Water repellency and its role in forming preferred flow paths in soils

Abstract

The role of water repellency in forming preferred how paths, and in accelerating solute transport, is explained on the basis of extensive field measurements in a grass-covered, extremely water-repellent sandy soil in the coastal region of the Netherlands. It was shown that following wetting of the 9-cm-thick humous topsoil to around 20–25 vol.%, preferred how paths, or fingers, were formed in the sandy subsoil. The fingers started at the layer interface at 'weak' spots with a relatively low degree of potential water repellency, and protruded vertically into the sandy subsoil. Flow through the fingers caused water-repellent substances to be leached from the topsoil to the subsoil, progressively fixating the finger positions. Laboratory analysis revealed that wetting of dry, water-repellent soil between the vertically directed fingers is an extremely slow process compared with wetting of dry, non-water-repellent, or slightly water-repellent, soil. Evidence was presented that fingers at the field site disappeared during dry periods and recurred at the same location during new rain events. Vertical solute leaching down the fingers was greater by a factor of 3 than that estimated on a piston flow basis. The development of new simulation models capable of predicting water and solute transport through water-repellent soils is urgently needed.