Fluorescence imaging of tracer distributions in soil profiles

To evaluate and parametrize transport models for the vadose (partially wate r-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 obt ain 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 d evice (CCO) 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 d istribution in a vertical soil profile. The fluorescence images (1242 by 11 52 pixels) were corrected for nonuniform lighting, changing surface roughne ss, and varying optical properties of the sail profile. The resulting two-d imensional relative concentration distributions were similar for BF and SB. The reason might be the fast transport regime, which prevents the establis hment of sorption equilibria. According to its higher sorption coefficient, OX was more strongly retarded. In this paper, we show that the fluorescenc e imaging technique is a powerful tool for the in-situ investigation of tra nsport processes of fluorescent solute tracers in soil profiles. Due to the high spatial resolution of the tracer concentration maps and the ability t o detect the flow field characteristics of differently reactive tracers sim ultaneously under field conditions, this technique provides valuable experi mental data for the test and development of theoretical models for heteroge neous solute transport in soils.