Interrelationship between Breakthrough Curves and X-ray Computed Tomography Analyzed Macropore Characteristics: An Update

The present study aimed to study the role of pore contact of micro-lysimeters in the soil column on tracer transport in relation to the structure, as assessed by X-ray CT under different initial conditions and irrigation. As it is not possible to carry out these investigations under field conditions, non-destructive CT data were used, making it possible for 3-D reconstruction of the structure of the investigated soil columns. Quantitative relationships between soil structure (especially macropore characteristics, namely their size, number, type, distribution and continuity) and soil hydraulic properties are essential for improving our ability to model flow and transport in structured soils. Ten undisturbed soil monoliths of clayey Pelosol at Gottingen, Germany covering the horizons Ah and P were collected. Some columns were left at natural humidity, some were oven-dried to simulate drought situations in forest soils in consequence of climate change. In seven columns, four micro-lysimeters, each, were installed at half height in order to obtain data for analysis of single solute pathways. A fixed amount of KBr tracer was applied to the humus layer. The columns were irrigated with CaCl2. Column output and lysimeter output were collected and analyzed to record breakthrough curves. Bimodal analytical convection dispersion equation (CDE) solutions were fitted for the column outputs using a non-linear least square fit. A simple CDE solution did not fit well. This supports the model of two overlaying transport phenomena. After breakthrough recording was complete, all columns were scanned using X-ray computed tomography (CT). From the CT data 3-D reconstructions of the porous system were created for visual inspection, and the exact pathways for macropores along the micro-lysimeters were determined. Additionally, indices of the pore structure were computed to compare with the slow and fast dispersivity values from the bimodal CDE fit. The three-dimensional reconstruction could be used to explain the difference in micro-lysimeter performance. Following the conclusion of irrigation, no statistically significant variations were found in the pore structure of the wet and dried columns. In general, there is a positive linear relationship between the pore index and slow dispersivity and a negative linear relationship with rapid dispersivity. In topsoil, these relationships are more robust. The CT images and 3-D reconstructions offer a fascinating look into the soil pore system and could be useful in understanding drought issues caused by climate change that are caused by humans.