Geophysical Research Abstracts, Vol. 6, 01835, 2004 SRef-ID: 1607-7962/gra/EGU04-A-01835 c European Geosciences Union 2004
ISOLATION AND CHEMICAL CHARACTERISTION OF ORGANIC COMPOUNDS FROM SANDY SOILS AND THEIR EFFECTS ON SOIL WATER REPELLENCY K.A. Mainwaring (1), S.H. Doerr (2), P. Douglas (1), C.T. Llewellyn (1), C.P. Morley (1) (1) Department of Chemistry, (2) Department of Geography, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK. (147427@swan.ac.uk)
\textbf{Water repellency} (or \textbf{hydrophobicity}) of soils can have serious environmental implications, including reduced seed germination and plant growth, accelerated surface runoff and soil erosion, and enhanced leaching of agrochemicals. It has been proposed that hydrophobic organic compounds, coating the normally \textbf{hydrophilic} (or \textbf{wettable}) soil minerals, are responsible. (However we have noted that the presence of such compounds does not always lead to water repellency). We have extracted, separated, and characterised by gas chromatography-mass spectrometry (GC-MS), organic compounds found in wettable and water repellent sandy soils from five countries (Australia, Greece, Portugal, the Netherlands and the United Kingdom). Although carboxylic acids ($C_{16}-C_{24})$, amides ($C_{14}-C_{24})$, alkanes ($C_{25}-C_{33})$, aldehydes/ketones ($C_{23}-C_{31})$ and complex ring-containing structures (e.g. cholesterols/sterols) were detected in all samples, there was a greater abundance of the higher molecular mass polar compounds (e.g. C$_{23/24 }$acids/amides and cholesterols/sterols) in the water repellent samples. We speculate that the presence of these higher molecular mass polar compounds in significant amounts,\textit{ together with} non-polar compounds, is necessary for water repellency to be exhibited. In order to investigate this hypothesis and also
the effects of functional group, molecular shape and packing efficiency on soil wettability we have studied the effect of applying organic compounds such as those found on soils i.e. long chain saturated and unsaturated (\textit{cis }and \textit{trans}) acids, a linear alkane and amide, linear and branched esters, a cholesterol and a sitosterol at different loadings, separately and as mixtures (acids and alkanes) to wettable acid washed sand. As speculated: (a) sand treated with polar organic compounds became water repellent; (b) sand treated with compounds bearing non-polar or weakly polar groups did not, \textbf{BUT} the combination of alkanes with carboxylic acids lead to much enhanced repellency when compared to the effect of acids alone. In addition, the ability of \textit{trans} but not \textit{cis} unsaturated acids to induce repellency suggests that molecules which can ``pack� well are more effective in causing repellency than those that do not.