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Hydrodynamic Dispersion in an Unsaturated Dune Sand

^a Dep. of Agricultural Sciences, Saga Univ., Saga 840-8502, Japan
^b Arid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680-0001, Japan
^c George E. Brown Jr. Salinity Laboratory, 450 West Big Springs Road, Riverside, CA 92507

View larger version (16K): [in a new window]   Fig. 1. Water retention curve of Tottori dune sand for drying from saturation. Fitted line is the van Genuchten retention function, ( - r)/(s - r) = [1 + (|h|)n]-1+1/n, with s = 0.35, r = 0.05, = 0.05, and n = 5.2.

View larger version (24K): [in a new window]   Fig. 2. Experimental setup for unsaturated displacement experiments.

View larger version (16K): [in a new window]   Fig. 3. Illustration of volumetric water content, , and soil water pressure, h, profiles during unit-gradient flow.

View larger version (14K): [in a new window]   Fig. 4. Volumetric water content, , as a function of log-scale unsaturated pore-water velocity, v, during unit-gradient flow.

View larger version (40K): [in a new window]   Fig. 5. Observed and fitted breakthrough curves at different depths: (a) saturated flow ( = 0.34), (b) unsaturated flow ( = 0.11).

View larger version (16K): [in a new window]   Fig. 6. Dispersivity, , as a function of log-scaled pore-water velocity, v, for saturated and unsaturated flow conditions.

View larger version (17K): [in a new window]   Fig. 7. Dispersivity, , as a function of volumetric water content, , for saturated and unsaturated flow conditions. Data from Padilla et al. (1999) for an unsaturated sand with an average particle size of 0.25 mm are also plotted.

View larger version (17K): [in a new window]   Fig. 8. Comparison between the effective dispersivity based on MIM parameters, MIM, according to Eq. [15], and for the CDE for unsaturated flow as a function of volumetric water content, . Dispersivities in the mobile phase, m, for unsaturated conditions are also plotted.

View larger version (18K): [in a new window]   Fig. 9. Mobile and immobile fraction, m/ and im/, as a function of volumetric water content, , for unsaturated flow conditions. Note that m/ + im/ = 1. Solid line was fitted by eye based on our measurements and dotted line corresponds to m/ = 0.853 as reported by De Smedt and Wierenga (1984) for unsaturated glass beads.

View larger version (12K): [in a new window]   Fig. 10. Time scale for exchange between the mobile and immobile phases, 1/, and the Peclet number of molecular diffusion, Pe, as a function of volumetric water content, , for unsaturated flow conditions.

View larger version (47K): [in a new window]   Fig. 11. Schematic illustration of flow domains in a nonaggregated sand for different ranges in volumetric water content, . For > 0.15: relatively homogeneous flow with rapid convective solute mixing in water-filled soil pores occurs. For < 0.15: slow solute mixing primarily by transverse diffusion in water film enveloping the soil particles.