Longitudinal and lateral dispersion in an unsaturated field soil

Lateral and longitudinal dispersion was quantified in a field soil under wa ter-unsaturated conditions. The relatively mobile dye tracer Brilliant Blue FCF was applied as a line source and leached into the soil at two differen t rates of infiltration, 4 and 24 mm d(-1), respectively. The resulting tra cer plume was photographically recorded at vertical soil profiles excavated perpendicularly to the line source after similar to 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 fitte d to optimize the agreement between measured and modeled averaged concentra tion profiles in both horizontal as well as vertical directions. Dispersivi ties showed a dependency on flow rates and amount of cumulative infiltratio n, but this dependency appeared to be related to the degree of irregulariti es of observed flow patterns. Large dispersivities were associated with hig her degree of irregularities in the flow patterns and vice versa. Layer bou ndaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large. This study demonstrates (1) th at more than just the vertical concentration profiles are needed to define the transport regime under unsaturated condition's, and (2) that even subtl e layer boundaries affect the lateral mixing regime and exert a marked infl uence on the transport in the main flow direction.