Migration of 3H in loess aerated zone under natural rainfall condition

It is one of the common methods to study water movement in geological media using 3H as tracer. Some characteristics of migration of 3H in aerated loess were obtained through analyzing the results of the field tracer migration tests of 3H under natural rainfall condition. The tests were conducted in test pits A and B. Area of each test pit is 2 x 2 m2. The depth of test pits A and B is 30 and 100 cm, respectively. Tracer 3H was mixed with an amount of loess to prepare a tracer source. The tracer source was put at the center of bottom of test pits A and B. The thickness and area of the tracer source layers were 0.7 cm and 1.5 x 1.5 m2, respectively. The tracer source layers were covered by original loess to level surface. Soil cores were periodically collected from test pits A and B from May 1989 to July 1991. The soil cores were cut into samples with thickness of 0.5-1.0 cm. The samples were heated and distilled under vacuum condition in order to separate water from loess. 3H content in the water samples was measured with low-level liquid scintillation analyzer to obtain concentration distributions of 3H in loess. It is found from the fitted results that the positions of the center of mass, X0(t), of concentration distributions of 3H moved down as a whole in loess aerated zone during the tests. Migration velocity of the mass center of the concentration distribution of 3H decreases with depth. During the test down-moving velocities of 3H were 5.4 x 10-2 cm/d (at depth of 30 cm in test pit A) and 1.6 x 10-2 cm/d (at depth of 100 cm in test pit B), respectively. Compared with the results in test pit A, the positions of the mass center of concentration distribution of 3H in test pit B moved up and down obviously. It can be seen from the fitted results that the longitudinal distribution variance of 3H, σx2(t) shows S-shape with sampling time, t. And the longitudinal distribution variance of 3H, σx2(t), can be expressed in different equation for different intervals