Identification of transport processes in soil cores using fluorescent tracers

To identify soil properties that control transport of adsorbing solutes in natural soil, we carried out leaching experiments in undisturbed soil cores taken from three soil layers of a Stagni-Humic Cambisol. Breakthrough curves (BTCs) of Cl- and two adsorbing fluorescent dye tracers, brilliant sulfaflavine (BF; 1H-Benz(de)isoquinoline-5-sulfonic acid, 2,3-dihydro-6-aniino-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), were measured. Three cores were scanned with x-rays to determine the three-dimensional (3-D) structure of large pores. After the leaching experiment, soil cores were horizontally sliced and dye concentration distributions on cross sections were derived from fluorescence signal images. Transport was investigated using BTCs and concentration maps, adsorption isotherms., and predictions by three different transport models: convection dispersion model (CDM), stream tube model (STM) and physical nonequilibrium model (PNEM). The dense network of large pores in the two upper soil layers induced a uniform lateral spreading of dyes and the CDM described the transport fairly well. In cores from the deeper layer, the large pore network was considerably less dense and dye patterns followed closely the few large pores without lateral mixing indicating preferential flow and explaining the fast dye breakthrough. Predictions by the STM revealed that the fast SB breakthrough could not be explained solely by preferential flow. Fitting the PNEM to breakthrough data and the low total dye concentration in the preferential flow region suggested a small sorption capacity of the preferential flow region for SB. Therefore, preferential leaching of dyes resulted from small-scale variations in physical and chemical soil properties.