Jiří Šimůnek

Using an inverse method to estimate the hydraulic properties of crusted soils from tension-disc infiltrometer data

b ( Laboratoire detude des Transferts en Hydrologie et En˝ironment LTHE, CNRS URA 1512, ´ ) Abstract An inverse procedure was used to estimate the soil hydraulic characteristics of a two-layered soil system—soil surface crust and subsoil—from data obtained during a tension-disc infiltration experiment. The inverse procedure combined the Levenberg-Marquardt nonlinear parameter optimization method with a numerical solution of the axisymmetric variably-saturated flow equation. The objective function was defined in terms of the cumulative infiltration curve and the final water content measured directly below the tension-disc infiltrometer at the end of the experiment; this final water content was assumed to correspond to the final supply pressure head. We analyzed two infiltration experiments carried out with a 25-cm diameter tension-disc infiltrom- eter. One experiment was carried out on a two-layered system, and a second after removal of the surface crust covering the sandy subsoil. Both experiments were performed with six consecutive supply tensions. We first analyzed the infiltration experiment for the subsoil only, thus yielding its hydraulic characteristics. Subsequent analysis of the infiltration experiment for the two-layered system with known hydraulic properties of the subsoil provided estimates of the hydraulic properties of the surface crust. We further compared the estimated hydraulic parameters of the w subsoil with those obtained using Woodings analytical method Wooding, R.A., 1968. Steady x

Kinetic modeling of virus transport at the field scale.

Bacteriophage removal by soil passage in two field studies was re-analyzed with the goal to investigate differences between one- and two-dimensional modeling approaches, differences between one- and two-site kinetic sorption models, and the role of heterogeneities in the soil properties. The first study involved removal of bacteriophages MS2 and PRDI by dune recharge, while the second study represented removal of MS2 by deep well injection. In both studies, removal was higher during the first meters of soil passage than thereafter. The software packages HYDRUS-ID and HYDRUS-2D, which simulate water flow and solute transport in one- and two-dimensional variably saturated porous media, respectively, were used. The two codes were modified by incorporating reversible adsorption to two types of kinetic sites. Tracer concentrations were used first to calibrate flow and transport parameters of both models before analyzing transport of bacteriophages. The one-dimensional one-site model did not fully describe the tails of the measured breakthrough curves of MS2 and PRD1 from the dune recharge study. While the one-dimensional one-site model predicted a sudden decrease in virus concentrations immediately after the peaks, measured data displayed much smoother decline and tailing. The one-dimensional two-site model simulated the overall behavior of the breakthrough curves very well. The two-dimensional one-site model predicted a more gradual decrease in virus concentrations after the peaks than the one-dimensional one-site model, but not as good as the one-dimensional two-site model. The dimensionality of the problem hence can partly explain the smooth decrease in concentration after peak breakthrough. The two-dimensional two-site model provided the best results. Values for k(att2) and k(det2) could not be determined at the last two of four monitoring wells, thus suggesting that either a second type of kinetic sites is present in the first few meters of dune passage and not beyond the second monitoring well, or that effects of soil heterogeneity and dimensionality of the problem overshadowed this process. Variations between single collector efficiencies were relatively small, whereas collision efficiencies varied greatly. This implies that the nonlinear removal of MS2 and PRD1 is mainly caused by variations in interactions between grain and virus surfaces rather than by physical heterogeneity of the porous medium. Similarly, a two-site model performed better than the one-site model in describing MS2 concentrations for the deep well injection study. However, the concentration data were too sparse in this study to have much confidence in the fitted parameters.

Parameter estimation of unsaturated soil hydraulic properties from transient flow processes

Abstract Three field methods recently proposed for estimating the soil hydraulic properties by numerical inversion of the Richards equation are discussed. The first method involves the use of tension disc permeameter data, while the second method uses data collected with modified cone penetrometer. The third method involves the use of a multiple-step field extraction device. Experimental data for each of the above three methods were analyzed by using the HYDRUS-2D code coupled with the Levenberg–Marquardt parameter estimation algorithm. Advantages and disadvantages of the three methods are discussed.

A THIRD-ORDER NUMERICAL SCHEME WITH UPWIND WEIGHTING FOR SOLVING THE SOLUTE TRANSPORT EQUATION

SUMMARY Solute transport in the subsurface is generally described quantitatively with the convection—dispersion transport equation. Accurate numerical solutions of this equation are important to ensure physically realistic predictions of contaminant transport in a variety of applications. An accurate third-order in time numerical approximation of the solute transport equation was derived. The approach leads to corrections for both the dispersion coeƒcient and the convective velocity when used in numerical solutions of the transport equation. The developed algorithm is an extension of previous work to solute transport conditions involving transient variably saturated fluid flow and non-linear adsorption. The third-order algorithm is shown to yield very accurately solutions near sharp concentration fronts, thereby showing its ability to eliminate numerical dispersion. However, the scheme does su⁄er from numerical oscillations. The oscillations could be avoided by employing upwind weighting techniques in the numerical scheme. Solutions obtained with the proposed method were free of numerical oscillations and exhibited negligible numerical dispersion. Results for several examples, including those involving highly non-linear sorption and infiltration into initially dry soils, were found to be very accurate when compared to other solutions. ( 1997 by John Wiley & Sons, Ltd.

Transport of copper oxychloride-based fungicide particles in saturated quartz sand.

Intensive use of copper-based fungicides in agriculture causes contamination of subsurface environment. While the transport of dissolved copper in porous media has been widely studied, transport mechanisms of particles of copper-based fungicides are poorly understood. This paper reports the results of tests involving the transport of colloid-size particles of a copper oxychloride-based fungicide (COF) in water-saturated quartz sand columns under varying electrochemical and hydrodynamic conditions. The effect of the ionic strength on colloid attachment and exclusion suggests that interactions of COF in water depend on the characteristics of the diffuse layer. Hydrodynamic shear influences the deposition of fungicide particles, indicating that attachment forces are weak. Particle deposition dynamics was well-fitted with the two-site kinetic attachment model, which reflects the heterogeneity of the quartz surface. Retention also occurs for unfavorable electrochemical conditions, which was attributed to stagnation zones arising from the physical and chemical heterogeneity of the quartz surface.