Transport Characteristics of Tritium in Unsaturated Loess of Semi-arid Area Under Natural Condition

Through analyses of the field tracer migration test results of tritium in unsaturated loess located in semi arid area, it is found that the longitudinal concentration distributions of 3H are symmetry at higher relative concentration and non symmetry at lower relative concentration. From processing of the migration test data the lateral concentration distribution variances ( σ 2 y ) of 3H are obtained. σ 2 y is 68.4 cm 2 (in pit A, at t =780 d and depth of 76 cm) and 138.5 cm 2(in pit B, at t =779 d and depth of 110 cm), respectively. The lateral diffusion parameter is about one third of the longitudinal diffusion parameter. It is found from analyzing faster component of diffusion of 3H that the changes of both the faster component fractions, f(t,U q), and the relative velocities, U q, with time in pit B are not significant. Under the prerequisite, the concentrations calculated by faster component at point concerned are larger than that by conventional calculation under some conditions. It shows that it is not conservative to use the velocity of the center of mass as representative to conduct conventional calculation under some conditions.