The Simulation of Uranium Diffusion Behavior in an Aquitard with Different Ionic Strength

Abstract:

We investigate uranium (U) transport behavior in the low-permeability zone with different ion strength (I). A 1-D analytical solution was used to simulate the uranium concentration profile. The low-permeability zone of 0.03 m thickness was considered a finite domain boundary at montmorillonite. Depending on ionic strength (I), the effective diffusion coefficient for the low-permeability zone were assumed to be 1.52 x 10^-8 m²/d (I = 0.01 M NaCl), 5.46 x 10^-8 m²/d (I = 0.1 M NaCl), and 2.35 x 10^-7 m²/d (I = 0.5 M NaCl), respectively. Uranium concentration profiles showed increasing solute penetration distance with time during solute loading. The solute penetration distance was different during the same period due to the difference in the effective diffusion coefficients depending on the ionic strengths. At 700 days, the penetration distance of uranium were 0.03 m at I = 0.5 M, and 0.01 m at I = 0.1 M was approximately three times larger than that of I=0.01 M. The diffusive front reached 0.03 m (the top of the aquitard) with C/C₀ = 0.01 at 0.6 yrs (I = 0.5 M), 6 yrs (I = 0.1 M), 15 yrs(I = 0.01 M). As the ionic strength increases, the higher mass of uranium migrates into the aquitard. In a natural environment, uranium can quickly reach the top of the aquitard, and uranium contamination persistence could become much longer.