A three‐dimensional field study of solute transport through unsaturated, layered, porous media: 2. Characterization of vertical dispersion

Abstract

Solute plumes were created in an unsaturated field soil with either flux application or by leaching an initial resident distribution (see Ellsworth et al., this issue). The spatial variance of the plumes initially increased with time between the soil surface and a depth of 2.5 m, within which the soil was a nearly structureless loamy sand. Below this depth, the plumes were observed to compress in the vertical direction as they moved into, and through, a region of subangular blocky structure and loam texture (between 2.5 and 4.0 m depth). As the solute moved below the layer of fine texture, the plume variance again increased with time. Using a transformed advection-dispersion equation description, two constant, field-averaged transport coefficients, V* and D}:, were determined in a scaled coordinate system from the moment equations. These two constant parameters were then used to predict the observed local, or plot scale, transport. Results indicate that the two constant parameters describe transport reasonably well at each plot site and over all sampling depths. INTRODUCTIONThe quest for an adequate description of downward chemical movement through unsaturated soil has been a major priority of soil and environmental scientists for many years. Early efforts at describing the transport process focused on characterizing movement under steady-state water flow through repacked soil columns whose lateral dimension was small compared to the vertical. Under such conditions, the advection-dispersion (or convection-dispersion) equation (ADE) with two constant coefficients (the velocity and longitudinal dispersion coefficient) became generally accepted as the consensus model for describing transport of mobile, nonreactive tracers [Nielsen and Biggar, 1962]. However, the basis for this selection was usually the degree of agreement shown between the model and the effluent concentrations of solute in miscible displacement experiments, which cannot evaluate whether the dispersion parameter in the model is constant without examining concentrations at different distances from the inlet end [Taylor, 1953].Although early field studies of solute movement at the plot scale used the ADE to describe experimental observations The ADE is a scale dependent model, whose representation of the solute dispersion process at the scale of observation is only valid after sufficient time has elapsed for lateral mixing to smooth out differences in concentration caused by advection of solute at different velocities [Taylor, 1953 [1989] indicate that the correlation length scale for saturated hydraulic conductivity may be an order of magnitude less in the vertical direction than in the horizontal, and may be short compared to the depth of interest for modelers of the unsaturated zone. In such cases, the parallel soil column approaches or transfer functions will not be able to estimate transport through the zone of heterogeneity.An example of this failure was reported by Butters and Jury [1989], who observed a linear growth over the first 2 m of ...

Methodological approach for evaluating the response of soil hydrological behavior to irrigation with treated municipal wastewater

“…In vertically heterogeneous soils, the steady state distributions of u and, consequently, v are spatially dependent. To eliminate the vertical dependency of v on u; Ellsworth and Jury (1991) introduced a coordinate-transformed depth or fluid coordinates:…”

Using time‐domain reflectometry to characterize shallow solute transport in an oak woodland hillslope in northern California, USA

“…This is the same response observed at probes 11b and 12b in plot 2. The process of solute moving into a zone where it is momentarily retained and dispersion decreases is called solute compression and it has been documented in field studies at soil horizon boundaries (Ellsworth and Jury, 1991). Solute compression is demonstrated in this plot by the smaller amount of solute spreading deeper in the soil compared with near the soil surface.…”

Simulation of the cumulative effects of chemical spills using a spatial–temporal dynamics analysis algorithm

“…The following general solute transport equation is used to describe the convection-dispersion representation adapted to the type of chemicals considered in this simulation [8][9][10][11][12][13][14][15][16]:…”