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Preferential Transport of Solute through Soil Columns Containing Constructed Macropores

Environmental Services, Chicago, IL 60606

Masoud Ghodrati^*

Dep. of Environment Science, Policy, and Management, Division of Ecosystem Sciences, Univ. of California, Berkeley, CA 94720

*Corresponding author (ghodrati{at}nature.berkeley.edu).

ABSTRACT

Implication of macropore flow in enhancing solute transport has prompted much research. The objective of this study was to characterize solute transport in constructed macropores in three soils of different water conductivities. The macropores were constructed by inserting metal rods into uniformly packed soil columns of 0.2-m i.d. and 0.3 m long. A set of nine soil columns was prepared and a total of 52 displacement experiments was conducted using NO₃ as a tracer. Significant preferential flow occurred in the silt loam containing seven continuous macropores of 3-mm diam. even at fluxes <0.5K, where K is saturated hydraulic conductivity of a column matrix. At fluxes far smaller than those causing saturation, no preferential flow took place and the breakthrough curves (BTCs) displayed single peaks, indicating that only the matrix contributed to solute transport. As fluxes increased, BTCs exhibited double peaks, and the relative magnitudes and positions of the two peaks were flux dependent. These two-curve or double-peak BTCs may represent solute transport through the macropores and the matrix, respectively. Due to the difficulty in keeping macropores open in silica sand, especially at fluxes K, little evidence of preferential flow was observed in this system. This was also ascribed to the porous and conductive nature of the medium. Preferential flow also occurred in the loamy sand, but a greater flux was needed to get the same degree of preferential flow as in the silt loam. In general, in the coarser soil, less preferential flow occurred even at similar normalized flow rates. Macropore continuity has been shown to be increasingly important in macropore flow when flux increases.

Received for publication June 5, 1996.

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