Quantifying fluorescent tracer distribution in allophanic soils to image solute transport
Résumé
The accurate prediction of solute transport through soils is a necessity to counter the worldwide degradation of aquifers. Dye tracers are widely used to visualize active flow paths in cross-sections of soil, but methods previously proposed to map concentrations have been very costly, demanding, or of coarse resolution and not always applicable in dark allophanic soils. We have developed a cheap and fairly easy experimental procedure and used multiple regression to map dye concentrations in two dimensions. We tested the method using the fluorescent dye, pyranine, in intact cores of an allophanic soil. The method requires a calibration step, which we made using eight dye concentrations. The main difficulty was to mix the soil homogeneously with the dye and to pack it evenly before acquisition of the images. The pyranine was infiltrated in soil cores under unsaturated conditions: its distribution on the vertical core faces was highly heterogeneous with fingered penetration. The maps of dye concentration obtained from each core section achieved fine spatial resolution (e.g. 0.25 mm2 per pixel) and satisfactory dye concentration localization and estimation. We could achieve better spatial resolution by sectioning the soil cores at finer intervals, and estimate the dye concentration more accurately by improving the correction for illumination variations.