Dirk Mallants

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

Spatial analysis of saturated hydraulic conductivity in a soil with macropores

Saturated hydraulic conductivity (KS) is an important soil hydraulic parameter for it establishes a limit on the rate of water and solute transmission through soil. However, its determination in the laboratory has been shown to be much influenced by column size. We evaluated the spatial variability of laboratory KS measurements using three different column sizes: firstly, sixty 5.1 cm long columns of 5 cm diameter were used (type-I), next, thirty 20 cm long and 20 cm diameter columns were considered (type-II), and finally, thirty columns 100 cm long and of 30 cm diameter (type-III) were studied. All columns were collected along a transect in a sandy loam soil with macropores. Estimates of macroporosity at three depths (2.5, 12.5, and 16.5 cm) for twenty-four of the type-II columns were calculated from stained dye patterns obtained during ponded infiltration. The geometric mean of KS decreased with increasing column size, i.e., from 2.24, 1.68 to 0.56 cm/h for type-I, -II, and -III columns, respectively. The coefficient of variation (CV) based on a log-normal distribution showed a similar trend: 619% for type-I, 217% for type-II, and 105% for type-III. Type-II and type-III columns were large enough to encompass a representative elementary volume (REV). The percentage of dye-staining (macropore cross-sectional area) decreased from 3% at 2.5 cm to 1.7% and 1.6% at 12.5 and 16.5 cm, respectively. Percentage of depth-averaged macropore area was moderately variable with CV = 51%. A geostatistical analysis revealed that a weak spatial structure existed for type-I KS measurements whereas type-II and type-III columns displayed better spatial correlation with a range of approximately 14 m and 11 m, respectively. Spatial correlation was also observed for depth-averaged macropore area with a range of 12 m. The cross-semivariogram calculated between type-II KS values and depth-averaged macropore area obtained from the same columns indicated positive spatial cross-correlation for all lags.

Estimating solute transport in undisturbed soil columns using time-domain reflectometry

Time-domain reflectometry (TDR) was used to monitor solute breakthrough curves (BTCs) in 30 saturated undisturbed soil columns collected along a 35-m-long transect in the field. The BTCs were obtained by relating the bulk soil electrical conductivity, ECa, to the relative concentration of a KCl solute pulse applied to the soil surface. Values of ECa were estimated by measuring the soils impedance to an electromagnetic wave generated by a cable tester. Parallel two-rod TDR probes inserted horizontally at a depth of 10 cm were used to monitor the soils impedance during transport of the KCl solute pulse. Calculated experimental time moments indicated that the BTC data were very variable in time and space. This variability was attributed in part to the relatively small volume of soil sampled with the TDR probes, and in part to the natural heterogeneity of the sandy loam soil. The observed BTCs were classified into three groups. One group showed bell-shaped curves consistent with the classical convection-dispersion equation (CDE). A second group was characterized by early breakthrough and long tailing. The BTCs in this group could be described by a mobile-immobile transport model (MIM). A third group of BTCs showed irregular shapes with several peaks. Time moments were used to compare the estimated (from the moments), fitted (CDE and MIM) and independently measured pore-water velocities. The disparities between the observed and fitted velocities suggest that for structured soil several TDR probes may be necessary in order to obtain reasonable estimates of column-scale solute transport behaviour.

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.

Determining local-scale solute transport parameters using time domain reflectometry (TDR)

The time domain reflectometry (TDR) technique was evaluated as a viable method for quick and accurate characterization of the solute transport parameters in controlled laboratory conditions. The method is able to measure steady-state solute breakthrough curves of inert solutes in soil columns. Transport of a solute was monitored in a column uniformly packed with a disturbed sandy material and columns filled with undisturbed soil material of three different soil horizons of a sandy soil profile. The measured solute breakthrough curves were used to assess the parameters of the classical two-parameter convection dispersion equation (CDE), in which it is assumed that the solute is completely miscible. Alternatively, a four-parameter two-region model was fitted to the data, assuming exchange between immobile and mobile water. The study reveals that transport of solutes in undisturbed sandy soil is much better described using the two-region model. In addition, it has been shown that the apparent dispersion coefficient of the CDE could be linearly related to the solute pore water velocity.

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

Evaluation of multimodal hydraulic functions in characterizing a heterogeneous field soil

Abstract Soil water retention curves are often used to estimate the hydraulic conductivity function. Unfortunately, single S-shaped functions cannot adequately describe water retention curves of structured soil, especially near saturation. The approach of superposition of two or more unimodal retention functions such as the van Genuchten model was used here to describe retention data of a macroporous soil. A total of 180 cores, 0.05 m diameter and 0.051 m long, were sampled along a 31-m-long transect in three overlying soil horizons. Use of unimodal retention curves leads to an underestimation of observed water contents both near saturation and in the midpore range, while an overestimation is found in the drier range. Superposition of two unimodal retention curves significantly improved the estimation over the entire pressure range. However, the predictions were still not ideal near saturation. With three unimodal curves, a perfect fit was obtained from saturation to residual water content. Most of the multimodal parameter values were moderately heterogeneous along the transect, with the surface horizon slightly more heterogeneous than the deeper layers. The coefficient of variation (CV) for multimodal parameters was generally in the range of 20 to 70%. Use of the multimodal van Genuchten model with the conductivity estimation model of Mualem resulted in conductivities that were generally much smaller than those estimated by the classical unimodal van Genuchten-Mualem model. A preliminary evaluation of the estimated bimodal and trimodal unsaturated hydraulic conductivity model was based on a comparison with independent conductivity measurements using a combination of crust test, hot-air method, and an unsteady drainage flux experiment on large columns. The crust and hot-air data compared best with the estimated trimodal conductivity function. The unsteady drainage data did not match well with the crust and hot-air data and could not be described with any of the estimated conductivity functions.

Cautionary notes on the use of pedotransfer functions for estimating soil hydraulic properties

Abstract The performance of published pedotransfer functions was evaluated in terms of predicted soil water content, pressure heads, and drainage fluxes for a layered profile. The pedotransfer functions developed by Vereecken et al. (1989), Vereecken et al. (1990) were used to determine parameters of the soil hydraulic functions θ(h) and K(h) which were then used as input to SWATRER, a transient one-dimensional finite difference soil water model with root uptake capability. The SWATRER model was used to simulate the hydraulic response of a multi-layered soil profile under natural climatic boundary conditions for a period of one year. The simulations were repeated by replacing the indirectly estimated water retention characteristic by (1) local-scale, and (2) field-scale mean observed θ(h) relationships. Soil moisture contents and pressure heads simulated at different depths in the soil profile were compared to measured values using these three different sets of hydraulic functions. Drainage fluxes at one meter below ground surface have also been simulated using the same three sets of hydraulic functions. Results show that simulations based on indirectly estimated moisture retention characteristics (obtained from pedotransfer functions) overpredict the observed moisture contents throughout the whole soil profile, but predict the pressure heads at shallow depths reasonably good. The results also show that the predicted drainage fluxes based on estimated retention functions are about four times as high compared to the drainage fluxes simulated using measured retention curves.

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.

Performance assessment of the disposal of vitrified high-level waste in a clay layer

Deep disposal is considered a safe solution to the management of high-level radioactive waste. The safety is usually demonstrated by means of a performance assessment. This paper discusses the methodological aspects and some of the results obtained for the performance assessment of the disposal of vitrified high-level waste in a clay layer in Belgium. The calculations consider radionuclide migration through the following multi-barrier components, all of which contribute to the overall safety: (1) engineered barriers and the host clay layer, (2) overlying aquifer, and (3) biosphere. The interfaces between aquifers and biosphere are limited to the well and river pathway. Results of the performance assessment calculations are given in terms of the time evolution of the dose rates of the most important fission and activation products and actinides. The role of the glass matrix in the overall performance of the repository is also discussed.