Solute Transport in Undisturbed Columns of an Aggregated Tropical Soil: Preferential Flow Effects

Authors:
  • MSM. S. Seyfried
    M. S. Seyfried
  • PRP. S. C. Rao
    P. S. C. Rao
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Abstract
Solute breakthrough curves (BTC) resulting from miscible displacement of 3 H 2 O in undisturbed soil columns under a range of soil‐water tensions were evaluated in terms of the mobile‐immobile (MIM) water model and the convective‐dispersive (CD) model. The BTC performed under tensions >0.1 kPa were approximately symmetric in shape and accurately described by the CD model, whereas BTC performed under tensions of 0 or 0.1 kPa were quite asymmetric and better described by the MIM model. Application of tension resulted in about a 10‐ to 20‐fold decrease in hydraulic conductivity [ K (θ)] with relatively little change in soil‐water content (θ). Highly asymmetric BTC were attributed to bypassing or preferential flow along macropores. Thus, in terms of model selection, bypassing was significant only when soil‐water contents were at or very near saturation. Rhodamine B dye patterns obtained under saturated conditions showed that soil‐water flow (and thus convective transport) was confined to small regions within the columns. However, easily identifiable, discrete channels were not observed in these regions. It appears that flow was conducted via a series of relatively large pores, or continuous pore sequences. Application of tension appears to have disconnected the most rapidly conducting pore sequences, thus reducing the asymmetry in measured BTC and compressing the dye “solute front.” Even so, dye patterns showed that flow was very heterogeneous, as was also evidenced by high dispersion coefficients in the 3 H 2 O experiments. Comparison of the frequency distributions of field‐measured saturated hydraulic conductivity values and measured rainfall intensities indicated that saturated conditions, and hence significant bypassing, are not expected to occur in this soil under field conditions. Observation of water table response to rainfall events supports this conclusion. From these experiments, we conclude that field‐scale models based on the CD model may adequately represent solute movement in soil at this field site.

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