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DIVISION S-1-SOIL PHYSICS

Small-Scale Variability in Solute Transport Processes in a Homogeneous Clay Loam Soil

^a Division of Ecosystem Sciences, Dep. of Environmental Science, Policy and Management, Univ. of California, Berkeley, CA 94720-3110 USA

ghodrati{at}nature.berkeley.edu

Small-scale variations in transport parameters may have a profound influence on larger scale flow processes. Fiber-optic miniprobes (FOMPs) provide the opportunity to continuously measure solute resident concentration in small soil volumes. A 20-channel multiplexed-FOMP system was used in repeated miscible displacements in a repacked clay loam soil column (20 cm long and 10-cm diam.) to examine small-scale, point-to-point variability in convective–dispersive transport processes. Transport parameters, measured 10 cm below the surface, were compared at two drip irrigation point densities and two fluxes. Irrigation densities of one irrigation drip point per 4 cm² and 11 cm² of column surface area produced similar results. The breakthrough curves measured at 0.10 cm h^-1 had a larger immobile phase than at a flux of 1.07 cm h^-1. In the clay loam soil the mobile–immobile model fit the breakthrough curves better than the convective–dispersive equation (CDE), with r² values of 99.6 and 97.1, respectively. This analysis demonstrated that dispersion and mass recovery were much more variable than pore water velocity in this repacked clay loam soil. However, even in the most variable transport conditions encountered, only 17 sampling points were necessary to describe the column average transport parameters within 20% of the mean.

Abbreviations: BTC, breakthrough curve • CDE, convective–dispersive equation • CV, coefficient of variation • FOMP, fiber-optic miniprobe • MIM, mobile–immobile water model • TDR, time domain reflectometry

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