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.

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To investigate transport processes in soils, detailed information about the spatial distribution of solutes is required. We describe a method to obtain concentration maps of fluorescent tracers on cross sections of soil cores with a high spatial resolution. The fluorescence signal of two dye tracers, brilliant sulfaflavine (BF; 1H‐Benz(de)isoquinoline‐5‐sulfonic acid, 2,3‐dihydro‐6‐amino‐1,3‐dioxo‐2‐(p‐tolyl)‐, monosodium salt) and sulforhodamine B (SB; xanthylium, 3,6‐bis(diethylamino)‐9‐(2,4‐disulfophenyl)‐, inner salt, sodium salt), was imaged on the exposed cross sections. The fluorescence signal was corrected for variable illumination light intensity and optical properties of the soil across the exposed surface. Correction factors for varying optical soil properties were derived from the image of the reflected excitation light at the exposed surface. Linear calibration relations related the corrected fluorescence image to the total tracer concentration (Ct) map, that is, mass of dye dissolved in the soil solution and sorbed to the soil particles per unit volume bulk soil. Corrections for varying optical properties of the soil surface were important to reduce the uncertainty of the concentration that was estimated from the fluorescence signal. For BF, the calibration relations were different for different soil materials and a soil specific calibration had to be used. Variations in background fluorescence were an important source of uncertainty of the BF concentration estimates but can be overcome by applying higher concentrations. For SB, variations in calibration relations and in the background fluorescence were considerably smaller, and so is the uncertainty of the estimated SB concentrations.

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