Longitudinal and lateral dispersion in an unsaturated field soil

Abstract

Lateral and longitudinal dispersion was quantified in a field soil under water-unsaturated conditions. The relatively mobile dye tracer Brilliant Blue FCF was applied as a line source and leached into the soil at two different rates of infiltration, 4 and 24 mm d-1, respectively. The resulting tracer plume was photographically recorded at vertical soil profiles excavated perpendicularly to the line source after ~50, 100, and 200 mm of cumulative infiltration. An image analysis technique was used to determine two-dimensional concentration distributions from the photographs. Average horizontal and vertical concentration distributions were analyzed using the two-dimensional advection-dispersion equation. Model parameters were fitted to optimize the agreement between measured and modeled averaged concentration profiles in both horizontal as well as vertical directions. Dispersivities showed a dependency on flow rates and amount of cumulative infiltration, but this dependency appeared to be related to the degree of irregularities of observed flow patterns. Large dispersivities were associated with higher degree of irregularities in the flow patterns and vice versa. Layer boundaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large. This study demonstrates (1) that more than just the vertical concentration profiles are needed to define the transport regime under unsaturated conditions, and (2) that even subtle layer boundaries affect the lateral mixing regime and exert a marked influence on the transport in the main flow direction.