Solute transport through large uniform and layered soil columns

Abstract

Solute transport experiments are often conducted with homogeneous soils, whereas transport in real situations takes place in heterogeneous soils. An experiment was conducted to compare unsaturated solute transport through uniform and layered soils. Pulse inputs of tritiated water, bromide and chloride were applied under steady flow conditions to the tops of two large (0.95 m diameter by 6 m deep) soil columns. One column was uniformly filled with loamy fine sand and the other filled with alternating 20‐cm‐thick layers of loamy fine sand and silty clay loam. Soil solution samples were collected during the experiment with suction candles installed at various depths in the columns. Solute transport parameters were estimated by fitting the convection‐dispersion equation to the observed breakthrough curves for each solute at various depths in each column. The match between the resulting calibrated curves and the experiment was better for the layered soil column than for the uniform soil column. The results displayed no clear relationship between the dispersion coefficients and depth for any of the tracers for either column. However, dispersivities were greater in the uniform column (3.5 cm) than in the layered column (1.2 cm), while retardation factors for bromide and chloride were similar (0.8 and 0.83, respectively, for the uniform and layered columns). A retardation factor less than one is attributed to anion exclusion. There was evidence of preferential flow in the uniform soil column. The peak concentrations at 5 m depth were greater than those observed at 4 m. Such behavior is inconsistent with one‐dimensional flow. Similar results were observed in an experiment performed 3.5 years earlier using the same soil column and approximately the same flow rates, but using a different tracer and associated chemical analysis, different soil saturation prior to the execution of the experiment, and different experimental personnel. This supports the thesis that the anomalous behavior is due to some form of preferential flow rather than due to experimental error.

Non‐Fickian transport in homogeneous unsaturated repacked sand

“…Plotting dispersivity (Table 2) as a function of column length (Figure 3) it is seen that dispersivity increases linearly with travel distance. While scale dependence has been observed for intact and heterogeneous columns [Khan and Jury, 1990;Bejat et al, 2000], repacked columns generally do not show scale dependence [Khan and Jury, 1990;Shukla et al, 2003], with exceptions observed by Huang et al [1995] and Porro et al [1993]. The dispersivity values are generally expected to be of the order of the mean grain size [Bear, 1972].…”

The contribution of old and new water to a storm hydrograph determined by tracer addition to a whole catchment

“…Scotter and Tillman, 1991;Porro et al, 1993;Jabro et al, 1994) there may be potential for the ion to become sorbed to organic soil or bound or electrostatically attracted at the mineral surface (Lange et al, 1996). As a consequence, the sorption strength of the Br À ion to soil, typical of that found in the study catchment, was assessed prior to undertaking the tracer experiments.…”

Time and space nonlocalities underlying fractional-derivative models: Distinction and literature review of field applications

“…Anomalous transport (despite the somewhat misleading nomenclature) is very often found at all scales -from pore scale simulations [144], soil cores [24,33,92,116], laboratory experiments [70,79,119,133], to fieldscale observations (e.g., [2,[11][12][13]45,65,83,87,143], among many others). Anomalous transport may be characterized by non-Gaussian leading or trailing edges (also called heavy tails) of a plume emanating from a point source, or nonlinear growth of the centered second moment.…”