breakdown of water repellency and solute transport through a hydrophobic soil

• Published in: Journal of hydrology (Amsterdam) 2000-05, Vol.231 (1-4), p.255-264
• Main Authors: Clothier, B.E, Vogeler, I, Magesan, G.N
• Format: Article
• Language: English
• Subjects: bulk density; clay; electrolytes; gravity; hydrophobicity; infiltration rate; magnesium; mycorrhizal fungi; organic matter; permeameters; potassium bromide; preferential flow; silt; solutes; steady flow; time domain reflectometry; water content; water repellent soils
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/s0022-1694(00)00199-2

Unsaturated infiltration into the Ramiha silt loam, an Andic Dystrochrept, follows the classic pattern. A rapid drop-off from a high flow rate, seemingly induced by capillary attraction, appears followed by an apparent steady-flow maintained by gravity at around 0.5 micrometer s-1. Beyond 100 min, however, the infiltration rate climbs nearly linearly to exceed 4 micrometer s-1 as the soil's water repellency breaks down. This is only evident after a period that might exceed the observer's attention span. The hydrophobicity in this case could be due to one, or a combination, of the many unusual characteristics of this soil-its low bulk density (0.8 Mg m-3), its strongly aggregated nature, the presence of mycorrhizal fungi, its high organic matter content (16.5%), or the presence of allophanic clay (4%). Our measurements of infiltration into undisturbed cores of Ramiha silt loam were made with disc permeameters set at the unsaturated pressure head of h0 = -40 mm. The permeameters contained a solution of electrolytic tracer (KBr) so that we could observe solute transport in this soil. Vertical three-wire rods for Time Domain Reflectometry (TDR) measurement were inserted directly through the base plate of the permeameter so that we could continuously monitor the soil's changing water content and resident concentration of electrolyte. The TDR measurements revealed the transient behaviour of fingered preferential flow into this soil during the breakdown of hydrophobicity. At the conclusion of the experiment, the soil cores were sectioned to permit measurement of the profiles in the resident concentration of the invading chemical. Near the surface, at the conclusion of the experiment, the resident concentration of bromide was found to be exactly that of the invading solution. So, despite the initial water repellency of the soil, the infiltrating bromide solution was subsequently able to invade the entire pore space-once the hydrophobicity had dissipated. Classic theory would then seem capable of describing solute transport after the effects of water repellency had faded.