Fluorescence Imaging of Tracer Distributions in Soil Profiles

Abstract

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.

Overview of inert tracer experiments in key belgian soil types: Relation between transport and soil morphological and hydraulic properties

“…Given the sizes of samples on which soil hydraulic properties are determined, it is questionable whether these small-scale velocity variations can be inferred indirectly from the spatial variability of measured soil hydraulic properties. However, using newly developed techniques, distributions of fluorescent tracer concentrations can be obtained with a very high spatial resolution (down to 1 mm 2) [Aeby et al, 2001]. …”

Sorption of Uranine on Forest Soils

“…Recently, concentrations of dyes have been evaluated to elucidate subsurface flux (Aeby et al 1997, Forrer et al 2000, in addition to the more traditional assessment of stained and unstained portions of soil profiles. In the past decade fluorescent dyes were also proposed as staining tracers and methods for quantifying tracer concentrations (Aeby et al 2001, Vanderborght et al 2002, Duwig et al 2008. While such techniques require more complicated equipment, they provide better possibilities to quantify vadose zone preferential flow, especially in dark (Duwig et al 2008) and heterogeneous soils where Brilliant Blue is difficult to detect or that would require very complex calibrations.…”

Investigación del flujo y transporte mediante experimentación a escala intermedia

“…Es de destacar que, en los extremos del tanque, para lograr una dispersión relativamente uniforme del trazador inyectado en el medio, se suele utilizar una capa corta de grava de sílice o arena muy gruesa (#8 Tyler o superior, que corresponde a diámetros de 2,5 a 3 mm). A fin de facilitar el muestreo automático al utilizar trazadores fluorescentes, y determinación de concentración mediante proceso de imágenes, se procura que el color de fondo de las diferentes arenas sea similar, aunque Aeby et al (2001) desarrollaron unos coeficientes de corrección para evitar este efecto en la imagen final.…”

“…Su principal ventaja es que resulta un método sencillo y barato, que permite realizar un seguimiento casi continuo de la evolución del trazador en el medio. (Aeby et al, 2001).…”

“…A pesar de que este procedimiento proporciona gran cantidad de información tanto en el espacio como en el tiempo, presenta dificultades respecto a la exactitud con la que se puede relacionar la intensidad del color con la concentración del soluto, especialmente en sedimentos reales en los que el color del fondo no es uniforme. Aeby y Schultze (2001) proporcionan un método para evitar estas dificultades en la práctica a través de la imagen digital. Este método incluye la toma de imáge-nes en la longitud de onda correspondiente a la de resonancia de un trazador fluorescente, y restando de ella (gracias al uso de imagen digital) la imagen del tanque sin trazador en el medio (imagen "blanco").…”