Water and Solute Transport in Field Soils and Lysimeters: The Effect of the Lower Boundary Condition

Contents

The Lysimeter Question

A lysimeter is a large soil block surrounded by a casing. The lower boundary of the lysimeter is shaved off from the parent soil and usually exposed to atmospheric pressure. This exposure results in a hydraulic barrier for water flow. The soil at the lower boundary has to be saturated with water before drainage outflow can occur. To overcome the problem of water saturation, a suction can be applied at the bottom of the lysimeter with porous ceramic plates, pipes, or fiberglass wicks. However, for lysimeters with a large surface area, the use of suction devices is impractical and often problematic. Consequently, most large lysimeters have a drainage system open to atmospheric pressure.

Lysimeters are intended to represent field conditions much better than laboratory columns, and have been widely used to investigate fate and behavior of chemicals in soils. Assuming that there is an undisturbed soil block in the lysimeter, the only difference between the lysimeter and the field soil is the lower boundary condition. It is unclear to what extent water and solute transport are affected by this difference in boundary conditions.

Several comparative studies between field soils and lysimeters have been reported in the literature. The comparisons included the temporal variation of temperature, water content, phosphorus fluxes, and pesticide concentrations in soil. The experimental evidence whether, and to what degree, a suction-free lysimeter represents field conditions adequately is not conclusive to date. The OECD is currently in the process of developing guidelines for the use of lysimeters for pesticide regulations. Such guidelines will have to be based on sound evaluation of the performance of lysimeters as regulatory tools.

Project Goals

The purpose of this project is to evaluate the effect of the lower boundary condition on solute in field soils and lysimeters. Specific objectives are to evaluate the effects of the boundary conditions under the following conditions:

Comparing Lysimeter and Field Soils

When comparing field soils and lysimeters we use numerical simulations as the research tool. With numerical simulations we can keep all the factors identical between field soil and lysimeter, except for the factor of interest, in our case this is the lower boundary condition. We assume that the lysimeter is characterized by a seepage boundary and the field soil by a free-drainage condition. The Figure 1 shows a schematic setup for the simulations.

Water flow is described by the Richards' Equation and solute transport by the advection-dispersion equation. The equations are solved with the finite element code CHAIN_2D of Simunek and van Genuchten (1994).

One-dimensional Simulations

Hydraulic Properties

Hydraulic properties of the two soils used are shown in the following Figure 2. Simulations are performed under steady-state, unsaturated water flow with three different fluxes q: 2, 0.5, and 0.1 cm/d. The depth L of the lysimeter is assumed to be 1.2~m. This depth is chosen according to guidelines issued by the European Community for testing fate and behavior of chemicals in the environment. For the field soil, the lower boundary conditions are approximated in the simulations by using a soil profile of 10 m depth. Test runs show that this depth was adequate to accurately simulate solute transport at 1.2 m depth. Solute concentrations are monitored over time at the bottom boundary of the lysimeter and at the corresponding depth in the field soil. All concentrations reported are relative concentrations, normalized by the input mass of solutes.

For steady-state water flow, the volumetric water contents and the matric potentials in the soil remain time-invariant. The Figure 3 shows the distributions of the volumetric water contents within the lysimeter. Because of the seepage boundary condition, the bottom boundary of the lysimeter is in all cases saturated, and accordingly the matric potential is zero. It can be seen that the coarser the soil is, the thinner the capillary fringe. In the sandy soil, the capillary fringe is about 5 cm in height, whereas in the clay loam soil the capillary rise extends upward to the soil surface. In contrast to the lysimeter, the water contents and matric potentials in the field soil with the unit-gradient boundary condition are constant throughout the soil profile, and there is no gradient along the vertical coordinate.

Non-Sorbing Solutes

The Figure 4 shows the breakthrough of a non-sorbing solute, e.g. bromide or chloride, at a depth of 1.2~m in the field soil and the lysimeter at different flow rates. The difference in solute breakthrough between field soil and lysimeter is more pronounced in the sand than in the loam soil. The two soils show the same characteristic features of solute transport between the field and the lysimeter simulations, even though these features are more distinct in the sand soil. In general, the differences between field soil and lysimeter increase as the flow rate decreases.

Sorbing Solutes

Linear Equilibrium Sorption

The effect of the lower boundary condition on the transport of a solute subject to linear equilibrium sorption is depicted in Figure 5 and Figure 6. An increasing distribution coefficient causes the breakthrough curves to approach each other. Whereas for no sorption there are more pronounced deviations between lysimeter and field soil in the sandy soil than in the loamy soil, the effect of the soil type diminishes for increasing sorption ( Figure 5). For a distribution coefficient of 2 mL/g, the breakthrough curves in the field soil and the lysimeter are almost identical for the sandy as well as for the loamy soil. To quantitatively assess these differences, the mean solute travel time, the variance of the solute travel time, and the maximum concentrations of the breakthrough curves were calculated for different sorption coefficients. The ratio of mean travel times between lysimeter and field soil is denoted as $\eta$, the ratio of the travel time variances as $\zeta$, and the ratio of maximum concentrations as $\kappa$. Figure 6 shows the ratios $\eta$, $\zeta$, and $\kappa$ plotted versus sorption coefficient for the sandy and the loamy soil. Mean and variance of the solute travel time are larger in the lysimeter than in the field soil, and correspondingly the contrary is true for the maximum concentrations. Two interesting points can be mentioned. First, the effect of the sorption is most pronounced at low K-values, indicated by the steep slope of the ratio curves, and as the sorption coefficient increases its effect decreases. Second, the sandy soil is more susceptible towards the sorption coefficient than the loamy soil.

Nonlinear Equilibrium Sorption

Nonlinearity of the sorption isotherm does not affect the difference between lysimeter and field soil, except for strongly convex isotherms. ( Figure 7) shows that the response of the ratio $\kappa$ is very sensitive for small values of the isotherm exponent $n$. In case of our simulations a large deviation between lysimeter and field soil occurred when the $n$-value was less than 0.8. A convex isotherm leads to a self-sharpening solute front in the soil profile. Since the lysimeter has a larger water content near the bottom, the aqueous concentration of the travelling front will decrease. Due to the nonlinearity of the isotherm, this leads to increased sorption, and thus to an enhanced retardation of the solutes. This is in contrast to the case of linear sorption, where sorption is independent of concentration. Strong convex nonlinearity in the sorption isotherm can lead to large differences between lysimeter and field soil.

Two-dimensional Simulations

Simulations will be carried out with media of homogeneous layers and with heterogeneous media. Heterogeneous porous media with specified spatial correlation lengths will be created with the turning bands method. Steady-state and transient water flow will be imposed.

Markus Flury
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