To evaluate and parametrize transport models for the vadose (partially water-unsaturated) zone, information about the spatial distributions of solutes is needed. We describe a technique for the simultaneous imaging of several fluorescent tracers in structured field soils. With this technique, we obtain information on local mixing under field conditions. Local dispersion is a decisive process that discriminates different flow regimes. The imaging device consists of a high-power xenon lamp and a sensitive charge coupled device (CCD) camera. The three fluorescent dyes Brilliant sulfaflavine (BF), Sulforhodamine B (SB), and Oxazine 170 (OX) were chosen as solute tracers for their spectroscopic properties and different sorption coefficients. We conducted a field experiment using these tracers and took images of their distribution in a vertical soil profile. The fluorescence images (1242 by 1152 pixels) were corrected for nonuniform lighting, changing surface roughness, and varying optical properties of the soil profile. The resulting two-dimensional relative concentration distributions were similar for BF and SB. The reason might be the fast transport regime, which prevents the establishment of sorption equilibria. According to its higher sorption coefficient, OX was more strongly retarded. In this paper, we show that the fluorescence imaging technique is a powerful tool for the in-situ investigation of transport processes of fluorescent solute tracers in soil profiles. Due to the high spatial resolution of the tracer concentration maps and the ability to detect the flow field characteristics of differently reactive tracers simultaneously under field conditions, this technique provides valuable experimental data for the test and development of theoretical models for heterogeneous solute transport in soils.

New methods are needed to quantify infiltration into frozen soil, an important issue for agricultural management in northern latitude regions. A dye tracer method that uses digital image analysis and fluorescence imaging is presented for visualizing and quantifying flow pathways in frozen soils. The method was applied to three soil columns in a cold chamber. Two of them were packed with sand and the third was an undisturbed soil monolith. After complete freezing to −5°C, the columns were irrigated at above‐freezing temperatures before they were vertically and horizontally sectioned to analyze pictures of the stained profiles and cross‐sections. Image analysis was done for either the visual or fluorescent spectral ranges of three different tracers. Small samples were taken from the profiles to calibrate the dye tracer concentration. This was achieved by means of second‐order polynomials of the R, G, and B values from the corresponding areas of the pictures with coefficients of determination of 0.92 to 0.99. This method results in concentration maps with a high spatial resolution reflecting the infiltration pattern. The experiment confirmed current hypotheses of infiltration mechanisms into frozen soil in that the infiltrability of the initially wet sand was restricted, whereas in the undisturbed soil monolith, the dye solution infiltrated through preferential pathways which were air filled at the time of freezing.

Dye tracers can be used to visualize flow paths in soils. We used digital image analysis to determine the spatial distribution of dye concentrations in a quartz sand column. Color slides and near‐infrared black‐and‐white photographs were taken from a sand column that was cut in halves longitudinally after a point‐source dye infiltration experiment. In a separate calibration experiment, sand samples were prepared with known water content and dye concentration. Photographs of such sample arrays were used as training areas for image analysis. Image analysis is a viable way for obtaining a high spatial resolution of tracer concentrations and provides a quantitative basis for describing different solute mixing regimes.

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