Noninvasive Observations of Flow Patterns in Locally Heterogeneous Mine Soils using Neutron Radiation

The description of water flow in mine soils is limited by heterogeneities due to the dumping of partly mixed overburden sediments resulting in a soil matrix with embedded lignitic or fine‐textured fragments. Previous attempts assuming a mobile–immobile type dual‐porosity model failed to explain soil core outflow experiments. Our objective in this study was to improve the understanding of flow in mine soils by visualizing flow paths with neutron radiography (NR) and neutron tomography (NT) using H 2 O as tracer. The methodological focus was to develop a setup for intact soil samples. Two‐dimensional NR technique was used to observe steady‐state infiltration in 2‐cm‐thick unsaturated (i.e., −5 and −30 hPa) soil slabs. In a three‐dimensional NT approach, we determined the distributions of infiltrating H 2 O in a soil core using a multistep in‐ and outflow experiment (i.e., at −30, −15, −3, and −30 hPa) with installed tensiometers. The NR time sequences allowed distinguishing millimeter‐scale flow paths in the soil slabs. The NT experiment revealed higher H 2 O contents in the vicinity of fragments; tensiometer data indicated local nonequilibrium in pressure heads between fragment and matrix pore regions. The results suggest that under near‐saturated conditions, water preferably flows around fragments within a more continuous pore network that exists locally in the matrix and that flow patterns depend on the structure of the embedded fragments. The separate consideration of this pore network may improve flow description in soils with comparable heterogeneity. Neutron imaging proved to be useful for complex natural soils as well, although quantification of H 2 O content dynamics was still limited here by residual water and lignitic carbon contents.