Flow path visualization in a lignitic mine soil using iodine–starch staining

A dye tracer visualization experiment was carried out in order to study the importance of preferential flow at forest-reclaimed mine soils. The objective was to evaluate iodine–starch staining for the identification of flow paths in such soils, for the commonly used dye tracer Brilliant Blue showed a poor contrast to the relatively dark lignitic mine soil. The experimental site was located in the Lusatian mining district, northeast Germany, and afforested with Pinus nigra. At the experimental plot area of 250×130 cm, 60 mm of iodide solution was applied at a rate of 7.5 mm/h. Flow path visualization was conducted at 10-cm increments down to 150 cm soil depth. The soil surface was powdered with starch and sprayed with a bleaching agent. Iodine–starch staining indicated the tracer conducting soil regions. Stained soil sections were documented by photography as well as by manual tracing on transparent window films. At each depth increment, 25 core samples were taken at a regular grid of 25×25 cm. For each core sample, the amount of residual iodide was determined. According to the visualized flow paths, the iodide solution infiltrated into the mine soil at distinct points. Beneath a redistribution zone at 10 cm, flow paths occurred as ‘fingers’, which constricted the hydraulically active soil volume to about 15% and persisted down to a depth of 60 cm, at one point to 150 cm. Only a part of the iodine–starch stainings spatially coincided with the vertical iodide concentration. For a closer relationship, the analysis of the complete soil volume seems to be necessary. Iodine–starch staining proved to be suitable for flow path visualization at dark lignitic mine soils under field conditions and seems to be feasible also for other dark soils. At lignitic forest-reclaimed mine soils, fingered flow has to be considered in water and solute budgets.