Soil water repellency characteristic curve of Spolic Technosols from the region of Maritsa-Iztok coal mine in Bulgaria

Soil water repellency is phenomenon which is caused by compounds of organic origin, type of clay minerals and wetting/drying cycles. Hydrophobicity is usually characterized by a great spatial and temporal variability, especially in mine spoils where different types of overburden materials are mixed with coal particles and other waste products. The aim of this study was to investigate the changes of hydrophobicity with soil moisture content of Heavy Clay textured Spolic Technosols in the region of Maritsa-Iztok open cast coal mine. The sampling was done in a stubble field at 8 points from two soil depths 10–15 cm and 30–35 cm. The samples were analyzed for particle size distribution, total organic carbon content, soil bulk density, soil moisture content at field sampling, soil particle density, soil moisture content and water-drop-penetration-time (WDPT) at different matric potentials (SWRCC). The irregular presence of coal particles and different overburden sediments lead to spatial variability of studied properties, especially of total organic carbon content. In depth the average clay content, organic carbon content, water retention properties and soil water repellency (SWR) increased and correspondingly the density properties decreased. The distribution of SWR classes of the studied samples at the applied suctions during the drying process showed that the hydrophilicity of all samples at saturation persisted at suction pF 1.0 in 73% of the samples Most of the samples at suctions pF 1.7 and pF 2.0 were with slight and strong water repellency, but still there were hydrophilic samples 27% and 18%, correspondingly. At pF 2.5, which is often accepted as field capacity of fine textured soils, all samples were water repellent. The further drainage of the samples at pF >3.3 led to domination of extreme SWR and above pF3.7 all samples were extremely water repellent. Besides the unfavorable consequences of water repellency, the dry hydrophobic topsoil at the time of sampling formed a capillary barrier that reduced the evaporation and led to nearly 3 times higher water content stored in the subsoil.