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Effect of Particle-Scale Heterogeneity on Uranium(VI) Transport in Unsaturated Porous Media

^a Pacific Northwest National Lab., 902 Battelle Blvd., P.O Box 999, Richland, WA 99354
^b Formerly of Dep. of Soil Water and Environmental Science, 429 Shantz Building no. 38, 1200 E. South Campus Dr., P.O. Box 210038, Univ. of Arizona, Tucson, AZ 85721-0038
^c Savannah River National Lab., Building 773-43A, Room 215, Aiken, SC 29808

View larger version (16K): [in this window] [in a new window]   FIG. 1. Sorption isotherms for U on separate silt loam (S/L) and coarse sand sediments [S/L(0.0)], and mixtures of the S/L and coarse sand sediments, S/L(0.7) and S/L(0.3), where the mass fraction of S/L is indicated in parentheses, se is the sorbed U equilibrium concentration, and ce is the aqueous U concentration.

View larger version (13K): [in this window] [in a new window]   FIG. 2. Conservative tracer transport in a mixture of 10% silt loam (S/L) and 90% coarse sand sediments at 20% saturation and an average pore-water velocity of 20 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. The solid line represents a curve fit with the two-region model, where the retardation factor R was fixed at 0.71.

View larger version (15K): [in this window] [in a new window]   FIG. 3. Uranium transport in a mixture of 10% silt loam (S/L) and 90% coarse sand sediments at 24% saturation and an average pore velocity of 1.6 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. A semi-independent prediction is shown for the two-region model where effective retardation Ref = 17.1 and the apparent equilibrium distribution coefficient Kd-ap = 0.96.

View larger version (16K): [in this window] [in a new window]   FIG. 4. Uranium transport in a mixture of 10% silt loam (S/L) and 90% coarse sand sediments at 18% saturation and an average pore velocity of 2 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. An independent prediction is shown for the two-region model where effective retardation Ref = 23.95 and the apparent equilibrium distribution coefficient Kd-ap = 0.96.

View larger version (14K): [in this window] [in a new window]   FIG. 5. Uranium transport in a mixture of 30% silt loam (S/L) and 70% coarse sand sediments at 32% saturation and an average pore velocity of 2 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. A semi-independent prediction is shown for the two-region model where effective retardation Ref = 39.0 and the apparent equilibrium distribution coefficient Kd-ap = 2.52.

View larger version (10K): [in this window] [in a new window]   FIG. 6. Uranium transport in a mixture of 27% silt loam (S/L) and 73% coarse sand sediments at 31% saturation and an average pore velocity of 2 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. An independent prediction is shown for the two-region model where effective retardation Ref = 42.60 and the apparent equilibrium distribution coefficient Kd-ap = 2.34.

View larger version (18K): [in this window] [in a new window]   FIG. 7. Uranium transport in a mixture of 70% silt loam (S/L) and 30% coarse sand sediments at 40% saturation and an average pore velocity of 2 cm h–1. The experimental breakthrough curve is represented by the open symbols; c/co is the effluent concentration. An independent prediction is shown for the two-region model where effective retardation Ref = 53.1 and the equilibrium distribution coefficient Kd = 4.77.