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DIVISION S-1-SOIL PHYSICS

Time Domain Reflectometry Measurements of Solute Transport across a Soil Layer Boundary

^a Environmental Engineering Lab., Dep. of Civil Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b Room 50, Murray Building, 3 Campus Drive, Univ. of Saskatchewan, Saskatoon, SK, Canada S7N 5A4

i5hn{at}civil.auc.dk

The mechanisms governing solute transport through layered soil are not fully understood. Solute transport at, above, and beyond the interface between two soil layers during quasi-steady-state soil water movement was investigated using time domain reflectometry (TDR). A 0.26-m sandy loam layer was packed on top of a 1.35-m fine sand layer in a soil column (0.15-m i.d.). Soil water content () and bulk soil electrical conductivity (EC_b) were measured by 50 horizontal and 2 vertical TDR probes. A new TDR calibration method that gives a detailed relationship between apparent relative dielectric permittivity (K_a) and was applied. Two replicate solute transport experiments were conducted adding a conservative tracer (KCl) to the surface as a short pulse. The convective lognormal transfer function model (CLT) was fitted to the TDR-measured time integral–normalized resident concentration breakthrough curves (BTCs). The BTCs and the average solute-transport velocities showed preferential flow occurred across the layer boundary. A nonlinear decrease in TDR-measured in the upper soil toward the soil layer boundary suggests the existence of a 0.10-m zone where water is confined towards fingered flow, creating lateral variations in the area-averaged water flux above the layer boundary. A comparison of the time integral–normalized flux concentration measured by vertical and horizontal TDR probes at the layer boundary also indicates a nonuniform solute transport. The solute dispersivity remained constant in the upper soil layer, but increased nonlinearly (and further down, linearly) with depth in the lower layer, implying convective-dispersive solute transport in the upper soil, a transition zone just below the boundary, and stochastic–convective solute transport in the remaining part of the lower soil.

Abbreviations: BTC, breakthrough curve • CD, convective–dispersive • CDE, convective–dispersion equation model • CLT, convective lognormal transfer function model • DC, direct current • EC_b, bulk soil electrical conductivity • EC_w, electrical conductivity in the soil solution • LSO, least squares optimization • pdf, probability density function • SC, stochastic–convective • TDR, time domain reflectometry

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