the different horizons. Model predictions showed only minor improvements over those previously obtained assuming homogeneous soil profiles. In an effort to improve the predictions, we also carried out a two–step, sequential optimization in which we first estimated the soil hydraulic parameters followed by estimation of the solute transport parameters. This approach allowed us to include additional parameters in the optimization process. A sensitivity analysis was performed to determine the most sensitive hydraulic and solute transport parameters. Soil water contents were found to be most sensitive to the n parameter in van Genuchten’s soil hydraulic model, followed by the saturated water content (.s), while solute concentrations were most affected by .s and DL. For these reasons, we estimated .s and n for the various soil horizons of the sequential optimization process during the first step, and only DL during the second step. Sequential estimation somewhat improved predictions of the cumulative infiltration rates during the first irrigation event. It also significantly improved descriptions of the soil water content, particularly of the upper horizons, as compared to those obtained using simultaneous estimation, whereas deep percolation rates of water did not improve. Solute concentrations in the soil profiles were predicted equally well with both optimization approaches.

Spatial variability of atrazine sorption parameters and other soil properties in a podzoluvisol

Abstract The spatial variability of the K f and n parameters of the Freundlich sorption isotherm for atrazine and their correlation with soil textural variables, cation exchange capacity and organic carbon content were studied in a stagnic podzoluvisol. Ninety-three sample points were organized on an irregular three-dimensional grid to a depth of 3.2 m. A trend in the vertical direction explains, for most variables, about 85% of the observed variance. This trend also significantly influences the observed correlation structure between the variables. The horizontal and vertical trends were removed from the data set with the median polish algorithm. The residuals resulting from this technique obey the intrinsic hypothesis. Organic carbon content, cation exchange capacity and n revealed spatial structure. The estimated correlation length scales in the vertical direction were between 0.63–0.81 m for n and the organic carbon content, and between 0.25–0.40 m for the cation exchange capacity. The variograms of sand, loam, clay and K f exhibited pure nugget. The correlation structures between the variables differ for different spatial increments. Variables appeared correlated at small spatial increments whereas they are not correlated if the spatial location of the sample points is neglected.

Probabilistic inference of multi-Gaussian fields from indirect hydrological data using circulant embedding and dimensionality reduction

© 2015. American Geophysical Union. All Rights Reserved. We present a Bayesian inversion method for the joint inference of high-dimensional multi-Gaussian hydraulic conductivity fields and associated geostatistical parameters from indirect hydrological data. We combine Gaussian process generation via circulant embedding to decouple the variogram from grid cell specific values, with dimensionality reduction by interpolation to enable Markov chain Monte Carlo (MCMC) simulation. Using the Matern variogram model, this formulation allows inferring the conductivity values simultaneously with the field smoothness (also called Matern shape parameter) and other geostatistical parameters such as the mean, sill, integral scales and anisotropy direction(s) and ratio(s). The proposed dimensionality reduction method systematically honors the underlying variogram and is demonstrated to achieve better performance than the Karhunen-Loeve expansion. We illustrate our inversion approach using synthetic (error corrupted) data from a tracer experiment in a fairly heterogeneous 10,000-dimensional 2-D conductivity field. A 40-times reduction of the size of the parameter space did not prevent the posterior simulations to appropriately fit the measurement data and the posterior parameter distributions to include the true geostatistical parameter values. Overall, the posterior field realizations covered a wide range of geostatistical models, questioning the common practice of assuming a fixed variogram prior to inference of the hydraulic conductivity values. Our method is shown to be more efficient than sequential Gibbs sampling (SGS) for the considered case study, particularly when implemented on a distributed computing cluster. It is also found to outperform the method of anchored distributions (MAD) for the same computational budget. Key Points: Joint Bayesian inference of Gaussian conductivity fields and their variograms A dimensionality reduction that systematically honors the underlying variogram Distributed multiprocessor implementation is straightforward

Benchmarks for multicomponent reactive transport across a cement/clay interface

The use of the subsurface for CO2 storage, geothermal energy generation, and nuclear waste disposal will greatly increase the interaction between clay(stone) and concrete. The development of models describing the mineralogical transformations at this interface is complicated, because contrasting geochemical conditions (Eh, pH, solution composition, etc.) induce steep concentration gradients and a high mineral reactivity. Due to the complexity of the problem, analytical solutions are not available to verify code accuracy, rendering code intercomparisons as the most efficient method for assessing code capabilities and for building confidence in the used model. A benchmark problem was established for tackling this issue. We summarize three scenarios with increasing geochemical complexity in this paper. The processes considered in the simulations are diffusion-controlled transport in saturated media under isothermal conditions, cation exchange reactions, and both local equilibrium and kinetically controlled mineral dissolution-precipitation reactions. No update of the pore diffusion coefficient as a function of porosity changes was considered. Seven international teams participated in this benchmarking exercise. The reactive transport codes used (TOUGHREACT, PHREEQC, with two different ways of handling transport, CRUNCH, HYTEC, ORCHESTRA, MIN3P-THCm) gave very similar patterns in terms of predicted solute concentrations and mineral distributions. Some differences linked to the considered activity models were observed, but they do not bias the general system evolution. The benchmarking exercise thus demonstrates that a reactive transport modelling specification for long-term performance assessment can be consistently addressed by multiple simulators.

Implementation and evaluation of permeability-porosity and tortuosity-porosity relationships linked to mineral dissolution-precipitation

Changes of porosity, permeability, and tortuosity due to physical and geochemical processes are of vital importance for a variety of hydrogeological systems, including passive treatment facilities for contaminated groundwater, engineered barrier systems (EBS), and host rocks for high-level nuclear waste (HLW) repositories. Due to the nonlinear nature and chemical complexity of the problem, in most cases, it is impossible to verify reactive transport codes analytically, and code intercomparisons are the most suitable method to assess code capabilities and model performance. This paper summarizes model intercomparisons for six hypothetical scenarios with generally increasing geochemical or physical complexity using the reactive transport codes CrunchFlow, HP1, MIN3P, PFlotran, and TOUGHREACT. Benchmark problems include the enhancement of porosity and permeability through mineral dissolution, as well as near complete clogging due to localized mineral precipitation, leading to reduction of permeability and tortuosity. Processes considered in the benchmark simulations are advective-dispersive transport in saturated media, kinetically controlled mineral dissolution-precipitation, and aqueous complexation. Porosity changes are induced by mineral dissolution-precipitation reactions, and the Carman-Kozeny relationship is used to describe changes in permeability as a function of porosity. Archie’s law is used to update the tortuosity and the pore diffusion coefficient as a function of porosity. Results demonstrate that, generally, good agreement is reached amongst the computer models despite significant differences in model formulations. Some differences are observed, in particular for the more complex scenarios involving clogging; however, these differences do not affect the interpretation of system behavior and evolution.