Mapping the 3D water dynamics in heterogeneous sands using thermal neutrons

The transport of dissolved pollutants and nutrients in soils depends on the spatial and temporal distribution of soil water. The water flow is strongly affected by the presence of air and the heterogeneity of the soil structure. To investigate the influence of structures and air on the soil water dynamics, the distribution of water in a sand column was tomographed using thermal neutrons. The experiment was conducted using a column filled with 5 × 5 × 6 sand cubes of 1 cm size. Each sand cube was filled with coarse or fine sand material. The volume fraction of the cubes filled with coarse sand was 33%. The cubes were arranged randomly. In the sample realization, a continuous path between top and bottom of the column within the coarse material existed. The continuity of this sand structure is expected to affect the water dynamics. At the beginning of the experiment, the pore space in the column was filled with water. Then, the column was drained by the application of a continuously changing suction force at the bottom of the column. After an equilibration period, the suction was released and water flowed back in the column. The water content in the sand cubes was mapped in intervals of one and a half minutes. A total tomography of the column was carried out in 53 s using an optimized flat panel. To increase the sensitivity of the thermal neutron tomography measurements, a mixture of normal water and heavy water was used. The drainage of the coarse sand was much faster than for the fine sand because the capillary forces are smaller and the hydraulic conductivity is high.