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Identification of Transport Processes in Soil Cores Using Fluorescent Tracers

Jan Vanderborght^*,a, Paul Gähwiller^,b and Hannes Flühler^b

^a Laboratory of Soil and Water, Katholieke Universiteit Leuven, Vital Decosterstraat 102, B-3000 Leuven, Belgium
^b Soil Physics, Inst. of Terrestrial Ecology, Swiss Federal Institute of Technology, ETHZ, Grabenstrasse 11a, CH-8952 Schlieren, Switzerland

View larger version (23K): [in a new window]   Fig. 1. Adsorption isotherms of brilliant sulfaflavine (BF) and sulforhodamine B (SB) of soil from the top (0- to 0.1-m layer) and subsoil (0.5- to 0.6-m layer). C, solute concentration in the soil solution; C0.5, dissolved equilibrium concentration at 50% of Smax; Kd, linear distribution coefficient. S, sorbed dye mass per mass of dry soil; and Smax, sorbed dye mass per mass of dry soil at maximum sorption.

View larger version (26K): [in a new window]   Fig. 2. Breakthrough curves of Cl-, brilliant sulfaflavine (BF), and sulforhodamine B (SB) in different soil core (# refers to soil core number) layers. Normalized concentrations, c = C/C0, in the outflow from soil cores are plotted vs. cumulative drainage. C is the concentration in the effluent; C0 is the input concentration. Lines are breakthrough curves predicted by the convection dispersion model that is calibrated to the Cl- breakthrough.

View larger version (34K): [in a new window]   Fig. 3. Depth profiles of normalized total concentrations, ct, of brilliant sulfaflavine (BF) and sulforhodamine B (SB) in soil cores (# refers to soil core number). Total concentrations, mass of dissolved and adsorbed dye per unit volume of bulk soil are normalized vs. the expected total dye concentration when the initial soil solution is completely replaced by the applied solution and in equilibrium with the adsorbed dye concentration (Eq. [10]). Lines are depth profiles predicted by the convection dispersion model that is calibrated to the Cl- breakthrough. The amount of water that drained from the cores since the start of the leaching experiment is given in parentheses.

View larger version (111K): [in a new window]   Fig. 4. Three dimensional reconstructs of the sulforhodamine B 20 mg L-1 total concentration isosurfaces and that of the large pores or less dense regions in two soil cores from the 0.5- to 0.6-m layer and one from the 0.2- to 0.3-m layer. The concentration isosurfaces were derived from concentration maps on horizontal core cross sections and the large pores or less dense regions from binarized x-ray computer tomography (CT) images.

View larger version (44K): [in a new window]   Fig. 5. (a) Original grey value x-ray computer tomography image (darker values correspond to less dense regions) of a horizontal core cross section (Core 1 of the 0.5- to 0.6-m layer), (b) binarized and opened x-ray CT image, (c) overlay of concentration map and binarized CT image (same cross section) from which identified unstained large pores are removed, and (d) map of distances to the nearest large pore (darker values correspond with larger distances). (e–h): same as (a–d) for a horizontal cross section in Core 1 from the 0.2- to 0.3-m layer.

View larger version (26K): [in a new window]   Fig. 6. (a) Average distribution of pixel distances from a large pore in a cross section. (b) Average normalized total dye concentration profile as a function of the distance from a large pore. Averages are taken of cross sections in a soil core with similar dye stained area. Dye concentrations are normalized by the average total concentration in the large pores.

View larger version (44K): [in a new window]   Fig. 7. Concentration maps of total brilliant sulfaflavine (BF) concentrations (mass of dissolved and adsorbed dye per unit volume of bulk soil) in core cross sections from the 0.5- to 0.6-m layer and from the 0.2- to 0.3-m layer. Heights of the cross sections from the bottom of the soil cores are indicated. The triangles on the concentration scale indicate the expected total tracer concentration when the initial soil solution is completely replaced by the applied solution and in equilibrium with the adsorbed dye concentration. Closed triangle is for the 0.2- to 0.3-m layer and open triangle for the 0.5- to 0.6-m layer.

View larger version (41K): [in a new window]   Fig. 8. Concentration maps of total sulforhodamine B (SB) concentrations (mass of dissolved and adsorbed dye per unit volume of bulk soil) in core cross sections from the 0.5- to 0.6-m layer and from the 0.2- to 0.3-m layer. Heights of the cross sections from the bottom of the soil cores are indicated. The triangles on the concentration scale indicate the expected total tracer concentration when the initial soil solution is completely replaced by the applied solution and in equilibrium with the adsorbed dye concentration. Closed triangle is for the 0.2- to 0.3-m layer and open triangle for the 0.5- to 0.6-m layer.

View larger version (24K): [in a new window]   Fig. 9. Predictions of the breakthrough of normalized brilliant sulfaflavine (BF) and sulforhodamine B (SB) concentrations, c = C/C0 (C is the concentration in the effluent; C0 is the input concentration) in the outflow from soil cores (soil core number is given in parentheses) using a stream tube model that is calibrated to the Cl- breakthrough.

View larger version (23K): [in a new window]   Fig. 10. Fits of the physical nonequilibrium model (PNEM) to the breakthrough of Cl-, brilliant sulfaflavine (BF), and sulforhodamine B (SB) concentrations (C) in the outflow from soil cores from the 0.5- to 0.6-m layer. Full lines are PNEM fits for the large m parameter set and dashed lines for the small m parameter set (see text and Table 4). The bottom right plot gives the outflow rate as a function of time in Core 2.

View larger version (30K): [in a new window]   Fig. 11. Mass fraction of brilliant sulfaflavine and sulforhodamine B on a horizontal core cross section plotted vs. the minimal fraction of the total area in which the mass fraction is contained. Smooth lines are mass fraction vs. area fraction plots that are derived from concentration maps on core cross sections (55–15 mm above the bottom of the soil cores, with gray values decreasing with height of the cross section). Discontinuous lines are mass fraction vs. area fraction plots at 45 mm above the bottom of the soil core predicted by the physical nonequilibrium model (PNEM). Full lines are PNEM predictions for the large m parameter set and dashed lines for the small m parameter set (see text and Table 4).