Heterogeneous Soil Columns Research Articles

Modeling solute transport in one-dimensional homogeneous and heterogeneous soil columns with continuous time random walk

In this paper, we used the continuous time random walk (CTRW) framework to characterize the transport process in 1250-cm long one-dimensional homogenous and heterogeneous soil columns at the experiments conducted by Huang et al. [Huang, K., Toride, N., van Genuchten, M.Th., 1995. Experimental investigation of solute transport in large, homogeneous and heterogeneous, saturated soil columns. Trans. Porous Media. 18, 283–302]. The transport process was also simulated by using the advection–dispersion equation (ADE) and the spatial fractional advection–dispersion equation (FADE) for comparison. In the homogeneous soil column, the non-Fickian behavior is found at the distances less than 1000 cm with β values larger than 1.60, but less than 2, and Fickian form transport is obtained at distances larger than 1000 cm with β values larger than 2. In the heterogeneous soil column, we found the most anomalous behavior at distances from 200 cm to 700 cm with β values ranging from 0.894 to 0.958, and non-Fickian transport process is observed at distances larger than 800 cm with β values in the range between 1 and 1.3. More significant non-Fickian behavior is found for transport in the heterogeneous soil column than that in the homogeneous soil column. The CTRW fits to the breakthrough curves (BTCs) have lower values of root mean square error (RMSE) and higher values of determination coefficient ( r 2), with respect to the fits of ADE and FADE. The CTRW model also is better captures the full evolution of BTCs, and especially their tails.

Evidence of one-dimensional scale-dependent fractional advection–dispersion

A semi-analytical inverse method and the corresponding program FADEMain for parameter estimation of the fractional advection–dispersion equation (FADE) were developed in this paper. We have analyzed Huang et al.'s [Huang, K., Toride, N., van Genuchten, M.Th., 1995. Experimental investigation of solute transport in large homogeneous and heterogeneous saturated soil columns. Trans. Porous Media 18, 283–302.] laboratory experimental data of conservative solute transport in 12.5-m long homogeneous and heterogeneous soil columns to test the non-Fickian dispersion theory of FADE. The dispersion coefficient was calculated by fitting the analytical solution of FADE to the measured data at different transport scales. We found that the dispersion coefficient increased exponentially with transport scale for the homogeneous column, whereas it increased with transport scale in a power law function for the heterogeneous column. The scale effect of the dispersion coefficient in the heterogeneous soil was much more significant comparing to that in the homogeneous soil. The increasing rate of dispersion coefficient versus transport distance was smaller for FADE than that for the advection–dispersion equation (ADE). Finite difference numerical approximations of the scale-dependent FADE were established to interpret the experimental results. The numerical solutions were found to be adequate for predicting scale-dependent transport in the homogeneous column, while the prediction for the heterogeneous column was less satisfactory.

Analytical solutions to steady state unsaturated flow in layered, randomly heterogeneous soils via Kirchhoff transformation

In this study, we derive analytical solutions of the first two moments (mean and variance) of pressure head for one-dimensional steady state unsaturated flow in a randomly heterogeneous layered soil column under random boundary conditions. We first linearize the steady state unsaturated flow equations by Kirchhoff transformation and solve the moments of the transformed variable up to second order in terms of σ Y and σ β , the standard deviations of log hydraulic conductivity Y=ln( K s) and of the log pore size distribution parameter β=ln( α). In addition, we also give solutions for the mean and variance of the unsaturated hydraulic conductivity. The analytical solutions of moment equations are validated via Monte Carlo simulations.

A Procedure to Calculate Biodegradation during Preferential Flow through Heterogeneous Soil Columns

Pesticides are found in groundwater sooner than commonly is predicted with traditional management models. These models do not account for preferential flow and assume uniform biodegradation. For more accurate modeling, the interaction of preferential flow and nonuniform biodegradation needs to be addressed. In this paper, we tested techniques to measure the rate of biodegradation in preferential flow paths by continuously applying p-nitrophenol (PNP) to the surface of packed soil columns that contained two types of preferential flow paths. The first type was an open macropore in a uniform soil matrix (macropore column), and the second type was a channel of coarser soil in a finer soil matrix (channel columns). Breakthrough curves were obtained from separate small areas of the macropore column with fiberglass wicks to collect and separate macropore flow from soil matrix flow. Breakthrough curves were also obtained from the channel columns for comparison to curves from homogeneous columns. Average biodegradation rates were calculated from the breakthrough curves. The PNP biodegradation occurred in both matrix and macropore flow regions of the macropore column, but the time and rate of biodegradation differed in the two flow regions, as well as between the channel columns and the homogeneous columns. The PNP was degraded rapidly despite short travel times through the path of preferential flow, indicating that rapid biodegradation may occur in macropores and other preferential flow paths. The data provide a basis for improved modeling of fate and transport of chemicals through vadose soil to groundwater.

Solute movement through homogeneous and heterogeneous soil columns : Renduo Zhang, Kangle Huang & Jiannan Xiang, Advances in Water Resources, 17(5), 1994, pp 317–324

Solute movement through homogeneous and heterogeneous soil columns : Renduo Zhang, Kangle Huang & Jiannan Xiang, Advances in Water Resources, 17(5), 1994, pp 317–324

PREDICTION OF SOLUTE TRANSPORT USING A TRANSFER FUNCTION MODEL AND THE CONVECTION-DISPERSION EQUATION

The convection-dispersion equation (CDE) is the most commonly used process representation for describing solute movement through soils. A transfer function is a model for studying complex soil systems in a simple way by characterizing the output flux as a function of the input flux. Extensive solute transport data from a laboratory experiment conducted through long soil columns were used to validate the intrinsic assumptions of the CDE and the transfer function. Analyses of the experimental data showed that the observed dispersion coefficient increased with the travel distance for both homogeneous and heterogeneous soils. The dispersion coefficients had a two-order change within the 1250-cm soil columns and were described using a linear function of the solute travel distance. At the same travel distance, the dispersion coefficients in the heterogeneous soil column were about 50 times higher than those in the homogeneous soil. The experimental data better supported the assumptions of the transfer function than those of the CDE. An excellent linear relationship was found between the mean of log of the travel time and log of the travel distance. In turn, the relationship well described the scale-dependent dispersion coefficients of the homogeneous and heterogeneous soil columns. The convection-dispersion equation and the transfer equation were utilized to predict solute transport in homogeneous and heterogeneous porous media. Compared with the experimental data, the transfer function predicted solute transport in the porous media much more accurately than did the CDE for these examples. In addition, the soil spatial variability had a less profound effect on the transfer function predictions than on the CDE predictions.

Experimental investigation of solute transport in large, homogeneous and heterogeneous, saturated soil columns

Laboratory tracer experiments were conducted to investigate solute transport in 12.5-m long, horizontally placed soil columns during steady saturated water flow . Two columns having cross-sectional areas of 10 x IOcm' were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular formations of various shapes and sizes. The heterogeneous soil column gradually changed, on average, from coarse-textured at one end to fine-textured at the other end . NaCl breakthrough curves (BTC's) in the columns were measured with electrical conductivity probes inserted a t 50- or 100-cm intervals. Observed BTC's in the homogeneous sandy column were relatively smooth and sigmoidal (S-shaped), while those in the heterogeneous column were very irregular, nonsigmoidal, and exhibited extensive tailing. Effective average pore-water velocities (tr,rr) and dispersion coefficients (&r) were estimated simultaneously by tilting an analytical solution of the convection-dispersionequation to the observe d BTC's. Velocity variations in the heterogeneousmedium were found to be much larger than those in the homogeneous sand. Values of the dispersivity, (Y = Derr/ecrrr for the homogeneous sandy column ranged from 0.1 to 5.0cm, while those for the heterogeneous column were as high as 200cm. The dispcrsivity for transport in both columns increased with travel distance or travel time, thus exhibiting scale- dependency. The heterogeneous soil column also showed the effects of preferential flow. i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spatial fluctuations in the dispersivity could bc explained qualitatively by the particular makeup of the heterogeneities in the column.

Solute movement through homogeneous and heterogeneous soil columns

The classical convection-dispersion equation (CDE) and the stochastical convection-dispersion equation (SCDE) were utilized to study solute transport in homogeneous and heterogeneous porous media. Extensive experimental data collected in 1250-cm soil columns were used for testing validity of the CDE and SCDE. Analyses of the experimental data showed that the observed dispersion coefficient increased with the travel distance for both homogeneous and heterogeneous soils. The range of the dispersion coefficients was from 0·05 to 20 cm 2 min −1 for the homogeneous soil, and from 3 to 200 cm 2 min −1 for the heterogeneous soil. A linear relationship between the dispersion coefficient and travel distance, assumed in the SCDE, described the scale-dependent dispersion coefficient reasonably well. Therefore, the SCDE predicted solute transport in the porous media more accurately than the CDE for these examples. In addition, the SCDE provided more consistent estimation of breakthrough curves than the CDE, using information obtained at a location either close to or far from the predicting place.

Fluid flow and solute transport processes in unsaturated heterogeneous soils: Preliminary numerical experiments

Preliminary numerical experiments are conducted to investigate scale-dependent macrodispersivity, relative to solute transport, in unsaturated heterogeneous porous media. Synthetic heterogeneous soil columns are generated with the turning-bands approach. Fluid flow and solute transport are simulated with a three-dimensional finite-element model. Our numerical experiments confirm a three-phase development of macrodispersivity as a plume of contaminant moves through a heterogeneous soil. The initial phase grows linearly, then becomes nonlinear, and eventually reaches an asymptotic constant value. We find that the development of macrodispersivity in an unsaturated heterogeneous soil column can be predicted successfully by Dagan's formulas which were originally developed for heterogeneous aquifers. For the study of contaminant transport in unsaturated soils, the travel distance one would normally deal with is much shorter than that required to reach asymptotic macrodispersivity; thus, the two pre-asymptotic phases should be the principal concern in modeling contaminant transport in unsaturated soils. The first phase of the macrodispersivity development corresponds to what is normally referred to as the advection period during which no significant lateral mixing takes place. The linear system model for contaminant transport, which uses the travel time probability density function as the model's impulse response function, is valid only for the first phase and should therefore be applied only to the simulation of near-field contaminant transport within the advection period.

The effect of soil compaction at various depths on root and shoot growth of oats.

The effect of a loose topsoil of 23-63 cm depth overlying a dense subsoil of 35-75 cm depth on root and shoot development was compared with the effect of a homogeneous column of loose soil of 98 cm depth. Shoot weights and, to a large extent root weights, were similar during early growth, but, at later stages, they were higher in the heterogeneous soil columns. Throughout growth, root weights were greater in loose topsoil of the heterogeneous soil columns than in the corresponding layer of the homogeneous columns. Total root weights were higher in the homogeneous columns up to May 23rd, but, at later dates they were higher in the heterogeneous columns. Decreases in root weight at the boundary zone between the layers were mainly due to decreased root branching and, secondarily, to decreased root number. Rooting depth in the dense subsoil increased with increasing thickness of the topsoil, and probably with increasing depth of the water table. Shoot/root ratios at later growth stages were highest for the heterogeneous columns with thin top-soil layers, indicating the presence of favourable growing conditions despite restricted root development. This was confirmed by water consumption data. (Abstract retrieved from CAB Abstracts by CABI’s permission)