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

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 (θ) and bulk soil electrical conductivity (ECb) 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 (Ka) and θ 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 θ 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.