Identification of transport processes from concentration patterns and breakthrough curves of fluorescent tracers

Transport of two differently sorbing fluorescent dye tracers, Sulphorhodamine B (SB), and Brilliant Sulfaflavine (BF), was monitored in undisturbed soil cores. Breakthrough curves were measured and concentration patterns of the dyes in the soil cores were derived from fluorescence images of horizontal slices. The adsorption isotherms of both dyes were determined in batch experiments. The effective mobility of the dyes due to adsorption to soil particles was determined by comparing the breakthrough curves of the dyes with the breakthrough of Cl-. The effective mobility was larger than the tracer mobility calculated from adsorption isotherms and was dramatically influenced by preferential transport. Large differences between effective and calculated mobility were reflected in concentration patterns in which the dyes were distributed over only a small fraction of the total soil volume. Inspection of preferential flow and transport models revealed that the larger mobility of dye tracers may be explained by a slower aqueous diffusion coefficient of the dyes than of the Cl- anion and/or by a smaller adsorption of the dyes in the mobile than in the immobile pore region. A comparison between the dye concentrations in a horizontal slice close to the bottom boundary of the soil cores and the concentrations in the outflow and inverse simulations using a physical non-equilibrium transport model, indicated a smaller adsorption of the dyes in the preferential flow regions than in the batch experiments.