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Physical and chemical aspects of water repellent soils affected by slashburning at Vancouver, British Columbia Henderson, Greg


The persistence and severity of water repellency in soils as affected by slashburning was examined in the municipal watersheds of Vancouver by the water drop penetration time (WDPT) and contact angle method, respectively. Also, hydrophobic materials were extracted from soil, separated by column adsorption chromatography, and analysed for functional groups by infrared absorption. Slashburning increased soil water repellency, as indicated by the WDPT method. Precise responses of soil repellency to slashburning were not consistent, as many site specific factors are involved, but increased repellency was evident in soil up to six years following burning. The severity of increased soil repellency caused by slashburning could not be assessed because contact angle determinations using the capillary rise equation and Darcy's Law were not reliable. The WDPT method was more consistent, reliable and simpler than the contact angle method, hence the WDPT method was considered best to indicate the presence of soil water repellency. A combination of polar and non-polar organic solvents, methanol and benzene respectively, extracted hydrophobic compounds from soil. The extractant separated into three fractions, using benzene and increasing proportions of acetone as eluting agents. Virtually all of the extract (90%) was recovered in Fraction I, indicating that the extraction was predominantly non-polar. Fraction II and III accounted for 4 and 6 per cent, respectively, of the applied extractables. Material in all Fractions induced repellency in wettable sand when 1 mg or more was applied to 5 gm of sand. Repellency was increased by heating the extracted materials in sand to 250°C for 10 minutes and was eliminated after heating to 300°C in inverse proportion to the mass applied. At 350°C, hydrophobic materials volatilized and the sand regained wettability. Analysis of an extraction by infrared absorption revealed that hydrophobic substances have hydrophilic and hydrophobic components. Adsorption of hydrophobic materials to soil particles is therefore likely initially by the hydrophilic end leaving the hydrophobic end of the organic molecule to form the outer surface thus preventing water from infiltrating. During slashburns, adsorption of hydrophobic materials is probably enhanced by optimization of close range van der Waals and London forces.

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