Characterization of field tracer transport using high-resolution images

Flow and transport in soils and groundwater are greatly affected by subsurface heterogeneity. We present results from infiltration experiments on four heterogeneous field plots (Orthic Luvisol) for plowed and nonplowed conditions. A 2-mm pulse of Br⁻ was applied, followed by a 40-mm application of a 5 g L-1 solution of the food dye Brilliant Blue FCF (Color Index 42090) in a 6-h period. Horizontal cross sections were photographed at 0.05- and 0.10-m depth intervals, representing the A(p) and B(t) horizons, respectively, either immediately or 90 d after the tracer application. High-resolution spatial maps of Brilliant Blue concentration were derived from the scanned photographs using one single calibration relationship between Brilliant Blue concentration and the color spectra for all plots and depths. No significant or consistent directional dependence was observed in the spatial correlation structure of the dye concentration for the horizontal cross sections. However, the integral scale showed a distinct depth dependency, partially caused by horizonation, with a larger value in the A(p) than in the B(t) horizon. Disturbed soil samples were taken at 15 locations for each cross section and analyzed for Br⁻. Although Brilliant Blue was retarded in the soil matrix with respect to Br⁻, both tracer concentrations showed an exponential decay with depth because of preferential flow enhanced by plowing. Only a small fraction of the dye was subjected to fast transport. The plot-scale information of the dye distribution revealed that our 15 sampling locations at each depth were sufficient to identify the averaged plot-scale transport behavior in the soil matrix, but failed to represent the conducting preferential flow pathways.